掲載種別：研究論文（学術雑誌）   共著区分：共著   国際・国内誌：国際誌  

Metal-encapsulating Si-16 cage clusters (M@Si-16) 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@Si16 SAs (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@Si-16 SAs are size-selectively soft-landed on organic substrates deposited with n-type fullerene (C-60) and p-type hexa-tert-butyl-hexa-peri-hexabenzocoronene (HB-HBC, C66H66), and the electronic states of M@Si-16 on the organic substrates are characterized by X-ray and ultraviolet photoelectron spectroscopy. On the C-60 substrate, all M@Si-16 are fixed to be cationic, forming M@Si-16(+) C-60(-) via a charge transfer interaction, while on an HB-HBC substrate, M@Si16-HB-HBC+ (M = Lu and Hf) is formed with anionic M@Si-16(-). Together with density functional theory calculations, the charge preference of the M@Si-16 SA is examined based on its chemical stability against O-2 gas exposure; Lu@Si-16 on HB-HBC is more robust toward O-2 than that on C-60, while Ta@Si-16 on HB-HBC is less robust than that on C-60. 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.

DOI： 10.1021/acs.jpcc.2c02196

掲載種別：研究論文（学術雑誌）   共著区分：共著   国際・国内誌：国際誌  

Cation-deficient copper chalcogenide nanocrystals (NCs) as a typical degenerated semiconductor have attracted great attention owing to their unique properties. However, the association between band structures and localized surface plasmon resonance (LSPR) in such NCs has not been thoroughly studied. Moreover, the synthesis of the colloidal Cu2-xSeyS1-y NCs with diverse crystal phases remains a challenge to date. Hence, we developed a facile method to synthesize a range of Cu2-xSeyS1-y-alloyed NCs with disparate crystal phases. We elucidated the tunable band structures and LSPR shift, and the results indicated that the modulation of the valance band maximum (VBM) position by Se/S alloying and the overlapping of the valence band and the Fermi level (E-F) dominate LSPR properties in alloyed NCs. Not only the variation of Cu vacancy along with the induced free carrier concentration but also the negative shift of VBM contribute to the LSPR shift toward higher energy.

DOI： 10.1021/acs.jpcc.2c01149

掲載種別：研究論文（学術雑誌）   共著区分：共著   国際・国内誌：国際誌  

DOI： 10.1021/acs.jpcc.1c10895

掲載種別：研究論文（学術雑誌）   共著区分：共著   国際・国内誌：国際誌  

Abstract

Aluminum nanoclusters (Al_n 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, Al_n 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 Al_n are characterized by X-ray photoelectron spectroscopy and chemical oxidative measurements. On the C₆₀ substrate, Al_n is fixed to be cationic but highly oxidative; however, on the HB-HBC substrate, they are stably fixed as anionic Al_n⁻ 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.

DOI： 10.1038/s41467-022-29034-9

その他URL： https://www.nature.com/articles/s41467-022-29034-9

掲載種別：研究論文（学術雑誌）   共著区分：共著   国際・国内誌：国際誌  

DOI： 10.1021/acs.jpcc.1c03591

掲載種別：研究論文（学術雑誌）  

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 Ag-n (n = 3-55)/Au-n (n = 21) and the organic C-60 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 Ag-n 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 twophoton excitation of an occupied state of the Ag-n 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 Ag-n NCs at the molecular heterointerface.

DOI： 10.1021/acsnano.0c08248

掲載種別：研究論文（学術雑誌）  

Nanocluster immobilization on an oxide surface is fundamentally important for forming nanocluster-assembled layered materials. We fabricated multilayered superatom films made of tantalum-encapsulating Si-16 cage nanocluster (Ta@Si-16) on a strontium titanate substrate (SrTiO3; STO) by the soft landing of size-selective Ta@Si-16. X-ray photoelectron spectroscopy (XPS) revealed that Ta@Si-16 survived at the interfacial oxide layer without oxidizing the central Ta atom, and pure Ta@Si-16 layers were formed successively on the modified Ta@Si-16/STO interfacial layer. The Ta@Si-16 superatoms in the multilayered Ta@Si-16 showed high chemical robustness against O-2 exposure, although the topmost Ta@Si-16 surface layer, including the central Ta atom, completely oxidized in ambient O-2 over several days. XPS depth analysis showed that Ta@Si-16 oxide formation was limited only at the topmost single layer, revealing that the middle Ta@Si-16 layers sandwiched between the top and bottom were protected by the formation of interfacial oxides.

