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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中嶋 敦

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中嶋　敦

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