研究者詳細 - 中嶋　敦

論文 - 中嶋　敦

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Tailoring Superatomic Stability with Transition Metals in Silicon Cages: Shrinking to M@Si₁₅ (M = Re, Os, Ir)

T. Ichikawa, K. Terasaka, A. Sasaki, A. Nakajima

Journal of Physical Chemistry Letters （American Chemical Society） 15 （ 46 ） 11678 - 11686 2024年11月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り

The design of materials with intriguing electronic properties is crucial for advancing nanoscale technologies, where precise control over atomic structure and electronic behavior is essential. Metal-encapsulating silicon cage superatoms (SAs) provide a new paradigm for molecular-scale material design, allowing fine-tuning of both structure and electronic characteristics. The formation of superatoms mimicking halogens, noble gases, and alkali metals has been well-studied, particularly with M@Si16, where early transition metals from groups 3 to 5 stabilize within a Si16 cage, achieving a 68-electron configuration. For late transition metals with excess electrons, a Si15 cage offers enhanced stability by fulfilling the 68-electron rule with one fewer Si atom. This research synthesizes Si15 cage-SAs with rhenium (Re) from group 7 and iridium (Ir) from group 9 on p-type and n-type organic substrates. The stability of Re@Si15 and Ir@Si15 is evaluated via oxidative reactivity with X-ray photoelectron spectroscopy and theoretical calculations, including osmium (Os) from group 8. Re@Si15-, Os@Si150, and Ir@Si15+ exhibit superatomic behaviors similar to halogens, noble gases, and alkali metals due to the 68-electron shell closure. Among them, Re@Si15- on p-type organic substrates shows superior electronic and geometric properties. These findings advance our understanding of M@Sin systems for transition metals, addressing longstanding questions about their properties at n = 15 and 16.

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Photoemission spectroscopy and microscopy for Ta@Si₁₆ superatoms and their assembled layers

M. Shibuta, T. Ohta, T. Kamoshida, K. Yamagiwa, H. Tsunoyama, T. Inoue, T. Masubuchi, A. Nakajima

Nanoscale （Royal Chemical Society） 16 （ 47 ） 21837 - 21846 2024年10月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 20403364

Superatoms (SAs) with specific compositions have the potential to significantly advance the field of nanomaterials science, leading to next-generation nanoscale functionalities. In this study, we fabricated assembled layers with tantalum metal-atom encapsulating silicon cage (Ta@Si16) SAs on an organic C60 substrate through deposition, and we characterized their electronic and optical properties by photoelectron spectroscopy and microscopy. The alkaline nature of Ta@Si16 SAs reveals their electronic behaviors, such as charge transfer and electromagnetic near-field sensing, through two-photon photoemission (2PPE) spectroscopy and microscopy with a femtosecond laser. The evolution of the work function for Ta@Si16 SAs on C60, observed by 2PPE spectroscopy, demonstrates charge transfer complexation between the topmost C60 layer and the first Ta@Si16 layer, consistent with the electron-donating alkaline characteristics of Ta@Si16 SAs. Specifically, a small amount of Ta@Si16 SA deposition leads to a dramatic increase in 2PPE intensity, attributable to electromagnetic near-field enhancements, suggesting applications as sensitizers for nonlinear imaging in photoemission microscopy. For the assembled Ta@Si16 SA layers, a plasmonic response of hνp = 17.9 eV is spectroscopically identified, including their valence and conduction band structures, and the plasmonic energetics are discussed in the context of metal doping in bulk silicon.

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Geometric and Electronic Properties of P Atom-Doped Al Nanoclusters: Alkaline-like Superatom of P@Al₁₂

M. Akutsu, K. Koyasu, K. Miyajima, M. Mitsui, T. Inoue, A. Nakajima

Journal of Physical Chemistry A （American Chemical Society） 128 （ 32 ） 6648 - 6657 2024年08月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 10895639

The geometric and electronic characteristics of phosphorus-atom doped aluminum nanoclusters, AlnPm (n = 7-17, m = 1 and 2), were investigated through a combination of experiments and theoretical calculations. The size dependences of the ionization energy (Ei) for AlnPm NCs exhibit a local minimum of 5.37 eV at Al12P1, attributed to an endohedral P@Al12 superatom (SA). This SA originates from an excess electron toward the 2P shell closing (40e), coexisting with an exohedral isomer featuring a vertex P atom. The stability of the endohedral P@Al12 is further enhanced in its cationic state compared to the exohedral isomer, when complexed with a fluorine (F) atom, forming an SA salt denoted as P@Al12+F- with an elevated Ei ranging from 6.42 to 7.90 eV. In contrast, for the anionic Al12P1-, the exohedral form is found to be more stable than the endohedral one using anion photoelectron spectroscopy and calculations. The geometric and electronic robustness of neutral P@Al12 SAs against electron donation and acceptance is discussed in comparison to rare-gas-like Si@Al12 SAs.

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Localized surface plasmon resonances of size-selected large silver nanoclusters (n = 70-100) soft-landed on a C₆₀ organic substrate

T. Inoue, K. Mizoguchi, M. Tokita, M. Shibuta, M. Nakaya, T. Eguchi, A. Nakajima

Physical Chemistry Chemical Physics （Royal Chemical Society） 26 （ 23 ） 16597 - 16602 2024年05月

研究論文（学術雑誌）, 最終著者, 責任著者, 査読有り, ISSN 14639076

Silver nanoclusters (Agn NCs) exhibit a remarkable optical property known as localized surface plasmon resonance (LSPR) in the visible to ultraviolet wavelengths. In this study, we address the size gap in LSPR responses between small NCs and nano-islands by synthesizing large Agn NCs with a countable number of atoms (n = 70-100) using a magnetron sputtering method, which were precisely size-selected and soft-landed onto substrates. The monodispersed Agn NCs were immobilized on a pre-decorated substrate with fullerene (C60) molecules, and their LSPR behaviors were characterized using two-photon photoemission (2PPE) spectroscopy. Due to the distinct polarization selectivity of incident light associated with LSPR, the intensity ratio between p- and s-polarized lights (Ip/Is) in 2PPE spectroscopy serves as a reliable indicator of LSPR and its structural correlations. From n = 70 to 100, the Ip/Is value gradually decreases as the cluster size increases. This decrease is attributed to the enhancement of s-polarized light (Is), indicating that large Agn NCs on a C60 substrate undergo a deformation from spherical to flattened geometries, particularly above approximately n = 55.

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Imaging of ultrafast photoexcited electron dynamics in pentacene nanocrystals on a graphite substrate

Shibuta M., Nakajima A.

Nanoscale （Royal Chemical Society） 16 （ 26 ） 12397 - 12405 2024年05月

研究論文（学術雑誌）, 査読有り, ISSN 20403364

Understanding molecular film growth on substrates and the ultrafast electron dynamics at their interface is crucial for advancing next-generation organic electronics. We have focused on studying the ultrafast photoexcited electron dynamics in nanoscale organic crystals of an aromatic molecule, pentacene, on a two-dimensional material of graphite substrate. Through the use of time-resolved two-photon photoelectron emission microscopy (2P-PEEM), we have visualized the ultrafast lateral evolution of photoexcited electrons. By resonantly tuning the incident photon to excite pentacene molecules, polarization-dependent 2P-PEEM has revealed that pentacene nanocrystals (sub- to several μm) on the substrate exhibit a preferential orientation, in which a molecular π-orbital contacts the substrate in a “lying flat” orientation, facilitating electron transfer to the substrate. The time-resolved 2P-PEEM captures the motion of excited electrons in a femto- to pico-second timescale, clearly imaging the ultrafast charge transfer and lateral expansion two-dimensionally on the graphite substrate. Moreover, we found that the lying-flat molecular orientation of pentacene nanocrystals is transformable into a “standing-up” one through gentle heating up to 50 °C. These experimental insights using time-resolved 2P-PEEM will be highly valuable in enhancing the photofunctionalities of organic electronic devices by controlled molecular deposition.

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Extended X-ray Absorption Fine Structure (EXAFS) Measurements on Alkali Metal Superatoms of Ta-Atom-Encapsulated Si₁₆ Cage

T. Inoue, T. Ina, H. Masai, N. Kondo, F. Matsui, T. Kinoshita, A. Nakajima

Journal of Physical Chemistry Letters （American Chemical Society） 15 （ 20 ） 5376 - 5381 2024年05月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 1948-7185

The silicon cage nanoclusters encapsulating a tantalum atom, termed Ta@Si16, exhibit characteristics of alkali metal “superatoms (SAs)”. Despite this conceptual framework, the precise structures of Ta@Si16 and Ta@Si16+ remain unclear in quantum calculations due to three energetically close structural isomers: C3v, Td, and D4d structures. To identify the geometrical structure of Ta@Si16 SAs, structural analysis was conducted using extended X-ray absorption fine structure (EXAFS) with a high-intensity monochromatic X-ray source, keeping anaerobic conditions. Focusing on “superordered” films, which constitute amorphous thin films composed solely of Ta@Si16 SAs, this analysis preserved locally ordered structures. Spectral comparisons between experimental and simulated Ta L3-edge EXAFS unveil that Ta@Si16 SAs on a substrate adopt a C3v-derived structure, while Si K-edge EXAFS introduces spectral ambiguity in structural identifications, attributed to both intracluster and intercluster scatterings. These findings underscore the significance of locally ordered structure analyses in understanding and characterizing novel nanoscale materials.

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Alkaline Earth Metal Superatom of W@Si₁₆: Characterization of Group 6 Metal Encapsulating Si₁₆ Cage on Organic Substrates

K. Terasaka, T. Kamoshida, T. Ichikawa, T. Yokoyama, M. Shibuta, M. Hatanaka, A. Nakajima

Journal of the American Chemical Society （American Chemical Society） 146 （ 14 ） 9605 - 9613 2024年03月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 00027863

Transition metal atom (M)-encapsulating silicon cage nanoclusters (M@Si16) exhibit a superatomic nature, depending on the central M atom owing to the number of valence electrons and charge state on organic substrates. Since M@Si16 superatom featuring group 4 and 5 transition metal atoms exhibit rare-gas-like and alkali-like characteristics, respectively, group 6 transition metal atoms are expected to show alkaline earth-like behavior. In this study, M@Si16, comprising a central atom from group 6 (MVI = Cr, Mo, and W) were deposited on C60 substrates, and their electronic and chemical stabilities were investigated in terms of their charge state and chemical reactivity against oxygen exposures. In comparison to alkali-like Ta@Si16, the extent of charge transfer to the C60 substrate is approximately doubled, while the oxidative reactivity is subdued for MVI@Si16 on C60, especially for W@Si16. The results show that a divalent state of MVI@Si162+ appears on the C60 substrate, which is consistently calculated to be a symmetrical cage structure of W@Si162+ in C3v, revealing insights into the “periodic law” of M@Si16 superatoms pertaining to the characteristics of alkaline earth metals.

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Oxidative Activation of Small Aluminum Nanoclusters with Boron Atom Substitution prior to Completing the Endohedral B@Al₁₂^- Superatom

T. Inoue, M. Hatanaka, A. Nakajima

Journal of the American Chemical Society （American Chemical Society） 145 （ 42 ） 23088 - 23097 2023年10月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 00027863

Elemental substitution and doping validate the optimization of chemical and physical properties of functional materials, and the composition ratio of the substituting atoms generally determines their properties by changing their geometric and electronic structures. For atomically precise nanoclusters (NCs) consisting of countable atom aggregates, the composition can be controlled accurately to provide an ideal model to study the heteroatom substitution effects. Since aluminum (Al) and boron (B) both belong to group 13 in the periodic table, the effect of B atom substitution on Aln NCs can be investigated while maintaining the total number of valence electrons in AlnBm NCs. In this study, oxidative reactivities of small Al NCs with B atom substitution are studied for AlnBm NCs (m = 1, n = 6-14 and m = 2, n = 11) supported on organic surfaces by using X-ray photoelectron spectroscopy and oxygen molecule (O2) exposure measurements. Before completing the endohedral B@Al12- superatomic NC, one B atom substitution in Al NCs (AlnB) enhances oxidative reactivities 3-20 times compared to those of Aln+1, particularly for n ≤ 11. When one Al atom of Al12B is further substituted by a B atom to form Al11B2, the reactivity drastically increases (6.6 × 102 times), showing that the B atom substitution makes the NC chemically active or inactive geometrically depending on the exohedral or endohedral site for the B atom in the Al NC. In addition, density functional theory calculations show that the electronegative B atom contributes to forming a locally positive Al site to facilitate O2 adsorption except in Al12B, in which the B atom is geometrically shielded by the surface of the Al12 cage in B@Al12

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Two-Photon Photoemission Spectroscopy and Microscopy for Electronic and Plasmonic Characterizations of Molecularly Designed Organic Surfaces

M. Shibuta, A. Nakajima

Journal of Physical Chemistry Letters （American Chemical Society） 14 （ 13 ） 3285 - 3295 2023年03月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 1948-7185

Functional surfaces decorated with organic molecules and/or nanoclusters (NCs) composed of several tens of atoms are promising for use in future photoelectronic substrates, whose functionalities are governed by molecular local electronic/plasmonic excitations at the interfaces. Here, we combine two-photon photoemission spectroscopy (2P-PES) and microscopy (2P-PEEM) to investigate the local excited-state dynamics at organic surfaces functionalized with NCs. The 2P-PES and 2P-PEEM for organic fullerene (C60) layers on graphite and Au substrates demonstrated photophysical characterization of electronic and plasmonic properties, including propagating surface plasmon polaritons (SPPs). The SPP propagation at the Au interface buried by overlayered C60 can be visualized by Agn NC deposition, which enhances plasmon-induced hot electrons, where the threshold number of Ag atoms (n ≥ 9) for the plasmonic response is revealed by the size dependence of 2P-PES for Agn NCs on C60 layers.

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Bridging the gas and condensed phases for metal-atom encapsulating silicon- and germanium-cage superatoms: electrical properties of assembled superatoms

T. Yokoyama, A. Nakajima

Physical Chemistry Chemical Physics （Royal Chemical Society） 25 （ 14 ） 9738 - 9752 2023年03月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 14639076

With the development of nanocluster (NC) synthesis methods in the gas phase, atomically precise NCs composed of a finite number of metal and semiconductor atoms have emerged. NCs are expected to be the smallest units for nanomaterials with various functions, such as catalysts, optoelectronic materials, and electromagnetic devices. The exploration of a stable NC called a magic number NC has revealed a couple of important factors, such as a highly symmetric geometric structure and an electronic shell closure, and a magic number behavior is often enhanced by mixing additional elements. A synergetic effect between geometric and electronic structures leads to the formation of chemically robust NC units called superatoms (SAs), which act as individual units assembled as thin films. The agglomeration of non-ligated bare SAs is desirable in fabricating the assembled SAs associated with intrinsic SA nature. The recent development of an intensive pulsed magnetron sputtering method opens up the scalable synthesis of SAs in the gas phase, enabling the fabrication of SA assembly coupled with the non-destructive deposition of a soft-landing technique. This perspective describes our recent progress in the investigation of the formation of binary cage SA (BCSA) assembled thin films composed of metal-atom encapsulating silicon-cage SAs (M@Si16) and germanium-cage SAs (M@Ge16), with a focus on their electrical properties associated with a conduction mechanism toward the development of new functional nanoscale materials.

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Superatom Generation and Deposition of Alkali-like Ta@Si₁₆ and Halogen-like Al₁₃ via Atomic Aggregation

T. Inoue, T. Ichikawa, K. Matsunaga, R. Birumachi, K. Nakamura, A. Nakajima

Journal of the Electrochemical Society （Electrochemical Society） 169 （ 10 ） 102511 2022年10月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 00134651

Surface modification with uniform nanostructures is a promising way to fabricate functionalized surface, and the synthetic electrode functionalization can be performed by depositing atomically precise nanoclusters with a specific number of atoms and compositions. During atomic nucleation and growth in gas phase, atomic aggregates exhibit novel electronic properties, mimicking atoms at a specific number of atoms and composition, which is called superatom (SA). Tantalum-encapsulating Si16 cage nanoclusters (Ta@Si16) and aluminum 13-mer (Al13) are promising SAs for designing their assembled materials with tunable properties, where one-electron-excessive Ta@Si16 and one-electron-deficient Al13 exhibit alkali- and halogen-like characteristics, respectively, owing to the electron shell closing. In this study, with an intense nanocluster ion source, Ta@Si16+ and Al13− were generated during the formation of TaSin + cations and Aln − anions, where Ta@Si16+ was remarkably generated compared with the neighboring species. SA deposition provided uniformly decorated surfaces exhibiting nanofunctional properties superior to those obtained by conventional atom deposition. With mass spectrometry, Ta@Si16+ and Al13− SAs were size-selectively soft-landed on the substrate surfaces predeposited with n-type fullerene (C60) and p-type hexa-tert-butyl-hexa-peri-hexabenzocoronene (HB-HBC, C66H66). The electronic states of Ta@Si16 and Al13 SAs immobilized on the organic substrates were characterized by X-ray photoelectron spectroscopy and oxidative reactivities under O2 exposure.

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Spectroscopic imaging of photoexcited states at a polycrystalline copper metal surface via two-photon photoelectron emission microscopy

M. Shibuta, A. Nakajima

Chemical Physics Letters （Elsevier） 804 139909 2022年07月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 00092614

The local photoexcited surface states of a polycrystalline-copper (Cu) substrate were observed via two-photon photoelectron emission microscopy (2P-PEEM) using a femtosecond laser. The 2P-PEEM results revealed local distributions of the photoexcited electrons, strongly depending on the crystal domains. With the energy-filtered images, the local 2P-photoemission spectra were selectively obtained for (1 1 1)- and (1 1 0)/(1 0 0)-crystal domains. Furthermore, the 2P-PEEM could visualize propagating Cu surface plasmon polaritons with wavevector of 7.61 × 106 m−1. These results indicate that the 2P-PEEM technique is a promising tool for studying the distribution of nonequivalent photoexcited surface states for various inhomogeneous functional nanomaterials.

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Molecularly Designed Cluster-Surface Interaction for Halogen-like and Alkali-like Metal-Encapsulating Silicon Cage Superatoms on n- and p-Type Organic Substrates

T. Kamoshida, M. Shibuta, T. Ohta, T. Eguchi, A. Nakajima

Journal of Physical Chemistry C （American Chemical Society） 126 （ 26 ） 10889 - 10899 2022年06月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 19327447

Metal-encapsulating Si16cage clusters (M@Si16) are promising superatoms (SAs) for designing tunable properties for their assembled materials by changing the central metal atom: halogen-like, rare-gas-like, and alkali-like characteristics appear for the central metal atom of groups 3, 4, and 5, respectively. To fabricate SA assemblies, metal-encapsulating M@Si16SAs (M = Lu, Hf, and Ta) must be controllably immobilized on a substrate. Substrates decorated with organic molecules can facilitate optimization of a cluster-surface interaction because the molecular local interactions between SAs and predeposited organic molecules govern the electronic properties through molecular complexation. In this study, M@Si16SAs are size-selectively soft-landed on organic substrates deposited with n-type fullerene (C60) and p-type hexa-tert-butyl-hexa-peri-hexabenzocoronene (HB-HBC, C66H66), and the electronic states of M@Si16on the organic substrates are characterized by X-ray and ultraviolet photoelectron spectroscopy. On the C60substrate, all M@Si16are fixed to be cationic, forming M@Si16+C60-via a charge transfer interaction, while on an HB-HBC substrate, M@Si16-HB-HBC+(M = Lu and Hf) is formed with anionic M@Si16-. Together with density functional theory calculations, the charge preference of the M@Si16SA is examined based on its chemical stability against O2gas exposure; Lu@Si16on HB-HBC is more robust toward O2than that on C60, while Ta@Si16on HB-HBC is less robust than that on C60. Depending on the SA properties, an appropriate selection of organic molecules for deposition provides a molecular designer concept for forming SA-assembled nanomaterials through the cluster-surface interaction.

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Al₁₃^- and B@Al₁₂^- superatoms on a molecularly decorated substrate

M. Shibuta, T. Inoue, T. Kamoshida, T. Eguchi, A. Nakajima

Nature Communications （Nature Portfolio） 13 （ 1 ） 1336 2022年03月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り

Aluminum nanoclusters (Aln NCs), particularly Al13− (n = 13), exhibit superatomic behavior with interplay between electron shell closure and geometrical packing in an anionic state. To fabricate superatom (SA) assemblies, substrates decorated with organic molecules can facilitate the optimization of cluster–surface interactions, because the molecularly local interactions for SAs govern the electronic properties via molecular complexation. In this study, Aln NCs are soft-landed on organic substrates pre-deposited with n-type fullerene (C60) and p-type hexa-tert-butyl-hexa-peri-hexabenzocoronene (HB-HBC, C66H66), and the electronic states of Aln are characterized by X-ray photoelectron spectroscopy and chemical oxidative measurements. On the C60 substrate, Aln is fixed to be cationic but highly oxidative; however, on the HB-HBC substrate, they are stably fixed as anionic Aln− without any oxidations. The results reveal that the careful selection of organic molecules controls the design of assembled materials containing both Al13− and boron-doped B@Al12− SAs through optimizing the cluster–surface interactions.

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Size-Dependent Oxidative Stability of Silicon Nanoclusters Mixed with a Tantalum Atom

M. Shibuta, M. Huber, T. Kamoshida, K. Terasaka, M. Hatanaka, G. Niedner-Schatteburg, A. Nakajima

Journal of Physical Chemistry C （American Chemical Society） 126 （ 9 ） 4423 - 4432 2022年03月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 19327447

Metal-encapsulating silicon cage (M@Si16) nanoclusters (NCs) are promising superatoms (SAs) as function-tunable nanomaterials, which exhibit a superior chemical stability owing to electronic and geometric closures. Here, we examine how the superatomic nature of an alkali-like Ta@Si16SA is staggered by the variation of the number of Si atoms in TaSinNCs (n = 6, 8, 12, 15, 17, and 18) and subsequent immobilization on a C60fullerene substrate. Using X-ray photoelectron spectroscopy, the size dependence of chemical robustness of TaSinNCs on C60against O2exposures is quantitatively evaluated: In addition to the most outstanding stability of a Ta@Si16SA as compared to TaSi15, some enhanced stability is also observed at Ta@Si17, which couples in a Si-adatom structure of (Ta@Si16)-Si to the support. While oxidative reactivities of TaSinare gradually suppressed from small NCs (n = 6) to larger ones (n = 18) with increasing number of Si atoms, the results show that (1) an enclosing Si cage around a Ta atom is completed at Ta@Si16on C60and (2) Ta@Si17NCs are robust against O2oxidation with assistance from the stability of Ta@Si16SA despite the Si-adatom.

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Enhanced oxygen reduction activity of size-selected platinum subnanocluster catalysts: Pt_n (n = 3-9)

A. Ohnuma, K. Takahashi, H. Tsunoyama, T. Inoue, P. Zhao, A. Velloth, M. Ehara, N. Ichikuni, M. Tabuchi, A. Nakajima

Catalysis Science and Technology （Royal Society of Chemistry (RSC)） 12 （ 5 ） 1400 - 1407 2022年01月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 20444753

Nanoclusters (NCs) are promising candidates to improve catalytic activity despite the controversial size specificity, because atomicity is a crucial parameter that determines the activity of platinum (Pt) NCs. This paper demonstrates the enhanced catalytic activity based on the charge redistribution in Pt sub-NCs containing three to nine Pt atoms (Ptn; n = 3-9) on a glassy carbon substrate. The sub-NCs show 1.6-2.2 times higher activity than the standard Pt/C catalysts with a Pt crystallite diameter of 2 nm. The geometric structures are identified using rigorous structure analyses by X-ray absorption fine structure spectroscopy and density functional theory, and the activity origin within the supported Pt sub-NCs is theoretically discussed from viewpoints of energetics for reaction intermediates in the electrochemical processes. It should be possible to use sub-NCs in future fuel cell technologies as an active catalyst with a high atomic efficiency.

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Platinum nanocluster catalysts supported on Marimo carbonviascalable dry deposition synthesis

N. Hirata, Y. Katsura, H. Gunji, M, Tona, K, Tsukamoto, M, Eguchi, T. Ando, A. Nakajima

RSC Advances （Royal Chemical Society） 11 （ 62 ） 39216 - 39222 2021年12月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り

The development of efficient fuel cells greatly promotes reducing the consumption of fossil energy, and it is crucial to enhance the platinum (Pt) catalytic activity by optimizing both the nanoparticle size and support effect. In this study, we generate a smaller and uniform size of naked Pt nanocluster (NC) catalystsviaa dry process in the gas phase, and using the direct powder embedded trapping method, the Pt NCs arespatioselectivelysupported on Marimo carbon (MC) that comprises a high density of carbon nanofilaments. At a minimum Pt loading of 0.05 mgPtcm−2for both the anode and cathode in a single cell configuration, a membrane-electrode assembly (MEA) formed by the dry-deposition Pt-NC(d)/MC exhibits superior power density (rated) of 0.105 gPtkW−1at a current density of 1.2 A cm−2, which is better output power density than the conventional MEA formed by Pt catalystsviaa wet process. The origin of the improved performance is investigated using transmission electron microscopy; dry-deposition Pt catalysts that are monodispersely loaded on the outer surface of MC can facilitate not only the gas reaction but also leaving the generated water. The present results demonstrate that the dry deposition of Pt NCs on MC can be used as a scalable catalyst synthesis method to reduce the Pt loading.

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Electrical Conduction of Superatom Thin Films Composed of Group-V-Metal-Encapsulating Silicon-Cage Nanoclusters

T. Yokoyama, T. Chiba, N. Hirata, M. Shibuta, A. Nakajima

Journal of Physical Chemistry C （American Chemical Society） 125 （ 33 ） 18420 - 18428 2021年08月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 19327447

Nanocluster assembled films have attracted great interest for designing nanostructured materials with unique electromagnetic properties through a bottom-up approach. Superatoms of group-5 metals (MV = V, Nb, and Ta) encapsulating silicon (Si) cage nanoclusters (MV@Si16), which are synthesized by high-power impulse magnetron sputtering technique, can be efficiently generated to form assembled films. Temperature-dependent current-voltage (I-V) characteristics of the MV@Si16 assembled films revealed that the electrical conduction mechanism is not band transport but hopping transport with Efros-Shklovskii variable range hopping for all central MV atoms. The results show that electrons involved in conduction are strongly correlated to localized electronic states; this correlation arises because of not only the geometrical disordering in noncrystalline assembled films but also the electronic nature of a superatomic 1H orbital with multiple nodes. The localization length depends on the specific MV and is several times the radius of MV@Si16 (0.45 nm); it is the largest for Ta (2.2 nm) and the smallest for Nb (0.8 nm), revealing a periodicity of superatoms.

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Confined Hot Electron Relaxation at the Molecular Heterointerface of the Size-Selected Plasmonic Noble Metal Nanocluster and Layered C₆₀

M. Shibuta, K. Yamamoto, T. Ohta, T. Inoue, K. Mizoguchi, M. Nakaya, T. Eguchi, A. Nakajima

ACS Nano （American Chemical Society） 15 （ 1 ） 1199 - 1209 2021年01月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 19360851

The plasmonic response of metallic nanostructures plays a key role in amplifying photocatalytic and photoelectric conversion. Since the plasmonic behavior of noble metal nanoparticles is known to generate energetic charge carriers such as hot electrons, it is expected that the hot electrons can enhance conversion efficiency if they are transferred into a neighboring molecule or semiconductor. However, the method of transferring the energized charge carriers from the plasmonically generated hot electrons to the neighboring species remains controversial. Herein, we fabricated a molecularly well-defined heterointerface between the size-selected plasmonic noble-metal nanoclusters (NCs) of Agn (n = 3-55)/Aun (n = 21) and the organic C60 film to investigate hot electron generation and relaxation dynamics using time-resolved two-photon photoemission (2PPE) spectroscopy. By tuning the NC size and the polarization of the femtosecond excitation photons, the plasmonic behavior is characterized by 2PPE intensity enhancement by 10-100 times magnitude, which emerge at n ≥ 9 for Agn NCs. The 2PPE spectra exhibit contributions from low-energy electrons forming coherent plasmonic currents and hot electrons with an excitation energy up to photon energy owing to two-photon excitation of an occupied state of the Agn NC below the Fermi level. The time-resolved pump-probe measurements demonstrate that plasmon dephasing generates hot electrons which undergo electron-electron scattering. However, no photoemission occurs via the charge transfer state forming Agn+C60- located in the vicinity of the Fermi level. Thus, this study reveals the mechanism of ultrafast confined hot electron relaxation within plasmonic Agn NCs at the molecular heterointerface.

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Interfacial oxidation of Ta-encapsulating Si₁₆ cage superatoms (Ta@Si₁₆) on strontium titanate substrates

M. Shibuta, R. Takano, A. Nakajima

Journal of Physical Chemistry C （American Chemical Society） 124 （ 51 ） 28108 - 28115 2020年12月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 19327447

Nanocluster immobilization on an oxide surface is fundamentally important for forming nanocluster-assembled layered materials. We fabricated multilayered superatom films made of tantalum-encapsulating Si16 cage nanocluster (Ta@Si16) on a strontium titanate substrate (SrTiO3; STO) by the soft landing of size-selective Ta@Si16. X-ray photoelectron spectroscopy (XPS) revealed that Ta@Si16 survived at the interfacial oxide layer without oxidizing the central Ta atom, and pure Ta@ Si16 layers were formed successively on the modified Ta@Si16/STO interfacial layer. The Ta@Si16 superatoms in the multilayered Ta@Si16 showed high chemical robustness against O2 exposure, although the topmost Ta@Si16 surface layer, including the central Ta atom, completely oxidized in ambient O2 over several days. XPS depth analysis showed that Ta@Si16 oxide formation was limited only at the topmost single layer, revealing that the middle Ta@Si16 layers sandwiched between the top and bottom were protected by the formation of interfacial oxides.

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Occupied and Unoccupied Levels of Half-Fluorinated and Perfluorinated Rubrene Thin Films Probed by One- And Two-Photon Photoemission

T. Inoue, M. Shibuta, T. Suzuki, A. Nakajima

Journal of Physical Chemistry C （American Chemical Society） 124 （ 23 ） 12409 - 12416 2020年05月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 19327447

We measured occupied and unoccupied energy levels of half-fluorinated and perfluorinated rubrenes (C42F14H14, F14-RUB, and C42F28, PF-RUB) thin films prepared on a highly oriented pyrolytic graphite substrate by a combination of one- and two-photon photoemission spectroscopies. It is revealed that the energy levels near the Fermi level (EF) decrease when the fluorination degree increases, where the energies of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are located at EF - 1.83 eV and EF + 0.9 eV for F14-RUB and EF - 2.51 eV and EF + 0.6 eV for PF-RUB, respectively. The HOMO-LUMO gaps corresponding to the energy for the creation of free photocarriers in molecular films - known as "transport gaps"- are evaluated by 2.7 eV for F14-RUB and 3.1 eV for PF-RUB.

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Visualization of Surface Plasmons Propagating at the Buried Organic/Metal Interface with Silver Nanocluster Sensitizers

K. Yamagiwa, M. Shibuta, A. Nakajima

ACS Nano （American Chemical Society） 14 （ 2 ） 2044 - 2052 2020年01月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 19360851

© 2020 American Chemical Society. Visualization of surface plasmon polariton (SPP) propagation at dielectric/metal interfaces is indispensable in providing opportunities for the precise designing and controlling of the functionalities of future plasmonic nanodevices. Here, we report the visualization of SPPs propagating along the buried organic/metal interface of fullerene (C60)/Au(111), through dual-colored two-photon photoemission electron microscopy (2P-PEEM) which precisely visualizes the SPP propagation of plasmonic metal nanostructures. Although SPPs excited by near-infrared photons at the few monolayer C60/Au(111) interface are clearly visualized as interference beat patterns between the SPPs and incident light, faithfully reflecting SPP properties modulated by the overlayer, photoemission signals are suppressed for thicker C60 films, due to less valence electrons participating in 2P-photoemission processes. With the use of silver (Agn (n = 21 and 55)) nanoclusters, which exhibit enhancement of overall photoemission intensities due to localized surface plasmons functioning as SPP sensitizers, it is revealed that the 2P-PEEM is applicable to the imaging of SPPs for thick C60/Au(111) interfaces, where SPP properties are hardly modulated by the added small amount (∼0.1 monolayer) of Agn sensitizers.

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Highly Dispersive Nearly Free Electron Bands at a 2D-Assembled C₆₀ Monolayer

M. Shibuta, K. Yamamoto, H. Guo, J. Zhao, A. Nakajima

Journal of Physical Chemistry C （American Chemical Society） 124 （ 1 ） 734 - 741 2019年12月

研究論文（学術雑誌）, 最終著者, 責任著者, 査読有り, ISSN 19327447

Superatomic molecular orbitals (SAMOs), which are atom-like diffuse orbitals formed at a molecule, play an important role in controlling the electronic functionality at one-to three-dimensional (1D-3D) assemblies of organic nanomaterials because they form highly dispersive and nearly free electron (NFE) bands, such as a bulk metal, by mixing their wavefunctions with neighbors. Herein, we identify a series of delocalized SAMOs at a two-dimensional assembled C60 fullerene monolayer utilizing resonant angle-resolved two-photon photoemission spectroscopy and theoretical calculations. SAMOs exhibit distinct NFE band dispersion characteristics to be hybridized between the diffuse orbitals at a 2D-assembly, unlike the well-known frontier unoccupied π∗ orbitals with localized wave functions at the carbon framework of C60. Density functional theory calculations for the NFE bands quantitatively reproduce the experimental observations including their energies and effective electron masses. These NFE bands comprising SAMOs above the Fermi level dominate charge transfer or photofunctional properties at the 1D-3D molecular assemblies, particularly when the band energies are lowered through a doping strategy.

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Vibrational Spectra of Thiolate-Protected Gold Nanocluster with Infrared Reflection Absorption Spectroscopy: Size- And Temperature-Dependent Ordering Behavior of Organic Monolayer

T. Yokoyama, N. Hirata, H. Tsunoyama, T. Eguchi, Y. Negishi, A. Nakajima

Journal of Physical Chemistry C （American Chemical Society） 124 （ 1 ） 363 - 371 2019年12月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 19327447

Vibrational spectra of thiolate-protected gold nanoclusters, prepared in a monolayer manner using the Langmuir-Blodgett method, were measured by means of infrared reflection absorption spectroscopy (IRAS). A transferred monolayer of gold nanoclusters ligated by dodecanethiolate or 2-phenylethane-1-thiolate onto a single-crystal gold (Au) surface of Au(111) exhibits worthy IRAS spectra that reveal temperature-dependent behaviors from 100 to 340 K as well as comprehensive peak assignments based on density functional theory calculations: the conformation change in ligands between all trans and gauche defect forms with temperature. In addition to the temperature dependence, the cluster size dependence of alkyl and phenyl moieties is discussed, compared with the IRAS spectra of the corresponding self-assembled monolayers (SAMs) on Au(111). Ligands spreading three-dimensionally from the Au core determine the coordination structure of the ligated Au nanoclusters.

