研究者詳細 - 中嶋　敦

論文 - 中嶋　敦

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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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