DOI： 10.1021/acs.jpcc.0c08813

掲載種別：研究論文（学術雑誌）  

DOI： 10.1103/PhysRevB.102.121401

掲載種別：研究論文（学術雑誌）  

We measured occupied and unoccupied energy levels of half-fluorinated and perfluorinated rubrenes (C42F14H14, F-14-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 (E-F) decrease when the fluorination degree increases, where the energies o f the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are located at E-F - 1.83 eV and E-F + 0.9 eV for F-14-RUB and E-F - 2.51 eV and E-F + 0.6 eV for PF-RUB, respectively. The HOMO-LUMO gaps corresponding to the energy for the creation o f free photocarriers in molecular films- known as "transport gaps"- are evaluated by 2.7 eV for F-14-RUB and 3.1 eV for PF-RUB.

DOI： 10.1021/acs.jpcc.0c01162

掲載種別：研究論文（学術雑誌）  

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 (C-60)/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 C-60/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 C-60 films, due to less valence electrons participating in 2P-photoemission processes. With the use of silver (Ag-n (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 C-60/Au(111) interfaces, where SPP properties are hardly modulated by the added small amount (similar to 0.1 monolayer) of Ag-n sensitizers.

DOI： 10.1021/acsnano.9b08653

掲載種別：研究論文（学術雑誌）  

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 C-60 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 pi* orbitals with localized wave functions at the carbon framework of C-60. 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.

DOI： 10.1021/acs.jpcc.9b10006

掲載種別：研究論文（学術雑誌）  

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 similar to 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.

DOI： 10.1021/acs.jpcc.8b08907

掲載種別：研究論文（学術雑誌）  

CONSPECTUS: 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@Si-16, 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 Si-16 cage; the replacement of M yields a series of M@Si(16)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@Si-16 have not yet been isolated. In this Account, we have focused on recent advances in synthesis and characterizations of M@Si-16 BCSs (M = Ti and Ta). A series of M@Si-16 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 C-60 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.

DOI： 10.1021/acs.accounts.8b00085

掲載種別：研究論文（学術雑誌）  

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.

DOI： 10.1039/C8CY01645C

掲載種別：研究論文（学術雑誌）  

<p>Stepwise oxidative reaction of a Ta-encapsulating Si₁₆ caged nanocluster superatom upon exposure to nitric oxide is investigated by monitoring N 1s core level signals.</p>

DOI： 10.1039/c8cp05580g

掲載種別：研究論文（学術雑誌）  

We have employed two-photon photoemission (2PPE) spectroscopy to observe photoinduced charge transfer at C-60 fullerene thin films formed on graphite and Au(111) substrates. From the analysis of the 2PPE intensities of molecular-derived unoccupied states, it is revealed that the photoexcited electron transfer from graphite substrate to C-60 is suppressed compared to C-60 on Au(111), owing to inert properties of graphite surfaces.

DOI： 10.1246/cl.170641

掲載種別：研究論文（学術雑誌）  

In this study, we have employed dual-color photoelectron emission microscopy (2P-PEEM) to visualize surface plasmon polaritons (SPPs) propagating along a chemically modified organic/metal interface of alkanethiolate self-assembled monolayers (Cn-SAMs; n is the number of alkyl carbon atoms) formed on Au(111). In dual-color 2P-PEEM, near-infrared photons around 900 nm generate SPPs at the Cn-SAMs/Au(111) interface, which interfere with the remaining light field. The resulting surface polarization beats are imaged as local distributions of 2P-photoelectrons probed by ultraviolet photons. Through dual-color 2P-PEEM for various alkyl chain lengths of Cn-SAMs, it is revealed that SPP properties are largely modified by an interfacial electronic state, particularly formed by the chemical interaction between surface Au atoms and adsorbate thiol molecules, thereby allowing the quantification of their group velocity at similar to 0.86 times the speed of light. Since the SPP properties are controllable in terms of their height as organic dielectric layers, a bottom-up tailored technique using SAMs exhibits designer capability in adjusting the dielectric properties toward applications in surface plasmonic devices.