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Enhanced oxygen reduction activity of platinum subnanocluster catalysts through charge redistribution

H. Tsunoyama, A. Ohnuma, K. Takahashi, A. Velloth, M. Ehara, N. Ichikuni, M. Tabuchi, A. Nakajima

Chemical Communications （Royal Society of Chemistry） 55 （ 84 ） 12603 - 12606 2019年09月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 13597345

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The stability for binary Al₁₂X nanoclusters (X = Sc and Ti): Superatom or Wade’s polyhedron

H. Tsunoyama, M. Akutsu, K. Koyasu, A. Nakajima

J. Phys. Condensed Matter （IOP Publishing） 30 （ 49 ） 494004 2018年11月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り

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Formation of Highly Ordered Semiconducting Anthracene Monolayer Rigidly Connected to Insulating Alkanethiolate Thin Film

Eguchi T., Hirata N., Shibuta M., Tsunoyama H., Nakajima A.

Journal of Physical Chemistry C （Journal of Physical Chemistry C） 122 （ 45 ） 26080 - 26087 2018年11月

研究論文（学術雑誌）, 査読有り, ISSN 19327447

© 2018 American Chemical Society. The formation and molecular structure of a self-assembled monolayer (SAM) of anthracene-substituted alkanethiol on Au(111) have been investigated by scanning tunneling microscopy and infrared reflection absorption spectroscopy. A clean and well-ordered SAM composed of densely packed "standing-up" molecules is formed by a wet-chemical process in air, followed by thermal annealing in vacuum. In the SAM, anthracene moieties are arranged in a bulklike in-plane herringbone structure and keep their crystalline ordering above room temperature, which is in contrast to the less ordering in Au nanoclusters ligated by the anthracene-alkanethiolates. In addition, hexagonal arrangement of anthracene dimer is found on the surface of the SAM. These structures remain after annealing at 400 K. Scanning tunneling spectroscopy performed on the surface shows an energy gap of ∼3.6 eV, similar to the band gap of bulk anthracene. The above results demonstrate that the highly ordered semiconducting anthracene monolayer is successfully formed on the insulating alkanethiolate thin film and has high thermal stability due to a strong chemical bonding across the interface.

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Liquid-phase catalysis by single-size palladium nanoclusters supported on strontium titanate: Size-specific catalysts for Suzuki-Miyaura coupling

H. Tsunoyama, H. Ito, M. Komori, R. Kobayashi, M. Shibuta, T. Eguchi, A. Nakajima

Catalysis Science and Technology （Royal Chemical Society） 8 （ 22 ） 5827 - 5834 2018年10月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 20444753

Metal nanoclusters comprising several to tens of atoms are strong candidates for advanced catalysis as they are highly size-specific, with unique chemical properties, which were revealed by gas-phase molecular beam experiments. The size-specificity of nanoclusters makes them potentially suitable for fabricating tailor-made catalysts optimized by chemical composition as well as size parameters. To exploit their full catalytic potential, complete understanding of size specificity in catalysis under realistic reaction conditions is essential, together with a suitable method for fabricating single-size nanocluster catalysts. We have demonstrated herein aqueous-phase catalysis by single-size palladium nanoclusters supported on a well-defined strontium titanate surface for the Suzuki-Miyaura coupling reaction, showing that a 13-mer exhibits highly size-specific activity, as well as a general increase in catalytic activity as the nanocluster size decreases. By comparison with the reactivity of heteroatom doped nanoclusters, the relationship between the size-specific activity and site-specific charge distribution is revealed by evaluating electronic structures using X-ray photoelectron spectroscopy.

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Nitric oxide oxidation of a Ta encapsulating Si cage nanocluster superatom (Ta@Si₁₆) deposited on an organic substrate; A Si cage collapse indicator

M. Shibuta, T. Niikura, T. Kamoshida, H. Tsunoyama, A. Nakajima

Physical Chemistry Chemical Physics （Royal Chemical Society） 20 （ 41 ） 26273 - 26279 2018年10月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 14639076

© 2018 the Owner Societies. The chemical reaction kinetics of an alkali-like superatom comprising a tantalum encapsulating Si16 cage nanocluster (Ta@Si16) deposited on an n-type organic substrate composed of overlayered C60 fullerene upon exposure to nitric oxide (NO) as a reactive gas are investigated. Core level X-ray photoelectron spectroscopy reveals that Ta@Si16 oxidation with NO proceeds stepwise from the outer Si16 cage to the central Ta atom; during the initial stage, NO is dissociatively chemisorbed by the cage surface of Ta@Si16 without penetrating the cage, while under extreme reaction conditions, the collapse of the Si16 cage leads to NO oxidation of the central Ta atom. In particular, molecular NO adsorption is associated with Ta oxidation only after the collapse of the Si16 cage of Ta@Si16. The reaction kinetics of M@Si16 with NO in the earlier stages of oxidation are discussed in conjunction with density functional theory calculations. Due to the superatomic nature of the shell closure with valence electrons coupled with metal encapsulation, surface oxidation of the caged Si in Ta@Si16 takes place gently compared to that of a naked Si surface, with molecularly physisorbed NO functioning as an indicator of Si cage collapse.

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Oxidative reactivity of alkali-like superatoms of group 5 metal-encapsulating Si₁₆ cage nanoclusters

M. Shibuta, T. Kamoshida, T. Ohta, H. Tsunoyama, A. Nakajima

Communications Chemistry （Springer Nature） 1 （ 1 ） 50 2018年09月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り

© 2018, The Author(s). It is crucial to control the reactivity of surface silicon atoms for applications in miniaturized silicon-based nanodevices. Here we demonstrate that reactive silicon atoms are made unreactive by forming a Si16 cage that encapsulates a metal atom. Specifically, group 5 metal-encapsulating Si16 nanoclusters (M@Si16: M = V, Nb, and Ta) exhibit alkali-like superatomic behavior on n-type C60 substrates, where charge transfer between M@Si16 and C60 satisfies the 68-electron shell closure as M@Si16+. The oxidation properties of M@Si16+ are investigated by X-ray photoelectron spectroscopy, revealing that the chemical stability of the caged silicon surface towards oxygen is enhanced by a factor of 104 compared to a crystalline silicon surface, and that M@Si16 are oxidized stepwise from the outer Si16 cage to the central metal atom. While the nanoclusters share a common Si16 cage, their chemical robustness depends on a superatomic “periodicity” (Ta@Si16 > V@Si16 > Nb@Si16) which is explained by the electron density distributions of M@Si16 investigated by DFT calculations.

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Characterization of floating-gate memory device with thiolate-protected gold and gold-palladium nanoclusters

T. Yokoyama, N. Hirata, H. Tsunoyama, Y. Negishi, A. Nakajima

AIP Advances （American Institute of Physics） 8 （ 6 ） 065002 2018年06月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 2158-3226

The floating-gate memory characteristics of thiolate-protected gold (Au:SR) and palladium doped Au (AuPd:SR) nanoclusters, Au25(SR)18, Au24Pd(SR)18, and Au38(SR)24 (R = C12H25), were investigated by capacitance-voltage (C-V) measurements in vacuum. Monolayer films of Au:SR nanoclusters were formed as floating-gate memory layers on p-type Si substrates by the Langmuir-Schaefer method with surface pressure - area (π-A) isotherm measurements. A fluoropolymer (CYTOP, ∼15 nm thick) was spin-coated on top to form a hydrophobic insulating layer. Using an Au pad (∼40 nm thick) as the gate electrode, C-V measurements exhibit clockwise hysteresis curves originating from the Au:SR and AuPd:SR nanoclusters against the reference measured in each sample, and the hysteresis widths were dependent on the composition and sizes of the Au:SR nanoclusters. The positive and negative voltage shifts in the hysteresis can be explained in terms of electronic structures in Au:SR and AuPd:SR-based devices.

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Synthesis and Characterization of Metal-Encapsulating Si₁₆ Cage Superatoms

H. Tsunoyama, M. Shibuta, M. Nakaya, T. Eguchi, A. Nakajima

Accounts of Chemical Research （American Chemical Society） 51 （ 8 ） 1735 - 1745 2018年05月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 00014842

Nanoclusters, aggregates of several to hundreds of atoms, have been one of the central issues of nanomaterials sciences owing to their unique structures and properties, which could be found neither in nanoparticles with several nanometer diameters nor in organometallic complexes. Along with the chemical nature of each element, properties of nanoclusters change dramatically with size parameters, making nanoclusters strong potential candidates for future tailor-made materials; these nanoclusters are expected to have attractive properties such as redox activity, catalysis, and magnetism. Alloying of nanoclusters additionally gives designer functionality by fine control of their electronic structures in addition to size parameters. Among binary nanoclusters, binary cage superatoms (BCSs) composed of transition metal (M) encapsulating silicon cages, M@Si16, have unique cage structures of 16 silicon atoms, which have not been found in elemental silicon nanoclusters, organosilicon compounds, and silicon based clathrates. The unique composition of these BCSs originates from the simultaneous satisfaction of geometric and electronic shell-closings in terms of cage geometry and valence electron filling, where a total of 68 valence electrons occupy the superatomic orbitals of (1S)2(1P)6(1D)10(1F)14(2S)2(1G)18(2P)6(2D)10 for M = group 4 elements in neutral ground state. The most important issue for M@Si16 BCSs is fine-tuning of their characters by replacement of the central metal atoms, M, based on one-by-one adjustment of valence electron counts in the same structure framework of Si16 cage; the replacement of M yields a series of M@Si16 BCSs, based on their superatomic characteristics. So far, despite these unique features probed in the gas-phase molecular beam and predicted by quantum chemical calculations, M@Si16 have not yet been isolated.In this Account, we have focused on recent advances in synthesis and characterizations of M@Si16 BCSs (M = Ti and Ta). A series of M@Si16 BCSs (M = groups 3 to 5) was found in gas-phase molecular beam experiments by photoelectron spectroscopy and mass spectrometry: formation of halogen-, rare-gas-, and alkali-like superatoms was identified through one-by-one tuning of number of total valence electrons. Toward future functional materials in the solid state, we have developed an intensive, size-selected nanocluster source based on high-power impulse magnetron sputtering coupled with a mass spectrometer and a soft-landing apparatus. With scanning probe microscopy and photoelectron spectroscopy, the structure of surface-immobilized BCSs has been elucidated; BCSs can be dispersed in an isolated form using C60 fullerene decoration of the substrate. The intensive nanocluster source also enables the synthesis of BCSs in the 100-mg scale by coupling with a direct liquid-embedded trapping method into organic dispersants, enabling their structure characterization as a highly symmetric "metal-encapsulating tetrahedral silicon-cage" (METS) structure with Frank-Kasper geometry.

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Size-Effect on Electrochemical Hydrogen Evolution Reaction by Single-Size Platinum Nanocluster Catalysts Immobilized on Strontium Titanate

H. Tsunoyama, Y. Yamano, C.-H. Zhang, M. Komori, T. Eguchi, A. Nakajima

Topics in Catalysis （Springer Nature） 61 （ 1 ） 126 - 135 2018年01月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 10225528

Size-specific chemical properties of metal nanoclusters (NCs) have motivated their applications in catalysis, whose properties were found to be dramatically different from bulk materials. We have reported herein the size-dependent activity of platinum (Pt) NCs for the electrochemical hydrogen-evolution-reaction (HER) in neutral water. The single-size NC catalysts were prepared by the soft-landing of mass-selected Pt NCs on a strontium titanate (SrTiO3) (100) surface using an intensive, size-selective NC source based on high-power impulse magnetron sputtering. Successful fabrications of single-size Pt NC catalysts were confirmed with scanning tunneling microscopy. It was found from electrochemical measurements that catalytic HER activity per unit Pt shows a maximum at Pt30 in the range of n = 1–45. Taking into account the electron affinities of Pt NCs measured by gas-phase anion photoelectron spectroscopy, the reaction mechanism of HER is deduced: the size-specific HER activity originates from the matching of energy levels of Pt NCs with the band structure of SrTiO3 surfaces.

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Energy Level Alignment of Organic Molecules with Chemically Modified Alkanethiolate Self-Assembled Monolayers

M. Shibuta, M. Ogura, T. Eguchi, A. Nakajima

Journal of Physical Chemistry C （American Chemical Society） 121 （ 49 ） 27399 - 27405 2017年11月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 1932-7447

We have employed two-photon photoemission spectroscopy to nondestructively resolve the unoccupied energy levels of fullerene C60 molecules deposited on alkanethiolate self-assembled monolayers (SAMs). By fluorine substitution of the hydrogen atoms in the alkyl chain, the work function (WF) increased from 4.3 eV for the alkanethiolate-SAM (H-SAM) to 5.7 eV for the fluorine-substituted SAM (F-SAM), owing to the formation of surface dipole layers. When C60 is deposited on the H-SAM and F-SAM, the energy positions of the unoccupied/occupied levels of C60 are pinned to the vacuum level (Fermi level (EF) + WF). As a result of the energy level alignment, on the F-SAM, the relative energy from EF of the highest occupied molecular orbital of C60 almost equals that of the lowest unoccupied molecular orbital, implying that the C60 film on the F-SAM exhibits both p- and n-type (ambipolar) charge transport properties, while C60 is known as a typical n-type semiconductor. The energetics are preserved even with multilayered C60 films at least up to ∼5 nm in thickness, showing that the dipole layers induced by SAMs are robust against molecular overlayers. Such a spectroscopic study on the energy levels for organic films will be of importance for further development of organic thin film devices.

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Anion Photoelectron Spectroscopy of Rubrene: Molecular Insights into Singlet Fission Energetics

H. Tsunoyama, A. Nakajima

Journal of Physical Chemistry C （American Chemical Society） 121 （ 38 ） 20680 - 20686 2017年08月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 19327447

Rubrene (C42H28, RUB) has been seen to be attractive as a promising building block for organic semiconductors. By means of gas-phase anion photoelectron spectroscopy, the adiabatic electron affinity for RUB molecules is determined to be 1.48 ± 0.03 eV, and the S0-T1 and S0-S1 transition energies of RUB are evaluated to be 1.16 ± 0.05 and 2.42 ± 0.05 eV, showing the possibility of singlet fission in terms of energy. The photoelectron spectra indicate that the vibronic coupling in RUB is similar in the neutral electronic states of S0, T1, and S1. Quantum chemistry calculation results demonstrate that the vibronic coupling in these states originates from their similarly restricted structural displacement upon photoexcitation. Molecular insights into energetics suggest the important role of a charge transfer state in singlet fission.

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Development of Integrated Dry-Wet Synthesis Method for Metal Encapsulating Silicon Cage Superatoms of M@Si₁₆ (M = Ti and Ta)

H. Tsunoyama, H. Akatsuka, M. Shibuta, T. Iwasa, Y. Mizuhata, N. Tokitoh, A. Nakajima

Journal of Physical Chemistry C （American Chemical Society） 121 （ 37 ） 20507 - 20516 2017年08月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 19327447

Nanoclusters (NCs) of several to hundreds of atoms in size are prospective functional units for future nanomaterials originating in their unique, size-specific properties. To explore the field of NC-based materials science, the development of large-scale, size-exclusive synthesis methods is in high demand, as one can see from the successful evolution of fullerene science. We have developed a large-scale synthesis method for main group-based NC compounds by scaling up the clean dry-process with a high-power impulse magnetron sputtering. The 100 mg scale synthesis of binary NCs of M@Si16 (M = Ti and Ta) stabilized by poly(ethylene glycol) dimethyl ether enables us to characterize their structures by an array of methods, for example, mass spectroscopy, X-ray photoemission spectroscopy, Raman spectroscopy, and 29Si nuclear magnetic resonance. Spectroscopic evidence indicates that the M@Si16 NCs are the metal-encapsulating tetrahedral silicon-cage structure satisfying the 68 electrons, closed-electronic-shell superatom.

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Photoexcited Electron Transfer Properties at C₆₀ Film on Graphite and on Au(111) Interfaces Studied by Two-Photon Photoemission Spectroscopy

M. Shibuta, K. Yamamoto, K. Yamagiwa, T. Eguchi, A. Nakajima

Chemistry Letters （Chemical Society of Japan） 46 （ 10 ） 1528 - 1531 2017年08月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り

Geometric and electronic properties of Si-atom doped Al clusters: Robustness of binary superatoms against charging

M. Akutsu, K. Koyasu, J. Atobe, K. Miyajima, M. Mitsui, H. Tsunoyama, A. Nakajima

Physical Chemistry Chemical Physics （Royal Society of Chemistry） 19 （ 31 ） 20401 - 20411 2017年07月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 14639076

The geometric and electronic properties of silicon-atom-doped aluminum clusters, AlnSim (n = 7-30, m = 0-2), were investigated experimentally. The size dependences of the ionization energy and electron affinity of AlnSim show that the stability of AlnSim is governed by the total number of valence electrons in the clusters, where Al and Si atoms behave as trivalent and tetravalent atoms, respectively. Together with theoretical calculations, it has been revealed that neutral Al10Si and Al12Si have a cage-like geometry with central Si atom encapsulation and closed electronic structures of superatomic orbitals (SAOs), and also that they both exhibit geometric robustness against reductive and oxidative changes as cage-like binary superatoms of Si@Al10 and Si@Al12. As well as the single-atom-doped binary superatoms, the effect of symmetry lowering was examined by doping a second Si atom toward the electron SAO closing of 2P SAO, forming Al11Si2. The corresponding anion and cation clusters keep their geometry of the neutral intact, and the ionization energy is low compared to others, showing that Al11Si2 is characterized to be, Si@Al11Si as an alkaline-like binary superatom. For Al21Si2, a face-sharing bi-icosahedral structure was identified to be the most stable as dimeric superatom clusters.

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A designer ligand field for blue-green luminescence of organoeuropium(II) sandwich complexes with cyclononatetraenyl ligands

K. Kawasaki, R. Sugiyama, T. Tsuji, T. Iwasa, H. Tsunoyama, Y. Mizuhata, N. Tokitoh, A. Nakajima

Chemical Communications （Royal Society of Chemistry） 53 （ 49 ） 6557 - 6560 2017年05月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 1359-7345

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Two-Photon Photoelectron Emission Microscopy for Surface Plasmon Polaritons at the Au(111) Surface Decorated with Alkanethiolate Self-Assembled Monolayers

K. Yamagiwa, M. Shibuta, A. Nakajima

Phys. Chem. Chem. Phys. （Royal Society of Chemistry） 19 （ 21 ） 13455 - 13461 2017年05月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り

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Photoexcited State Confinement in Two-Dimensional Crystalline Anthracene Monolayer at Room Temperature

M. Shibuta, N. Hirata, T. Eguchi, A. Nakajima

ACS Nano （American Chemical Society） 11 （ 4 ） 4307 - 4314 2017年04月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 1936-0851

Organic thin film electronics place a high demand on bottom-up technology to form a two-dimensionally (2D) functional unit consisting of a single molecular crystalline layer bound to a layered structure. As the strong interaction between a substrate and molecules makes it difficult to evaluate the electronic properties of organic films, the nature of electronic excited states has not been elucidated. Here, we study a 2D crystalline anthracene monolayer electronically decoupled by alkanethiolates on a gold substrate using scanning tunneling microscopy and time-resolved two-photon photoemission spectroscopy and unravel the geometric/electronic structures and excited electron dynamics. Our data reveal that dispersive 2D excited electrons on the surface can be highly coupled with an annihilation of nondispersive excitons that facilitate electron emission with vibronic interaction. Our results provide a fundamental framework for understanding photoexcited anthracene monolayer and show how the coupling between dispersive and nondispersive excited states may assist charge separation in crystalline molecular layers.

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Highly Ordered Self-Assembled Monolayers of Carboxy- and Ester-Terminated Alkanethiols on Au(111): Infrared Absorption and Hyperthermal-Deposition Experiments with Cr(benzene)₂ Ions

N. Hirata, S. Suga, Y. Noguchi, M. Shibuta, H. Tsunoyama, T. Eguchi, A. Nakajima

Journal of Physical Chemistry C （American Chemical Society） 121 （ 12 ） 6736 - 6747 2017年03月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 19327447

The physical and chemical properties of carboxy- and ester-terminated alkanethiolate self-assembled monolayers (SAMs) on a gold (Au) substrate (Au(111)), which were prepared in a one-pot reaction, were investigated by means of infrared reflection absorption spectroscopy (IRAS) and hyperthermal-deposition experiments with Cr(benzene)2 ions. IRAS spectra showed that the vibrational modes of hydrogen-bonded COOH and CH2 wagging are sensitive to the flatness and cleanness of Au substrates for COOH-SAM and COOMe-/COOEt-SAM, respectively. The intensities of CH2 wagging modes are temperature dependent, which is relevant to the alkyl chain ordering in the highly ordered SAM. The terminal groups of the highly ordered SAM are dissociated by hyperthermal collision events of gas-phase-synthesized Cr(benzene)2 sandwich complex ions, and the adsorption properties of deposited Cr(benzene)2 are discussed in relation to IRAS and temperature-programmed desorption (TPD) measurements.

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Adiabatic electron affinity of pentacene and perfluoropentacene molecules studied by anion photoelectron spectroscopy: Molecular insights into electronic properties

T. Masubuchi, Y. Sugawara, A. Nakajima

Journal of Chemical Physics （American Institute of Physics） 145 （ 24 ） 244306 2016年12月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 00219606

Pentacene (C22H14, PEN) and perfluoropentacene (C22F14, PFP) are considered promising building blocks of organic semiconductors. Using gas-phase anion photoelectron spectroscopy, the adiabatic electron affinity of PEN and PFP molecules is determined to be 1.43 ± 0.03 and 2.74 ± 0.03 eV, respectively, and the S0-T1 transition energies of PEN and PFP are evaluated to be 0.96 ± 0.06 and 0.72 ± 0.05 eV, respectively. Photoelectron spectra indicate that the vibronic coupling in PFP is stronger than that in PEN. Quantum chemistry calculations demonstrate that the strong vibronic coupling originates from significant structural displacement upon electron injection to PFP.

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Fabrication and Characterization of Floating Memory Devices Based on Thiolate-Protected Gold Nanoclusters

N. Hirata, M. Sato, E. Tsunemi, Y. Watanabe, H. Tsunoyama, M. Nakaya, T. Eguchi, Y. Negishi, A. Nakajima

Journal of Physical Chemistry C （American Chemical Society） 121 （ 20 ） 10638 - 10644 2016年11月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 1932-7447

The floating memory properties of thiolate-protected gold (Au:SR) nanoclusters, Au25(SR)18, Au38(SR)24, Au144(SR)60, and Au333(SR)79 (R = C12H25), and Au colloids were investigated using four-probe measurements in vacuum. Uniform and monolayer films of Au:SR nanoclusters or Au colloids were formed as floating memory layers on p-type Si substrates by Langmuir-Blodgett method. The fluoropolymer (CYTOP, ∼15 nm thick) was spin-coated on top to form a hydrophobic insulating layer. Using Au dot (∼40 nm thick) as the anode electrode, the capacitance-voltage (C-V) measurements were performed using the Au and Cu plate electrodes as contact points for the two probes. Clockwise hysteresis curves originating from the Au:SR nanoclusters or Au colloids were observed, and the hysteresis width was dependent on the size of the Au:SR nanoclusters and the sample temperature. In particular, for the Au38(SR)24 nanocluster, multiple phases were observed in the C-V curve, implying their application in multivalued memory devices.

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Imaging and spectromicroscopy of photocarrier electron dynamics in C₆₀ fullerene thin films

M. Shibuta, K. Yamagiwa, T. Eguchi, A. Nakajima

Applied Physics Letters （American Institute of Physics） 109 （ 20 ） 203111 2016年11月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 00036951

We have employed a two-photon photoelectron emission microscopy (2P-PEEM) to observe the photocarrier electron dynamics in an organic thin film of fullerene (C60) formed on a highly oriented pyrolytic graphite with a spatial resolution of ca. 135 nm. In this approach, photocarrier electrons in C60 single-layer islands generated by the first pump photon are detected by the second probe photon. These spectromicroscopic observations conducted over a 100 × 100 nm2 region of C60 islands consistently reproduced the macroscopic two-photon photoemission spectrum of fully covered C60 monolayer film, where the energy of photocarrier electron in the islands was +0.9 eV relative to the Fermi level. Time-resolved 2P-PEEM revealed that the photocarrier electron decayed from the monolayered C60 islands into the substrate with a time constant of 470 ± 30 fs.

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Direct observation of photocarrier electron dynamics in C₆₀ films on graphite by time-resolved two-photon photoemission

M. Shibuta, K. Yamamoto, T. Ohta, M. Nakaya, T. Eguchi, A. Nakajima

Scientific Reports （Springer Nature） 6 35853 2016年10月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 2045-2322

Time-resolved two-photon photoemission (TR-2PPE) spectroscopy is employed to probe the electronic states of a C60 fullerene film formed on highly oriented pyrolytic graphite (HOPG), acting as a model two-dimensional (2D) material for multi-layered graphene. Owing to the in-plane sp 2-hybridized nature of the HOPG, the TR-2PPE spectra reveal the energetics and dynamics of photocarriers in the C60 film: after hot excitons are nascently formed in C60 via intramolecular excitation by a pump photon, they dissociate into photocarriers of free electrons and the corresponding holes, and the electrons are subsequently detected by a probe photon as photoelectrons. The decay rate of photocarriers from the C60 film into the HOPG is evaluated to be 1.31 × 1012 s-1, suggesting a weak van der Waals interaction at the interface, where the photocarriers tentatively occupy the lowest unoccupied molecular orbital (LUMO) of C60. The photocarrier electron dynamics following the hot exciton dissociation in the organic thin films has not been realized for any metallic substrates exhibiting strong interactions with the overlayer. Furthermore, the thickness dependence of the electron lifetime in the LUMO reveals that the electron hopping rate in C60 layers is 3.3 ± 1.2 × 1013 s-1.

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Multiple-decker and ring sandwich formation of manganese-benzene organometallic cluster anions: Mn_nBz_n^- (n = 1-5 and 18)

T. Masubuchi, T. Iwasa, A. Nakajima

Physical Chemistry Chemical Physics （Royal Chemical of Society） 18 （ 37 ） 26049 - 26056 2016年09月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 14639076

Organometallic multiple-decker sandwich clusters are topics of great interest due to their unique electronic and magnetic properties originating from anisotropic structures. We report a joint anion photoelectron spectroscopic and computational study on a new family of manganese (Mn)-benzene (Bz) anionic clusters MnnBzn-. In stark contrast to the most widely studied vanadium-Bz sandwich clusters, it is found that MnnBzn- (n = 1-5) clusters exhibit unprecedented multiple-decker structures with a tilted Mn-Bz stacking and a monotonically increasing behavior of their high spin multiplicities. Furthermore, a couple of closed ring forms of Mn18Bz18- and its neutral state are computationally anticipated as an intriguing "cluster of Mn1Bz1 clusters" in which the neutral Mn18Bz18 has extremely high C18h symmetry with an uncommon spin state of 2S + 1 = 55. The extensively delocalized electron environment of Mn18Bz18 allows the simple Hückel model to reveal the strong intra-atomic exchange interactions within the Mn 3d electrons.

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Charge Transfer Complexation of Ta-Encapsulating Ta@Si₁₆ Superatom with C₆₀

T. Ohta, M. Shibuta, H. Tsunoyama, T. Eguchi, A. Nakajima

Journal of Physical Chemistry C （American Chemical Society） 120 （ 28 ） 15265 - 15271 2016年07月

研究論文（学術雑誌）, 最終著者, 責任著者, 査読有り, ISSN 19327447

The tantalum-encapsulating Si16 cage nanocluster superatom (Ta@Si16) has been a promising candidate for a building block of nanocluster-based functional materials. Its chemical states of Ta@Si16 deposited on an electron acceptable C60 fullerene film were evaluated by X-ray and ultraviolet photoelectron spectroscopies (XPS and UPS, respectively). XPS results for Si, Ta, and C showed that Ta@Si16 combines with a single C60 molecule to form the superatomic charge transfer (CT) complex, (Ta@Si16)+C60-. The high thermal and chemical robustness of the superatomic CT complex has been revealed by the XPS and UPS measurements conducted before and after heat treatment and oxygen exposure. Even when heated to 720 K or subjected to ambient oxygen, Ta@Si16 retained its original framework, forming oxides of Ta@Si16 superatom.

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Extended Smoluchowski Model for the Formation of Size-Selected Silver Nanoclusters Generated via Modulated Pulsed Power Magnetron Sputtering

C.-H. Zhang, H. Tsunoyama, Y. Feng, A. Nakajima

Journal of Physical Chemistry C （American Chemical Society） 120 （ 10 ） 5667 - 5672 2016年03月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 1932-7447

Size-selected silver nanoclusters (NCs) were generated using a modulated pulsed power magnetron sputtering (MPP-MSP) system coupled with a mass spectrometer, and their formation mechanism was investigated by applying an extended Smoluchowski model. Attractive interactions between NC cations and anions were incorporated into the conventional Smoluchowski equations to explain the origin of the size tuning of Ag NCs prepared via MPP-MSP. Bunches overlapped as the repetition rate (f) and peak power (Pp) increased, resulting in the initial formation of large ionic nanoclusters, ultimately causing a reduction in the size of the NC ions due to neutralization via NC cation-anion charge recombination and the formation of aggregates.

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Interplay between experiments and calculations for organometallic clusters and caged clusters

A. Nakajima

AIP Conference Proceedings （American Institute of Physics） 1702 090031-1 - 090031-6 2015年12月

研究論文（国際会議プロシーディングス）, 単著, 筆頭著者, 最終著者, 責任著者, 査読有り, ISSN 0094-243X

Clusters consisting of 10-1000 atoms exhibit size-dependent electronic and geometric properties. In particular, composite clusters consisting of several elements and/or components provide a promising way for a bottom-up approach for designing functional advanced materials, because the functionality of the composite clusters can be optimized not only by the cluster size but also by their compositions. In the formation of composite clusters, their geometric symmetry and dimensionality are emphasized to control the physical and chemical properties, because selective and anisotropic enhancements for optical, chemical, and magnetic properties can be expected. Organometallic clusters and caged clusters are demonstrated as a representative example of designing the functionality of the composite clusters. Organometallic vanadium-benzene forms a one dimensional sandwich structure showing ferromagnetic behaviors and anomalously large HOMO-LUMO gap differences of two spin orbitals, which can be regarded as spin-filter components for cluster-based spintronic devices. Caged clusters of aluminum (Al) are well stabilized both geometrically and electronically at Al12X, behaving as a "superatom".

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Chemical Characterization of an Alkali-Like Superatom Consisting of a Ta-Encapsulating Si₁₆ Cage

M. Shibuta, T. Ohta, M. Nakaya, H. Tsunoyama, T. Eguchi, A. Nakajima

Journal of the American Chemical Society （American Chemical Society） 137 （ 44 ） 14015 - 14018 2015年10月

研究論文（学術雑誌）, 最終著者, 責任著者, 査読有り, ISSN 00027863

Chemical characterization was performed for an alkali-like superatom consisting of a Ta-encapsulating Si16 cage, Ta@Si16, deposited on a graphite substrate using X-ray photoelectron spectroscopy (XPS) to element-specifically clarify the local electronic structure of the cage atoms. The XPS spectra derived from Ta 4f and Si 2p core levels have been well modeled with a single chemical component, revealing the formation of a symmetric Si cage around the Ta atom in the deposited nanoclusters. On chemical treatments by heating or oxygen exposure, it is found that the deposited Ta@Si16 is thermally stable up to 700 K and is also exceptionally less reactive toward oxygen compared to other Ta-Si nanoclusters, although some heat degradation and oxidation accompany the treatments. These results show the promising possibility of applying Ta@Si16 as a building block to fabricate cluster-assembled materials consisting of naked nanoclusters.

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Excitation and Relaxation Dynamics of Two-Dimensional Photoexcited Electrons on Alkanethiolate Self-Assembled Monolayers

M. Shibuta, N. Hirata, R. Matsui, M. Nakaya, T. Eguchi, A. Nakajima

Journal of Physical Chemistry C （American Chemical Society） 119 （ 40 ） 22945 - 22953 2015年09月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 1932-7447

The electron dynamics in alkanethiolate self-assembled monolayers (Cn-SAMs; n = 6-18, where n is the number of alkyl carbons) formed on Au(111) surfaces has been investigated by time- and angle-resolved two-photon photoemission spectroscopy. The time evolution of photoexcited electrons flowing down into image potential states (IPSs) formed on standing-up structure of SAMs is resolved two-dimensionally; the electron lifetime in the IPS increases with chain length, from sub-ps to 100 ps. The chain length dependence of the IPS lifetime is particularly marked at shorter chain lengths of n = 6-10, whereas it becomes milder at chain lengths above n = 10, whose alkyl layer thickness is ≈10 Å. This thickness dependence can be explained by two competitive channels for the decay of IPS electrons: one is electronic coupling of IPS with unoccupied bulk Au states and an interfacial state localized at the Au-S linkage, and the other is IPS electron decay to the Au substrate through a tunneling barrier of insulating alkyl chains. The former is most influential at shorter chain lengths, while the latter is solely dominant at longer chain lengths. In addition, the photon energy dependence of the IPS intensity revealed that electron injection into the IPS is mediated effectively by an electron excitation into interfacial resonance formed in the alkyl layer.

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Hard x-ray photoelectron spectroscopy using an environmental cell with silicon nitride membrane windows

E. Tsunemi, Y. Watanabe, H. Oji, Y. T. Cui, J. Y. Son, A. Nakajima

Journal of Applied Physics （American Institute of Physics） 117 （ 23 ） 234902 2015年06月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 0021-8979

We applied hard x-ray photoelectron spectroscopy (HAXPES) to a sample under ambient pressure conditions using an environmental cell with an approximately 24 nm-thick SiNx membrane window. As a model chemical substance, europium (II) iodide (EuI2) sealed in the cell with argon gas was investigated with HAXPES to identify the chemical species present inside the cell. The optical and morphological properties of the sample within the cell were measured with optical and fluorescent microscopy, scanning electron microscopy, cathodoluminescence, and energy dispersive x-ray spectrometry. We confirmed the effectiveness of the gas barrier properties of the cell with the SiNx window and demonstrated its applicability to various other optical and electron measurements as well as HAXPES.

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Heterodimerization via the covalent bonding of Ta@Si₁₆ nanoclusters and C₆₀ molecules

M. Nakaya, T. Iwasa, H. Tsunoyama, T. Eguchi, A. Nakajima

Journal of Physical Chemistry C （American Chemical Society） 119 （ 20 ） 10962 - 10968 2015年02月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 19327447

Initial products prepared via the surface immobilization of Ta-atom-encapsulated Si16 cage (Ta@Si16) nanoclusters on solid surfaces terminated with monolayer films of C60 molecules were investigated using scanning tunneling microscopy (STM). The STM results indicated that marked aggregation and desorption of surface-immobilized Ta@Si16 nanoclusters were not induced, even after thermal annealing at -500 K, whereas the local vertical and lateral positions of the Ta@Si16 nanoclusters with respect to neighboring adsorption sites in the C60 film were modified. This local positional transition occurred on C60 monolayer films weakly bonded (via van der Waals forces) to substrates such as highly oriented pyrolytic graphite (HOPG) but did not occur on C60 monolayer films covalently bonded to substrates such as Si(111)7 × 7. These results indicated that the heterodimer consisting of a Ta@Si16 nanocluster and a C60 molecule, Ta@Si16-C60, was formed as an initial product via covalent bonding, which inhibited wide-range surface migration of the Ta@Si16 nanoclusters but allowed them to locally change their positions via thermally activated precessional motion. In addition, the transition temperature of the local positional shift was found to decrease as the area density of the surface-immobilized Ta@Si16 nanoclusters increased, indicating that the barrier height of the precessional motion of the Ta@Si16-C60 heterodimer was decreased due to accumulation of the elastic strain energy generated in the C60 films.

