研究者詳細 - 中嶋　敦

論文 - 中嶋　敦

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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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