DOI： 10.1039/c7cp01693j

掲載種別：研究論文（学術雑誌）  

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.

DOI： 10.1021/acsnano.7b01506

掲載種別：研究論文（学術雑誌）  

We have employed two-photon photoemission (2PPE) spectroscopy to observe photoinduced charge transfer at C-60 fullerene thin films formed on graphite and Au(111) substrates. From the analysis of the 2PPE intensities of molecular-derived unoccupied states, it is revealed that the photoexcited electron transfer from graphite substrate to C-60 is suppressed compared to C-60 on Au(111), owing to inert properties of graphite surfaces.

DOI： 10.1246/cl.170641

掲載種別：研究論文（学術雑誌）  

The physical and chemical properties of carboxyand 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 hyperthermaldeposition 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.

DOI： 10.1021/acs.jpcc.7b00292

掲載種別：研究論文（学術雑誌）  

We have employed two-photon photoemission spectroscopy to nondestructively resolve the unoccupied energy levels of fullerene C-60 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 alkanethfolate-SAM (H-SAM) to 5.7 eV for the fluorine-substituted SAM (F-SAM), owing to the formation of surface dipole layers. When C-60 is deposited on the H-SAM and F-SAM, the energy positions of the unoccupied/occupied levels of C-60 are pinned to the vacuum level (Fermi level (E-F) + WF). As a result of the energy level alignment, on the F-SAM, the relative energy from E-F of the highest occupied molecular orbital of C-60 almost equals that of the lowest unoccupied molecular orbital, implying that the C-60 film on the F-SAM exhibits both p- and n-type (ambipolar) charge transport properties, while C-60 is known as a typical n-type semiconductor. The energetics are preserved even with multilayered C-60 films at least up to similar 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.

DOI： 10.1021/acs.jpcc.7b07955

掲載種別：研究論文（学術雑誌）  

Nanoclusters (NCs) of several to hundreds of atoms in size are prospective functional units for future nanomaterials originating in their unique, size-specific properties. To explore the field of NC-based materials science, the development of large-scale, size-exclusive synthesis methods is in high demand, as one can see from the successful evolution of fullerene science. We have developed a large-scale synthesis method for main group-based NC compounds by scaling up the clean dry-process with a high-power impulse magnetron sputtering. The 100 mg scale synthesis of binary NCs of M@Si-16 (M = Ti and Ta) stabilized by poly(ethylene glycol) dimethyl ether enables us to characterize their structures by an array of methods, for example, mass spectroscopy, X-ray photoemission spectroscopy, Raman spectroscopy, and Si-29 nuclear magnetic resonance. Spectroscopic evidence indicates that the M@Si-16 NCs are the metal-encapsulating tetrahedral silicon-cage structure satisfying the 68 electrons, closed-electronic-shell superatom.

DOI： 10.1021/acs.jpcc.7b06449

掲載種別：研究論文（学術雑誌）  

DOI： 10.1063/1.4967380

掲載種別：研究論文（学術雑誌）  

DOI： 10.1038/srep35853

掲載種別：研究論文（学術雑誌）  

The tantalum-encapsulating Si-16 cage nanocluster superatom (Ta@Si-16) has been a promising candidate for a building block of nanocluster-based functional materials. Its chemical states of Ta@Si-16 deposited on an electron acceptable C-60 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@Si-16 combines with a single C-60 molecule to form the superatomic charge transfer (CT) complex, (Ta@Si-16)+C-60(-). 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@Si-16 retained its original framework, forming oxides of Ta@Si-16 superatom.

DOI： 10.1021/acs.jpcc.6b04955

掲載種別：研究論文（学術雑誌）  

DOI： 10.1021/jacs.5b08035

掲載種別：研究論文（学術雑誌）  

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 approximate to 10 angstrom. 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.