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Experimental and theoretical studies of the structural and electronic properties of vanadium-benzene sandwich clusters and their anions: V_nBz_n^0/- (n = 1-5) and V_nBz_n-1^0/- (n = 2-5)

T. Masubuchi, T. Iwasa, A. Nakajima

Journal of Chemical Physics （American Institute of Physics） 141 （ 21 ） 214304 2014年12月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 00219606

One end open VnBzn- (n = 1-5; Bz = benzene) and both ends open VnBzn-1- (n = 2-5) vanadium-benzene cluster anions were studied using anion photoelectron spectroscopy and density functional calculations. The smaller (n ≤ 3) VnBzn and VnBzn-1 clusters and corresponding anions were found to have structural isomers, whereas full-sandwiched VnBzn+1 clusters preferred to form multiple-decker sandwich structures. Several isomeric V2Bz2 structures were identified theoretically and the anion photoelectron spectra of V2Bz20/- were explained well by the coexistence of two isomeric structures: (1) a V2-core structure sandwiched between benzene molecules and (2) an alternating sandwich structure with the spin state strongly dependent on the structure. The adiabatic electron affinity of both VnBzn and VnBzn-1 was found to increase with the cluster size at larger sizes (n = 4 or 5) and approaches to that of VnBzn+1. The evolution of the structural and electronic properties of VnBzm and VnBzm- (m = n and n - 1) with size is discussed in comparison with VnBzn+1 and VnBzn+1-.

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Formation of a superatom monolayer using gas-phase-synthesized Ta@Si₁₆ nanocluster ions

M. Nakaya, T. Iwasa, H. Tsunoyama, T. Eguchi, A. Nakajima

Nanoscale （Royal Society of Chemistry） 6 （ 24 ） 14702 - 14707 2014年10月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 20403364

The controlled assembly of superatomic nanocluster ions synthesized in the gas phase is a key technology for constructing a novel series of functional nanomaterials. However, it is generally difficult to immobilize them onto a conductive surface while maintaining their original properties owing to undesirable modifications of their geometry and charge state. In this study, it has been shown that this difficulty can be overcome by controlling the donor-acceptor interaction between nanoclusters and surfaces. Cations of Ta-atom-encapsulated Si16 cage nanoclusters (Ta@Si16) behaving as rare-gas-like superatoms are synthesized in the gas phase and deposited on conductive surfaces terminated with acceptor-like C60 and donor-like α-sexithiophene (6T) molecules. Scanning tunneling microscopy and spectroscopy have demonstrated that Ta@Si16 cations can be densely immobilized onto C60-terminated surfaces while retaining their cage shape and positive charge, which is realized by creating binary charge transfer complexes (Ta@Si16+-C60-) on the surfaces. The Ta@Si16 nanoclusters exhibit excellent thermal stability on C60-teminated surfaces similar to those in the gas phase, whereas the nanoclusters destabilize at room temperature on 6T-terminated surfaces owing to the loss of electronic closure via a change in the charge state. This journal is

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Development of ultrafine multichannel microfluidic mixer for synthesis of bimetallic nanoclusters: Catalytic application of highly monodisperse AuPd nanoclusters stabilized by poly(N-vinylpyrrolidone)

N. Hayashi, Y. Sakai, H. Tsunoyama, A. Nakajima

Langmuir （American Chemical Society） 30 （ 34 ） 10539 - 10547 2014年08月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 07437463

On account of their novel properties, bimetallic nanoparticles and nanoclusters (NCs) are strong potential candidates for optical, magnetic, and catalytic functional materials. These properties depend on the chemical composition and size (number of constituent atoms) of the NCs. Control of size, structure, and composition is particularly important for fabricating highly functional materials based on bimetallic NCs. Size- and structure-controlled synthesis of two-element alloys can reveal their intrinsic electronic synergistic effects. However, because synergistic enhancement of activity is strongly affected by composition as well as by size and structure, controlled synthesis is a challenging task, particularly in catalytic applications. To investigate catalytic synergistic effects, we have synthesized highly monodisperse, sub-2 nm, solid-solution AuPd NCs stabilized with poly(N-vinylpyrrolidone) (AuPd:PVP) using a newly developed ultrafine microfluidic mixing device with 15 m wide multiple lamination channels. The synergistic enhancement for catalytic aerobic oxidation of benzyl alcohol exhibited a volcano-shaped trend, with a maximum at 20-65 at. % Pd. From X-ray photoelectron spectroscopic measurements, we confirmed that the enhanced activity originates from the enhanced electron density at the Au sites, donated by Pd sites. © 2014 American Chemical Society.

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Electronic states of titanyl phthalocyanine films on alkanethiolate self-assembled monolayers probed by two-photon photoemission

M. Ogura, M. Shibuta, T. Eguchi, A. Nakajima

Journal of Electron Spectroscopy and Related Phenomena （Elsevier Science BV） 195 272 - 277 2014年07月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 0368-2048

We studied both occupied and unoccupied states of titanyl phthalocyanine (TiOPc) films formed on an octanethiolate self-assembled monolayer (SAM) on an Au(1 1 1) surface fabricated in a wet chemical process using two-photon photoemission (2PPE) spectroscopy. A 1.90-nm-thick layer of TiOPc formed an unoccupied state at 2.9 eV above the Fermi level. This state was resonantly enhanced at 4.3-4.4 eV photon energy, suggesting that electrons were excited from the highest occupied molecular orbital of TiOPc. The 2PPE measurements of photon energy dependence and light polarization selectivity revealed that the unoccupied state originates from a charge transfer exciton not observed in organic thin films deposited on bare metallic substrates. The formation of a charge transfer exciton implies that the SAM strongly insulates the molecular monolayer.

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Investigation of the electronic structures of organolanthanide sandwich complex anions by photoelectron spectroscopy: 4f orbital contribution in the metal-ligand interaction

N. Hosoya, K. Yada, T. Masuda, E. Nakajo, S. Yabushita, A. Nakajima

Journal of Physical Chemistry A （American Chemical Society） 118 （ 17 ） 3051 - 3060 2014年04月

研究論文（学術雑誌）, 最終著者, 責任著者, 査読有り, ISSN 10895639

The electronic structures of lanthanide (Ln) ions sandwiched between 1,3,5,7-cyclooctatetraene (COT), Ln(COT)2-, have been investigated by anion photoelectron spectroscopy. Complexes of 12 Ln atoms were investigated (excluding promethium (Pm), europium (Eu), and ytterbium (Yb)). The 213 nm photoelectron (PE) spectra of Ln(COT)2- exhibit two peaks assignable to the highest occupied molecular orbital (HOMO; e 2u) and the next HOMO (HOMO-1; e2g) approximately at 2.6 and 3.6 eV, respectively, and their energy gap increases as the central metal atom progresses from lanthanum (La) to lutetium (Lu). Since lanthanide contraction shortens the distance between the Ln atom and the COT ligands, the widening energy gap represents the destabilization of the e2u orbital as well as the stabilization of the e2g orbital. Evidence for 4f orbital contribution in the metal-ligand interaction has been revealed by the Ln atom dependence in which the same e2u orbital symmetry enables an interaction between the 4f orbital of Ln atoms and the π orbital of COT. © 2014 American Chemical Society.

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Formation and electronic structures of organoeuropium sandwich nanowires

N. Hosoya, R. Takegami, J. Suzumura, K. Yada, K. Miyajima, M. Mitsui, M. B. Knickelbein, S. Yabushita, A. Nakajima

Journal of Physical Chemistry A （American Chemical Society） 118 （ 37 ） 8298 - 8308 2014年04月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 1089-5639

Organoeuropium sandwich clusters, comprising europium (Eu) and 1,3,5,7-cyclooctatetraene (COT) (Eun(COT)m), were produced in the gas phase using a laser vaporization synthesis method. Photoionization mass spectra revealed an exclusive Eun(COT)m formation with three compositions: m = n + 1, m = n, and m = n - 1, which, we propose, correspond to full-sandwich, half-sandwich, and inverted-sandwich structures, respectively. The charge distributions, metal-ligand bonding characteristics, and electronic structures of the clusters were comprehensively investigated by photoionization measurements of Eun(COT)m neutrals and by photoelectron spectroscopy of Eun(COT)m- and isoelectronic Ban(COT)m- anions. The results confirmed that (1) highly ionic metal-ligand bonding is formed between Eu2+ and COT2- within the sandwich structure (at the termini, ionic forms are Eu+ and COT-) and (2) size dependence of orbital energy can be explained by the Coulombic interaction of simple point charge models between the detaching electrons and dipoles/quadrupoles. When the terminus of the sandwich clusters is Eu2+, COT2-, or Eu0, the orbital energy of the electron detachment channel at the opposite terminus strongly depends on the cluster size. In this case, the molecular stack behaves as a one-dimensionally aligned dipole; otherwise, it behaves as a quadrupole, and the relationship between cluster size and electron detachment energy is much weaker. The study also reports on the 4f orbital energy in Eu ions and the formation mechanism of organoeuropium sandwich nanowires up to 12 nm in length. The nanowires are formed by successive charge transfer at the terminal part, Eu+ and COT-, which reduces the ionization energy and increases the electron affinity, respectively. (Graph Presented).

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Physical properties of mononuclear organoeuropium sandwich complexes ligated by cyclooctatetraene and bis(trimethylsilyl)cyclooctatetraene

T. Tsuji, S. Fukazawa, R. Sugiyama, K. Kawasaki, T. Iwasa, H. Tsunoyama, N. Tokitoh, A. Nakajima

Chemical Physics Letters （Elsevier Science） 595-596 144 - 150 2014年03月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 0009-2614

Mononuclear organoeuropium sandwich complexes were synthesized from ligands of 1,3,5,7-cyclooctatetraene (COT) and 1,4- and 1,6-bis(trimethylsilyl)- cyclooctatetraenes (COT″) in a liquid ammonia of europium (Eu) and lithium (Li). Elemental analyses revealed the chemical compositions as [(DME)Li] 2Eu(COT)2 (1) and [(DME)Li]2Eu(COT″) 2 (2). Vibrational analyses and magnetic measurements revealed a ligand-Eu-ligand sandwich structure. The Eu2+ ions induce paramagnetic behaviors at 6-300 K. Although UV-irradiated 1 and 2 emit light of similar color, the quantum yields of their toluene solution widely differ (2.4% and 21%, respectively). The effects of the trimethylsilyl group are evaluated from quantum chemical calculations. © 2014 Elsevier B.V. All rights reserved.

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Liquid-phase synthesis of multidecker organoeuropium sandwich complexes and their physical properties

T. Tuji, N. Hosoya, S. Fukazawa, R. Sugiyama, T. Iwasa, H. Tsunoyama, H. Hamaki, N. Tokitoh, A. Nakajima

Journal of Physical Chemistry C （American Chemical Society） 118 （ 11 ） 5896 - 5907 2014年03月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 1932-7447

Multidecker organoeuropium sandwich complexes were synthesized with the organic ligands 1,3,5,7-cyclooctatetraene (COT) and 1,4- and 1,6-bis(trimethylsilyl)cyclooctatetraenes (COT″) in a liquid ammonia solution of europium (Eu) metal. Infrared (IR) absorption and Raman spectroscopies and magnetic measurements were used to identify local structures and charge distributions. It was found that the organoeuropium complexes have Eu2+-COT2- (Eu2+-COT″2-) charge distributions, including a local structure of ligand-Eu-ligand sandwiches. Together with elemental analysis by inductively coupled plasma atomic emission spectroscopy, it is shown that the organoeuropium complexes form a multidecker sandwich structure that is insouble in many solvents. In addition to their paramagnetic behaviors at 150-300 K, the organoeuropium complexes of Eu-COT and Eu-COT″ showed red and orange-red emissions, respectively, under UV irradiation. The introdution of bulky trimethylsilyl groups onto COT prominently enhanced the emission intensity by more than a factor of 10 compared to that of Eu-COT, and similar emission enhancement was observed when ethylenediamine vapor was contacted with Eu-COT solid. The intensity enhancement and the shifts in the emission wavelength are discussed from the viewpoint of orbital interactions between Eu2+ and COT2- (COT″2-). © 2014 American Chemical Society.

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Formation and control of ultrasharp metal/molecule interfaces by controlled immobilization of size-selected metal nanoclusters onto organic molecular films

M. Nakaya, T. Iwasa, H. Tsunoyama, T. Eguchi, A. Nakajima

Advanced Functional Materials （Wiley-VCH Verlag GMBH） 24 （ 9 ） 1202 - 1210 2014年03月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 1616-301X

The formation of metallic layers on ultrathin molecular films via a well-controlled interface is essential for constructing organic nanodevices composed of metal/molecule/metal sandwich junctions. The scanning tunneling microscopy and spectroscopy studies demonstrate that an ultrasharp metal/molecule interface is realizable by depositing size-selected Ag nanoclusters (Agn) from the gas phase on few-layer films of C 60 molecules. It is also demonstrated that Agn nanoclusters can be immobilized on monolayer films of oligothiophene molecules via C60 molecules, although they three-dimensionally aggregate on bare oligothiophene films. It is also shown that electrons and holes are injected into the topmost layer of C60 films via the Ag n/C60 interface. Moreover, the barrier height for carrier injection at the Agn/C60 interface can be modified depending on the size of Agn nanoclusters and the kinetic energy during the deposition. The present results demonstrate that the controlled immobilization of metallic nanoclusters on molecular films can be used as a fabrication technology for metal/molecule/metal junctions. Metal/molecule heterojunctions are constructed by the controlled immobilization of size-selected Ag nanoclusters (Agn) from the gas phase on ultrathin C60 films. In such heterojunctions, electrons and holes can be precisely injected into the topmost C60 molecules in films via an atomically abrupt metal/molecule interface. The barrier height for carrier injection can be modified. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

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Wavelength dependence of water microdroplet shattering with dual IR lasers and dye sensitization

A. Sugiyama, A. Nakajima

Zeitschrift fur Physikalische Chemie （Walter de Gruyter GMBH） 228 （ 4-5 ） 459 - 470 2014年02月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 0942-9352

We have used time-resolved imaging to examine the effect of incident radiation wavelength in the range 2.75-3.75 μm on the shattering of water microdroplets by single and dual lasers. Dye sensitization of microdroplets was induced by using the water-soluble dye sulforhodamine G; this enabled us to observe both the forward and backward components of fragmented droplets and splashes. Compared with employing a single IR laser, simultaneous illumination by two IR lasers can effectively compress the spatial distribution of small fragments around the absorption maximum for the OH-stretching vibration of liquid water (y=2.75-3.25 μm). Scattering and compression of the fragmented microdroplets after single and dual IR laser irradiation can be explained by conservation of momentum.

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Probing of an adsorbate-specific excited state on an organic insulating surface by two-photon photoemission spectroscopy

M. Shibuta, H. Hirata, T. Eguchi, A. Nakajima

Journal of the American Chemical Society （American Chemical Society） 136 （ 5 ） 1825 - 1831 2014年01月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 0002-7863

In this study, we investigate the photoexcited electronic states of ferrocene (Fc) molecules adsorbed on an organic insulating surface by two-photon photoemission spectroscopy. This insulating layer, composed of a decanethiolate self-assembled monolayer formed on an Au(111) substrate, enables us to probe the electronically excited states localized at the adsorbed Fc molecules. The adsorbate-specific state is resonantly excited by photons at 4.57 eV, which is 0.5 eV smaller than the energy of the first molecular Rydberg state of free Fc in the gas phase. This result indicates that the electrons are bound to both the excited hole formed in the adsorbate and the positive image charge induced in the substrate. The hybridized electronic characteristics of the adsorbate-specific state are responsible for the strong transition selectivity and short lifetime of the excited state. © 2014 American Chemical Society.

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Electronic and optical properties of vertex-sharing homo- and hetero-biicosahedral gold clusters

T. Iwasa, K. Nobusada, A. Nakajima

Journal of Physical Chemistry C （American Chemical Society） 117 （ 46 ） 24586 - 24591 2013年11月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 1932-7447

Electronic and optical properties of vertex sharing homo- and heterobiicosahedral gold clusters [M1M2Au 23(PH3)10(SCH3)5Cl 2]2+ (M1/M2 = Au/Au, Pt/Hg) are investigated by means of density functional computations, focusing on superatom assembly. In the homo clusters, the projected density of states onto the spherical harmonics reveals that each icosahedral unit behaves as a superatom with the Au(6s6p) electrons and electron shell closings of S and P orbitals followed by unoccupied D orbitals; each unit satisfies an 8-electron superatom condition. Because the Au(5d) band appears between the S and P orbitals, the electronic structure is characterized as S, Au(5d)-band, and P. The lowest electronic excitation in the homo cluster is the P to D transition. In the hetero biicosahedral cluster, the central Au atoms of each icosahedron are replaced by Pt and Hg. The HOMO and LUMO are well localized to the PtAu 12 and HgAu12 units, respectively, forming a cluster-assembled heterojunction in a single stable structure. The absorption spectrum of the heterobiicosahedral cluster is characterized by charge transfer from PtAu12 to HgAu12 in the visible region, accompanied by a weak charge transfer in the opposite direction. Despite low numbers of charge carriers, the heterobiicosahedral cluster shows a large dipole moment. © 2013 American Chemical Society.

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Geometric, electronic, and optical properties of monomer and assembly of endohedral aluminum superatomic clusters

T. Iwasa, A. Nakajima

Journal of Physical Chemistry C （American Chemical Society） 117 （ 41 ） 21551 - 21557 2013年10月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 1932-7447

Density functional computations are used to evaluate the geometric, electronic, and optical properties of endohedral aluminum clusters X@Al 12 (X = B, Al, Si, P) and their assemblies. All X@Al 12+/0/- clusters are perfect or slightly distorted icosahedral structures, with the exception of Al13+, which is highly distorted. The projected density of states (PDOS) onto the spherical harmonics of monomers clearly reveals superatom behavior and electron shell closings of F orbitals in a 40-electron species. The electronic absorption spectrum of SiAl12 is analyzed in terms of the superatom orbitals. The optimized structures of X@Al12-Y@Al12 (X-Y = Si-Si, B-P, Al-P) dimers are constructed by facing the sides of the monomers in a staggered fashion. The PDOS of the dimers mostly exhibit five hybridizations: S, P, SD, PF, and SDG. The exceptions are HOMO, which possesses a DFG hybridized character and lies between the PF and SDG regions, and LUMO, which possesses a DG hybridized character. By analyzing the simulated absorption spectra of the B@Al12-P@Al12 and Al13-P@Al12 heterodimers, charge transfers from B/Al@Al12 to P@Al12 are found in the visible region, weakly accompanying the opposite charge transfer. The heterodimers have substantial charge carriers, estimated as the difference in electron counts from the closed-shell Si@Al12, with slight charge depletions (∼0.2). The charge distributions in B@Al 12 and P@Al12 are essentially unaltered by the insertion of Si@Al12 into the heterodimer, resulting in that the heterotrimer possesses a larger dipole moment than the heterodimer. © 2013 American Chemical Society.

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Advanced nanocluster ion source based on high-power impulse magnetron sputtering and time-resolved measurements of nanocluster formation

C.-H. Zhang, H. Tsunoyama, H. Akatsuka, H. Sekiya, T. Nagase, A. Nakajima

Journal of Physical Chemistry A （American Chemical Society） 117 （ 40 ） 10211 - 10217 2013年09月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 1089-5639

We developed a new nanocluster (NC) ion source based on the high-power impulse magnetron sputtering (HiPIMS) technique coupled with a gas flow cell reactor. Silver NC anions (Agn-) with a maximum intensity of 5.5 nA (Ag11-) are generated with the size ranging from the atomic anion to the 70-mer, which is well-controlled by simply adjusting the peak power and repetition rate of the HiPIMS. By time-resolved density profiles of Agn-, we find that the ion beam generated by HiPIMS is characterized by individual 100 ms duration "bunches" below a repetition rate of 10 Hz, which is well-Thermalized with a group velocity of 5 m/s. The high intensity of the NCs is attributable to the high ionization fraction by this HiPIMS ion source, while the underlying mechanism of the flexible size tuning of the ion source is understood by time-resolved mass spectrometry coupled with the sequential growth mechanism; the increment of the density of the target species in the bunches with the peak power and the overlapping of the bunches with the repetition rate cause the formation of large NCs. © 2013 American Chemical Society.

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Geometric, electronic, and optical properties of a boron-doped aluminum cluster of B₂ Al₂₁^-: A density functional theory study

T. Iwasa, A. Nakajima

Chemical Physics Letters （Elsevier Science BV） 582 100 - 104 2013年09月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 00092614

We present the physicochemical properties for the lowest-energy isomer of a boron-doped aluminum cluster of B2Al21-. The isomer was obtained by basin-hopping minimization based on the density functional theory, starting from a face-sharing bi-icosahedral structure in which two boron atoms were endohedrally doped to each icosahedron. The lowest-energy isomer is a triangular form in which an aluminum cage encapsulates two boron atoms endohedrally. The electronic structure was analyzed by projecting Kohn-Sham orbitals onto the spherical harmonics; occupied and unoccupied frontier orbitals are dominantly G- and H-symmetries, respectively. Optical absorption is mainly assigned to G to H transitions. © 2013 Elsevier B.V. All rights reserved.

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Development of high-flux ion source for size-selected nanocluster ions based on high-power impulse magnetron sputtering

H. Tsunoyama, C.-H. Zhang, H. Akatsuka, H. Sekiya, T. Nagase, A. Nakajima

Chemistry Letters （Chemical Society of Japan） 42 （ 8 ） 857 - 859 2013年08月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 0366-7022

In this study, we have developed a highly intensive ion source of size-selected nanoclusters based on a high-power impulse magnetron sputtering method. The maximum intensity of the size-selected silver nanocluster anions exceeded that of standard ion sources by more than ten times. The high intensity in the present method was achieved by the high throughput of the ion optics and the high ionization fraction. Fine tuning of the nanocluster size distributions was also demonstrated. © 2013 The Chemical Society of Japan.

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Development of cluster ion source based on modulated pulse power magnetron sputtering technique

C.-H. Zhang, H. Tsunoyama, H. Akatsuka, H. Sekiya, T. Nagase, A. Nakajima

The 8th Annual IEEE International Conference on Nano/Micro Engineered and Molecular Systems （IEEE） 428 - 431 2013年07月

研究論文（国際会議プロシーディングス）, 共著, 最終著者, 責任著者, 査読有り

A new ion source based on modulated pulse power (MPP) magnetron sputtering (MSP) was developed and demonstrated for cluster production. By employing MPP-MSP, both silver and silicon cluster beams were produced and analyzed by a quadrupole mass spectrometer. It is found that the maximum intensity of silver cluster anions reaches 500 pA, which is considerably higher than that with a conventional DC-MSP. For silicon cluster cation, the overall ion intensity of the cluster beam by MPP-MSP is about three times higher than that generated by DC-MSP. © 2013 IEEE.

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Study on electronic properties of composite clusters toward nanoscale functional advanced materials

A. Nakajima

Bulletin of the Chemical Society of Japan （Chemical Society of Japan） 86 （ 4 ） 414 - 437 2013年04月

研究論文（学術雑誌）, 単著, 筆頭著者, 最終著者, 責任著者, 査読有り, ISSN 0009-2673

Clusters consisting of 101000 atoms size-dependently exhibit novel electronic and geometric properties. In particular, composite clusters composed of several elements and/or components provide a promising way for a bottom-up approach for designing functional advanced materials, because the functionality of the composite clusters can be optimized not only by the cluster size but also by their compositions. This account surveys recent investigations of composite clusters focusing on the efforts to create a new functional composite cluster by a fine doping or hybridization based on their size-specific electronic properties. Organometallic clusters and caged clusters are demonstrated as a representative example of designing the functionality of magnetism and electronic state structures. In order to create functional nanomaterials, furthermore, a fine controlling methodology of the soft-landing technique has been developed to fix the composite clusters onto a surface decorated with a self-assembled monolayer. The embedded isolation mechanism on the substrate is discussed form the viewpoint of self-assembly phenomena with the molecular ordering of σ σ interaction, and also the electronic structures characterized by molecular ordering of π- π interaction is intrinsically revealed by molecular clusters of -conjugate polyacenes as a model for self-assembled aggregates. © 2013 The Chemical Society of Japan.

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Imaging and Characterizing Long-Range Surface Plasmon Polaritons Propagating in a Submillimeter Scale by Two-Color Two-Photon Photoelectron Emission Microscopy

M. Shibuta, T. Eguchi, A. Nakajima

Plasmonics （Springer Nature） 8 （ 3 ） 1411 - 1415 2013年04月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 1557-1955

Long-range surface plasmon polaritons (SPPs), which propagate along metal/dielectric interfaces to submillimeter distances in the range of near-infrared (NIR) excitation wavelength, were examined by two-color two-photon photoelectron emission microscopy (2P-PEEM). Interferences between incident NIR photons and SPPs excited by the NIR photons at surface defects were imaged by detecting photoelectrons emitted from a gold surface, assisted by simultaneously irradiated ultraviolet photons which are to overcome the workfunction of the surface. The wavelength of the interference beat depends sensitively on the NIR wavelength. By analyzing the interference beat, the dispersion curve as well as phase and group velocities of SPP's were experimentally obtained. The results closely match the theoretical one based on the Drude free electron model, indicating that two-color 2P-PEEM is applicable not only to the visualization of NIR-excited SPPs but also to the quantitative analysis of its physical properties. This method will be widely used to observe SPPs for various artificial plasmonic devices. © 2013 Springer Science+Business Media New York.

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Excited electron dynamics at ferrocene-terminated self-assembled monolayers on Au(111): Lengthened lifetime of image potential state

N. Hirata, M. Shibuta, T. Eguchi, A. Nakajima

Chemical Physics Letters （Elsevier Science BV） 561-562 131 - 136 2013年03月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 00092614

Ultraviolet photoemission spectroscopy (UPS) and two-photon photoemission (2PPE) spectroscopy have been employed to investigate the electronic states and the excited electron dynamics of ferrocene (Fc)-terminated self-assembled monolayers (SAMs) on Au(1 1 1). For the 11-ferrocenyl-1-undecanethiol (FcC11-) SAM, an image potential state (IPS) located at EF +3.8 eV was observed by time-resolved 2PPE. Its lifetime, 90 fs, was found to be much shorter than that of decanethiol (C10-) SAM, 11 ps. By forming the C10-SAM beforehand, the IPS lifetime for FcC11-&C10-SAM was lengthened to ∼9 ps; this can enable improvement in molecular ordering and substantial formation of C10 domains. © 2013 Elsevier B.V. All rights reserved.

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Size and structure dependence of electronic states in thiolate-protected gold nanoclusters of Au₂₅(SR)₁₈, Au₃₈(SR)₂₄, and Au₁₄₄(SR)₆₀

T. Ohta, M. Shibuta, H. Tsunoyama, Y. Negishi, T. Eguchi, A. Nakajima

Journal of Physical Chemistry C （American Chemical Society） 117 （ 7 ） 3674 - 3679 2013年02月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 19327447

The electronic states of three different sizes of compositionally precise thiolate-protected gold nanoclusters, Au25(SR)18, Au 38(SR)24, and Au144(SR)60 (R = C12H25), have been evaluated by X-ray photoemission spectroscopy. The Au 4f core-levels of the nanoclusters are well reproduced by two spectral components derived from centered core-Au and positively charged shell-Au atoms, the numbers of which are determined based on the atomic structures of the nanoclusters. The spin-orbit splitting of Au 5d5/2 and 5d3/2 in the valence band becomes narrower than that for bulk Au, depending on the cluster size, which is quantitatively characterized by a reduction in the average coordination number of Au. The Au 5d valence-band spectra also show that the charge reorganization of 5d electrons induced by interaction with thiol molecules is more significant for the 5d5/2 than the 5d3/2 orbital. © 2013 American Chemical Society.

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Experimental and theoretical studies on the electronic properties of vanadium-benzene sandwich cluster anions, V_nBz_n+1^- (n = 1-5)

T. Masubuchi, K. Ohi, T. Iwasa, A. Nakajima

Journal of Chemical Physics （American Institute of Physics） 137 （ 22 ） 224305 2012年12月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 00219606

Vanadium-benzene cluster anions, VnBzn+1- (Bz = C6H6) were generated by laser ablation and supersonic jet methods, and studied using photoelectron spectroscopy. The density functional theory was employed to compute their geometric and electronic structures. It is concluded that the V nBzn+1- anions exhibit multiple-decker sandwich structures similar to their corresponding neutrals, and the adiabatic electron affinity increases with the cluster size. Our computation shows that the excess electron of the anion occupies the d orbitals of the vanadium atoms and that it is delocalized one-dimensionally. Furthermore, a very large HOMO-LUMO gap difference between majority and minority spin orbitals is observed for both the neutrals and the anions, and the V nBzn+10/- clusters are found to be completely spin-polarized. These facts confirm the possibility of using V nBzn+1 clusters as spin filters. © 2012 American Institute of Physics.

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Probing buried organic-organic and metal-organic heterointerfaces by hard x-ray photoelectron spectroscopy

M. Shibuta, T. Eguchi, Y. Watanabe, J. Son, H. Oji, A. Nakajima

Applied Physics Letters （American Institute of Physics） 101 （ 22 ） 221603 2012年11月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 0003-6951

We present a nondestructive characterization method for buried hetero-interfaces for organic/organic and metal/organic systems using hard x-ray photoelectron spectroscopy (HAXPES) which can probe electronic states at depths deeper than ∼10 nm. A significant interface-derived signal showing a strong chemical interaction is observed for Au deposited onto a C60 film, while there is no such additional feature for copper phthalocyanine deposited onto a C60 film reflecting the weak interaction between the molecules in the latter case. A depth analysis with HAXPES reveals that a Au-C 60 intermixed layer with a thickness of 5.1 nm is formed at the interface. © 2012 American Institute of Physics.

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Wavelength dependence of IR laser for dual-laser shattering of a water microdroplet

A. Sugiyama, A. Nakajima

Chemistry Letters （Chemical Society of Japan） 41 （ 11 ） 1481 - 1483 2012年11月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 0366-7022

For the single- and dual-laser shattering of water microdroplets, the wavelength dependence of an infrared (IR) laser was experimentally examined in the range 2.253.75 μm using ion current measurements coupled with time-resolved imaging. Compared to a single IR laser, simultaneous illumination by dual IR lasers effectively compresses the spatial distribution of small fragments around the absorption maximum for the OHstretching vibration of liquid water (λ=2.753.25 μm). The effect can be explained by the small penetration depth of the IR laser toward water microdroplets. © 2012 The Chemical Society of Japan.

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Molecular-scale and wide-energy-range tunneling spectroscopy on self-assembled monolayers of alkanethiol molecules

M. Nakaya, M. Shikishima, M. Shibuta, N. Hirata, T. Eguchi, A. Nakajima

ACS Nano （American Chemical Society） 6 （ 10 ） 8728 - 8734 2012年09月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 1936-0851

The electronic properties of alkanethiol self-assembled monolayers (alkanethiolate SAMs) associated with their molecular-scale geometry are investigated using scanning tunneling microscopy and spectroscopy (STM/STS). We have selectively formed the three types of alkanethiolate SAMs with standing-up, lying-down, and lattice-gas phases by precise thermal annealing of the SAMs which are conventionally prepared by depositing alkanethiol molecules onto Au(111) surface in solution. The empty and filled states of each SAM are evaluated over a wide energy range covering 6 eV above/below the Fermi level (E F) using two types of STS on the basis of tunneling current-voltage and distance-voltage measurements. Electronic states originating from rigid covalent bonds between the thiol group and substrate surface are observed near E F in the standing-up and lying-down phases but not in the lattice-gas phase. These states contribute to electrical conduction in the tunneling junction at a low bias voltage. At a higher energy, a highly conductive state stemming from the alkyl chain and an image potential state (IPS) formed in a vacuum gap appear in all phases. The IPS shifts toward a higher energy through the change in the geometry of the SAM from the standing-up phase to the lattice-gas phase through the lying-down phase. This is explained by the increasing work function of alkanethiolate/Au(111) with decreasing density of surface molecules. © 2012 American Chemical Society.

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Dual IR laser shattering of a water microdroplet

A. Sugiyama, A. Nakajima

Applied Physics A: Materials Science and Processing （Springer Nature） 109 （ 1 ） 31 - 37 2012年07月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 0947-8396

Ion desorption from the infrared (IR) laser shattering of water microdroplets (ø90 μm in diameter) was experimentally examined by ion current measurements coupled with time-resolved imaging by a charge-coupled-device camera. When a microdroplet was shattered by simultaneous illumination by two IR lasers (λ=2.9 μm) from both the left- and right-hand sides, the time-resolved imaging shows that a lot of small fragments of splash spread around the droplet. The spatial distributions of the small fragments were symmetrically compressed. The resulting fragment swarm was effectively introduced into a vacuum chamber through an inlet skimmer ø0.3-0.4 mm in diameter. The ion current measured from a 10 -6 mol/m 3 NaCl water solution microdroplet using two lasers was considerably enhanced compared to that by single IR laser shattering. When one of the two IR lasers was delayed by 0-1000 μs, the ion current gradually decreased with the delay time, and dropped substantially at delays longer than 100 ns. The results are ascribed to dynamical processes following the multi-photon excitation. The dual IR laser ablation of a liquid droplet can enhance the efficiency of ion formation with a lower dispersion velocity, which can be conveniently combined with time-of-flight mass spectrometry. © 2012 Springer-Verlag.

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Geometric, electronic, and optical properties of a superatomic heterodimer and trimer: Sc@Si₁₆-V@Si₁₆ and Sc@Si₁₆-Ti@Si₁₆-V@Si₁₆

T. Iwasa, A. Nakajima

Journal of Physical Chemistry C （American Chemical Society） 116 （ 26 ） 14071 - 14077 2012年06月

研究論文（学術雑誌）, 最終著者, 責任著者, 査読有り, ISSN 19327447

The geometric, electronic, and optical properties of a heterodimer and trimer consisting of metal-encapsulating silicon cage clusters, M@Si 16 (M = Sc, Ti, V) with D 4d symmetry, are studied using density functional computations to explore the possibility of using these clusters as building blocks for a nanometer scale heteroassembly. In this study, among the possible low-lying geometries, the linear-form of the hetero-oligomers is adopted as a model system, where the D 4d monomers are covalently bonded by facing their squares in an eclipsed fashion. The heterodimer consisting of halogen-like Sc@Si 16 and alkaline-like V@Si 16 has a dipole moment of 7.63 D, and its occupied and virtual frontier orbitals are localized to V@Si 16 and to Sc@Si 16, respectively. Some of the inner molecular orbitals exhibit superatomic bonding and antibonding character. The electronic excitations involve charge-transfer states mainly from V@Si 16 to Sc@Si 16 in the optical energy region. The linear heterotrimer of Sc@Si 16-Ti@Si 16- V@Si 16, formed by inserting the rare-gas-like Ti@Si 16, has a larger dipole moment of 15.6 D and one or more localized frontier orbitals compared to the dimer. We propose possible formation routes to realize the present hetero-oligomers using photoexcitation or energy-selective electron injection into several LUMOs of the monomers that are suitable for linear-oligomerization. © 2012 American Chemical Society.

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Electronic states of alkanethiolate self-assembled monolayers on Au(111) studied by two-photon photoemission spectroscopy

N. Hirata, M. Shibuta, R. Matsui, A. Nakajima

Journal of Physical Chemistry C （American Chemical Society） 116 （ 25 ） 13623 - 13628 2012年06月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 1932-7447

Two-photon photoemission (2PPE) spectroscopy has been employed to probe the electronic states of n-alkanethiolate self-assembled monolayers (SAMs) on an Au(111) substrate fabricated in a wet chemical process. Electronic states newly formed in the SAM formation were observed below and above the Fermi level (E F) for various alkyl chain lengths of C12-, C18-, and C22-SAMs. At 3.5-3.7 eV above E F, an unoccupied state originating from a bond between gold and sulfur atoms appears, a state that shows little dispersion with parallel to the surface. The peak position of the unoccupied state depends on the substrate temperature, and it was stabilized with increasing temperature; E F +3.7 eV at 85 K and E F +3.5 eV at 330 K for C18-SAM. The stabilization of the state is attributed to the increase of intermolecular interaction at sulfur atoms with their neighboring S atoms, which is caused by a change in the tilt angle of the alkyl chains in the SAM: on increasing the temperature, the interaction between S atoms in the SAM is promoted by the more upright alkyl chains. © 2012 American Chemical Society.

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Charge separation at the molecular monolayer surface: Observation and control of the dynamics

M. Shibuta, N. Hirata, R. Matsui, T. Eguchi, A. Nakajima

Journal of Physical Chemistry Letters （American Chemical Society） 3 （ 8 ） 981 - 985 2012年03月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 1948-7185

Charge separation dynamics relevant to an electron transfer have been revealed by time- and angle-resolved two-photon photoemission spectroscopy for an n-alkanethiolate self-assembled monolayer (SAM) on a Au(111) surface fabricated by a chemical-wet process. The electron was photoexcited into an image potential state located at 3.7 eV above the Fermi level (E F), and it survived well for more than 100 ps on dodecanethiolate (C12)-SAM. The degree of electron separation is precisely controlled by selecting the length of the alkyl chain (C10-C18). We have also evaluated molecular conductivity at the specific electron energy of E F + 3.7 eV. The tunneling decay parameter, β, was fitted by β 90 K = 0.097 Å -1 and β RT = 0.13 Å -1. These values were one order smaller than that at around E F by conventional contact probe methods. © 2012 American Chemical Society.

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Investigation of lanthanide sandwich nanoclusters encapsulated with a cyclo-olefin polymer as a gas barrier

E. Tsunemi, T. Tsuji, S. Fukazawa, H. Tsunoyama, Y. Watanabe, A. Nakajima

Applied Physics Express （Japan Society of Applied Physics） 5 （ 3 ） 035202 2012年03月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 1882-0778

Lanthanide nanoclusters ligated by planar organic molecules have one-dimensional sandwich structures and are air/moisture sensitive. To investigate their optical properties, originating from the lanthanide elements, for novel nanodevices, we preserved the nanoclusters with a cycloolefin polymer as a gas barrier. The technique enabled us to conduct various spectroscopic analyses without oxidation of the samples, such as fluorescence microscopy, photoluminescence, and X-ray absorption fine structure determination, thus revealing that lanthanide sandwich nanoclusters containing europium (Eu) atoms exhibit orange fluorescence attributed to divalent Eu 2+. © 2012 The Japan Society of Applied Physics.

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Anion photoelectron spectroscopy of germanium and tin clusters containing a transition- or lanthanide-metal atom; MGe_n^- (n = 8-20) and MSn_n^- (n = 15-17) (M = Sc-V, Y-Nb, and Lu-Ta)

J. Atobe, K. Koyasu, S. Furuse, A. Nakajima

Physical Chemistry Chemical Physics （Royal Society of Chemistry） 14 （ 26 ） 9403 - 9410 2012年02月

研究論文（学術雑誌）, 最終著者, 責任著者, 査読有り, ISSN 14639076

The electronic properties of germanium and tin clusters containing a transition- or lanthanide-metal atom from group 3, 4, or 5, MGe n (M = Sc, Ti, V, Y, Zr, Nb, Lu, Hf, and Ta) and MSn n (M = Sc, Ti, Y. Zr, and Hf), were investigated by anion photoelectron spectroscopy at 213 nm. In the case of the group 3 elements Sc, Y, and Lu, the threshold energy of electron detachment of MGe n- exhibits local maxima at n = 10 and 16, while in the case of the group 4 elements Ti, Zr, and Hf, it exhibits a local minimum only at n = 16, associated with the presence of a small bump in the spectrum. A similar behavior is observed for MSn n- around n = 16, and these electronic characteristics of MGe n and MSn n are closely related to those of MSi n. Compared to MSi n, however, the larger cavity size of a Ge n cage allows metal atom encapsulation at a smaller size n. A cooperative effect between the electronic and geometric structures of clusters with a large cavity of Ge 16 or Sn 16 is discussed together with the results of experiments that probe their geometric stability via their reactivity to H 2O adsorption. © 2012 the Owner Societies.

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Physical properties of aliphatic monolayer on indium-tin oxide and SnO₂(110) relevant to thermal stability of soft-landed Cr(benzene)₂

S. Nagaoka, K. Horiuchi, M. Shikishima, A. Nakajima

Journal of Physical Chemistry C （American Chemical Society） 115 （ 49 ） 24215 - 24220 2011年11月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 19327447

The physical properties of surface modified indium-tin oxide (ITO) and SnO 2(110) with an adsorbed aliphatic chain monolayer were studied by soft-landing experiments coupled with infrared (IR) spectroscopy, temperature programmed desorption (TPD) spectroscopy, and scanning probe microscopy. The IR spectra showed that the aliphatic chains of palmitic acid on both the metal oxide surfaces form a crystalline-phase monolayer with an all-trans conformation at room temperature. Gas-phase synthesized Cr(benzene) 2 cations deposited with a hyperthermal energy (20 eV) were used to probe the surface morphology. The IR spectrum after deposition showed that the cations soft-landed after being neutralized and oriented to cant the molecular axes far from the surface normal direction through penetration into the monolayer matrix. The TPD spectrum showed that the desorption activation energy of Cr(benzene) 2 from an aliphatic monolayer on ITO is much lower than that from the aliphatic monolayer on SnO 2 or a self-assembled monolayer of alkanethiol on Au(111). The lower thermal stability of Cr(benzene) 2 on the aliphatic monolayer on ITO can be ascribed to considerable surface roughness of the ITO itself, which can be characterized by scanning probe microscopy. © 2011 American Chemical Society.

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Soft-and reactive-landing of Cr(aniline)₂ sandwich complexes onto self-assembled monolayers: Separation between functional and binding sites

S. Nagaoka, K. Ikemoto, K. Horiuchi, A. Nakajima

Journal of the American Chemical Society （American Chemical Society） 133 （ 46 ） 18719 - 18727 2011年10月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 00027863

Soft-and reactive-landing of gas-phase synthesized cationic Cr(aniline)2 complexes onto self-assembled monolayers of methylterminated (CH3-SAM) and carboxyl-terminated (COOH-SAM) organothiolates coated on gold were performed at hyperthermal collision energy (5-20 eV). The properties of the Cr(aniline)2 complexes on the SAM surfaces were characterized using infrared reflection absorption spectroscopy (IRAS) and temperature-programmed desorption (TPD), together with theoretical calculations based on density functional theory (DFT). For the CH 3-SAM, the Cr(aniline)2 complexes were embedded inside the SAM matrix in a neutral charge state, keeping a sandwich structure. For the COOH-SAM, the IRAS and TPD study revealed that the amine-containing Cr(aniline)2 complexes were bound to the SAM surface in two forms of physisorption and chemical linking through an amide bond. In the desorption, the latter form appeared as the reaction product between organothiolates and Cr(aniline)2 above 400 K, where the organothiolate molecules, forming the SAM, were desorbed from the gold surface. The results show that the hyperthermal depositions onto a COOH-SAM bring about reactive-landing followed by covalent linking of an amide bond between the amine-containing Cr(aniline)2 complexes to the carboxyl-terminated SAM surface, in which the binding sites can be separated from the functional sites of the d-π interaction. © 2011 American Chemical Society.

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Theoretical investigation of a titanium-aniline complex with and without an alkyl chain

T. Iwasa, K. Horiuchi, M. Shikishima, Y. Noguchi, S. Nagaoka, A. Nakajima

Journal of Physical Chemistry C （American Chemical Society） 115 （ 33 ） 16574 - 16582 2011年07月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 1932-7447

Density functional theory computations are employed to investigate the geometric, electronic, and vibrational properties of a neutral Ti(aniline) experimentally soft-landed onto an alkanethiol self-assembled monolayer (CH-SAM) matrix. Five optimized structures of a bare Ti(aniline) are obtained: Three of the five optimized structures show Ti-phenyl ring bonds with singlet, triplet, and quintet spin states and the other two show Ti-N bonds with triplet and quintet states. For their calculated IR spectra in the range 0-2000 cm -1, the peak position and intensity of the NH2 scissor mode (∼1600 cm-1) are insensitive to the Ti binding and spin states, whereas those of the NH2-C6H5 intergroup stretching (∼1200 cm-1) and NH2 inversion (500-1000 cm-1) modes are sensitive to these factors. To study Ti(aniline) in a CH-SAM matrix, geometric structures of a modeled system of Ti(aniline)-C3H8 are optimized and their infrared reflection absorption spectra are directly calculated by projecting the normal mode derivatives of the dipole moment of the system onto the surface normal. C3H8 stabilizes Ti(aniline) by about 8-10 kcal/mol but causes little change in the IR spectrum in the range 1300-1700 cm-1, except for the quintet state. From a comparison of the predicted IRAS spectra with the experiment, Ti(aniline) in CH-SAM is concluded to be in a triplet state with a Ti-phenyl ring bond. © 2011 American Chemical Society.

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Photoemission enhancement induced by near-fields via local surface plasmon resonance of silver nanoparticles on a hydrogen-terminated Si(111) surface

T. Nakamura, N. Hirata, Y. Sekino, S. Nagaoka, A. Nakajima

Journal of Physical Chemistry C （American Chemical Society） 114 （ 39 ） 16270 - 16277 2010年09月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 1932-7447

The photoemission enhancement with local surface plasmon resonance (LSPR) was studied by two-photon photoemission (2PPE) spectroscopy for size-selected silver (Ag) and gold (Au) metal nanoparticles (NPs) deposited on a hydrogen-terminated Si(111)-(1 × 1) [H-Si(111)] surface. At 0.0015 monolayer equivalents (MLE) of Ag NPs, the photoemission enhancement was observed at a photon energy of 3.10 eV, which corresponds to the peak energy for the LSPR of isolated Ag NPs. A surface with around 0.005 MLE LSPR of size-selected Ag NPs exhibited three- and four-photon photoemission processes, implying monodispersed Ag NPs on H-Si(111). This enhancement could not be observed for Au NP deposition, even at 1.0 MLE in the photon energy range of 2.90-3.23 eV. Taking into account the polarization and photoemission-angle dependences, the photoemission enhancement could be accounted for by a mechanism involving the near-fields induced by the LSPR of Ag NPs. This mechanism is consistent with an analysis based on the effect of the interparticle distance between Ag NPs on the near-field intensity and polarization. © 2010 American Chemical Society.

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Two-photon photoemission spectroscopy for silver nanoparticles on a hydrogen-terminated Si(111) surface: Metal nanoparticle-enhanced photoemission

T. Nakamura, N. Hirata, S. Nagaoka, A. Nakajima

Chemical Physics Letters （Elsevier Science BV） 489 （ 1-3 ） 69 - 74 2010年04月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 00092614

Two-photon photoemission (2PPE) spectroscopy was employed for size-selected silver nanoparticles (Ag NPs) deposited on a hydrogen-terminated Si(1 1 1) [H-Si(1 1 1)] to reveal photoemission enhancement through local surface plasmon resonance (LSPR). At low coverage of Ag NPs around 0.005 monolayer equivalents (MLE), the photoemission from the H-Si(1 1 1) substrate was enhanced by ∼2 × 102 times. Above 0.10 MLE, in contrast, photoemission from the Ag NPs themselves was observed. From the experimental results of polarization and the photoemission angle dependence, the photoemission enhancement, depending on the coverage of Ag NPs, is attributed to near-fields from LSPR of Ag NPs. © 2010 Elsevier B.V. All rights reserved.

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Size-dependent magnetic properties of naked and ligand-capped nickel nanoparticles

A. Sugiyama, Y. Taguchi, S. Nagaoka, A. Nakajima

Chemical Physics Letters （Elsevier Science BV） 485 （ 1-3 ） 129 - 132 2010年01月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 0009-2614

Ligand-free naked and oleylamine-capped nickel nanoparticles (Ni NPs) were size-selectively prepared in the size ranges of 5-17 nm (naked) and 13-19 nm (capped) in diameter through gas- and liquid-phase syntheses, respectively. As for the magnetic properties of both the naked and oleylamine-capped Ni NPs, the blocking temperatures, TB, increase with the size, where TB for the naked Ni NPs are significantly higher than that for the capped Ni NPs. The results indicate that the ferromagnetic core region of Ni NPs is reduced by the formation of nonmagnetic layers at the surface of the capped NPs, where their thickness is estimated to be 1-2 nm. © 2009 Elsevier B.V. All rights reserved.

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Electronic structure of hydrogen-terminated silicon surfaces [H-Si(111)] studied by two-photon photoemission

T. Nakamura, K. Miyajima, N. Hirata, T. Matsumoto, Y. Morikawa, H. Tada, A. Nakajima

Applied Physics A: Materials Science and Processing （Springer Nature） 98 （ 4 ） 735 - 743 2009年12月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 09478396

The surface and bulk electronic states of hydrogen-terminated Si(111)-(1 × 1) [H-Si(111)] were studied by two-photon photoemission (2PPE) spectroscopy. With the polarization and emission-angle dependence of the 2PPE spectra, five series of electronic structures for H-Si(111) can be identified as A, B, C, D, and E in the 2PPE spectra by fine tuning the exciting photon energy, hv, from 4.23 to 4.90 eV. For hv < 4.6 eV, peak A arises from a surface resonance at a 1.01 eV from EF, and at the photon energy higher than 4.6 eV, peak A indicates photoemission from an intermediate state, corresponding to an image-potential state at 3.61 eV from EF. Other structures, B, C, and D arise from bulk states, while structure E occurs from an electronhole excitation process. This work has revealed the surface resonance and the image-potential state on H-Si(111) for the first time. © Springer-Verlag 2009.

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Characterization of alkyl monolayer covalently bonded to Si(111) and soft-landing of vanadium-benzene sandwich clusters onto the alkyl monolayer substrate

T. Matsumoto, S. Nagaoka, K. Ikemoto, M. Mitsui, M. Ara, H. Tada, A. Nakajima

European Physical Journal D （SPRINGER） 52 （ 1-3 ） 99 - 102 2009年03月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 1434-6060

A hexadecyl monolayer covalently attached to Si(111) surfaces (C 16-Si(111)) was prepared at 200 °C from 1-hexadecene. Formation of the monolayer was characterized by water contact angle measurement, attenuated total reflection infrared (ATR-IR) spectroscopy, and X-ray photoelectron spectroscopy (XPS). Gas phase synthesized vanadium (V)-benzene (Bz) 1:2 (VBz2) sandwich clusters were size-selectively deposited onto the C16-Si(111) substrate thus prepared and an oxidized Si substrate. Investigation of the resultant clusters was implemented by thermal desorption spectroscopy (TDS). About 30 K increase in threshold desorption temperature of the landed clusters was observed on going from the oxidized Si to the C16-Si(111) substrate, a result indicating that the clusters are more strongly bound to the C16-Si(111) than to the oxidized Si. This result was explained by the penetration of the landed clusters into the hexadecyl monolayer. © 2009 EDP Sciences, SIF, Springer-Verlag Berlin Heidelberg.

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An atomic force microscope study of vanadium-benzene sandwich clusters soft-landed on self-assembled monolayers

S. Nagaoka, K. Ikemoto, K. Fujio, K. Hiehata, A. Sasahara, M. Mitsui, H. Onishi, A. Nakajima

European Physical Journal D （Springer Nature） 52 （ 1-3 ） 103 - 106 2009年03月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 1434-6060

Multiple-decker vanadium-benzene sandwich clusters Vn(benzene) n+1 produced by a laser-vaporization synthesis method were soft-landed onto self-assembled monolayers of alkanethiol (C18H-SAM) and fluorinated alkanethiol (C10F-SAM) at 200 K. Noncontact atomic force microscopy has been used to examine the resulting adsorption states of the clusters landed on the SAMs at room temperature. For each SAM substrate, the aggregates of the deposited clusters were observed at the vacancy islands and near the steps of the SAM surface. The result indicates that, at room temperature, the clusters landed on the SAM substrate thermally diffuse on the surface to form columnar-shape islands around the defect sites of the SAM surface. © 2009 EDP Sciences, SIF, Springer-Verlag Berlin Heidelberg.

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Soft-landing experiments of Cr(benzene)₂ sandwich complexes onto a carboxyl-terminated self-assembled monolayer matrix

K. Ikemoto, S. Nagaoka, T. Matsumoto, M. Mitsui, A. Nakajima

Journal of Physical Chemistry C （American Chemical Society） 113 （ 11 ） 4476 - 4482 2009年02月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 19327447

Gas-phase synthesized Cr(benzene)2 sandwich complexes were soft-landed onto a carboxyl-terminated selfassembled monolayer (C 15COOH-SAM) on a gold substrate at hyperthermal collision energy (5-20 eV). The deposition process and adsorption properties of the soft-landed complexes were investigated by means of infrared reflection absorption spectroscopy (IRAS) and temperature programmed desorption (TPD) measurements. With the soft-landing, the Cr(benzene)2 complexes are adsorbed at the SAM surface while keeping their native sandwich structure intact even after the hyperthermal collision event. Moreover, the complexes tend to orient their molecular axes reasonably or comparatively close to the surface normal direction to the SAM surface. This orientation might arise from chemical interactions between the benzene ring in the sandwich complexes and carboxyl groups on the SAM surface. An Arrhenius analysis of the TPD spectra reveals the desorption activation energy of soft-landed complexes as 133 ± 10 kJ/mol, a value which is comparable to typical adsorption heat of chemisorptions. Chemical interactions between the Cr(benzene)2 complexes and carboxyl groups would provide firm trapping of the complexes at the SAM surface, preventing the supported complexes from being desorbed until near room temperature. © 2009 American Chemical Society.

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Anion photoelectron spectroscopy of transition metal- and lanthanide metal-silicon clusters: MSi_n^- (n = 6-20)

K. Koyasu, J. Atobe, S. Furuse, A. Nakajima

Journal of Chemical Physics （American Institute of Physics） 129 （ 21 ） 214301 - (7 pages) 2008年12月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 00219606

The electronic properties of silicon clusters containing a transition or lanthanide metal atom from group 3, 4, or 5, M Sin, (M=Sc, Ti, V, Y, Zr, Nb, Lu, Tb, Ho, Hf, and Ta) were investigated by anion photoelectron spectroscopy at 213 nm. In the case of the group 3 elements Sc, Y, Lu, Tb, and Ho, the threshold energy of electron detachment exhibits local maxima at n=10 and 16, while in case of the group 4 elements Ti, Zr, and Hf, the threshold energy exhibits a local minimum at n=16, associated with the presence of a small bump in the spectrum. These electronic characteristics of M Sin are closely related to a cooperative effect between their geometric and electronic structures, which is discussed, together with the results of experiments that probe their geometric stability via their reactivity to H2O adsorption, and with theoretical calculations. © 2008 American Institute of Physics.

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Soft-landing isolation of gas-phase-synthesized transition metal-benzene complexes into a fluorinated self-assembled monolayer matrix

S. Nagaoka, K. Ikemoto, T. Matsumoto, M. Mitsui, A. Nakajima

Journal of Physical Chemistry C （American Chemical Society） 112 （ 40 ） 15824 - 15831 2008年09月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 1932-7447

Gas-phase-synthesized chromium-benzene 1:2 sandwich cation complexes [Cr+(benzene)2] were soft-landed on a self-assembled monolayer (SAM) of fluorinated alkanethiol (C10F-SAM) at a hyperthermal collision energy of ∼20 eV. The adsorption properties and thermal stability of the soft-landed complexes were studied with infrared reflection absorption spectroscopy (IRAS) and temperature-programmed desorption (TPD). The landed complexes were neutralized due to charge transfer from the SAM substrate, but their native sandwich structure remained intact. The hyperthermal collision event resulted in the penetration of the incoming complexes into the C10F-SAM matrix. The embedded complexes then tended to orient their molecular axes approximately along the surface normal. This orientational preference is measurably different from that of complexes isolated in alkanethiol SAM matrices, a discrepancy that might be caused by a repulsive interaction between the n cloud of the capping benzene rings of the complex and the side-chain CF2 groups of the fluorocarbon chains in the C10F-SAM matrix. The thermal desorption study showed that the complexes supported inside the C10F-SAM could resist thermal desorption until the high-temperature region of ∼320 K, a persistence revealing a large desorption activation energy (∼190 kJ/mol). © 2008 American Chemical Society.

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Infrared Spectroscopy and Thermal Desorption Study of Vanadium-Mesitylene 1:2 Sandwich Clusters Soft-Landed onto a Long-Chain n-Alkanethiolate Self-Assembled Monolayer

M. Mitsui, S. Doi, K. Ikemoto, S. Nagaoka, A. Nakajima

J.-P. Connerade and A. Solov’yov (eds.) "Latest Advances in Atomic Cluster Collisions (the Proceedings of ISACC 2007)" （Imperial College Press, London.） 199 - 204 2008年09月

研究論文（国際会議プロシーディングス）, 共著, 最終著者, 責任著者, 査読有り

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Mass spectroscopic study of metal-rich K_2-5(KI)_n⁺ nanoclusters

S. H. Huh, H. S. Jeong, K. Koyasu, K. Miyajima, M. Mitsui, A. Nakajima

Journal of Molecular Structure （Elsevier Science BV） 886 （ 1-3 ） 39 - 42 2008年08月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 00222860

Metal-rich K1 - 5 (KI)n+ (n ∼27) clusters were produced by the supersonic cooling expansion of laser-induced KI plasma. The obtained mass spectroscopic peaks show the unique magic numbers at n = 9, 13, and 22 for K1(KI)n; n = 8, 12, and 21 for K2(KI)n; n = 8, 12, 16, and 21 for K3(KI)n; n = 7, 9, 11, and 13 for K4(KI)n; and n = 12 and 21 for K5(KI)n. These magic numbers are fairly different from those of other reported metal-rich Na1 - 9 (NaF)n+ and Cs1 - 5 (CsI)n+ clusters. It is interesting to note that K2, 3, 5 (KI)n+ clusters have the same magic numbers at n = 12 and 21 and possess structures that are closely related to the 3 × 3 × 3 and 3 × 3 × 5 cubic structures with the excess electron at the side anion vacancy or the center anion vacancy. © 2007 Elsevier B.V. All rights reserved.

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Experimental and theoretical characterization of MSi₁₆^-, MGe₁₆^-, MSn₁₆^-, and MPb₁₆^- (M=Ti, Zr, and Hf): The role of cage aromaticity

S. Furuse, K. Koyasu, J. Atobe, A. Nakajima

Journal of Chemical Physics （American Institute of Physics） 129 （ 6 ） 064311 - (6 pages) 2008年08月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 00219606

Silicon (Si), germanium (Ge), tin (Sn), and lead (Pb) clusters mixed with a group-4 transition metal atom [M=titanium (Ti), zirconium (Zr), and hafnium (Hf)] were generated by a dual-laser vaporization method, and their properties were analyzed by means of time-of-flight mass spectroscopy and anion photoelectron spectroscopy together with theoretical calculations. In the mass spectra, mixed neutral clusters of M Si16, M Ge16, and M Sn16 were produced specifically, but the yield of M Pb16 was low. The anion photoelectron spectra revealed that M Si16, M Ge16, and M Sn16 neutrals have large highest occupied molecular orbital-lowest unoccupied molecular orbital gaps of 1.5-1.9 eV compared to those of M Pb16 (0.8-0.9 eV), implying that M Si16, M Ge16, and M Sn16 are evidently electronically stable clusters. Cage aromaticity appears to be an important determinant of the electronic stability of these clusters: Calculations of nucleus-independent chemical shifts (NICSs) show that Si16 4-, Ge16 4-, and Sn16 4- have aromatic characters with negative NICS values, while Pb16 4- has an antiaromatic character with a positive NICS value. © 2008 American Institute of Physics.

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Mass spectrometry and photoelectron spectroscopy of o-, m-, and p-ternhenyl cluster anions: The effect of molecular shape on molecular assembly and ion core character

M. Mitsui, N. Ando, A. Nakajima

Journal of Physical Chemistry A （American Chemical Society） 112 （ 25 ） 5628 - 5635 2008年05月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 10895639

Mass spectrometry and photoelectron spectroscopy of o-, m-, and p-terphenyl cluster anions, (0-TP)n- (n = 2-100), (m-TP) n- (n = 2-100), and (p-TP)n- (n = 1-100), respectively, are conducted to investigate the effect of molecular shape on the molecular aggregation form and the resultant ion core character of the clusters. For (o-TP)n- and (m-TP)n-, neither magic numbers nor discernible isomers are observed throughout the size range. Furthermore, their vertical detachment energies (VDEs) increase up to large n and depend linearly on n-1/3, implying that they possess a three-dimensional (3D), highly reorganized structure encompassing a monomelic anion core. For (p-TP)n-, in contrast, prominent magic numbers of n = 5, 7, 10, 12, and 14 are observed, and the VDEs show pronounced irregular shifts below n = 10, while they remain constant above n = 14 (isomer A). These results can be rationalized with two-dimensional (2D) orderings of p-TP molecules and different types of 2D shell closure at n = 7 and 14, the monomelic and multimeric anion core, respectively. Above n = 16, the new feature (isomer B) starts to appear at the higher binding side of isomer A, and it becomes dominant with n, while isomer A gradually disappears for larger sizes. In contrast to isomer A, the VDEs of isomer B continuously increase with the cluster size. This characteristic size evolution suggests that the transition to modified 2D aggregation forms from 2D ones occurs at around n = 20. © 2008 American Chemical Society.

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Nanogranular structures formed by combinatorial control processing of size-selected metal nanoparticles

S. H. Huh, D. Riu, C. Kim, Y. Naono, T. Nakamura, A. Sugiyama, A. Nakajima

Journal of Applied Physics （American Institute of Physics） 103 （ 9 ） 094317 - (7 pages) 2008年05月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 0021-8979

Nanogranular films and lines, which are composed of size-selected Au, Ni, and Pt nanoparticles (NPs) and which exhibit an intermediate structure between that of the individual NPs and that of polycrystalline ones, were produced by a fully physical process. By using a combination of techniques such as lithography, lift-off, size selection, and aerosol deposition, well-designed Au lines (linewidth W=80 nm) and thickness-controlled rectangular patterns could be produced, while Ni and Pt NPs did not form continuous lines if the linewidth was less than W=150 nm. This difference might be explained by the extent of collision-induced surface melting during NP deposition onto the substrate. © 2008 American Institute of Physics.

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Photoelectron spectroscopy of cluster anions of naphthalene and related aromatic hydrocarbons

N. Ando, M. Mitsui, A. Nakajima

Journal of Chemical Physics （American Institute of Physics） 128 （ 15 ） 154318 - (8 pages) 2008年04月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 0021-9606

The electronic structures and structural morphologies of naphthalene cluster anions, (naphthalene) n- (n=3-150), and its related aromatic cluster anions, (acenaphthene) n- (n=4-100) and (azulene) n- (n=1-100), are studied using anion photoelectron spectroscopy. For (naphthalene) n- clusters, two isomers coexist over a wide size range: isomers I and II-1 (28≤n≤60) or isomers I and II-2 (n∼60). Their contributions to the photoelectron spectra can be separated using an anion beam hole-burning technique. In contrast, such an isomer coexistence is not observed for (acenaphthene) n- and (azulene) n- clusters, where isomer I is exclusively formed throughout the whole size range. The vertical detachment energies (VDEs) of isomer I (7≤n≤100) in all the anionic clusters depend linearly on n-13 and their size-dependent energetics are quite similar to one another. On the other hand, the VDEs of isomers II-1 and II-2 produced in (naphthalene) n- clusters with n∼30 remain constant at 0.84 and 0.99 eV, respectively, 0.4-0.6 eV lower than those of isomer I. Based upon the ion source condition dependence and the hole-burning photoelectron spectra experiments for each isomer, the energetics and characteristics of isomers I, II-1, and II-2 are discussed: isomer I is an internalized anion state accompanied by a large change in its cluster geometry after electron attachment, while isomers II-1 and II-2 are crystal-like states with little structural relaxation. The nonappearance of isomers II-1 and II-2 for (acenaphthene) n- and (azulene) n- and a comparison with other aromatic cluster anions indicate that a highly anisotropic and symmetric π -conjugated molecular framework, such as found in the linear oligoacenes, is an essential factor for the formation of the crystal-like ordered forms (isomers II-1 and II-2). On the other hand, lowering the molecular symmetry makes their production unfavorable. © 2008 American Institute of Physics.

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Thermal and hyperthermal collision-energy depositions of transition metal-benzene sandwich complexes onto a self-assembled n-octadecanethiol monolayer

S. Nagaoka, K. Ikemoto, T. Matsumoto, M. Mitsui, A. Nakajima

Journal of Physical Chemistry C （American Chemical Society） 112 （ 17 ） 6891 - 6899 2008年04月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 1932-7447

Gas-phase synthesized transition metal-benzene sandwich complexes of M(benzene)2 (M = Ti, V, and Cr) are soft-landed onto a self-assembled monolayer of n-octadecanethiol (C18-SAM) at a collision energy of 10-20 eV. The resulting adsorption states and thermal desorption kinetics of the soft-landed complexes are studied with infrared reflection absorption spectroscopy and temperature-programmed desorption. The complexes keep their native sandwich structure intact on the SAM substrate even after the "hyperthermal" deposition event. The soft-landed complexes are oriented with their molecular axes largely tilted off the surface normal of the SAM substrate and exhibit unusually large desorption activation energies (E d = ∼130 kJ/mol). For comparison, thermal deposition (∼25 meV) of Cr(benzene)2 vapor onto the C18-SAM, carried out using a physical vapor deposition technique, showed that the complexes are weakly physisorbed (Ed = ∼70 kJ/mol) on the SAM with a random orientation. Only a hyperthermal collision event allows the incident complexes to penetrate into the SAM matrix. The desorption of the embedded complexes in the SAM is then suppressed to around room temperature and may be associated with the crystal-rotator phase transitions of the SAM matrix. © 2008 American Chemical Society.

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Nanosurface-confined anisotropic growth and magnetism of ellipsoidal α-Fe nanogranules

S. H. Huh, C. Kim, D. Riu, Y. Taguchi, A. Nakajima

Journal of Nanoscience and Nanotechnology （American Scientific Publishers） 8 （ 4 ） 1959 - 1964 2008年04月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 1533-4880

We have investigated the nanosurface-confined anisotropic growth of ordered-ellipsoidal Fe nanogranules when an Fe plume was deposited at a slanting angle onto an anodized aluminum oxide (AAO) film. Layer-by-layer growth was also investigated. This growth is driven by two critical factors: (1) a new rhombic AAO cell and (2) the slanting deposition of the Fe plume. During slanting deposition, the rhombic AAO cell induces strong restrictions in the nucleation site, growth direction, and granular size; therefore, the degree of freedom during growth is restricted. The magnetic dipoles of the ordered Fe nanogranules are placed along the long axis of the ellipsoid at an angle of 180° (antiparallel) due to the demagnetizing field, shape anisotropy, and magnetic dipole-to-dipole interactions. Copyright © 2008 American Scientific Publishers All rights reserved.

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Stern-Gerlach study of multidecker lanthanide-cyclooctatetraene sandwich clusters

K. Miyajima, M. B. Knickelbein, A. Nakajima

Journal of Physical Chemistry A （American Chemical Society） 112 （ 3 ） 366 - 375 2008年01月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 1089-5639

Multilayer lanthanide-cyclooctatetraene organometallic clusters, Ln n(C8H8)m, (Ln = Eu, Tb, Ho, Tm; n = 1 - 7; m = n - 1, n, n + 1) were produced by a laser vaporization synthesis method. The magnetic deflections of these organometallic sandwich clusters were measured by a molecular beam magnetic deflection technique. Most of the sandwich species displayed one-sided deflection, while some of smaller Ln-C 8H8 clusters showed symmetric broadening without or with only very small (or absent) net high-field deflection. In general, the total magnetic moments, calculated from the magnitude of the beams deflections, increase with the number of lanthanide atoms (i.e., with increasing sandwich layers); however for Tb-, Ho-, and Tm-C8H8 clusters with n > 3, the suppression of the magnetic moments was observed, possibly through antiferromagnetic interactions. For Eu-C8H8 clusters, we observe a linear increase of the magnetic moments with the number of Eu atoms up to n = 7, with average magnetic moment per Eu atom around 7 μB - similar to that displayed by conventionally synthesized mononuclear Eu IIC8H8 complexes, indicating that Eu atoms exist as Eu2+ ions in the full sandwich Eun(C 8H8)n+1 clusters. These results suggest that Eun(C8H8)n+1 is a promising candidate for a high-spin, one-dimensional building block in organometallic magnetic materials. © 2008 American Chemical Society.

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Comprehensive photoelectron spectroscopic study of anionic clusters of anthracene and its alkyl derivatives: Electronic structures bridging molecules to bulk

N. Ando, M. Mitsui, A. Nakajima

Journal of Chemical Physics （American Institute of Physics） 127 （ 23 ） 234305 - (13 pages) 2007年12月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 00219606

The evolution of the electronic structure of molecular aggregates is investigated using anion photoelectron (PE) spectroscopy for anionic clusters of anthracene (Ac) and its alkyl derivatives: 1-methylanthracene (1MA), 2-methylanthracene (2MA), 9-methylanthracene (9MA), 9,10-dimethylanthracene (DMA), and 2-tert-butylanthracene (2TBA). For their monomer anions (n=1), electron affinities are confined to the range from 0.47 to 0.59 eV and are well reproduced by density functional theory calculations, showing the isoelectronic character of these molecules. For cluster anions (n=2-100) of Ac and 2MA, two types of isomers I and II coexist over a wide size range: isomers I and II-1 (4≤n<30) or isomers I and II-2 (n∼40 for Ac and n∼55 for 2MA). However, for the other alkyl-substituted Ac cluster anions (i.e., 1MA, 9MA, DMA, and 2TBA), only isomer I is exclusively formed, and neither isomer II-1 nor II-2 is observed. The vertical detachment energies (VDEs) of isomer I in all the anionic clusters depend almost linearly on n-13. In contrast, the VDEs of isomers II-1 (n14) and II-2 (n=40-100), appeared only in Ac and 2MA cluster anions, remain constant with n and are ∼0.5 eV lower than those of isomer I. The PE spectra revealed the characteristics of each isomer: isomer I possesses a monomeric anion core that is gradually embedded into the interior of the cluster with increasing n. On the other hand, isomers II-1 and II-2 possess a multimeric (perhaps tetrameric) anion core, but they differ in the number of layers from which they are made up; monolayer (isomer II-1) and multilayers (isomer II-2) of a two-dimensionally ordered, finite herringbone-type structure, in which electron attachment produces only little geometrical rearrangement. Moreover, the agreement of the constant VDEs of isomer II-2 with the bulk data demonstrates the largely localized nature of the electronic polarization around the excess charge in a crystal-like environment, where about 50 molecules provide a charge stabilization energy comparable to the bulk. © 2007 American Institute of Physics.

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Geometric and electronic structures of terbium-silicon mixed clusters (TbSi_n; 6 ≤ n ≤ 16)

M. Ohara, K. Miyajima, A. Pramann, A. Nakajima, K. Kaya

Journal of Physical Chemistry A （American Chemical Society） 111 （ 42 ） 10884 - 10884 2007年09月

研究論文（学術雑誌）, 共著, 責任著者, 査読有り, ISSN 1089-5639

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Mass spectrometry and photoelectron spectroscopy of tetracene cluster anions, (tetracene)_n^- (n = 1-100): Evidence for the highly localized nature of polarization in a cluster analogue of oligoacene crystals

M. Mitsui, N. Ando, A. Nakajima

Journal of Physical Chemistry A （American Chemical Society） 111 （ 39 ） 9644 - 9648 2007年08月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 10895639

Photoelectron spectroscopy of tetracene cluster anions, (tetracene) n- (n = 1 -100), reveals the coexistence of two types of isomers, designated as isomers I and II-1 (n = 10-50) or isomers I and II-2 (n > 60), in a wide size range. The vertical detachment energies (VDEs) of isomer I increase persistently due to polarization and structural relaxation effects, where a monomelic anion core is encompassed with geometrically reorganized neutral molecules. Conversely, a characteristic ion distribution in the mass spectrum of (tetracene)n- ensues from the two-dimensional (2D) herringbone-type ordering of isomer II-1, whose VDEs remain constant at 1.80 eV for n ≥ 14. Also, isomer II-2, presumably adopting multilayered structural motifs, exhibits invariable VDEs of 2.0 eV, a manifestation of significant charge screening effects in these isomers. The invariable nature of the VDEs of isomers II-1 and II-2 unambiguously demonstrates a largely localized nature of polarization induced by the excess charge residing in microscopic crystal-like environments. Surprisingly, only 14 tetracene molecules within a 2D herringbone-type layer including an excess charge can provide the charge stabilization energy corresponding to ∼80% of that of the crystal, and the rest of the energy is provided by polarization of neutral molecules in adjacent layers. © 2007 American Chemical Society.

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Generation of high-quality lines and arrays using nanoparticle controlling processes

S. H. Huh, D. Riu, Y. Naono, Y. Taguchi, S. Kawabata, A. Nakajima

Applied Physics Letters （American Chemical Society） 91 （ 9 ） 093118 2007年08月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 0003-6951

Highly reliable lines and arrays comprising the smallest metallic nanoparticles (NPs) were produced by using a combination of techniques such as lithography, lift-off, size selection, aerosol deposition, and NP colloids. The narrowest 80 nm Au lines were produced by the deposition of only 4 nm Au NPs, driven by two factors-NP kinetics and collision-induced surface melting. An array of unsupported free 30-100 Ni NPs and one of 20-40 nm Ni NPs supported in holes were produced by the principles of hole-size-induced filtering and the binding of thiol groups. © 2007 American Institute of Physics.

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Stern-Gerlach experiments of one-dimensional metal-benzene sandwich clusters: M_n(C₆H₆)_m (M = Al, Sc, Ti, and V)

K. Miyajima, S. Yabushita, M. B. Knickelbein, A. Nakajima

Journal of the American Chemical Society （American Chemical Society） 129 （ 27 ） 8473 - 8480 2007年06月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 00027863

A molecular beam of multilayer metal-benzene organometallic clusters M n(C6H6)m (M = Al, Sc, Ti, and V) was produced by a laser vaporization synthesis method, and their magnetic deflections were measured. Multidecker sandwich clusters of transition-metal atoms and benzene Scn(C6H6)n+1 (n = 1, 2) and Vn(C6H6)n+1 (n = 1-4) possess magnetic moments that increase monotonously with n. The magnetic moments of Al(C6H6), Scn(C6H 6)n+1, and Vn(C6H6) n+1 are smaller than that of their spin-only values as a result of intracluster spin relaxation, an effect that depends on the orbital angular momenta and bonding characters of the orbitals containing electron spin. While Ti(C6H6)2 was found to be nonmagnetic, Ti n(C6H6)n+1 (n = 2, 3) possess nonzero magnetic moments. The mechanism of ferromagnetic spin ordering in M 2(C6H6)3 (M = Sc, Ti, V) is discussed qualitatively in terms of molecular orbital analysis. These sandwich species represent a new class of one-dimensional molecular magnets in which the transition-metal atoms are formally zerovalent. © 2007 American Chemical Society.

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Designable formation of metal nanoparticle array with the deposition of negatively charged nanoparticles

S. Kawabata, Y. Naono, Y. Taguchi, S. H. Huh, A, Nakajima

Applied Surface Science （Elsevier Science BV） 253 （ 16 ） 6690 - 6696 2007年06月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 0169-4332

Ultrafine gold and platinum nanoparticles (NPs) were fabricated by pulsed laser ablation in helium gas and the NPs 2-15 nm in diameter were selectively classified by an electrostatic size-selection technique employing a low-pressure differential mobility analyzer (LP-DMA). The spherical NPs obtained showed a narrower distribution of diameters of anionic NPs over cationic NPs. With this knowledge, the anionic NPs were deposited onto silicon substrates designed by electron beam lithography processing, and designable patterns of arrayed NPs were produced by removing the photoresist layer. © 2007 Elsevier B.V. All rights reserved.

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Formation of Large molecular cluster anions and elucidation of their electronic structures

M. Mitsui, A. Nakajima

Bulletin of the Chemical Society of Japan （Chemical Society of Japan） 80 （ 6 ） 1058 - 1074 2007年06月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 0009-2673

The present account describes our efforts to understand the microscopic aspects of condense phase phenomena, such as ion solvation in molecular liquids and charge carrier localization in molecular solids, by viewing finite-size molecular clusters as their embryonic forms. In this effort, we efficiently prepared supersonically cooled, isolated molecular cluster anions with up to more than 100 constituent molecules and size-selectively investigated their electronic structures using anion photoelectron spectroscopy. Large anionic clusters of two different types of organic molecule; acetonitrile and naphthalene reported as examples of the noteworthy results obtained in our studies. In these two systems, we found that energetically close anionic isomers coexist over a broad range in size, and their contribution in the photoelectron spectra could be separated using anion beam hole-burning technique. A detailed inspection into the electronic states and size-dependent energetics of each isomer has enabled us to establish a link from the large finite cluster to the infinite bulk system. © 2007 The Chemical Society of Japan.

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Fabrication and characterization of a granular film consisting of size-selected silver nanoparticles: Application to a SERS substrate

S. Kawabata, N. Ishikawa, M. Mitsui, A. Nakajima

European Physical Journal D （Springer Nature） 43 （ 1-3 ） 155 - 158 2007年05月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 14346060

Uniform-sized silver nanoparticles with average diameter of 13.7 nm have been prepared in the gas-phase by combining a pulsed laser ablation method with a low pressure-differential mobility analyzer (LP-DMA). By depositing the silver nanoparticles onto a silicon substrate, a granular film consisting of size-selected silver nanoparticles has been fabricated and its morphology and electronic properties have been examined using transmission electron microscopy (TEM) and UV-visible absorption spectroscopy. This granular silver film serves as a highly active substrate for surface-enhanced Raman scattering (SERS). © EDP Sciences/Società Italiana di Fisica/Springer-Verlag 2007.

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Magnetic trapping of nanometer- and micrometer-sized particles

S. H. Huh, C. Y. Kim, H. S. Jeong, A. Nakajima

Journal of Applied Physics （American Institute of Physics） 101 （ 10 ） 104317 2007年05月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 0021-8979

Nanometer- and micrometer-sized particles (NPs and MPs) of gas-phase Co, Fe, and Fe-Nd-Dy-B were magnetically trapped on a substrate. For the magnetic trapping of NPs, two stepwise evolutions were investigated: (1) the formation of NP aggregates above a critical diameter of 8 nm and (2) increase in the length to a few micrometers, which is the result of the equilibrium between the magnetostatic and thermal energies of NPs and the granular linear chains such as granular nanorods and nanofibers. The magnetic trapping of MPs or MPsNPs is particularly interesting because of the sequential pearl-necklace-like growth and magnetic separation in the linear chains. © 2007 American Institute of Physics.

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Soft-landing isolation of multidecker V₂(benzene)₃ complexes in an organic monolayer matrix: An infrared spectroscopy and thermal desorption study

S. Nagaoka, T. Matsumoto, K. Ikemoto, M. Mitsui, A. Nakajima

Journal of the American Chemical Society （American Chemical Society） 129 （ 6 ） 1528 - 1529 2007年01月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 00027863

Gas-phase synthesized multidecker V2(benzene)3 sandwich complexes were soft-landed onto a self-assembled monolayer of an n-octadecanethiol (C18-SAM). The infrared absorption spectrum of this complex supported on the C18-SAM was obtained by infrared reflection absorption spectroscopy (IRAS). The vibrational assignment of the IRAS spectrum for the V2(benzene)3 sandwich was based on a harmonic frequency analysis that employed density functional theory. The close similarity between the experimental and calculated results for the IR absorption frequencies demonstrates that the V2(benzene)3 complexes have a multidecker sandwich structure on the SAM substrate. Moreover, our thermal desorption study established that the sandwich complexes can be isolated on the SAM substrate up to a high temperature of ∼350 K. Copyright © 2007 American Chemical Society.

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Electronic properties of Si and Ge atoms doped In clusters: In_nSi_m and In_nGe_m

M. Akutsu, K. Koyasu, J. Atobe, K. Miyajima, M. Mitsui, A. Nakajima

Journal of Physical Chemistry A （American Chemical Society） 111 （ 4 ） 573 - 577 2007年01月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 10895639

Electronic properties of silicon and germanium atom doped indium clusters, InnSim and InnGem, were investigated by photoionization spectroscopy of the neutrals and photoelectron spectroscopy of the anions. Size dependence of ionization energy and electron affinity for InnSi1 and InnGe1 exhibit pronounced even-odd alternation at cluster sizes of n = 10-16, as compared to those for pure Inn clusters. This result shows that symmetry lowering with the doped atom of Si or Ge results in undegeneration of electronic states in the Id shell formed by monovalent In atoms. © 2007 American Chemical Society.

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Electronic and geometric stabilities of clusters with transition metal encapsulated by silicon

K. Koyasu, J. Atobe, M. Akutsu, M. Mitsui, A. Nakajima

Journal of Physical Chemistry A （American Chemical Society） 111 （ 1 ） 42 - 49 2006年12月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 1089-5639

Silicon clusters mixed with a transition metal atom, MSia, were generated by a double-laser vaporization method, and the electronic and geometric stabilities for the resulting clusters with transition metal encapsulated by silicon were examined experimentally. By means of a systematic doping with transition metal atoms of groups 3, 4, and 5 (M = Sc, Y, Lu, Ti, Zr, Hf, V, Nb, and Ta), followed by changes of charge states, we explored the use of an electronic closing of a silicon caged cluster and variations in its cavity size to facilitate metal-atom encapsulation. Results obtained by mass spectrometry, anion photoelectron spectroscopy, and adsorption reactivity toward H2O show that the neutral cluster doped with a group 4 atom features an electronic and a geometric closing at n = 16. The MSi16 cluster with a group 4 atom undergoes an electronic change in (i) the number of valence electrons when the metal atom is substituted by the neighboring metals with a group 3 or 5 atom and in (ii) atomic radii with the substitution of the same group elements of Zr and Hf. The reactivity of a halogen atom with the MSi 16 clusters reveals that VSi16F forms a superatom complex with ionic bonding. © 2007 American Chemical Society.

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Experimental and theoretical characterization of aluminum-based binary superatoms of Al₁₂X and their cluster salts

M. Akutsu, K. Koyasu, J. Atobe, N. Hosoya, K. Miyajima, M. Mitsui, A. Nakajima

Journal of Physical Chemistry A （American Chemical Society） 110 （ 44 ） 12073 - 12076 2006年10月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 10895639

The geometric and electronic structures of aluminum binary clusters, AlnX (X = Si and P), have been investigated, using mass spectrometry, anion photoelectron spectroscopy, photoionization spectroscopy, and theoretical calculations. Both experimental and theoretical results show that Al 12Si has a high ionization energy and low electron affinity and Al12P has a low ionization energy, both with the icosahedral structure having a central Si or P atom, revealing that Al12Si and Al12P exhibit rare-gas-like and alkali superatoms, respectively. Experiments confirmed the possibility that the change in the total number of valence electrons on substitution could produce ionically bound binary superatom complexes, the binary cluster salts Al12P+F- and Al12B-Cs+. © 2006 American Chemical Society.

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Adiabatic electron affinities of oligophenyls: Anion photoelectron spectroscopy and density functional theory study

Nakamura T., Ando N., Matsumoto Y., Furuse S., Mitsui M., Nakajima A.

Chemistry Letters （CHEMICAL SOC JAPAN） 35 （ 8 ） 888 - 889 2006年08月

研究論文（学術雑誌）, 最終著者, 責任著者, 査読有り, ISSN 03667022

Adiabatic electron affinities (EA a's) of seven oligophenyls: biphenyl (BP), 4-cyanobiphenyl (CNBP), o-terphenyl (o-TP), m-terphenyl (m-TP), p-terphenyl (p-TP), p-quaterphenyl (p-QP), and 1,3,5-triphenylbenzene (TPB), have been evaluated by anion photoelectron spectroscopy and density functional theory calculations. The result provides the direct determination of the EA a values of the isolated CNBP, p-TP, p-QP, and TPB molecules, while EA a of BP, o-TP, and m-TP were estimated from the extrapolation of EA a vs cluster size. The excellent agreement between the experimental and calculated EA a were obtained. Copyright © 2006 The Chemical Society of Japan.

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Room-temperature isolation of V(benzene)₂ sandwich clusters via soft-landing into n-alkanethiol self-assembled monolayers

S. Nagaoka, T. Matsumoto, E. Okada, M. Mitsui, A. Nakajima

Journal of Physical Chemistry B （American Chemical Society） 110 （ 32 ） 16008 - 16017 2006年07月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 15206106

The adsorption state and thermal stability of V(benzene)2 sandwich clusters soft-landed onto a self-assembled monolayer of different chain-length n-alkanethiols (Cn-SAM, n -8, 12, 16, 18, and 22) were studied by means of infrared reflection absorption spectroscopy (IRAS) and temperature-programmed desorption (TPD). The IRAS measurement confirmed that V(benzene)2 clusters are molecularly adsorbed and maintain a sandwich structure on all of the SAM substrates. In addition, the clusters supported on the SAM substrates are oriented with their molecular axes tilted 70-80° off the surface normal. An Arrhenius analysis of the TPD spectra reveals that the activation energy for the desorption of the supported clusters increases linearly with the chain length of the SAMs. For the longest chain C22-SAM, the activation energy reaches ∼150 kJ/mol, and the thermal desorption of the supported clusters can be considerably suppressed near room temperature. The clear chain-length-dependent thermal stability of the supported clusters observed here can be explained well in terms of the cluster penetration into the SAM matrixes. © 2006 American Chemical Society.

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Photoelectron spectroscopy of binary Au cluster anions with a doped metal atom: Au_nM^- (n = 2-7), M = Pd, Ni, Zn, Cu, and Mg

K. Koyasu, Y. Naono, M. Akutsu, M. Mitsui, A. Nakajima

Chemical Physics Letters （Elsevier Science BV） 422 （ 1-3 ） 62 - 66 2006年04月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 00092614

The electronic structures of binary gold cluster anions: AunM- (M = Pd, Ni, Zn, Cu, Mg) were investigated by anion photoelectron spectroscopy at the photon energy of 4.66 eV. The spectra exhibit even-odd alternation of electron affinities and characteristic spectral changes with n, which are accountable in terms of the interactions between the Au clusters and doped metal atoms. For the binary clusters containing Pd, Ni, and Cu, their valence electrons are shared with those of the Au clusters, whereas for Mg the doped atom is weakly bound to Au clusters. A size-dependent spectral change is clearly observed for AunPd- at n ≈ 5. © 2006 Elsevier B.V. All rights reserved.

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Electronic properties of Cs-atom doped aluminum and silicon clusters: Al_nCs_m and Si_nCs_m

K. Koyasu, M. Akutsu, J. Atobe, M. Mitsui, A. Nakajima

Chemical Physics Letters （Elsevier Science BV） 421 （ 4-6 ） 534 - 539 2006年04月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 00092614

The effect of Cs atom doping on metallic aluminum clusters and covalent silicon clusters, AlnCsm (n = 5-14, m = 0-3) and SinCsm (n = 5-16, m = 0-3), was examined by mass spectrometry and anion photoelectron spectroscopy. For clusters containing Cs atom(s), the electron affinities of both clusters are generally decreased and the following characteristic features are observed: for AlnCsm, Cs-atom doping causes (1) electron filling into the electronic shell structure of the Aln clusters and (2) geometrical packing of icosahedral 13-atoms, while for SinCsm Cs-atom doping enhances electronic stability to be ascribed to pure Sin clusters, particularly at (n, m) = (10, 3) and (13, 1). © 2006 Elsevier B.V. All rights reserved.

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Laser synthesis and magnetism of amorphous iron and cobalt carbide nanoparticles with carbon onion

S. H. Huh, A. Nakajima

Journal of Applied Physics （American Institute of Physics） 99 （ 6 ） 064302 - (5 pages) 2006年03月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 0021-8979

Amorphous iron or cobalt carbide nanoparticles with a carbon onion structure were produced using a pulsed laser ablation of metallocene powders at room temperature. While they are ferromagnetic at room temperature, they exhibit a drastic change in magnetic spin moments at 20-30 K. These magnetic properties are ascribed to a considerable magnetic contribution of superparamagnetism for the amorphous iron or cobalt carbide nanoparticles with a carbon onion and the origin of the magnetic properties is discussed. © 2006 American Institute of Physics.

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Classification and characterization of gold and nickel nanoparticles with a differential mobility analyzer

Y. Naono, S. Kawabata, S. H. Huh, A. Nakajima

Science and Technology of Advanced Materials （National Institute for Material Science） 7 （ 2 ） 209 - 215 2006年03月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 1468-6996

Ultrafine gold and nickel nanoparticles (NPs) were fabricated by pulsed laser ablation in helium gas, and the NPs 2-15 nm in diameter were selectively classified by an electrostatic size-selection technique employing a low-pressure differential mobility analyzer (LP-DMA). In the classification of the NPs, a chain of several gold NPs or doubly charged NPs were produced as well as singly charged spherical NPs. The morphological selection was performed with the LP-DMA, using a relationship in which the cross section of the spherical NP chains was governed by the product of the diameter and the length, instead of by the square of the diameter. Below a blocking temperature of about 100 K, the Ni NPs that were 3.1 nm in diameter exhibited ferromagnetism. © 2006 Elsevier Ltd. All rights reserved.

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Soft-landing isolation of vanadium-benzene sandwich clusters on a room-temperature substrate using n-alkanethiolate self-assembled monolayer matrixes

M. Mitsui, S. Nagaoka, T. Matsumoto, A. Nakajima

Journal of Physical Chemistry B （American Chemical Society） 110 （ 7 ） 2968 - 2971 2006年02月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 1520-6106

Gas-phase synthesized vanadium-benzene 1:2 (VBz2) sandwich clusters were size-selectively deposited onto bare gold and long-chain n-alkanethiolate [-S-(CH2)n-1-CH3; n = 16, 18, and 22] self-assembled monolayer (SAM)-coated gold substrates under ultrahigh vacuum (UHV) conditions. Investigation of the resulting deposited clusters was performed by infrared reflection absorption spectroscopy (IRAS) and thermal desorption spectroscopy (TDS). The IR frequencies of the soft-landed VBz 2 clusters show excellent agreement with the fundamentals reported in IR data of VBz2 in an argon matrix. The analysis of IRAS spectra reveals that while there was no orientational preference of the VBz2 clusters on a bare gold substrate, the VBz2 clusters deposited onto the SAM substrates were highly oriented with the molecular axis 70-80° tilted off the surface normal. In addition, analysis of TDS spectra revealed unusually large adsorption heats of the physisorbed VBz2 clusters. The present results are explained by cluster penetration into the long-chain alkanethiolate SAM and for the first time demonstrate the matrix isolation of gas-phase organometallic clusters around room temperature. © 2006 American Chemical Society.

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Determination of the excited-state structure of 7-azaindole-water cluster using a Franck-Condon analysis

R. Brause, D. Krügler, M. Schmitt, K. Kleinermanns, A. Nakajima, T. A. Miller

Journal of Chemical Physics （American Institute of Physics） 123 （ 22 ） 224311 2005年12月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 0021-9606

The change of the 7-azaindole-water cluster structure upon electronic excitation was determined by a Franck-Condon analysis of the intensities in the fluorescence emission spectra obtained via excitation of five different vibronic bands. A total of 105 emission band intensities were fitted, together with the changes of rotational constants of one isotopomer. These rotational constants have been obtained from a fit to the rovibronic contour of the cluster. The geometry change upon electronic excitation to the π π* state can be described by a strong and asymmetric shortening of the hydrogen bonds and a deformation of both the pyridine and the pyrrole rings of 7-azaindole. The resulting geometry changes are interpreted on the basis of ab initio calculations. © 2005 American Institute of Physics.

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New world in soft-nano-materials

K. Kaya, A. Nakajima, M. Mitsui

Proceedings - 2005 International Conference on MEMS, NANO and Smart Systems, ICMENS 2005 （IEEE COMPUTER SOC） 231 - 232 2005年11月

研究論文（国際会議プロシーディングス）, 共著, 査読有り

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Magnetic properties of lanthanide organometallic sandwich complexes produced in a molecular beam

K. Miyajima, M. B. Knickelbein, A. Nakajima

Polyhedron （Pergamon-Elsevier Science Ltd.） 24 （ 16-17 ） 2341 - 2345 2005年11月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 0277-5387

The magnetic properties of gas-phase terbium-cyclooctatetraene multi-decker sandwich complexes, Tbn(C8H8)n+1 were measured using a Stern-Gerlach type magnetic deflection approach. Beams of Tbn(C8H8)n+1 complexes displayed one-sided deflection toward high field - indicating that fast spin relaxation occurs within the complexes as they pass through the magnetic field. The magnetic moment for Tbn(C8H8)n+1 (n = 1-5) was evaluated using the Langevin model. Evolution of magnetic moment with the complex size is discussed with electronic structures for oxidation states of Tb3+/2+ ions, implying the possibility of antiferromagnetic interaction of two adjacent Tb2+ ions. © 2005 Elsevier Ltd. All rights reserved.

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Preparation of a SERS substrate using vacuum-synthesized silver nanoparticles

N. Horimoto, N. Ishikawa, A. Nakajima

Chemical Physics Letters （Elsevier Science BV） 413 （ 1-3 ） 78 - 83 2005年09月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 0009-2614

Vacuum-synthesized Ag nanoparticles were dispersed and immobilized on an organosilane-functionalized glass surface in water. Nearly spherical particles with diameters of ≈ 200 nm were used for a substrate for surface enhanced Raman scattering (SERS). Molecules deposited on this substrate in both gas and liquid phases were detected by SERS. The SERS efficiency of this substrate was compared with that of Ag colloidal substrate and the morphological difference of these SERS substrates was studied by scanning electron microscopy. The aggregates of 5-10 Ag nanoparticles formed at the spread area exhibited prominent SERS activity. © 2005 Elsevier B.V. All rights reserved.

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Formation and photoelectron spectroscopy of nanoscale cluster anions of biphenyl, (BP)_n^- (n = 2-100)

M. Mitsui, Y. Matsumoto, N. Ando, A. Nakajima

Chemistry Letters （Chemical Society of Japan） 34 （ 9 ） 1244 - 1245 2005年08月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 03667022

Evolution of electronic structures of nanoscale biphenyl cluster anions, (BP) n- (n = 2-100), have been studied by photoelectron spectroscopy in a size-selective fashion. The energetics of vertical detachment energy (VDE) revealed negative electron affinity of a biphenyl molecule and substantial cluster reorganizations via electron attachment. Comparison of the VDE energetics with naphthalene cluster anions implied that the freezing (and/or melting) temperatures of the BP clusters may be much lower than those of naphthalene clusters. Copyright © 2005 The Chemical Society of Japan.

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Trapping of V(benzene)₂ sandwich clusters in a n-alkanethiol self-assembled monolayer matrix

S, Nagaoka, E. Okada, S. Doi, M. Mitsui, A. Nakajima

European Physical Journal D （Springer Nature） 34 （ 1-3 ） 239 - 242 2005年07月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 14346060

V(benzene)2 sandwich cluster cations produced in the gas phase were size-selectively deposited onto a self-assembled monolayer of n-hexadecanethiols (HDT-SAM) chemisorbed on a Au(111) surface as well as onto a bare Au(111) surface. The thermal chemistry of the neutralized clusters on each substrate was studied with temperature programmed desorption (TPD). From the analyses of the threshold in the TPD, the desorption activation energies of the clusters deposited were determined to be 64.4 ±12.8 kJ/mol for the Au(111) and 130 ±10 kJ/mol for the HDT-SAM. The remarkably large desorption activation energy from the SAM suggests that the deposited clusters are incorporated into the SAM matrix and firmly trapped inside the alkyl chains of the SAM. © EDP Sciences, Società Italiana di Fisica, Springer-Verlag 2005.

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Stern-Gerlach studies of organometallic sandwich clusters

K. Miyajima, M. B. Knickelbein, A. Nakajima

European Physical Journal D （Springer Nature） 34 （ 1-3 ） 177 - 182 2005年07月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 1434-6060

Stern-Gerlach type magnetic deflection measurements were performed for two types of multiple sandwich clusters: vanadium-benzene Vn(C 6H6)n+1 and terbium-cyclooctatetraene Tb n(C8H8)n+1. Beams of V n(C6H6)n+1 clusters (n = 1-4) showed symmetric broadening induced by the inhomogeneous field, indicating free spin behavior similar to that displayed by isolated paramagnetic atoms. By contrast, beams of Tbn(C8H8)n+1 clusters displayed one-sided deflection, indicating that fast spin relaxation occurs within the clusters. The difference in the magnetic deflection behavior exhibited by these two systems is explained by their electronic structures, specifically the bonding characteristics between metal atoms and ligand molecules. © EDP Sciences, Società Italiana di Fisica, Springer-Verlag 2005.

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Negative ion photoelectron spectroscopy of 2,2'-bithiophene cluster anions, (2T)_n^- (n = 1-100)

M. Mitsui, Y. Matsumoto, N. Ando, A. Nakajima

European Physical Journal D （Springer Nature） 34 （ 1-3 ） 169 - 172 2005年07月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 14346060

Cluster anions of 2,2'-bithiophene, (2T)n-, were produced up to n ∼ 500 in the gas-phase. The energetics of the excess electron in the (2T)n- clusters with n =1-100 were explored by negative ion photoelectron spectroscopy. When the vertical detachment energies (VDEs) obtained from the photoelectron spectra were analyzed by a plot against n-1/3, it has been revealed that the excess electron trapping level thus extrapolated is located at ∼0.8 eV below the conduction band minimum (i.e. LUMO) of the 2T thin film. The large slope of the VDEs vs. n-1/3 plot suggests that the neutral 2T molecules surrounding the anion core take non-planar twisted conformations with permanent dipole moments, resulting in the exceedingly deep trapping of the excess electron in the 2T cluster anions. © EDP Sciences, Società Italiana di Fisica, Springer-Verlag 2005.

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Gold-nanoparticle-dispersed boron-doped diamond electrodes for electrochemical oxidation of oxalic acid

T. A. Ivandini, Y. Naono, A. Nakajima, Y. Einaga

Chemistry Letters （Chemical Society of Japan） 34 （ 8 ） 1086 - 1087 2005年07月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 0366-7022

Electrochemical behavior of gold-nanoparticle-dispersed diamond electrodes have been studied for the oxidation of oxalic acid in phosphate buffer solution. Comparison is made with gold, as-deposited diamond and gold-implanted diamond. The electrodes combine the high catalytic activity of gold and the low background current of diamond electrodes. Moreover, the electrodes can overcome the problem of stability as it is found when using other electrodes. Copyright © 2005 The Chemical Society of Japan.

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Selective formation of MSi₁₆ (M = Sc, Ti, and V)

K. Koyasu, M. Akutsu, M. Mitsui, A. Nakajima

Journal of the American Chemical Society （American Chemical Society） 127 （ 14 ） 4998 - 4999 2005年03月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 00027863

Metal-encapsulated silicon cage clusters are a new class of clusters and are opening up new avenues for silicon-based nanoscale materials. We present experimental evidence for a highly stable cluster corresponding to M@Si16 (M = Sc, Ti, and V). Mass spectrometry and anion photoelectron spectroscopy show that the cluster features an electronically closed TiSi16 neutral core which undergoes a change in the number of valence electrons involving (i) substitution of neighboring metals with Sc and V, or (ii) addition of a halogen atom to the TiSi16 anion, and that VSi16F is predicted to form an ionically bound superatom complex. Copyright © 2005 American Chemical Society.

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Geometric and electronic structures of multiple-decker one-end open sandwich clusters: Eu_n(C₈H₈)_n^- (n = 1-4)

R. Takegami, N. Hosoya, J. Suzumura, A. Nakajima, S. Yabushita

Journal of Physical Chemistry A （American Chemical Society） 109 （ 11 ） 2476 - 2486 2005年02月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 10895639

We have measured the photoelectron spectra of the multiple-decker 1:1 sandwich clusters of Eun(COT)n- (n = 1-4; COT = 1,3,5,7-cyclooctatetraene), synthesized in the gas phase, and studied theoretically the bonding scheme, charge distribution, valence orbital energies, and photodetachment energies. We calculated the ground electronic state X - and the first excited electronic state A-, both of which have strong ionic bonding and a characteristic charge distribution. Moreover, we found that the valence orbital energies of Eu (6s) and COT (L δ) depend strongly on cluster size and their positions in the clusters. With the calculated vertical detachment energies for these valence orbitals, we assigned the peaks in the experimental photoelectron spectra and analyzed the origin of their interesting behavior by employing simple point charge models. From these analyses, it became clear that the characteristic behavior of the spectra is due to the strong ionic bonding and the charge distribution. In addition, using the point charge models, we estimated the vertical detachment energies of the one-dimensional polymer [Eu(COT)]. © 2005 American Chemical Society.

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Ionization energies and electron distributions of one-end open sandwich clusters: Eu_n(C₈H₈)_n (n = 1-4)

R. Takegami, N. Hosoya, J. Suzumura, K. Yada, A. Nakajima, S. Yabushita

Chemical Physics Letters （Elsevier Science BV） 403 （ 1-3 ） 169 - 174 2005年02月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 00092614

The ionization energies of Eun(COT)n (n = 1-4) were found to decrease asymptotically with the cluster size. The low-spin state X and the high-spin state a were characterized with DFT calculations; their adiabatic energy difference was found to decrease with the cluster size. The calculated ionization energies of the X states reproduced the experimental size dependence. Those of the a states also agreed except for n = 1. These features were explained by the significant variation of the electrostatic potential at ionization sites. The state-specific permanent dipole moments were calculated to clarify the difference in the electronic states. © 2005 Elsevier B.V. All rights reserved.

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Electron-beam-induced surface quasimelting of Co granular nanowires

S. H. Huh, A. Nakajima

Applied Physics Letters （American Institute of Physics） 85 （ 25 ） 6149 - 6151 2004年12月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 0003-6951

Electron-beam-induced surface quasimelting of Co granular nanowires (GNWs) of 6-10 nm in diameter and ~30 nm in length has been studied using a high-resolution electron microscope. From the time evolution of electron microscope images as a function of the irradiation time, it has been revealed that the morphological change in the Co GNW is quite different from that in a Co nanocluster (NC) in terms of structures and phase transitions. For example, the allotropic Β→α transition, inhibited in the Co NC, can proceed in the Co GNW through the lattice softening of fcc (111) facets without dimensional collapse. © 2004 American Institute of Physics.

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Lanthanide organometallic sandwich nanowires: Formation mechanism

N. Hosoya, R. Takegami, J. Suzumura, K. Yada, K. Koyasu, K. Miyajima, M. Mitsui, M. B. Knickelbein, S. Yabushita, A. Nakajima

Journal of Physical Chemistry A （American Chemical Society） 109 （ 1 ） 9 - 12 2004年12月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 10895639

A molecular beam of europium-cyclooctatetraene sandwich nanowires Eu n(COT) m was produced by a laser vaporization synthesis method. The formation mechanism of the nanowires was quantitatively revealed by photoelectron and photoionization spectroscopies of the Eu-COT species, together with supporting theoretical calculations. From these results, it is confirmed that growth processes extending the length of Eu-COT nanowires involve a series of elementary reactions in which efficient charge transfer occurs at the terminal reaction sites. In every elementary step, the reaction proceeds between one reactant having low ionization energy and the other reactant having high electron affinity, probably via a "harpoon" mechanism.

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Negative ion photoelectron spectroscopy of acridine molecular anion and its monohydrate

S. Kokubo, N. Ando, K. Koyasu, M. Mitsui, A. Nakajima

Journal of Chemical Physics （American Institute of Physics） 121 （ 22 ） 11112 - 11117 2004年12月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 0021-9606

Negative ion photoelectron spectroscopy was employed to investigate the electronic structure of the acridine molecular anion and its monohydrated anion in the gas phase. Their adiabatic electron affinities were measured to be 0.896±0.010 and 1.18±0.05 eV, and the low-lying electronic excited states in both neutral acridine and in its monohydrate were revealed. The photoelectron spectra clearly exhibit the presence of low-lying singlet and triplet states having a (π, π*) configuration in an uncomplexed acridine molecule. Comparison of the photoelectron spectrum of acridine with that of anthracene shows that photodetachment processes into the excited states of (n, π*) configuration have little intensity, implying a relatively large intramolecular structural relaxation in the (n, π*) states. © 2004 American Institute of Physics.

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Coexistence of two different anion states in polyacene nanocluster anions

M. Mitsui, S. Kokubo, N. Ando, Y. Matsumoto, A. Nakajima, K. Kaya

Journal of Chemical Physics （American Institute of Physics） 121 （ 16 ） 7553 - 7556 2004年10月

研究論文（学術雑誌）, 共著, 責任著者, 査読有り, ISSN 0021-9606

The coexistence of two types of anion sates in nanometer-scale polyacene cluster anions, was analyzed. Nanoclusters of naphthalene and anthracene having single excess electron were synthesized in the gas-phase. The photoelectron spectra of size-selected cluster anions containing 2 to 100 molecules exhibited rigid 'crystal-like' cluster anion in which surrounding neutral molecules reorganize around the charge core. The results show that two anion states appear to correlate to negative polaronic states formed in corresponding crystals.

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Ferromagnetism in one-dimensional vanadium-benzene sandwich clusters

K. Miyajima, A. Nakajima, S. Yabushita, M. B. Knickelbein, K. Kaya

Journal of the American Chemical Society （American Chemical Society） 126 （ 41 ） 13202 - 13203 2004年09月

研究論文（学術雑誌）, 共著, 責任著者, 査読有り, ISSN 0002-7863

A molecular beam of multilayer vanadium-benzene organometallic complexes V n(C 6H 6) m was produced by a laser vaporization synthesis method. The magnetic moments of the complexes were measured by a molecular beam magnetic deflection technique, and were found to increase with the number of vanadium atoms in the cluster, showing that the unpaired electrons, which occupy the nonbonding d σ orbitals localized on the metal atoms, couple ferromagnetically. These sandwich species represent a new class of one-dimensional molecular magnets in which the transition metal atoms are formally zerovalent. Copyright © 2004 American Chemical Society.

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Electronic structures of exohedral lanthanide-C₆₀ clusters

J. Suzumura, N. Hosoya, S. Nagao, M. Mitsui, A. Nakajima

Journal of Chemical Physics （American Institute of Physics） 121 （ 6 ） 2649 - 2654 2004年08月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 00219606

The electronic structures of several gas phase exohedral lanthanide (Ln)-C60, LnnC60 with n=1-4, clusters were discussed. it was shown that most of the Ln atoms preferably take +3 oxidation states, while Eu atoms alone assume +2 oxidation states, and that C60 accepts up to twelve donated electrons in LnnC60. The photoionization examination of the oxygen atom mixing into the Ln nC60 clusters showed that each oxygen atom reduces two electrons from C60. It was suggested that the number of accepted electrons in C60 can be varied by a suitable choice of the number of Ln atoms and O atoms.

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Photoelectron spectroscopy of pyrene cluster anions, (pyrene)_n^- (n = 1-20)

N. Ando, S. Kokubo, M. Mitsui, A. Nakajima

Chemical Physics Letters （Elsevier Science BV） 389 （ 4-6 ） 279 - 283 2004年05月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 00092614

Pyrene molecular anion and its homogeneous clusters, (pyrene) n- (n=2-20), have been studied by anion photoelectron spectroscopy. The adiabatic electron affinity (EAa) of pyrene was directly determined to be 0.406±0.010 eV. This EAa value agrees with that calculated using the method of density functional theory, showing that the excess electron occupies the lowest unoccupied molecular orbital of pyrene molecule. Photoelectron spectra of its cluster anions suggest that the excess electron in the pyrene dimer anion is delocalized over the two pyrene moieties, whereas in larger clusters the excess electron is predominantly localized on a single pyrene molecule. © 2004 Elsevier B.V. All rights reserved.

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Magnetic trapping of Co_coreCoO_shell nanoparticles produced by gas phase reaction

S. H. Huh, A. Nakajima

Chemistry Letters （Chemical Society of Japan） 33 （ 4 ） 428 - 429 2004年03月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 03667022

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Fabrication of ferromagnetic nanocluster rods by magnetic trapping

S. H. Huh, A. Nakajima, K. Kaya

Journal of Applied Physics （American Institute of Physics） 95 （ 5 ） 2732 - 2736 2004年03月

研究論文（学術雑誌）, 共著, 最終著者, 査読有り, ISSN 0021-8979

The fabrication of ferromagnetic nanocluster rods by magnetic trapping was discussed. The morphology of the produced nanomaterials was analyzed by electron microscopy and the composition by electron energy-loss spectroscopy. It was found that one dimensional structured soft magnets, up to ∼ 150 nm in length were formed consisting of magnetic clusters a few nanometers in diameter. The mechanism and driving behind their growth process and their magnetic evolution was also proposed.

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Coexistence of Solvated Electrons and Solvent Valence Anions in Negatively Charged Acetonitrile Clusters, (CH₃CN)_n^- (n = 10-100)

M. Mitsui, N. Ando, S. Kokubo, A. Nakajima, K. Kaya

Physical Review Letters （American Institute of Physics） 91 （ 15 ） 1530021 - 1530024 2003年10月

研究論文（学術雑誌）, 共著, 責任著者, 査読有り, ISSN 00319007

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Band gap shiftings in Co-doped Nb_n (n = 3-15) clusters: Influence of Co 3d electrons on the electronic structure

A. Pramann, K. Koyasu, A. Nakajima, K. Kaya

International Journal of Mass Spectrometry （Elsevier Science BV） 229 （ 1-2 ） 77 - 82 2003年07月

研究論文（学術雑誌）, 共著, 責任著者, 査読有り, ISSN 13873806

The photoelectron spectra of bimetallic, mass-selected Nb nCo- clusters (n = 3-15) measured at 4.66eV photon energy are reported. The electronic structure changes of the binary clusters are discussed in comparison with the electronic structures of pure Nb n- clusters as a function of size evolution. Spectroscopic parameters like electron affinities (EAs) and vertical detachment energies (VDEs) are determined. The addition of one Co atom to the Nb n cluster core significantly perturbs the electronic structures of pure Nbn clusters at small cluster sizes. Electronic state merging can be explained by hybridization of Co 3d and 4s as well as Nbn 4d-derived valence orbitals. Tentative geometric structure postulations are carried out revealing the influence of the incoming Co atom toward the intact Nbn cluster core. In strong contrast to pure Nbn clusters, a highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gap closing of NbnCo at n = 6 indicates a bulk-like evolution of electronic structures induced by charge transfer of Co 3d electrons. © 2003 Elsevier B.V. All rights reserved.

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Geometric and electronic structures of metal (M)-doped silicon cluster (M = Ti, Hf, Mo and W)

M. Ohara, K. Koyasu, A. Nakajima, K. Kaya

Chemical Physics Letters （Elsevier Science BV） 371 （ 3-4 ） 490 - 497 2003年03月

研究論文（学術雑誌）, 共著, 責任著者, 査読有り, ISSN 0009-2614

We have studied geometric and electronic structures of metal (M) atom doped silicon (Si) clusters, MSin (M = Ti, Hf, Mo and W), using mass spectrometry, a chemical-probe method and photoelectron spectroscopy. In the mass spectra for all of the mixed cluster anions, MSin-, both MSi15- and MSi16- were abundantly produced compared to neighbors. Together with the result of the adsorption reactivity and photoelectron spectroscopy, it has been revealed that one metal atom can be encapsulated inside a Sin cage at n ≥ 15. © 2003 Elsevier Science B.V. All rights reserved.

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The electronic structures of terbium-phthalocyanine sandwich clusters

K. Miyajima, E. Okada, A. Nakajima, K. Kaya

Chemistry Letters （Chemical Society of Japan） 32 （ 3 ） 280 - 281 2003年02月

研究論文（学術雑誌）, 共著, 責任著者, 査読有り, ISSN 0366-7022

Terbium (Tb)-phthalocyanine (pc) sandwich clusters, Tbn(pc)n+1 (n = 1-4), and their Na adducts, TbNak(pc)2 (k = 1-4) were produced by dual laser vaporization of a Tb rod and the molded rod of metal-free or disodium phthalocyanine (H2pc/Na2pc) in gas phase. The minimum number of Na atoms attached to the cationic/neutral/anionic Tb1(pc)2 clusters is 2, 1, and 0, respectively. Together with the ionization energies of the Na adducts of Tb(Na)k(pc)2 (k = 1 and 2), it is revealed that the Tb-pc clusters are formed through fairly ionic bondings, where each component exists as Tb3+, Na+, and pc2-, respectively.

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Negative ion photoelectron spectroscopy of (benzene)_n^- (n = 53-124) and (toluene)_n^- (n = 33-139): Solvation energetics of an excess electron in size-selected aromatic hydrocarbon nanoclusters

M. Mitsui, A. Nakajima, K. Kaya

Journal of Chemical Physics （American Institute of Physics） 117 （ 21 ） 9740 - 9749 2002年12月

研究論文（学術雑誌）, 共著, 責任著者, 査読有り, ISSN 00219606

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Quasi-band electronic structure of V_n(benzene)_n+1 clusters

K. Miyajima, K. Muraoka, M. Hashimoto, T. Yasuike, S. Yabushita, A. Nakajima, K. Kaya

Journal of Physical Chemistry A （American Chemical Society） 106 （ 45 ） 10777 - 10781 2002年08月

研究論文（学術雑誌）, 共著, 責任著者, 査読有り, ISSN 10895639

Photoionization efficiency (PIE) curves were measured for sandwich vanadium-benzene clusters, Vn(benzene)n+1 (n = 1-5), which were produced by the reaction of laser-vaporized vanadium atoms with benzene vapor. A two-rod laser vaporization source was used for enhancing the production of the larger clusters. The second lowest ionization energies of the sandwich clusters were probed experimentally for the first time. The PIE curves of the sandwich clusters clearly present evidence that the clusters take a quasi-band electronic structure which results from a one-dimensional sandwich structure.

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Electronic structures of Ho_nC₆₀ clusters (n = 1-5): High electron acceptability of C₆₀

S. Nagao, Y. Negishi, A. Kato, Y. Nakamura, A. Nakajima, K. Kaya

Journal of Chemical Physics （American Institute of Physics） 117 （ 7 ） 3169 - 3172 2002年08月

研究論文（学術雑誌）, 共著, 責任著者, 査読有り, ISSN 00219606

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Photoelectron spectroscopy of palladium-doped gold cluster anions; Au_nPd^- (n = 1-4)

K. Koyasu, M. Mitsui, A. Nakajima, K. Kaya

Chemical Physics Letters （Elsevier Science BV） 358 （ 3-4 ） 224 - 230 2002年05月

研究論文（学術雑誌）, 共著, 責任著者, 査読有り, ISSN 00092614

Palladium-doped gold clusters, AunPd- (n = 1-4), were investigated using anion photoelectron spectroscopy at 4.66 eV photon energy. Electron affinities (EAs) and vertical detachment energies (VDEs) are determined, and the electronic structures of Pd-doped Aun clusters are compared to those of pure Aun clusters. A peak shape analysis reveals electronic and geometric similarity between Aun- and Aun-1Pd- clusters and it is found that (1) an electron promotion occurs from 4d to 5s orbital in the Pd atom, and that (2) the bond of Au-Pd is formed through σ orbital between 6s of Au and 5s of Pd. © 2002 Published by Elsevier Science B.V.

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Photoelectron spectroscopy of cobalt oxide cluster anions

A. Pramann, K. Koyasu, A. Nakajima, K. Kaya

Journal of Physical Chemistry A （American Chemical Society） 106 （ 19 ） 4891 - 4896 2002年04月

研究論文（学術雑誌）, 共著, 責任著者, 査読有り, ISSN 1089-5639

Photoelectron spectroscopy of size-selected ConOm- clusters (n = 4-20; m = 0-2) is performed in a pulsed molecular beam experimental at photon energies of 3.49 and 4.66 eV. Cobalt oxide cluster anions are generated in a laser vaporization source and the photoelectron spectra are measured with a magnetic bottle photoelectron spectrometer. The size-dependent evolution of the electronic structure of the oxide clusters is discussed in terms of cluster composition. Threshold binding energy features are mainly contributed from Co d-derived orbitals throughout the complete series, indicating that the incoming oxygen atoms cause a minor influence on the electronic structures. Although for n > 13 EAs and VDEs of Con and ConOm clusters overlap within the experimental resolution, at smaller cluster sizes, the values increase in the order of Con < ConO < ConO2. For Co4O3- and Co5O3-, the spectra indicate the presence of isomers with low EAs peroxo and ozonide isomers. To compare the different channels of oxygen chemisorption, relative dissociation energies are determined via thermochemical cycles.

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Anion photoelectron spectroscopy of V_nO_m^- (n = 4-15; m = 0-2)

A. Pramann, K. Koyasu, A. Nakajima, K. Kaya

Journal of Chemical Physics （American Institute of Physics） 116 （ 15 ） 6521 - 6528 2002年04月

研究論文（学術雑誌）, 共著, 責任著者, 査読有り, ISSN 00219606

Anion photoelectron spectroscopy was used to study small negatively charged vanadium oxide clusters at 4.66 eV photon energy. The electronic structure of the vanadium clusters was investigated as a function of size n and composition m. The changes of the electronic structure of the vanadium cluster cores after adding two oxygen atoms were examined. Thermomechanical cycles were used to determine relative dissociation energies for different oxygen-loss channels from the parent clusters. Electron affinities and vertical detachment energies were found to increase with increasing cluster size.

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Behavior of silicon and germanium clusters on a C₆₀ fullerene

M. Ohara, Y. Nakamura, Y. Negishi, K. Miyajima, A. Nakajima, K. Kaya

Journal of Physical Chemistry A （American Chemical Society） 106 （ 18 ） 4498 - 4501 2002年04月

研究論文（学術雑誌）, 共著, 責任著者, 査読有り, ISSN 10895639

Behavior of silicon (Si) and germanium (Ge) atoms (or clusters) on the surface of a C60 fullerene has been revealed by using time-of-flight mass spectroscopy and photoelectron spectroscopy. The mixed clusters of C60Sin and C60Gem were generated by two-lasers vaporization of a molded C60 rod and a Si (or a Ge) rod in He carrier gas. The size distributions of anionic and cationic C60Sin/C60Gem clusters monotonically decreased with the number of Si/Ge atoms, and were limited only up to n = 4 and m = 3. The photoelectron spectra of C60Sin- and C60Gem- are similar to that of pure C60- at m = 3, 4, whereas the spectra at n, m = 1, 2 are different from that of C60-. These results show that Si/Ge atoms assemble together into Si/Ge clusters on the C60 cage with an increase in the number of Si/Ge atoms.

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Anion photoelectron spectroscopy of vanadium-doped cobalt clusters

Pramann A., Koyasu K., Nakajima A., Kaya K.

Journal of Physical Chemistry A （AMER CHEMICAL SOC） 106 （ 11 ） 2483 - 2488 2002年03月

研究論文（学術雑誌）, 査読有り, ISSN 1089-5639

Bimetallic cobalt (Co)-vanadium (V) clusters, ConVm- (n = 4-14, m = 0-2), are investigated using anion photoelectron spectroscopy at 4.66 eV photon energy. Electron affinities (EA) and vertical detachment energies (VDE) are determined, and the electronic structures of V- and V2-doped Con clusters are compared to those of pure Con clusters. Bonding and hybridization effects can be explained using a charge-transfer model. A peak shape analysis confirms the high stability of the Co12V cluster with a most plausible icosahedral structure with the V atom in the cage center. Together with ionization potentials (IP), the electronic structures of the clusters are compared to H2 chemisorption rate coefficients reported previously. Thus, this study completes the investigation of electronic properties and reactivity of the bimetallic ConVm clusters as a function of charge, size, and distribution. Although the reactivity of ConVm reveals a better correlation toward the EA rather than toward IP, no improved correlation is found between promotion energies and reactivity. This result clearly shows that the chemisorption reactivity of the heterogeneous clusters is largely governed by geometric factors.

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Geometric and electronic structures of terbium-silicon mixed clusters (TbSi_n; 6 ≤ n ≤ 16)

M. Ohara, K. Miyajima, A. Pramann, A. Nakajima, K. Kaya

Journal of Physical Chemistry A （American Chemical Society） 106 （ 15 ） 3702 - 3705 2002年03月

研究論文（学術雑誌）, 共著, 責任著者, 査読有り, ISSN 10895639

The geometric and the electronic structures of terbium-silicon anions, TbSin- (6 ≤ n ≤ 16) were investigated by using photoelectron spectroscopy (PES) and a chemical-probe method. The clusters were produced by a double-rod laser vaporization technique. From trends observed in the electron affinities (EAs), the TbSin- clusters were categorized into three groups of (I) 6 ≤ n ≤ 9, (II) n = 10, 11, and (III) n ≥ 12. Together with adsorption reactivity toward H2O it is concluded that a Tb atom is encapsulated inside a Sin cage at n ≥ 10; Tb@Sin.

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A Novel Network Structure of Organometallic Clusters in Gas Phase

A. Nakajima, K. Kaya

E. Buzaneva and P. Scharff (eds.) “Frontiers of Multifunctional Nanosystems” (NATO Science Series, Series II. Vol. 57) Kluwer Academic Publishers （Springer Netherlands）（ 57 ） 173 - 190 2002年

研究論文（研究会，シンポジウム資料等）, 共著, 筆頭著者, 責任著者, 査読有り

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Comparison of hydrogen chemisorption rates and electronic structures of small Nb_nAl^- clusters: A photoelectron spectroscopic study

A. Pramann, A. Nakajima, K. Kaya

Chemical Physics Letters （Elsevier Science BV） 347 （ 4-6 ） 366 - 372 2001年10月

研究論文（学術雑誌）, 共著, 責任著者, 査読有り, ISSN 00092614

Photodetachment photoelectron spectroscopy experiments on NbnAl- clusters have been carried out in a size range n=4 to 12 at 3.49 eV photon energy. The evolution of the electronic structure is discussed as a function of cluster size and composition. Electron affinities (EA) and vertical detachment energies are determined. One central point of this study is the direct comparison of the electronic structure of pure and Al-doped Nbn- clusters in the threshold binding energy region with regard to hybridization effects. Direct correlation between electronic structure and H2 chemisorption rate constants reported by Nonose et al. [Chem. Phys. Lett. 164 (1989) 427], is found for n=5 to 8 and n>9. © 2001 Elsevier Science B.V.

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Photoelectron spectroscopy of V_x(coronene)_y and Ti_x(coronene)_y anions

M. A. Duncan, A. Knight, Y. Negishi, S. Nagao, K. Judai, A. Nakajima, K. Kaya

Journal of Physical Chemistry A （American Chemical Society） 105 （ 44 ） 10093 - 10097 2001年10月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 10895639

Gas-phase anion complexes are produced containing one or more vanadium or titanium atoms bound to one or more molecules of the polycyclic aromatic hydrocarbon (PAH) coronene (C24H12). These species are prepared with coincident laser vaporization of a metal rod and a pressed pellet of coronene in a pulsed-nozzle cluster source. The anion complexes produced are mass-selected with a time-of-flight spectrometer and photoexcited with various Nd:YAG laser harmonics. A magnetic bottle instrument is employed for time-of-flight photoelectron spectroscopy measurements on these complexes. Electron affinities (EA's) are observed to be greater for the organometallic complexes than they are for the isolated metal clusters or the isolated coronene molecule. EA's increase with the number of metal atoms in the complex. The spectra observed for V-coronene complexes are similar in appearance to those seen previously for V-benzene and V-C60, suggesting that the metal binds as a π complex in a symmetric ring site. V-(coronene)2 complexes exhibit different spectra under different conditions attributed to the formation of sandwich and external-metal isomers.

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Non-destructive selection of geometrical isomers of the Al(C₆H₆) cluster by a 2 m electrostatic hexapole field

K. Imura, T. Kawashima, H. Ohoyama, T. Kasai, A. Nakajima, K. Kaya

Physical Chemistry Chemical Physics （Royal Chemical Society） 3 （ 17 ） 3593 - 3597 2001年10月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 14639076

A supersonic cluster beam containing isomers of Al(C6H6) complexes was generated by laser evaporation, and non-destructively selected using a 2 m long electrostatic hexapole. The focusing curve shows clear evidence that there are two kinds of Al(C6H6) isomers which are slightly different from each other in geometry; one is an asymmetric 1,2-complex and the other nearly C6v symmetric 1,4-complex. The electric dipole moments of the two isomers are found to be 1.5 ± 0.1 and 1.4 ± 0.1 D, respectively. We carried out computation using density functional theory in order to estimate their structures. We found that the 1,2-complex is more stable than the 1,4-complex. The present work confirms that the electrostatic hexapole technique is useful for non-destructive selection of geometrical isomers in a beam.

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Mass spectra and photoelectron spectroscopy of negatively charged benzene clusters, (benzene)_n^- (n = 53-124)

M. Mitsui, A. Nakajima, K. Kaya, U. Even

Journal of Chemical Physics （American Institute of Physics） 115 （ 13 ） 5707 - 5710 2001年10月

研究論文（学術雑誌）, 共著, 責任著者, 査読有り, ISSN 00219606

The threshold size and photoelectron study of neat negatively charged benzene clusters was carried out using a high-temperature and high-pressure valve to generate cold, large, negatively charged benzene clusters. It was found that there is an abrupt onset of formation of the (benzene)n- clusters occurring at size n=53, and the cluster distribution extended to above n>160. Benzene clusters smaller than the threshold size were also observed. Photoelectron spectroscopy revealed that the VDES of (benzene)n- with n = 53-124 fell within 0.4-0.6 eV.

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Photoelectron spectroscopy of bimetallic aluminum cobalt cluster anions: Comparison of electronic structure and hydrogen chemisorption rates

A. Pramann, A. Nakajima, K. Kaya

Journal of Chemical Physics （American Institute of Physics） 115 （ 12 ） 5404 - 5410 2001年09月

研究論文（学術雑誌）, 共著, 責任著者, 査読有り, ISSN 00219606

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Photoelectron spectroscopy of gold-silver binary cluster anions (Au_nAg_m^-; 2 ≤ n + m ≤ 4)

Y. Negishi, Y. Nakamura, A. Nakajima, K. Kaya

Journal of Chemical Physics （American Institute of Physics） 115 （ 8 ） 3657 - 3663 2001年08月

研究論文（学術雑誌）, 共著, 責任著者, 査読有り, ISSN 00219606

The photoelectron spectra of the small Au-Ag mixed clusters produced by the laser vaporization of their alloy rod were observed. It was found that in their anions, the replacement of an Ag atom by an Au atom accompanies a preferable substitution to accept the excess charging electron. In triatomic Au-Ag clusters, Au atoms existed as a terminal atom. Evidence was obtained that in tetratomic Au-Ag clusters, isomers can coexist due to the contribution from an ionic bonding character.

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Photoelectron spectroscopy of yttrium oxide cluster anions: Effects of oxygen and metal atom addition

A. Pramann, Y. Nakamura, A. Nakajima, K. Kaya

Journal of Physical Chemistry A （American Chemical Society） 105 （ 32 ） 7534 - 7540 2001年07月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 10895639

The photoelectron spectra of small gas-phase YnOm- clusters (n = 2-10, m = 1-3) are measured at photon energies of 3.49 and 4.66 eV. Cluster generation is performed with the aid of laser vaporization, whereas the kinetic energy of electrons is measured using a magnetic bottle photoelectron spectrometer. The evolution of the electronic structure of neutral yttrium oxide clusters is discussed in terms of both sequential oxygen addition to the metal cluster core and sequential metal atom increase. Electron affinities and vertical detachment energies are measured and compared to ionization potentials of the monoxide cluster series [Knickelbein, M. J. Chem. Phys. 1995, 102, 1]. A general trend in the observed photoelectron spectra is a shifting of threshold energies to higher binding energies with increasing cluster size n. At low electron binding energies, main spectral features are contributed from metal d-band orbitals whereas the O 2p contribution becomes apparent with increasing oxygen content due to a higher degree of hybridization of yttrium s, d, and oxygen 2p orbitals. The differences of yttrium-oxygen dissociation energies are determined and compared.

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Electronic and geometric properties of exohedral sodium- and gold-fullerenes

B. Palpant, Y. Negishi, M. Sanekata, K. Miyajima, S. Nagao, K. Judai, D. M. Rayner, B. Simard, P. A. Hackett, A. Nakajima, K. Kaya

Journal of Chemical Physics （American Institute of Physics） 114 （ 19 ） 8459 - 8466 2001年05月

研究論文（学術雑誌）, 共著, 責任著者, 査読有り, ISSN 0021-9606

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Electronic properties of substituted aluminum clusters by boron and carbon atoms (Al_nB_m^-/Al_nC_m^-); New insights into s-p hybridization and perturbed shell structures

H. Kawamata, Y. Negishi, A. Nakajima, K. Kaya

Chemical Physics Letters （Elsevier Science BV） 337 （ 4-6 ） 255 - 262 2001年04月

研究論文（学術雑誌）, 共著, 最終著者, 査読有り, ISSN 00092614

Substituted Al clusters with B atom, which is trivalent but smaller than Al atom, or with covalent C atom were investigated using photoelectron spectroscopy (PES). In Aln-mBm- (n≥5 for m=1, n≥10 for m=2), three valence electrons of each B atom as well as Al atom contribute to electron shell structures, while the C atom strongly binds two free electrons from Aln- in AlnC- (n=5-30). The substitution with B atom allowed us to determine that s-p mixing starts at n=6 in pure Al clusters. Perturbed electron shells and the geometry of Al12B1-,Al11B2-, and Al12C- are discussed. © 2001 Elsevier Science B.V.

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Novel Electronic and Geometric Properties in Sub-nanometer Clusters

K. Kaya, M. Hashimoto, A. Kato, Y. Nakamura, K. Miyajima, Y. Negishi, S. Nagao, A. Nakajima

Proceeding of Nobel Symposium “The Physics and Chemistry of Clusters” （World Scientific） 278 - 290 2001年04月

研究論文（研究会，シンポジウム資料等）, 共著, 最終著者, 査読有り

In the gas phase, novel network structures were found in organometallic clusters between metal atoms and C-60. Lanthanide organometallic clusters, Ln(n)(C-60) consisting of lanthanide atoms (Ln; Ln = Eu and Ho) and C60 molecules were produced predominantly at compositions of (n, m)=(n, n+3) [n= 1 -4). The cluster having (1, 4) composition is proposed to form a tetrahedral structure, in which a Ln atom is surrounded by four C60 molecules. Together with the results of sequential chemisorption of Cl atoms on cluster cations, photoelectron spectroscopy of cluster anions leads us to conclude that Lnn(C-60)(m) clusters are charge transfer complexes expressed as Ln(n)(x+)(C-60)(m)(x-), and that Eu and Ho atoms exist as Eu2+ and Ho3+ ions. Furthermore, the electronic and geometric properties of gas-phase exohedral NanC60 and AunC60 cluster are investigated. For NanC60, the high stability of the n=3, 6, 9, and 12 anions was observed, which suggests a "wet" structure having these atoms packed into stable trimers on the surface. In contrast, the electron affinity of AunC60 clearly shows even-odd alternation with the number of Au atoms, which suggests a "dry" structure where An atoms are segregated from each other.

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A soft-landing experiment on organometallic cluster ions: Infrared spectroscopy of V(benzene)₂ in Ar matrix

K. Judai, K. Sera, S. Amatsutsumi, K. Yagi, T. Yasuike, S. Yabushita, A. Nakajima, K. Kaya

Chemical Physics Letters （Elsevier Science BV） 334 （ 4-6 ） 277 - 284 2001年02月

研究論文（学術雑誌）, 共著, 責任著者, 査読有り, ISSN 00092614

Vanadium (V)-benzene cluster ions, produced by laser ablation with reaction toward benzene vapor, were size-selected and deposited into a low-temperature Ar matrix. Infrared spectrum of V1(benzene)2 in the Ar matrix was measured after one-hour deposition with the deposition energy of 20 eV. The spectrum was in agreement with both the reported spectrum and our theoretical calculations, showing that (1) V1(benzene)2, prepared in the gas-phase reaction, takes a sandwich structure and that (2) the ions were soft-landed onto the Ar matrix and were neutralized by charge transfer from a metal substrate without fragmentation. © 2001 Elsevier Science B.V.

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Photoelectron spectroscopy of scandium-arene complex anions

K. Judai, Y. Nakamura, M. Tachibana, Y. Negishi, A. Nakajima, K. Kaya

Chemistry Letters （Chemical Society of Japan） 30 （ 2 ） 114 - 115 2001年02月

研究論文（学術雑誌）, 共著, 責任著者, 査読有り, ISSN 0366-7022

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Visible Photoluminescence from Deposited Germanium-Oxide Clusters

A. Nakajima, Y. Negishi, Y. Nakamura, K. Kaya

B. K. Rao, S. M. Bose, M. P. Das, and S. N. Sahu (eds.) “Science and Technology of Nanostructured Materials” （Nova Science Publishers, Inc.） 175 - 190 2001年

研究論文（研究会，シンポジウム資料等）, 共著, 筆頭著者, 責任著者, 査読有り

Optical properties of the deposited germanium-oxide (Ge-O) prepared from precursors of the Ge-O clusters were investigated. The strong visible photoluminescence peaked at 500 nm was observed under the excitation of 325 nm laser light, when the GenOn clusters were deposited onto a substrate as a precursor. The XPS spectrum of the deposited Ge-O on the substrate shows the contribution from the oxidation state of Ge2+, which shows that 1:1 composition of Ge2+-O2- is a key factor to exhibit the visible emission. Correspondingly, the electronic structures of the germanium-oxide cluster precursors produced by a laser vaporization were studied by using photoelectron spectroscopy (PES) for GenOm- and GenOmF- (n=2-6, m=1-5) in the gas phase. Our development of the halogen atom doping method enables us to determine the HOMO-LUMO gap of the corresponding neutral GenOn clusters experimentally, and it was found that the gap of the GenOn (n=2-5) clusters reasonably corresponds to the energy of the visible light. These results indicate that the GenOn clusters can be ascribed to the center of the visible photoluminescence.

Visible photoluminescence of the deposited germanium-oxide prepared from clusters in the gas phase

Y. Negishi, S. Nagao, Y. Nakamura, A. Nakajima, S. Kamei, K. Kaya

Journal of Applied Physics （American Institute of Physics） 88 （ 10 ） 6037 - 6043 2000年11月

研究論文（学術雑誌）, 共著, 責任著者, 査読有り, ISSN 0021-8979

Optical properties of the deposited germanium-oxide (Ge-O) prepared from the laser ablation of GeO2 were investigated. When the GenOm clusters, having the compositions mainly at n = m, were deposited onto a substrate as a precursor, strong visible photoluminescence peaked at 500 nm was observed under the excitation of 325 nm laser light. X-ray photoelectron spectroscopy for the deposited Ge-O on the substrate consistently shows the substantial component of the oxidation state of Ge2+, which shows the contributions from 1:1 composition of Ge2+-O2-. Correspondingly, the electronic structures of the germanium-oxide cluster were studied by using photoelectron spectroscopy in gas phase. Our developed method of the halogen atom doping enables us to determine the highest occupied molecular orbital-the lowest unoccupied molecular orbital gap of the corresponding neutral GenOn clusters experimentally, and it was found that the gap of the GenOn (n = 2-5) clusters reasonably corresponds to the energy of the visible light. These results imply that the GenOn clusters can be ascribed to the origin of the visible photoluminescence. © 2000 American Institute of Physics.

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A Novel Network Structure of Organometallic Clusters in Gas Phase

K. Kaya, S. Nagao, Y. Negishi, K. Judai, A. Kato, Y. Nakamura, A. Nakajima

P. Jena, et al. (eds.) “Proceeding of International Symposium on Cluster and Nanostructure Interfaces" （World Scientific, Singapore.） 301 - 309 2000年08月

研究論文（国際会議プロシーディングス）, 共著, 最終著者, 責任著者, 査読有り

Photoelectron spectroscopy of copper cyanide cluster anions: on the possibility of linear and ring structures

Y. Negishi, T. Yasuike, F. Hayakawa, M. Kizawa, S. Yabushita, A. Nakajima, K. Kaya

Journal of Chemical Physics （American Institute of Physics） 113 （ 5 ） 1725 - 1731 2000年08月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 00219606

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Electronic and Geometric Structures of Metal-Silicide Clusters

M. Sanekata, T. Koya, S. Nagao, Y. Negishi, A. Nakajima, K. Kaya

Transactions of the Materials Research Society of Japan （MRS-J） 25 （ 4 ） 1003 - 1006 2000年07月

研究論文（学術雑誌）, 共著, 査読有り

Gas Phase Synthesis of Transition Metal-Ferrocene Clusters by a Two-lasers Ablation Method

S. Nagao, A. Kato, A. Nakajima, K. Kaya

Transactions of the Materials Research Society of Japan （MRS-J） 25 （ 4 ） 959 - 962 2000年07月

研究論文（学術雑誌）, 共著, 査読有り

Electronic States of Germanium-Oxide Clusters and Their Visible Emission

Y. Negishi, S. Nagao, Y. Nakamura, S. Kamei, A. Nakajima, K. Kaya

Transactions of the Materials Research Society of Japan （MRS-J） 25 （ 4 ） 999 - 1002 2000年07月

研究論文（学術雑誌）, 共著, 査読有り

Multiple-decker sandwich poly-ferrocene clusters

S. Nagao, A. Kato, A. Nakajima, K. Kaya

Journal of the American Chemical Society （American Chemical Society） 122 （ 17 ） 4221 - 4222 2000年04月

研究論文（学術雑誌）, 共著, 責任著者, 査読有り, ISSN 0002-7863

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Visible and Infrared Photoluminescence from Deposited Germanium-Oxide Clusters and from Ge Nanocrystals

A. Nakajima, M. Fujii, S. Hayashi, K. Kaya

L. Pavesi and E. Buzaneva (eds.)“Frontiers of Nano-Optoelectronic Systems: Molecular Scale Engineering and Processes” (Proceeding of NATO/EC Advanced Research Workshop) （World Scientific） 303 - 317 2000年

研究論文（研究会，シンポジウム資料等）, 共著, 筆頭著者, 責任著者, 査読有り

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Photoelectron spectroscopy of tin and lead cluster anions: Application of halogen-doping method

Y. Negishi, H. Kawamata, A. Nakajima, K. Kaya

Journal of Electron Spectroscopy and Related Phenomena （Elsevier Science BV） 106 （ 2-3 ） 117 - 125 1999年12月

研究論文（学術雑誌）, 共著, 責任著者, 査読有り, ISSN 03682048

Electronic properties of tin (Sn) and lead (Pb) cluster anions were studied using photoelectron spectroscopy (PES) with a magnetic bottle-type electron spectrometer, where the feasibility of a halogen-doping method we recently developed was explored. The binary cluster anions containing a chlorine atom (Cl) were produced by a laser vaporization of a target rod in a He carrier gas mixed with a small amount of CCl4 gas. Comparing the PES of SnnCl- and those of Sn-n, it was found that the doped Cl atom in SnnCl- deprives each Sn-n cluster of the excess electron without any serious rearrangement of the Snn framework, which is similar to our previous work on SinF- and GenF-. The halogen-doping method enables us to reveal the HOMO-LUMO gap of the corresponding neutral cluster of the Snn clusters, showing that the Snn clusters are semiconductor clusters, especially below n=30, similar to the Sin and the Gen clusters. For the Pbn clusters, however, the halogen-doping method was inapplicable, which implies that the Pbn clusters are different from the other group-14 clusters, probably exhibiting a metallic nature. © 2000 Elsevier Science B.V. All rights reserved.

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A Novel Network Structure of Organometallic Clusters in the Gas Phase

A. Nakajima, K. Kaya

Journal of Physical Chemistry A （American Chemical Society） 104 （ 2 ） 176 - 191 1999年12月

研究論文（学術雑誌）, 共著, 筆頭著者, 査読有り, ISSN 1089-5639

In the gas phase, novel network structures were found in organometallic clusters between metal atoms produced by laser ablation and organic ligand molecules. For 3d metal-benzene, MnBzm, two kinds of structures of multiple sandwich and rice-ball were formed, depending on the metal elements. Early transition metals (ME) of Sc, Ti, and V form the multiple-decker sandwich structure of (n, m) = (n, n + 1) in which metal atoms and benzene are alternately piled up, while late transition metals (ML) of Fe, Co, and Ni form the rice-ball structure in which central metal clusters were fully covered by benzene molecules. The ionization energy of ME-Bz drops significantly with increasing layers, which can be explained by delocalization of d electrons along the molecular axis. M-C60 binary clusters were also generated by a two-laser vaporization method; ME-C60 clusters efficiently form a chain or a ring structure consisting of a dumbbell unit, in which metal atoms and C60 are alternately connected. For ML-C60 clusters, however, the metal atom is tricapped by C60 and a face-centered tetrahedron structure is formed at (n, m) = (4, 4). A similar multiple-decker sandwich structure is formed also between lanthanide metal atoms (Ln) and an organic ligand of cyclooctatetraene (COT). The Ln-COT cluster is a charge transferred cluster consisting of positively charged Ln and negatively charged COT. Their electronic structure is fairly ionic and is localized around each metal atom. These novel structures of organometallic clusters should inspire new thoughts in material science because it is hoped that the regular arrangement of metal ions can introduce useful properties such as electroconductivity and magnetism.

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Generation and electronic properties of lanthanide-cyclooctatetraene organometallic clusters in gas phase

T. Kurikawa, Y. Negishi, F, Hayakawa, S. Nagao, K. Miyajima, A. Nakajima, K. Kaya

European Physical Journal D （Springer） 9 （ 1-4 ） 283 - 287 1999年12月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 1434-6060

Organometallic clusters of lanthanide (Ln = Nd, Er, Eu, and Yb) and 1,3,5,7-cyclooctatetraene (C8H8) were produced by a combination of laser vaporization and molecular beam methods. In the mass spectra of [Lnn,(C8H8)m], compositions of m = n + 1 were magic numbers. From mass spectrometry, photo-ionization spectroscopy, and photoelectron spectroscopy, we have concluded that these magic-numbered clusters take multiple-decker sandwich structures in which Ln atoms and C8H8 molecules are alternatedly piled up. These sandwich clusters are formed through ionic bonds composed of multiply charged cations [Lnk+ (k = 2 and 3)] and multiply charged anions [C8Hh-8 (h = 1, 1.5, and 2)].

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Photoelectron spectroscopy of 7-azaindole-water cluster anions

A. Nakajima, Y. Negishi, R. Hasumi, K. Kaya

European Physical Journal D （Springer） 9 （ 1-4 ） 303 - 307 1999年12月

研究論文（学術雑誌）, 共著, 筆頭著者, 査読有り, ISSN 1434-6060

The 7-azaindole (7-AzI) cluster anions and their H2O adducts were generated by electron attachment to the 7-AzI/7-AzI-H2O clusters, and their electronic properties studied with mass spectrometry and photoelectron spectroscopy. In the mass spectra of the 7-AzI cluster anions, (7-AzI)-n, monomer anions were missing due to negative electron affinity (EA), while clusters (n ≥ 2) have positive EA, and those having even n were more abundant than those at odd n. EA increased with cluster size from 0.88 eV at n = 2, and an even-odd alternation in the EA was seen. The even-odd alternation showed that the 7-AzI dimer acts as a unit for cluster formation. In the photoelectron spectrum of (7-AzI)-2 at 266 nm, the second peak was observed 2.4 eV above the first peak; this is attributed to the triplet state of the neutral (7-AzI)2. Depending on the detachment energy, a movable peak, with the detachment of 420-300 nm (2.9-4.1 eV), was additionally observed between the first and the second peaks; this was thought to be caused by a delayed photoemission via an electronic excited state of the (7-AzI)-2 anion.

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Pyramidal Structures of Lanthanide-C₆₀ Clusters (Ln_n(C₆₀)_m: Ln = Eu and Ho)

S. Nagao, Y. Negishi, A. Kato, Y. Nakamura, A. Nakajima, K.Kaya

Journal of Physical Chemistry A （American Chemical Society） 103 （ 45 ） 8909 - 8914 1999年10月

研究論文（学術雑誌）, 共著, 責任著者, 査読有り, ISSN 10895639

Novel lanthanide organometallic clusters, Lnn(C60)m, consisting of lanthanide atoms (Ln; Ln = Eu and Ho) and C60 molecules were produced in gas phase by a two-laser vaporization method. The Lnn(C60)m clusters were produced predominantly at compositions of (n, m) = (n, n + 3) [n = 1-4]. These stoichiometries showed no chemisorption reactivity toward CCl4 and O2 reactants. The cluster having (1,4) composition is proposed to form a tetrahedral structure, in which a Ln atom is surrounded by four C60 molecules. In larger clusters with (n, n + 3) composition, each Ln atom is found in a local (1,4) group. Together with the results of sequential chemisorption of Cl atoms on cluster cations, photoelectron spectroscopy of cluster anions leads us to conclude that Ln1(C60)m clusters are charge-transfer complexes expressed as Ln1x+(C60)mx-, and that Eu and Ho atoms exist as Eu2+ and Ho3+ ions, respectively.

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Photoelectron spectroscopy of sodium-coated C₆₀ and C₇₀ cluster anions

B. Palpant, A. Otake, F. Hayakawa, Y. Negishi, G. H. Lee, A. Nakajima, K. Kaya

Physical Review B - Condensed Matter and Materials Physics （American Institute of Physics） 60 （ 7 ） 4509 - 4512 1999年08月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 10980121

A systematic study of photoelectron spectroscopy for C60Na-N (N=0–33) and C70Na-N (N=0–15) clusters is reported. These clusters are produced by a laser vaporization source and their photoelectron spectrum is measured using a magnetic-bottle time-of-flight spectrometer. For N < 13, the adiabatic electron affinity and the vertical detachment energy display strong regular features with increasing N. A packing of the sodium atoms in stable Na3 trimers is inferred, with each atom lying above a pentagonal ring. For larger clusters (N>13), a very different trend is observed, signing the appearance of a metallic behavior for the sodium layer at the fullerene surface. The specific case of C70 is also discussed. © 1999 The American Physical Society.

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Production of jet-cooled coronene and coronene cluster anions and their study with photoelectron spectroscopy

M. A. Duncan, A. M. Knight, Y. Negishi, S. Nagao, Y. Nakamura, A. Kato, A. Nakajima, K. Kaya

Chemical Physics Letters （Elsevier Science BV） 309 （ 1-2 ） 49 - 54 1999年08月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 0009-2614

Anions of coronene (C24H12) and its clusters are produced by laser ablation of a pressed pellet sample in a double rod, pulsed nozzle source. Coronene anions are produced efficiently only when transition metal vapor is present in the same plasma from co-vaporization of a metal sample. Anion monomers and clusters of coronene are size-selected and studied with photoelectron spectroscopy in a magnetic bottle spectrometer. Spectra at 532, 355 and 266 nm probe electron affinities and neutral electronic states. The electron affinity of coronene is 0.47±0.09 eV and that of coronene dimer is 0.67±0.09 eV.

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Charge distributions in multiple-decker sandwich clusters of lanthanide-cyclooctatetraene: Application of Na-atom doping

K. Miyajima, T. Kurikawa, M. Hashimoto, A. Nakajima, K. Kaya

Chemical Physics Letters （Elsevier Science BV） 306 （ 5-6 ） 256 - 262 1999年06月

研究論文（学術雑誌）, 共著, 責任著者, 査読有り, ISSN 00092614

In gas phase, Na-atom adducts were produced for organometallic lanthanide complexes of Lnn(C8H8)n+1 [Ln = europium (Eu) and holmium (Ho); C8H8 = 1,3,5,7-cyclooctatetraene (COT)] by a combination of two-laser vaporization and a molecular beam method. The number of attached Na atoms can be reasonably explained by charge distributions of the complex, where Eu and Ho atoms exist as Eu2+ and Ho3+ , respectively, and C8H8 molecules as COT2-, accepting two excess electrons. The behavior of the Na adducts chemically demonstrates that the organometallic lanthanide complex is formed through ionic bonding, implying that it is possible to be isolated as its Na salt. © 1999 Elsevier Science B.V. All rights reserved.

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Electronic properties of organometallic metal-benzene complexes [M_n(benzene)_m (M = Sc-Cu)]

T. Kurikawa, H. Takeda, M. Hirano, K. Judai, T. Arita, S. Nagao, A. Nakajima, K. Kaya

Organometallics （American Chemical Society） 18 （ 8 ） 1430 - 1438 1999年03月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 02767333

Neutral metal-benzene complexes, Mn(benzene)m (M = Sc to Cu), are produced for all of the 3d transition metals in the gas phase by using the laser vaporization method. These species are characterized by mass spectrometry, photoionization spectroscopy, and chemical probe experiments. Depending on the metal, there are two types of structures for Mn(benzene)m: multiple-decker sandwich structures and metal clusters fully covered with benzene molecules (rice-ball structures). The former sandwich structure is characteristic of the complexes for early transition metals (Sc-V), whereas the latter is formed for late transition metals (Fe-Ni). Electronic structures of M1(benzene)x (x = 1, 2) complexes are investigated through systematic measurements of ionization energies (Ei's).

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Photoelectron spectra of small nanophase W metal cluster anions

G. H. Lee, S. H. Huh, Y. C. Park, F. Hayakawa, Y. Negishi, A. Nakajima, K. Kaya

Chemical Physics Letters （Elsevier Science BV） 299 （ 3-4 ） 309 - 314 1999年01月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 0009-2614

We have recorded photoelectron spectra of Wn- (n=20-90) using a magnetic bottle type photoelectron spectrometer. Each photoelectron spectrum consists of a single, broad peak, shifted successively towards a higher electron binding energy (EBE) with increasing n. We estimated the threshold EBEs from the spectra. The threshold EBE versus n-1/3 plot showed a straight line and was extrapolated to 4.50 eV which is consistent with the W bulk work function, 4.55 eV. We also estimated the bond dissociation energies of Wn- cluster anions using a simple energetic model.

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Multiple-decker sandwich complexes of lanthanide 1,3,5,7- cyclooctatetraene [Ln_n(C₈H₈)_m] (Ln = Ce, Nd, Eu, Ho, and Yb); localized ionic bonding structure

T. Kurikawa, Y. Negishi, F. Hayakawa, S. Nagao, K. Miyajima, A. Nakajima, K. Kaya

Journal of the American Chemical Society （American Chemical Society） 120 （ 45 ） 11766 - 11772 1998年10月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 00027863

Organometallic lanthanide complexes of Ln(n)(C8H8)(m) (Ln = lanthanide metals of Ce, Nd, Eu, Ho, and Yb; C8H8 := 1,3,5,7-cyclooctatetraene) were produced by a combination of laser vaporization and molecular beam methods. The complexes having (n, m) = (n, n + 1) for n = 1-5 were prominently produced as magic numbers in the mass spectra. Using mass spectrometry, photoionization spectroscopy, and photoelectron spectroscopy, it is concluded that these magic-numbered complexes form multiple decker sandwich structures in which Ln atoms and C8H8 molecules are alternately piled up. The obtained spectroscopic data show that these sandwich complexes are formed through ionic bonds and are charge transfer complexes, where the Ln atoms are multiply charged cations of Ln(k+) (k = 2 and 3) and C8H8 molecules are multiply charged anions.

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The infrared HOMO-LUMO gap of germanium clusters

Y. Negishi, H. Kawamata, F. Hayakawa, A. Nakajima, K. Kaya

Chemical Physics Letters （Elsevier Science BV） 294 （ 4-5 ） 370 - 376 1998年09月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 0009-2614

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Formation and structures of transition metal-C₆₀ clusters

S. Nagao, T. Kurikawa, K. Miyajima, A. Nakajima, K. Kaya

Journal of Physical Chemistry A （American Chemical Society） 102 （ 24 ） 4495 - 4500 1998年05月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 10895639

Transition metal (Sc, Ti, V, and Cr)-C60 binary clusters were produced in the gas phase by a two-laser vaporization method. In the mass spectra of cationic Mn(C60)m+, for M = Sc, Ti, and V, clusters having the composition of (n, m) = (1, 1)+, (1, 2)+, (2, 3)+, (3, 4)+, (4, 4)+, and (5, 5)+ were predominantly produced. The peaks having (n, n + 1)+ compositions are explicable in terms of multiple dumbbell structures, in which metal atoms and C60 are alternately stacked. Clusters with composition (4, 4)+ and (5, 5)+ are presumed to be ring structures. For Crn(C60)m+, however, a Cr atom is surrounded by three C60, forming a tricapped structure, and the structure acts as a unit for larger stable clusters of (2, 4)+, (3, 4)+, and (4, 4)+. Ionization energy measurements for neutral Mn(C60)m and a chemical probe method using CO, O2, and C2H4 as reactants gave information concerning the geometrical and electronic structures.

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Geometric and electronic structures of fluorine bound silicon clusters

R. Kishi, Y. Negishi, H. Kawamata, S. Iwata, A. Nakajima, K. Kaya

Journal of Chemical Physics （American Institute of Physics） 108 （ 19 ） 8039 - 8058 1998年05月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 0021-9606

Geometries and energies of SinF- and SinF (n=1-7) were investigated with ab initio MO calculations; the Møller-Plesset perturbation and coupled cluster methods were used to take into account the electron correlation. The F atom is bound to the apex atom in the Sin frame, and the electronic structures of the Sin part in SinF- and SinF are similar to those of the corresponding Sin and Si+n, respectively. The calculated electron affinities (EAs) of SinF are in agreement with the experimental values. The size dependence of EAs of SinF is similar to that of the ionization energies of Sin at n≥5, whereas it is different at n≤4. In the HOMO of SinF (n≤4), the weak antibonding nature between F and Si greatly makes the SinF- anions unstable, resulting in small EA. The vibrationally resolved photoelectron spectra (PES) of SinF- (n=1-3) were measured, and were completely reproduced theoretically. © 1998 American Institute of Physics.

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Formation of cobalt-C₆₀ clusters: Tricapped Co(C₆₀)₃ unit

T. Kurikawa, S. Nagao, K. Miyajima, A. Nakajima, K. Kaya

Journal of Physical Chemistry A （American Chemical Society） 102 （ 10 ） 1743 - 1747 1998年02月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 10895639

Cobalt (Co)-C60 mixed clusters were produced by the combination of laser vaporization and molecular beam methods. Cationic Con(C60)m+ clusters were produced predominantly with the compositions (n, m) = (0-1, 1), (1, 2), (1, 3), (2-4, 4), and (5, 5). Both mass spectrometry and a chemical probe method have revealed that compositions of (1, 3) and (4, 4) correspond to a tricapped planar structure and a tricapped trigonal pyramid structure, respectively, in which each Co atom is surrounded by three C60.

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Geometric and electronic structures of silicon-sodium binary clusters. II. Photoelectron spectroscopy of Si_nNa_m^- cluster anions

R. Kishi, H. Kawamata, Y. Negishi, S. Iwata, A. Nakajima, K. Kaya

Journal of Chemical Physics （American Institute of Physics） 107 （ 23 ） 10029 - 10043 1997年12月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 00219606

Electronic properties of silicon-sodium cluster anions (SinNam-,; 4≤n≤11, 1≤m≤3) were investigated by photoelectron spectroscopy using a magnetic-bottle type electron spectrometer. The electron affinities (EAs) of SinNam were determined from the threshold energies in the photoelectron spectra of SinNam-. Besides, the geometries and energies of SinNa- (n=1-7) were investigated with ab initio MO (molecular orbital) calculations; Møller-Plesset perturbation and coupled cluster methods were used to take into account the electron correlation. By an Na adsorption on Sin, the EA decreases for n=4-6, but increases for n≥7. The calculations for SinNa- show that the Sin framework for n=4-6 accepts at least two excess electrons. For Si7Na-, however, the charge transfer from the Na atom scarcely occurs to avoid distorting the close-packed Si7 framework. Moreover, the ionization energy (Ei) and EA of SinNam, show several patterns with the number of the Na atoms, which reflect the charge transfer mechanism in the cluster reflecting the nature of the electron-accepting orbitals of the corresponding Sin and the adsorption types of the Na atoms. © 1997 American Institute of Physics.

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Ab initio MO studies of neutral and anionic SiC_n clusters (n = 2-5)

M. Gomei, R. Kishi, A. Nakajima, S. Iwata, K. Kaya

Journal of Chemical Physics （American Institute of Physics） 107 （ 23 ） 10051 - 10061 1997年12月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 00219606

The geometries and energies of SiCn and SiCn (n=2-5) were investigated with ab initia calculations including electron correlation effects with the MP2/6-31G* method, followed by MP4 and CCSD(T) single-point calculations to determine the most stable isomers. The adiabatic electron affinities (AEA) were evaluated with the ΔMP4 method. Because of the complexity of the electronic structure of SiC2, its AEA was calculated at the CCSD(T)/aug cc-pVTZ//CCSD(T)/aug cc-pVDZ level of approximation. For the neutral SiCn clusters, the isomer having a triplet ground state has large EA, whereas the isomer having a singlet ground state has small EA. This is attributed to the bonding character between Si and C atoms in the orbital occupied by the extra electron. The calculated EA was used to assign the photoelectron spectra of SiCn- reported previously. Furthermore, the effects of Renner-Teller splitting for the linear isomers of anions are discussed. © 1997 American Institute of Physics.

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Study on iron-sulfur cluster in gas phase: Electronic structure and reactivity

A. Nakajima, T. Hayase, F. Hayakawa, K. Kaya

Chemical Physics Letters （Elsevier Science BV） 280 （ 3-4 ） 381 - 389 1997年12月

研究論文（学術雑誌）, 共著, 筆頭著者, 査読有り, ISSN 00092614

In gas phase, electronic and geometric properties of iron-sulfur cluster (FenSm, n=1-8, m=2-6) have been investigated using photoelectron spectroscopy (PES) of their anions and a chemical probe method for their cations. The electron affinities of FenSm greatly change at the composition of n=m, and the reactivity of FenSm+ toward NH3 and C2H2 is enhanced around equally mixed compositions. These results show that the FenSm clusters produced in gas phase seemingly take the structure having alternate bonds between Fe and S atoms, likewise that of an active site of iron-sulfur proteins.

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Multiple dumbbell structures of vanadium-C₆₀ clusters

A. Nakajima, S. Nagao, H. Takeda, T. Kurikawa, K. Kaya

Journal of Chemical Physics （American Institute of Physics） 107 （ 16 ） 6491 - 6494 1997年10月

研究論文（学術雑誌）, 共著, 筆頭著者, 査読有り, ISSN 00219606

Vanadium (V)-C60 mixed clusters were produced by two laser vaporization methods. In the mass spectra of cationic Vn(C60)m+, the abundant clusters were produced at the composition of (n,m) = (1,1), (1,2) (2,3), (3-4,4), and (5,5). This pattern is explicable in terms of a multiple dumbbell structure V atom and C60 are alternatively piled up. The ionization energy and the reactivity of the dumbbell clusters toward O2 and CO show V1(C60)2 takes a structure of V1(η6 -C60)2. © 1997 American Institute of Physics.

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Geometric and electronic structures of silicon-sodium binary clusters. I. Ionization energy of Si_nNa_m

R. Kishi, S. Iwata, A. Nakajima, K. Kaya

Journal of Chemical Physics （American Institute of Physics） 107 （ 8 ） 3056 - 3070 1997年08月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 00219606

Sodium doped silicon clusters (SinNam; 1≤n≤14, 1≤m≤5) produced by two types of laser vaporization were studied. The adsorption of Na atoms on the Sin clusters leads the substantial lowering of the ionization energy, Ei, of SinNam clusters. Their reactivity toward NO molecules was measured with a fast flow reactor, and the anticorrelation between the Eis and the reactivity was clearly observed; species having low Ei exhibit high reactivity and vice versa. Moreover, the clear parallelism between the Eis of SinNa and the EAs of Sin is found. This is consistent with the fact that the structure of SinNa clusters keeps the frame of the corresponding Sin cluster unchanged and that the electronic structure of SinNa is similar to that of the corresponding negative ion Sin-. In addition to the experimental studies, the geometries, adsorption energies, and vertical ionization energies of SinNa (n = 1-7) were investigated with ab initio MO calculations including electron correlation; The Møller-Plesset perturbation theory was used and the configuration interaction (CI) calculation was carried out, particularly for a diatomic molecule, SiNa. © 1997 American Institute of Physics.

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Why do vanadium atoms form multiple-decker sandwich clusters with benzene molecules efficiently?

T. Yasuike, A. Nakajima, S. Yabushita, K. Kaya

Journal of Physical Chemistry A （American Chemical Society） 101 （ 29 ） 5360 - 5367 1997年07月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 1089-5639

Transition-metal benzene clusters, Mn(benzene)m (M = Ti, V, and Cr), were synthesized by the reaction of laser-vaporized metal atoms with benzene vapor. All the clusters exhibit magic number behavior at m = n + 1, which is rationalized by the structure of a multiple-decker sandwich, but V atoms can efficiently take the sandwich structure (up to n = 5) in particular. This metal specificity of the V atoms and their growth mechanism were examined by quantum chemical calculations, the full valence configurational interaction (FVCI) method with configuration-averaged SCF Orbitals. The calculation results imply that (1) total spin conservation in growth process plays an important role and (2) the production in the sandwich clusters particularly favors a process through lower spin states. The combination between experimental and theoretical investigations leads us to a better comprehension of both the bonding scheme in the sandwich clusters and the growth mechanism, and accordingly, a more efficient production method is proposed generally for the transition-metal sandwich complexes.

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Effect of ring substituents on formation rates for vanadium-arene clusters

M. Hirano, K. Judai, A. Nakajima, K. Kaya

Journal of Physical Chemistry A （American Chemical Society） 101 （ 27 ） 4893 - 4899 1997年07月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 1089-5639

Vanadium-arene complexes, Vn(arene)m (arene = benzene (C6H6), toluene (C6H5CH3), and fluorobenzene (C6H5F)), were synthesized by the reaction of laser-vaporized vanadium atoms with arene vapor. Both the analysis of the mass spectra and the measurements of their ionization energies indicate that both Vn(C6H5-CH3)m and Vn(C6H5F)m, clusters take a multiple-decker sandwich structure similar to the structure of Vn-(C6H6)m clusters. The relative reactivity between benzene and toluene toward vanadium was also determined; toluene is 4 times as reactive as benzene for V1(C6H5CH3)1/V 1(C6H6)1 production and 2 times for V1(C6H5-CH3)2/V 1(C6H6)2 production because of reactivity enhancement of the electron-donating ring substituent. In the case of the Vn(C6H5F)m cluster, it is found that the electron-withdrawing ring substituent of fluorobenzene results in much less reactivity than is observed for benzene and toluene.

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Formation of vanadium-arene complex anions and their photoelectron spectroscopy

K. Judai, M. Hirano, H. Kawamata, S. Yabushita, A. Nakajima, K. Kaya

Chemical Physics Letters （Elsevier Science BV） 270 （ 1-2 ） 23 - 30 1997年05月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 0009-2614

Vanadium-arene complex anions, V1(arene)-m (m =1,2; arene = benzene, fluorobenzene, and toluene) were synthesized by the gas phase reaction of laser vaporized metal atoms with aromatic compound vapors. According to the 18-electron rule, it was anticipated that bis(benzene)vanadium should be stabilized by an electron attachment. However, it was experimentally concluded that bis(benzene)vanadium has negative electron affinity. The photoelectron spectra for the observed complex anions were measured, and the electronic configurations of metal-arene complexes and their structures are discussed.

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Photoelectron spectroscopy of germanium-fluorine binary cluster anions: The HOMO-LUMO gap estimation of Ge_n clusters

Y. Negishi, H. Kawamata, T. Hayase, M. Gomei, R. Kishi, F. Hayakawa, A. Nakajima, K. Kaya

Chemical Physics Letters （Elsevier Science BV） 269 （ 3-4 ） 199 - 207 1997年05月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 00092614

Electronic properties of germanium-fluorine cluster anions (GenF-m; n = 1-11, m = 1-3) were studied using photoelectron spectroscopy (PES) with a magnetic-bottle type electron spectrometer. Compared between the PES of GenF- and those of the Ge-n, it was found that the doped F atom in GenF- deprives each Ge-n cluster of the excess electron without any serious rearrangement of the Gen framework. The F doping method enables us to estimate the HOMO-LUMO gap of the corresponding neutral Gen clusters (n = 4-11). For a diatomic GeF- cluster, furthermore, the vibrational structures could be resolved to determine its vibrational frequency.

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Structures and stabilities of 3d-transition metal-benzene organometallic clusters

T. Kurikawa, H. Takeda, A. Nakajima, K. Kaya

Zeitschrift fur Physik D-Atoms Molecules and Clusters （Springer） 40 （ 1-4 ） 65 - 69 1997年03月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 0178-7683

In the gas phase, we have successfully synthesized organometallic clusters, Mn(benzene)m (M = 3d transition metal atoms), by using a laser vaporization method. The measurements of mass spectra and ionization energies (Ei) have revealed that the organometallic clusters can take two types of structures; layered sandwich structures (m = n + 1) and metal clusters saturatedly covered with benzenes. For early transition metals of Sc, Ti, and V, only the multiple decker sandwich structure clusters were preferentially produced, in which benzene and metal atoms are alternately piled up. For late transition metals of Co and Ni, the metal clusters saturatedly surrounded by benzenes were also produced as well as the sandwich clusters. Furthermore, the Eis of M1(benzene)2 (M = Sc-Ni) were systematically measured and their electronic properties will be discussed. © Springer-Verlag 1997.

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Photoelectron spectroscopy of transition metal-sulfur cluster anions

A. Nakajima, H. Kawamata, T. Hayase, Y. Negishi, K. Kaya

Zeitschrift fur Physik D-Atoms Molecules and Clusters （Springer） 40 （ 1-4 ） 17 - 21 1997年03月

研究論文（学術雑誌）, 共著, 筆頭著者, 査読有り, ISSN 0178-7683

Metal (M)-sulfur cluster anions (M = Ag, Fe and Mn) have been studied using photoelectron spectroscopy (PES) with a magnetic-bottle type time-of-flight electron spectrometer. The MnS-m cluster anions were formed in a laser vaporization cluster source. For Ag-S, the largest coordination number of Ag atoms (nmax) is generally expressed as nmax=2m-1 in each series of the number of S atoms (m). For Fe-S and Mn-S, it was found that the stable cluster ions are the ones with compositions of n = m and n = m±1. Their electron affinities were measured from the onset of the PES spectrum. For Ag-S, the EAs of Ag1Sm are small and around 1 eV, whereas those of AgnSm (n ≥ 2) become large above 2 eV. The features in the mass distribution and PES suggest that Ag2S unit is preferentially formed with increasing the number of Ag atoms. For Fe-S and Mn-S, the PES spectra of FenS-m/MnnS-m show a unique similarity at n ≥ m, indicating that the Fe/Mn atom addition to FenS-n/MnnS-n has little effect on the electronic property of FenSn/MnnSn. The PES spectra imply that the FenSn cluster is the structural framework of these clusters, as similarly as the determined structure of the FenSn cluster in nitrogenase enzyme. © Springer-Verlag 1997.

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Photoelectron spectroscopy of silicon- and germanium-fluorine binary cluster anions (Si_nF_m^-,Ge_nF_m^-)

K. Kaya, H. Kawamata, Y. Negishi, T. Hayase, R. Kishi, A. Nakajima

Zeitschrift fur Physik D-Atoms Molecules and Clusters （Springer） 40 （ 1 ） 5 - 9 1997年03月

研究論文（学術雑誌）, 共著, 最終著者, 査読有り, ISSN 01787683

Electronic properties of silicon-fluorine and germanium-fluorine cluster anions (SinF-m; n = 1-9, m = 1-3, GenF-m; n=1-9, m=1-3) were investigated by photoelectron spectroscopy using a magnetic-bottle type electron spectrometer. The binary cluster anions were generated by a laser vaporization of a silicon/germanium rod in an He carrier gas mixed with a small amount of SiF4 or F2 gas. Comparison between photoelectron spectra of SinF-/GenF- and Si-n/Ge-n (n=4-9) gives the insight that the doped F atom can remove one electron from the corresponding Si-n/Ge-n cluster without any serious rearrangement of Sin/Gen framework, because only the first peak of Si-n/Ge-n, corresponding singly occupied molecular orbital (SOMO), disappears and other successive spectral features are unchanged with the F atom doping © Springer-Verlag 1997.

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High-resolution laser-induced fluorescence spectra of 7-azaindole-water complexes and 7-azaindole dimer

A. Nakajima, M. Hirano, R. Hasumi, K. Kaya, H. Watanabe, C. C. Carter, J. M. Williamson, T. A. Miller

Journal of Physical Chemistry A （American Chemical Society） 101 （ 4 ） 392 - 398 1997年01月

研究論文（学術雑誌）, 共著, 筆頭著者, 査読有り, ISSN 1089-5639

The geometric structures of 7-azaindole-water complexes, (7-AzI)-(H2O)n (n = 1-3), and 7-AzI dimer were investigated by laser-induced fluorescence (LIF) spectroscopy with high resolution (∼0.01 cm-1). For the 7-AzI-(H2O)n complexes (n = 1-3), the LIF spectra show partially resolved rotational structure, which has been analyzed in combination with theoretical calculations. This analysis yields the rotational constants and characterizes the structures. In 7-AzI-(H2O)1, the H2O molecule is located in the molecular plane, forming a six-membered ring with two hydrogen bonds. Analyses for 7-AzI-(H2O)2 and -(H2O)3 show, furthermore, that both the second and the third H2O are located in the molecular plane of 7-AzI, forming a network ring of hydrogen bonds. For the 7-AzI dimer, the LIF spectrum shows an unresolved rotational envelope. The rotational contour has been analyzed, and we discuss the dimer's likely structure.

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Geometric and electronic structure of binary clusters containing silicon atoms

K. Kaya, A. Nakajima

Materials Science and Engineering: A （Elsevier Science BV） 217/218 7 - 11 1996年10月

研究論文（学術雑誌）, 共著, 最終著者, 査読有り, ISSN 09215093

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Photoelectron spectroscopy of silicon-fluorine binary cluster anions (Si_nF_m^-)

H. Kawamata, Y. Negishi, R. Kishi, S. Iwata, A. Nakajima, K. Kaya

Journal of Chemical Physics （American Institute of Physics） 105 （ 13 ） 5369 - 5376 1996年10月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 0021-9606

Electronic properties of silicon-fluorine cluster anions (Si_nF_m^-; n=1-11, HZ = 1-3) were investigated by photoelectron spectroscopy using a magnetic-bottle type electron spectrometer. The binary cluster anions were generated by a laser vaporization of a silicon rod in an He carrier gas mixed with a small amount of SiF₄ or F₂ gas. The highly abundant clusters are SiF_m^- (m=3 and 5) and Si_nF^- (n=6, 7, and 10) in their mass spectra. In the photoelectron spectra of SiF_m^- (m = 1-5), the clusters having odd m have higher electron affinity (EA) than those having even in, indicating that the even/odd alternation in EA is attributed to their electronic structures of a closed/open valence shell. Comparison between photoelectron spectra of Si_nF^- and Si_n^- (n=4-11) gives the insight that the doped F atom can remove one electron from the corresponding Si_n^- cluster without any serious rearrangement of Si_n^- framework, because only the first peak of Si_n^-, corresponding singly occupied molecular orbital (SOMO), disappears and other successive spectral features are unchanged with the F atom doping. In some clusters, furthermore, the vibrational structures could be resolved to determine a vibrational frequency and to presume the geometry with ab initio molecular orbital calculations.

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Theoretical study of carbon doped small silicon clusters: Electron affinities of Si_nC (n = 2-5)

R. Kishi, M. Gomei, A. Nakajima, S. Iwata, K. Kaya

Journal of Chemical Physics （American Institute of Physics） 104 （ 21 ） 8593 - 8604 1996年06月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 00219606

The geometries and energies of SinC and SinC- (n=2-5) were investigated with ab initio MO calculations including electron correlation. The Møller-Plesset perturbation theory was used for n=3-5. The configuration interaction (CI) calculations were carried out for n=2 and 3. The most stable isomers of the neutral SinC clusters have the structure in which an Si atom having the largest sp hybridization in the corresponding Sin+1 cluster is replaced by a C atom. For their anions, the structures in which an excess electron is distributed avoiding a C atom become stable. Moreover, the adiabatic electron affinities of SinC and vertical detachment energies of anionic SinC- were computed. They reproduced the trends of photoelectron spectra of SinC- reported previously. © 1996 American Institute of Physics.

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Theoretical Study of Silicon-M Binary Clusters（M=C and Na）

R. Kishi, A. Nakajima, S. Iwata, K. Kaya

T. Kondow et al.（eds.）”Structures and Dynamics of Clusters” frontiers science series （Universal Academy Press）（ 16 ） 607 - 612 1996年05月

論文集(書籍)内論文, 共著

Organometallic Clusters studied Under the Molecular Beam

K. Kaya, K. Hoshino, T. Kurikawa, H. Takeda, M. Hirano, T. Yasuike, S. Yabushita, A. Nakajima

T. Kondow et al.（eds.）”Structures and Dynamics of Clusters” Frontiers Science Series （Universal Academy Press）（ 16 ） 129 - 134 1996年05月

論文集(書籍)内論文, 共著, 最終著者

Photoelectron Spectroscopy of Binary Silicon Cluster Anions

A. Nakajima, N. Zhang, H. Kawamata, T. Hayase, K. Kaya

T. Kondow et al.（eds.）”Structures and Dynamics of Clusters” frontiers science series （Universal Academy Press）（ 16 ） 377 - 382 1996年05月

論文集(書籍)内論文, 共著, 筆頭著者

Photoelectron spectroscopy of iron-sulfur cluster anions

N. Zhang, T. Hayase, H. Kawamata, K. Nakao, A. Nakajima, K. Kaya

Journal of Chemical Physics （American Institute of Physics） 104 （ 10 ） 3413 - 3419 1996年03月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 0021-9606

Iron-sulfur cluster anions (FenSm-, n=1-6, m=1-6) have been studied using photoelectron spectroscopy (PES) with a magnetic-bottle-type time-of-flight electron spectrometer. The FenSm-. cluster anions were formed in a laser vaporization cluster source. It was found that the stable cluster ions are the ones with compositions of n=m and n=m±1. The electron affinities were measured from the onsets of the PES spectra. Three low-lying electronic states were observed for FeS. The PES spectra of FenS- (n=1-4) series show a unique similarity, indicating that the Fe atom addition to FeS- has little effect on the electronic property of FeS. The PES spectra of FenS2- series show a similarity among the cluster anions with n=2-4, showing that Fe2S2- is the structural framework of these clusters. For FenS3- series, Fe3S3- is proposed to be the structural framework. The electronic properties and geometrical structures of the clusters are discussed. © 1996 American Institute of Physics.

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Structures of organometallic clusters: V_n(benzene)_m and Co_n(benzene)_m

K. Hoshino, T. Kurikawa, H. Takeda, A. Nakajima, K. Kaya

Surface Review and Letters （World Scientific Publishing） 3 （ 1 ） 183 - 185 1996年02月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 0218625X

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Photoelectron spectroscopy of silicon-carbon cluster anions

A. Nakajima, T. Taguwa, K. Nakao, M. Gomei, R. Kishi, S. Iwata, K. Kaya

Surface Review and Letters （World Scientific Publishing） 3 （ 1 ） 411 - 415 1996年02月

研究論文（学術雑誌）, 共著, 筆頭著者, 査読有り, ISSN 0218-625X

Photoelectron spectra of CnSim- cluster anions (3≤ n, m ≤6) were measured at the photon energy of 3.49 eV, by using a magnetic-bottle electron spectrometer having ∼60-meV resolution. The CnSim- clusters were produced by laser vaporization of a carbon-silicon mixture rod in a He carrier gas. The spectra of the C1Sim-1- clusters are similar to those of pure Si-m clusters in the peak positions and their envelopes, which is attributed to the similarity in electronic structure of Si and C atoms, leading to a similar geometry. In contrast, the similarity in the photoelectron spectra is not observed between C-n and Cn-1Si1- clusters, which is attributed to a change in their geometry; from chain to ring.

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Electron-correlation effects on geometrical and electronic structures of Si_nNa clusters

R. Kishi, A. Nakajima, S. Iwata, K. Kaya

Surface Review and Letters （World Scientific Publishing） 3 （ 1 ） 365 - 369 1996年02月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 0218-625X

The geometries and energies of silicon-sodium binary clusters (Si_nNa, n = 2-7) were investigated with the inclusion of electron-correlation effects using the second-order Møller-Plesset perturbation theory (MP2) with the 6-31G basis set. In addition, the fourth-order (MP4) energies were evaluated to determine the ground-state isomers. Si₄Na cluster was calculated with the polarized 6-31G* basis set. For all of the Si_nNa clusters, the most stable isomers have bridge-site type structures. Apart from the bridge-site type Si₆Na, the structure of the most stable isomer of Si_nNa keeps the frame of the corresponding Si_n cluster unchanged, and the electronic structure of Si_nNa is similar to that of the corresponding negative ion Si^-
_n. The silicon framework of the bridge-site type of Si₆Na is distorted from the compressed octahedron to the face-capped trigonal bipyramid.

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Photoelectron spectroscopy of binary cluster anions

K. Kaya, A. Nakajima

Surface Review and Letters （World Scientific Publishing） 3 （ 1 ） 637 - 642 1996年02月

研究論文（学術雑誌）, 共著, 最終著者, 査読有り, ISSN 0218-625X

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Photoelectron spectroscopy of binary-metal cluster anions containing sulfur atom

A. Nakajima, T. Taguwa, K. Nakao, K. Hoshino, S. Iwata, K. Kaya

Surface Review and Letters （World Scientific Publishing） 3 （ 1 ） 417 - 421 1996年02月

研究論文（学術雑誌）, 共著, 筆頭著者, 査読有り, ISSN 0218-625X

Photoelectron spectra of MnS-1 (M = Al, Cu, Ag, and Au) cluster anions were measured at the photon energies of 3.49 and 4.66 eV using a magnetic-bottle electron spectrometer having ∼60-meV resolution. The electron affinities of AlS-, CuS-, AgS-, and AuS- are 2.60(3), 2.10(3), 2.09(3), and 2.44(3) eV, respectively. The large EA of AlS- is attributed to large stability by six bonding electrons, and a new electronic state of A′2Π can be found 0.4 eV above the AlS ground state, which is in agreement with a theoretical calculation. In the photoelectron spectrum of AuS-, two peaks due to a splitting by the L-S coupling could be observed, and the amount of the splitting is 0.18(2) eV. The results of aluminum clusters containing sulfur atom will also be presented.

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Photoelectron spectroscopy of manganese-sulfur cluster anions

N Zhang, H. Kawamata, A. Nakajima, K. Kaya

Journal of Chemical Physics （American Institute of Physics） 104 （ 1 ） 36 - 41 1996年01月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 0021-9606

Manganese-sulfur cluster anions (MnnSm-, n = 1-10, m = 1-10) have been studied using a magnetic-bottle type photoelectron spectroscopy (PES). The MnnSm- cluster anions were formed in a laser vaporization cluster source. The most stable cluster anions were found to have the compositions of n=m and n=m±1. The electron affinities of neutral manganese sulfide clusters were measured from the onsets of the PES spectra. A new electronically excited state at 0.75 eV above the ground state was found for MnS. From the size dependence of the PES spectra, it was found that Mn2S2-, Mn3S3-, and Mn4S4- are structural frameworks in MnnS2-, MnnS3-, and MnnS4- series, respectively. The electronic properties and geometrical structures of the clusters are discussed. © 1996 American Institute of Physics.

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Structures and ionization energies of cobalt-benzene clusters (Co_n(benzene)_m)

T. Kurikawa, M. Hirano, H. Takeda, K. Yagi, K. Hoshino, A. Nakajima, K. Kaya

Journal of Physical Chemistry （American Chemical Society） 99 （ 44 ） 16248 - 16252 1995年11月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 00223654

Cobalt-benzene clusters, Con(benzene)m, were synthesized by the reaction of laser-vaporized Co atoms with benzene vapor. The clusters exhibit magic-number behavior not only at m = n + 1 (n = 1-3) but also at (n-m) = (3-3), (4-4), (5-4), (6-4), (7-4), (8-5), and (9-6). Observed magic-number behavior and ionization energies are consistently explained by assuming the structure of the clusters to be multiple-decker sandwiches for n-m (m = n + 1, n = 1 - 3) and to be an n atom metal cluster coated by m benzene molecules for n-m (n ≥ 4). In this paper, we will present the properties of Con(benzene)m clusters from the viewpoints of mass distribution, reactivity, and ionization energies. The structure and formation mechanism of the Con(benzene)m clusters will also be discussed. © 1995 American Chemical Society.

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Photoelectron spectroscopy of silicon-carbon cluster anions (Si_nC_m^-)

A. Nakajima, T. Taguwa, K. Nakao, M. Gomei, R. Kishi, S. Iwata, K. Kaya

The Journal of Chemical Physics （American Institute of Physics） 103 （ 6 ） 2050 - 2057 1995年08月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 00219606

Photoelectron spectra of Sin,Cm- cluster anions (1≤n≤7 and 1≤m≤5) were measured at the photon energies of 3.49 eV, by using a magnetic bottle electron spectrometer. The Si nCm- clusters were produced either by a laser vaporization of a silicon-carbon mixed rod or by two laser vaporizations of carbon and silicon rods in a He carrier gas. The spectra of the Si nC1- (3≤n≤7) clusters are similar to those of pure Sin+1- clusters in the peak positions and their envelopes, which is attributed to the isovalent electronic structure of Si and C atoms as well as to the similar geometrical structure. In contrast, the similarity in the photoelectron spectra is not observed between Cm+1 and Si1Cm (2≤m≤5) clusters, which is attributed to a change in their geometry; from chain to ring. These experimental conclusions are discussed with the results of our theoretical calculations. © 1995 American Institute of Physics.

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Photoelectron spectroscopy and mass distributions of aluminum-sulfur cluster anions (Al_nS_m^-)

A. Nakajima, N. Zhang, H. Kawamata, T. Hayase, K. Nakao, K. Kaya

Chemical Physics Letters （Elsevier Science BV） 241 （ 4 ） 295 - 300 1995年07月

研究論文（学術雑誌）, 共著, 筆頭著者, 査読有り, ISSN 00092614

Photoelectron spectra of AlnSm- cluster anions (0 ≤ n, m ≤ 10) were measured at photon energies of 3.49 and 4.66 eV, by using a magnetic bottle electron spectrometer. The AlnSm-. clusters were produced by laser vaporization of an aluminum-sulfur mixed rod in He carrier gas. The abundant AInSM clusters are three types of AlS-(Al2S3)k, AlS2-(Al2S3)k, and (Al2S3)k-, where an unit of Al2S3 aggregates with ion cores of AlS-, AlS2-, and Al2S3, respectively. Al3S3 is exceptionally stable with its geometrical structure. © 1995.

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Ionization energies of cobalt-vanadium bimetallic clusters (Co_nV_m)

K. Hoshino, T. Naganuma, K. Watanabe, Y. Konishi, A. Nakajima, K. Kaya

Chemical Physics Letters （Elsevier Science BV） 239 （ 4-6 ） 369 - 372 1995年06月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 00092614

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Structures and ionization energies of sandwich clusters (V_n(benzene)_m)

K. Hoshino, T. Kurikawa, H. Takeda, A. Nakajima, K. Kaya

Journal of Physical Chemistry （American Chemical Society） 99 （ 10 ） 3053 - 3055 1995年03月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 00223654

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Photoelectron spectroscopy of Al_nS₁^- clusters (n = 1-9)

A. Nakajima, T. Taguwa, K. Nakao, K. Hoshino, S. Iwata, K. Kaya

The Journal of Chemical Physics （American Institute of Physics） 102 （ 2 ） 660 - 665 1995年01月

研究論文（学術雑誌）, 共著, 筆頭著者, 査読有り, ISSN 00219606

Photoelectron spectra of AlnS1 (1≤n≤9) cluster anions were measured at the photon energies of 3.49 and 4.66 eV, using a magnetic bottle electron spectrometer having ∼60 meV resolution. AlS - has an electron affinity of as large as 2.6 eV, attributed to large stability by six bonding electrons. A new electronic state, A′ 2Π, was found 0.4 eV above the AlS ground state, which is in agreement with theoretical calculations. AlnS1- cluster anions are produced efficiently up to n=5, but the production of larger cluster sizes is found difficult. The mass distribution of AlnS 1- cluster anions and electron binding energy can be explained by a geometric structure obtained by theoretical calculation. © 1995 American Institute of Physics.

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Spectroscopic Study of Hydrogen Bonded7-Azaindole Clusters

A. Nakajima, F. Ono, Y. Kihara, A. Ogawa, K. Matsubara, K. Ishikawa, M. Baba, K. Kaya

Laser Chemistry （Wiley-VCH Verlag GMBH） 15 （ 2-4 ） 167 - 181 1995年01月

研究論文（学術雑誌）, 共著, 筆頭著者, 査読有り, ISSN 0278-6273

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Ionization energies of aluminum-cesium bimetallic clusters (Al_nCs_m)

K. Hoshino, K. Watanabe, Y. Konishi, T. Taguwa, A. Nakajima, K. Kaya

Chemical Physics Letters （Elsevier Science BV） 231 （ 4-6 ） 499 - 503 1994年12月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 00092614

Aluminum-cesium bimetallic clusters (AlnCsm) were produced by the combination of laser vaporization and a high-temperature pulsed nozzle. To investigate their electronic structures, ionization energies (EiS) of the AlnCsm clusters were measured using a tunable ultraviolet laser combined with a time-of-flight mass spectrometer. In general, their Eis decrease by 0.4-1.0 eV with increasing number of cesium atoms, but discontinuous changes in Eis can be also observed, which are attributed to electronic shell structures; completeness of electron shell by the addition of a Cs atom induces high Ei, and an excess electron from the Cs atom results in large Ei, decrement. The electronic stability of the AlnCsmclusters can be also examined by their abundance. © 1994.

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Photoelectron Spectroscopy of AlS^- Diatomic Anion

A. Nakajima, T. Taguwa, K. Nakao, K. Hoshino, S. Iwata, K. Kaya

Chemistry Letters （Chemical Society of Japan） 1994 （ 8 ） 1525 - 1528 1994年08月

研究論文（学術雑誌）, 共著, 筆頭著者, 査読有り, ISSN 0366-7022

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Theoretical study of silicon sodium binary clusters. Geometrical and electronic structures of Si_nNa (n = 1-7)

R. Kishi, A. Nakajima, S. Iwata, K. Kaya

Chemical Physics Letters （Elsevier Science BV） 224 （ 1-2 ） 200 - 206 1994年07月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 00092614

The geometries, ionization energies and adsorption energies of siliconsodium binary clusters (SinNa, n=1-7) were studied with ab initio MO calculations. All the calculations were carried out at the restricted Hartree-Fock level of approximation for both closed and open shell systems. The structure of the most stable isomer of the SinNa clusters keeps the frame of the corresponding Si cluster unchanged. The electronic structure of SinNa is similar to that of the corresponding negative ions Si-n. The ionization energies of SinNa were evaluated with the ΔSCF method, and they become smaller than that of the corresponding Sin as was reported experimentally. © 1994.

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Photoionization electronic spectroscopy of AlNa

A. Nakajima, K. Hoshino, K. Watanabe, Y. Konishi, T. Kurikawa, S. Iwata, K. Kaya

Chemical Physics Letters （Elsevier Science BV） 222 （ 4 ） 353 - 357 1994年05月

研究論文（学術雑誌）, 共著, 筆頭著者, 査読有り, ISSN 0009-2614

Two electronic transitions are observed for a new heteronuclear dimer, AlNa, using two-color two-photon ionization spectrocopy (2R2PI). The combination of the analysis of vibronic structure and ab initio calculation enables us to assign the two transitions as D 1Π←X 1Σ+ and C 1Σ+←X 1Σ+; the origins of the D and C states are located at 16427 and 15422 cm-1, and the vibrational frequencies being 151 cm-1 in the D state, 115 cm-1 in the C state, and 186 cm-1 in the X state, respectively. © 1994.

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Photoelectron spectroscopy of iron carbonyl cluster anions (Fe_n(CO)_m^-, n = 1-4)

A. Nakajima, T. Taguwa, K. Kaya

Chemical Physics Letters （Elsevier Science BV） 221 （ 5-6 ） 436 - 440 1994年04月

研究論文（学術雑誌）, 共著, 筆頭著者, 査読有り, ISSN 00092614

Photoelectron spectra of iron-carbonyl cluster anions (Fen(CO)-m) were measured in the gas phase at the photon energy of 4.66 eV, using a magnetic bottle electron spectrometer. The iron-carbonyl cluster anions were produced by slow electron attachment of less than 1 eV to a supersonic expansion of Fe(CO)5 seeded in a He carrier gas. The photoelectron spectra of the Fen(CO)- 5n-1 anions show unresolved broad features, and the threshold energy (ET) and the vertical detachment energy (VDE) were determined: The ET of Fen(CO)-5n-1 gradually increases by 0.1-0.3 eV with the cluster size. The study suggests that the iron carbonyl cluster anions produced are van der Waals clusters, in which Fe(CO)-4 works as an ion core. © 1994.

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Electronic Properties of Silicon-M Binary Clusters（M = C & Na）

A. Nakajima, K. Nakao, M. Gomei, R. Kishi, S. Iwata, K. Kaya

Material Research Society Symposium Proceedings （Springer Nature） 358 61 - 66 1994年

研究論文（国際会議プロシーディングス）, 共著, 筆頭著者, 査読有り

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Photoelectron spectroscopy of (C₆F₆)_n^- and (Au-C₆F₆)^- clusters

A. Nakajima, T. Taguwa, K. Hoshino, T. Sugioka, T. Naganuma, F. Oho, K. Watanabe, K. Nakao, Y. Konishi, R. Kishi, K. Kaya

Chemical Physics Letters （Elsevier Science BV） 214 （ 1 ） 22 - 26 1993年10月

研究論文（学術雑誌）, 共著, 筆頭著者, 査読有り, ISSN 00092614

Photoelectron spectra of hexafluorobenzene (C6F6) cluster anions and its Au complex anion were measured in the gas phase at the photon energy of 3.49 eV, using a magnetic bottle electron spectrometer having ≈ 80 meV resolution. The spectra show unresolved broad features: The vertical detachment energies (VDEs) were determined from the maxima of the spectra. The VDE of (C6F6)-n gradually increases by 0.1-0.2 eV with the cluster size (n=1-8) from 1.56(3) to 2.54(6) eV, and the VDE of (Au-C6F6)- anion is 1.99(5) eV. © 1993.

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Ionization potentials of gold-sodium (Au_nNa_m) bimetallic clusters

K. Hoshino, T. Naganuma, K. Watanabe, A. Nakajima, K. Kaya

Chemical Physics Letters （Elsevier Science BV） 211 （ 6 ） 571 - 574 1993年08月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 00092614

Ionization potentials of gold-sodium (AunNam) bimetallic clusters were measured by a tunable ultraviolet laser combined with a time-of-flight mass spectrometer. The ionization potentials of AunNam clusters generally decrease with the number of Na atoms, but discontinuities in the ionization potentials of AunNam clusters were observed when the total number of valence electrons in the clusters fills an electronic shell. This result indicates that valence electrons of both Au atoms and Na atoms are delocalized in the clusters, forming the electronic shell. © 1993.

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Sodium doped binary clusters II: Electron shell of In_nNa_m and Au_nNa_m clusters

A. Nakajima, K. Hoshino, T. Naganuma, K. Watanabe, K. Kaya

Zeitschrift für Physik D Atoms, Molecules and Clusters （Springer Nature） 26 89 - 91 1993年03月

研究論文（学術雑誌）, 共著, 筆頭著者, 査読有り, ISSN 0178-7683

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Sodium doped binary clusters I: Ionization potentials of Si_nNa_m clusters

K. Kaya, T. Sugioka, T. Taguwa, K. Hoshino, A. Nakajima

Zeitschrift für Physik D Atoms, Molecules and Clusters （Springer Nature） 26 201 - 203 1993年03月

研究論文（学術雑誌）, 共著, 最終著者, 査読有り, ISSN 01787683

Sodium doped silicon clusters (SinNam, 3≤n≤11, 1≤m≤4) were produced by two independent laser vaporization methods and their ionization potentials were measured by scanning the wavelength of the UV dye laser. The IPs of most SinNam clusters decrease monotonously with the number of Na atoms, but IPs of Si7Nam and Si10Nam clusters show an apparent even-odd alternation; odd numbers of Na atoms efficiently decrease the IP but even numbers of Na atoms never significantly decrease the IPs. In addition, the reactivity of SinNam clusters for NO molecules was investigated with a fast flow reactor, and an anti-correlation between IP and the reactivity was clearly observed; clusters having high IP show low reactivity and vice versa. © 1993 Springer-Verlag.

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Electronic shell structure of indium-sodium (In_nNa_m) bimetallic clusters examined by their ionization potentials and mass distributions

A. Nakajima, K. Hoshino, T. Sugioka, T. Naganuma, T. Taguwa, Y. Yamada, K. Watanabe, K. Kaya

Journal of Physical Chemistry （American Chemical Society） 97 （ 1 ） 86 - 90 1993年01月

研究論文（学術雑誌）, 共著, 筆頭著者, 査読有り, ISSN 00223654

Indium-sodium (InnNam) bimetallic clusters were produced by two independent laser vaporization methods. Ionization potentials (IPs) of the InnNam clusters were measured up to m = 2 using a tunable ultraviolet laser combined with a time-of-flight (TOF) mass spectrometer. At small sizes (n = 3-15), the ionization potentials decrease by 0.1-0.8 eV with the addition of Na atom(s), whereas the IPs of larger Inn (15 ≤ n ≤ 27) clusters do not decrease with Na addition. Moreover, IPs of In7Na1 and In13Na1 clusters are higher than those of In7 and In13, and the IP increments can be explained by electronic shell closings of the 1p (8e) and 2p shell (40 e), where In atoms in the clusters are monovalent and trivalent, respectively. The electronic shell structure was also examined by a magic number in mass distributions of InnNam- cluster anions; the In12Na3- cluster can be observed as magic numbers, corresponding to the 2p shell closing. In contrast, no electronic shell structure is observed in pure Inn clusters around n = 13. These results indicate that Na atom addition can induce s/p hybridization in the Inn clusters. We also present mass distributions of aluminum-sodium cluster anions, AlnNam-, whose feature can be understood by the electronic shell model. © 1993 American Chemical Society.

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Electronic spectra of p-dicyanobenzene (p-DCNB), p-DCNB-H₂O complex, and p-DCNB dimer in a supersonic jet

K. Fujita, T. Fujiwara, K. Matsunaga, F. Ono, A. Nakajima, H. Watanabe, T. Koguchi, I. Suzuka, H. Matsuzawa, S. Iwata, K. Kaya

Journal of Physical Chemistry （American Chemical Society） 96 （ 26 ） 10693 - 10697 1992年12月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 00223654

S1-S0 electronic spectra of jet-cooled p-dicyanobenzene (p-DCNB), its H2O complex, and the dimer ((p-DCNB)2) were recorded using laser-induced fluorescence and two-color multiphoton ionization methods. Several vibrational bands in S1 of the monomer were assigned to the corresponding vibrational modes in the ground S0 state. Geometric structures of p-DCNB-H2O and (p-DCNB)2 were determined by the analyses of rotational envelopes of the vibronic bands of these complexes. © 1992 American Chemical Society.

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Ionization potentials of cobalt-sodium bimetallic clusters (Co_nNa_m)

K. Hoshino, T. Naganuma, Y. Yamada, K. Watanabe, A. Nakajima, K. Kaya

The Journal of Chemical Physics （American Institute of Physics） 97 （ 5 ） 3803 - 3807 1992年09月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 00219606

Cobalt-sodium bimetallic clusters (ConNam, n = 3-48) were produced by a two independent laser-vaporization method. Ionization potentials of the ConNam clusters were measured up to m = 3 using a tunable ultraviolet laser combined with a time-of-flight (TOF) mass spectrometer. In general, the ionization potentials decrease monotonically with the number of sodium atoms, and the ionization potentials of Co nNam+1 decrease by 0.2-0.8 eV compared to those of the corresponding ConNam cluster. However, the amount of IP decrement by the Na doping is relatively large at n ≤ 17 whereas it is small and constant at n ≥ 18. This feature can be explained by a geometric rearrangement; at small n, the Na doping induces a large geometric rearrangement of the cluster, but at large n, the geometric change is small. Reactivity of ConNam cluster toward H2 was also measured and the effect of the Na doping was studied. The reactivity also suggests the geometric change induced by the Na doping. Moreover, we examined the anticorrelation between IP and reactivity of the ConNam clusters, and no anticorrelation between them could be revealed. © 1992 American Institute of Physics.

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Magic Numbers of Bimetallic Cluster Ions Consisted of Group-Ⅲ Metals: the Icosahedron Structure of 13 Atoms Clusters

A. Nakajima, T. Sugioka, T, Kishi, K. Kaya

Physics and Chemistry of Finite Systems: From Clusters to Crystals （Kluwer Academic Publishers） 1 99 - 104 1992年09月

研究論文（学術雑誌）, 共著, 筆頭著者, 査読有り

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Electronic Structure and Reactivity of Bimetallic Clusters Containing Sodium Atoms

K. Kaya, A. Nakajima, T. Naganuma, K. Hoshino

Physics and Chemistry of Finite Systems: From Clusters to Crystals （Kluwer Academic Publishers） 1 229 - 234 1992年09月

研究論文（学術雑誌）, 共著, 査読有り

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Polarized laser induced fluorescence of BaO (X ¹Σ⁺) produced in a crossed-beam reaction of Ba+SO₂

A. Nakajima, T. Munakata, T. Nagata, T. Kondow, T. Kasuya

The Journal of Chemical Physics （American Institute of Physics） 97 （ 2 ） 991 - 996 1992年07月

研究論文（学術雑誌）, 共著, 筆頭著者, 査読有り, ISSN 00219606

Rotational alignment of the product BaO in the crossed-beam reaction Ba+SO2 → BaO+SO was studied as a function of collision energy. The collision energy was varied from 3.0 to 8.0 kcal/mol by changing the stagnation condition of the supersonic SO2 beam. The rotational alignment was probed by means of the polarized laser-induced fluorescence (PLIF) of the product. The distribution of the rotational vector was isotropic at the collision energy of 3.0 kcal/mol, but anisotropic at a higher collision energy, with the most probable alignment being perpendicular to the relative velocity vector. These results indicate that the reaction proceeds via a long-lived complex at a low collision energy, whereas at a higher collision energy, the lifetime of the complex is short enough to couple the rotational angular momentum to the total angular momentum. © 1992 American Institute of Physics.

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Mass distributions of negative cluster ions of sodium oxide

A. Nakajima,T. Sugioka, K. Hoshino, K. Kaya

Chemical Physics Letters （Elsevier Science BV） 189 （ 4-5 ） 455 - 459 1992年02月

研究論文（学術雑誌）, 共著, 筆頭著者, 査読有り, ISSN 0009-2614

Negative cluster ions of sodium oxide, NanO−m (m = 1 and 2), were generated by laser vaporization of a sodium rod, and their mass distribution was measured. In the mass spectra, a number of intense peaks compared to neighbors were observed; in NanO−1, the “magic numbers” were observed at n = 9, 21, 27, 35, 41, and 59, and in NanO−2, they were observed at n = 11, 17, 23, 29, 37, 43, and 61. Compared with the magic numbers predicted by the jellium shell model, two and four sodium atoms are increased for the magic numbers by adding one and two oxygen atom(s), respectively. Therefore, it is concluded that (1) no valence electrons of oxygen atoms contribute to form the electronic shells of clusters, and that (2) two sodium atoms in the NanO−m clusters are bound by one oxygen atom, forming an Na2O unit as a spectator. © 1992, All rights reserved.

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Electronic and geometric structures of aluminum-boron negative cluster ions (Al_nB_m^-)

A. Nakajima, T. Kishi, T. Sugioka, K. Kaya

Chemical Physics Letters （Elsevier Science BV） 187 （ 3 ） 239 - 244 1991年12月

研究論文（学術雑誌）, 共著, 筆頭著者, 査読有り, ISSN 00092614

Aluminum-boron negative cluster ions (AlnB-m, n=5-30, m=1-3) were generated by two independent laser-vaporization sources. The mass distribution of AlnB-2 exhibits one peak at n=12 and one step between n=22 and 23 as magic numbers, whereas that of AlnB-2 exhibits only one step between n=21 and 22. The loss of the magic number of Al11B-2 can be explained by the geometric structures: Al12B-1 is stabilized by both an electronic shell closing of the 2p shell and a close-packing structure of icosahedron. In contrast, Al11B-2 can satisfy the electronic shell closing by cannot take the stable geometric structure because the diameters of Al and B atoms differ largely. The structure of Al12B-1 can be an icosahedron with a B atom located at the center. © 1991.

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Ionization potentials of aluminum-sodium bimetallic clusters (Al_nNa_m)

A. Nakajima, K. Hoshino, T. Naganuma, Y. Sone, K. Kaya

The Journal of Chemical Physics （American Institute of Physics） 95 （ 10 ） 7061 - 7066 1991年11月

研究論文（学術雑誌）, 共著, 筆頭著者, 査読有り, ISSN 00219606

Aluminum-sodium bimetallic clusters (AlnNam, n = 2-26) were produced by two independent laser vaporization methods. To investigate the electronic structures, ionization potentials of the Al nNam clusters were measured up to m = 3 using a tunable ultraviolet laser combined with time-of-flight (TOF) mass spectrometer. In general, the ionization potentials monotonically decrease with the number of sodium atoms, and the ionization potentials of AlnNa1 generally decrease by 0.2-0.6 eV compared to those of the corresponding Al n cluster. In contrast, the ionization potentials of Al 13Na1 and Al23Na1 are higher than (or equal to) those of Al13 and Al23. The anomalous change in the ionization potentials of Al13Na1 and Al 23Na1 can be explained by the electronic shell model: By the addition of one sodium atom to the bare aluminum clusters, the total number of their valence electrons strictly satisfies the shell closing of 2p and 3s shells, respectively, and therefore, the clusters can be stabilized electronically. © 1991 American Institute of Physics.

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Structure and reactivity of bimetallic Co_nV_m⁺ cluster ions

A. Nakajima, T. Kishi, T. Sugioka, Y. Sone, K. Kaya

Journal of Physical Chemistry （American Chemical Society） 95 （ 18 ） 6833 - 6835 1991年09月

研究論文（学術雑誌）, 共著, 筆頭著者, 査読有り, ISSN 00223654

The reaction of bimetallic ConVm+ cluster ions, generated by laser vaporization, toward hydrogen was investigated by using a fast-flow reactor. Compared with pure Con+ cluster ions, the reactivity of Con-1V+ cluster ions exhibits a drastic decrease at n ≥ 13 similar to that of neutral ConVm clusters. The result can be explained in terms of their geometrical structure; at n ≥ 13, the V atom is located at the center of the cluster and is surrounded by Co atoms. © 1991 American Chemical Society.

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Production of bimetallic clusters containing manganese atoms by laser-vaporization method

Y. Sone, K. Hoshino, T. Naganuma, A. Nakajima, K. Kaya

Journal of Physical Chemistry （American Chemical Society） 95 （ 18 ） 6830 - 6832 1991年09月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 0022-3654

Bimetallic clusters containing manganese atoms were generated by applying a laser-vaporization method. The bimetallic clusters composed of Ta and Mn, TanMnm, were produced and could be mixed in any ratio, whereas those composed of Co and Mn, ConMnm, could only be produced with more than four cobalt atoms; Co4 plays an important role in the production of ConMnm clusters. The difference of mass distribution between TanMnm and ConMnm can be qualitatively explained by an analogy of their phase diagrams in the bulk. © 1991 American Chemical Society.

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Reactivity of positively charged cobalt cluster ions with CH₄, N₂, H₂, C₂H₄, and C₂H₂

A. Nakajima, T. Kishi, Y. Sone, S. Nonose, K. Kaya

Zeitschrift für Physik D Atoms, Molecules and Clusters （Springer Nature） 19 （ 4 ） 385 - 387 1991年03月

研究論文（学術雑誌）, 共著, 筆頭著者, 査読有り, ISSN 0178-7683

Reactivity of positively charged cobalt cluster ions (Co_n⁺, n=2-22), produce by laser vaporization, with various gas samples (CH₄, N₂, H₂, C₂H₄, and C₂H₂) were systematically investigated by using a fast-flow reactor. The reactivity of Co_n⁺ with the various gas samples is qualitatively consistent with the adsorption rate of the gas to cobalt metal surfaces. Co_n⁺ highly reacts with C₂H₂ as characterized by the adsorption rate to metal surfaces, and it indicates no size dependence. In contrast, the reactions of Co_n⁺ with the other gas samples indicate a similar cluster size dependence; at n=4, 5, and 10-15, Co_n⁺ highly reacts. The difference can be explained by the amount of the activation energy for chemisorption reaction. Compared with neutral cobalt clusters, the size dependence is almost similar except for Co₄⁺ and Co₅⁺. The reactivity enhancement of Co₄⁺ and Co₅⁺ indicates that the cobalt cluster ions are presumed to have an active site for chemisorption at n=4 and 5, induced by the influence of positive charge.

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Mass distributions of aluminum negative cluster ions and their binary cluster ions mixed with a carbon atom

A. Nakajima, T. Kishi, T. Sugioka, Y. Sone, K. Kaya

Chemical Physics Letters （Elsevier Science BV） 177 （ 3 ） 297 - 300 1991年02月

研究論文（学術雑誌）, 共著, 筆頭著者, 査読有り, ISSN 0009-2614

The mass distributions of aluminum negative cluster ions and their binary cluster ions mixed with a carbon atom, generated by laser vaporization, were measured. In the range from n=5 to 70, the mass distribution of pure aluminum negative cluster ions, Al-n, exhibits one peak at n = 13 and one step between n=23 and 24. The mass distribution of the series of AlnC-, however, exhibits one peak at n=7 and one step between n=24 and 25. The absence of a peak in the series of AlnC- around n=13 can be explained by the electronic structure in relation to the electronic shell model, and both the peak and the step in the mass distribution of AlnC- clusters can be explained by their geometric structure. © 1991.

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Photodissociation dynamics of NH₃, NH₂D, NHD₂, and ND₃: Rovibronic absorption analysis of the Ã-X̃ transition

A. Nakajima, K. Fuke, K. Tsukamoto, Y. Yoshida, K. Kaya

Journal of Physical Chemistry （American Chemical Society） 95 （ 2 ） 571 - 574 1991年01月

研究論文（学術雑誌）, 共著, 筆頭著者, 査読有り, ISSN 00223654

The photodissociation dynamics of Ã state ammonia molecules has been further investigated for deuterium-substituted derivatives NH2D, NHD2, and ND3 measuring the homogeneous line widths on action spectra. The line widths of the 201 vibronic band were determined to be 19, 12, and 1.3 cm-1 fwhm for NH2D, NHD2, and ND3, respectively. In consideration of our previously reported value of 26 cm-1 for NH3, the spectral line width decreases in proportion to the number of D atoms. This result shows that the dissociation rate in the Ã state is additive as for the number of H atoms, and it is explained by the fact that the zero-point energies on the potential surface of Ã state are equally shifted among deuterium-substituted ammonias. © 1991 American Chemical Society.

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Electronic spectrum of jet-cooled cyanocyclopentadienyl radical

T. Ishida, H. Abe, A. Nakajima, K. Kaya

Chemical Physics Letters （Elsevier Science BV） 170 （ 5-6 ） 425 - 429 1990年07月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 0009-2614

The D1-D0 electronic spectrum of jet-cooled cyanocyclopentadienyl (C5H4CN) radical, produced by excimer laser photolysis of 1,3-cyclopentadiene-1-carbonitrile (C5H5CN), was measured by laser-induced fluorescence (LIF). The LIF spectrum was identical to that of radicals produced by photolysis of phenyl isocyanate and o-chloroaniline, which had been assigned to phenyl nitrene. This result shows that the nascent radical product in the photolysis of o-chloroaniline and phenyl isocyanate is isomerized to C5H5CN and the final product of C5H4CN radical is derived via the second photolysis. The rotational contour of the 0-0 band of the C54CN radical, at 27143 cm-1 (368.3 nm), is also presented. The electronic transition is A-type and π electron transition, and the symmetry of both the ground and excited electronic state is 2B1 (of the C2v point group). © 1990.

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Rotational distributions of CO₂⁺ (X̃ ²Π_g) produced by electron-impact ionization of supercooled CO₂

A. Nakajima, T. Nagata, T. Kondow, K. Kuchitsu

Chemical Physics Letters （ELSEVIER SCIENCE BV） 151 （ 6 ） 511 - 515 1988年10月

研究論文（学術雑誌）, 共著, 筆頭著者, 査読有り, ISSN 00092614

Laser-induced fluorescence of CO2+ (X̃ 2Πg) produced by electron impact on CO2 in a supersonic beam was measured. An analysis of the CO2+ (Ã-X̃) spectra has revealed that the rotational distribution of CO2+ is essentially Boltzmann and that the effective rotational temperature increases uniformly with decreasing impact energy, 26± 1 K at 300 eV and 31 ± 1 K at 45 eV. This trend of rotational excitation, which is similar to that observed in the N2 + e system, is interpreted in terms of the rotational transitions in electron-impact ionization. © 1988.

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Nascent rotational distributions of N₂⁺(X ²Σ_g⁺) produced by electron-impact ionization of N₂ in a supersonic beam

T. Nagata, A. Nakajima, T. Kondow, K. Kuchitsu

The Journal of Chemical Physics （American Institute of Physics） 87 （ 11 ） 6507 - 6512 1987年12月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 00219606

Laser-induced fluorescence from nascent N2+(X 2Σg+) ions produced by electron impact on a N2 supersonic beam was observed. An analysis of the B 2Σu+-X 2Σ g+ (0,0) band shows that the rotational state distributions cannot be represented by a single Boltzmann function, higher N″ levels being overpopulated. Experimental and analytical efforts were made to minimize the influence of cascading and relaxation on the rotational distributions. The rotational energy of N2+ (X) thus estimated increases with decreasing electron energy from 2.26±0.16 meV at 300 eV to 4.24±0.27 meV at 25 eV. This trend is explained qualitatively in terms of angular momentum transfer through multipole electron-molecule interactions. © 1987 American Institute of Physics.

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Polarized laser induced fluorescence of N₂⁺（X²Σ_g⁺) produced in the electron impact ionization of N₂

T. Nagata, A. Nakajima, T. Kondow, K. Kuchitsu

Chemistry Letters （Chemical Society of Japan） 16 （ 10 ） 1911 - 1914 1987年03月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 0366-7022

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