DOI： 10.1021/acs.jpcc.5b06549

掲載種別：研究論文（学術雑誌）  

DOI： 10.1021/ja4088456

掲載種別：研究論文（学術雑誌）  

DOI： 10.1021/jp4097875

掲載種別：研究論文（学術雑誌）  

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 4.3-4.4eV 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. (C) 2014 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.elspec.2014.06.011

掲載種別：研究論文（学術雑誌）  

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

DOI： 10.1021/ja4088456

掲載種別：研究論文（学術雑誌）  

The electronic states of three different sizes of compositionally precise thiolate-protected gold nanoclusters, Au-25(SR)(18), Au-38(SR)(24), and Au-144(SR)(60) (R = C12H25), have been evaluated by X-ray photoemission spectroscopy. The Au 4f core-levels of the nanoclusters are well reproduced by two spectral components derived from centered core-Au and positively charged shell-Au atoms, the numbers of which are determined based on the atomic structures of the nanoclusters. The spin-orbit splitting of Au 5d(5/2) and 5d(3/2) in the valence band becomes narrower than that for bulk Au, depending on the cluster size, which is quantitatively characterized by a reduction in the average coordination number of Au. The Au 5d valence-band spectra also show that the charge reorganization of 5d electrons induced by interaction with thiol molecules is more significant for the 5d(5/2) than the 5d(3/2) orbital.

DOI： 10.1021/jp400785f

掲載種別：研究論文（学術雑誌）  

The electronic states of three different sizes of compositionally precise thiolate-protected gold nanoclusters, Au-25(SR)(18), Au-38(SR)(24), and Au-144(SR)(60) (R = C12H25), have been evaluated by X-ray photoemission spectroscopy. The Au 4f core-levels of the nanoclusters are well reproduced by two spectral components derived from centered core-Au and positively charged shell-Au atoms, the numbers of which are determined based on the atomic structures of the nanoclusters. The spin-orbit splitting of Au 5d(5/2) and 5d(3/2) in the valence band becomes narrower than that for bulk Au, depending on the cluster size, which is quantitatively characterized by a reduction in the average coordination number of Au. The Au 5d valence-band spectra also show that the charge reorganization of 5d electrons induced by interaction with thiol molecules is more significant for the 5d(5/2) than the 5d(3/2) orbital.

DOI： 10.1021/jp400785f

掲載種別：研究論文（学術雑誌）  

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

DOI： 10.1007/s11468-013-9554-6

掲載種別：研究論文（学術雑誌）  

60ZnCl(2)-20KCl-20BaCl(2)-xTbCl(3) glasses (x = 0.10, 0.25, 0.50, 0.75, 1.00, and 1.25) were prepared by melt-quenching method, and Tb3+ fluorescence properties were investigated under 355 nm excitation. Regardless of x values, the electrons that were relaxed from the D-5(3) to D-5(4) level of Tb3+ ions by the multiphonon relaxation, were repressed to 28% of all the excited electrons because the ZnCl2-based glass had much lower phonon energy than oxide glasses. For 0 < x <= 0.34, the cross relaxation, (D-5(3) -> D-5(4)) -> ((7)Fo <- F-7(6)), was repressed, and consequently 72% and 28% of all the excited electrons were radiatively relaxed by the D-5(3) -> F-7(J) (J = 6, 5, 4, 3, and 2) and D-5(4) -> F-7(J) (J = 6, 5, 4, and 3) transitions, respectively. The lifetimes of the D-5(3) and D-5(4) initial levels were obtained to be 1.1 and 2.1 ms, respectively. (C) 2013 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.materresbull.2013.07.030

掲載種別：研究論文（学術雑誌）  

Ultraviolet photoemission spectroscopy (UPS) and two-photon photoemission (2PPE) spectroscopy have been employed to investigate the electronic states and the excited electron dynamics of ferrocene (Fc)terminated self-assembled monolayers (SAMs) on Au(111). For the 11-ferrocenyl-1-undecanethiol (FcC11-) SAM, an image potential state (IPS) located at E-F +3.8 eV was observed by time-resolved 2PPE. Its lifetime, 90 fs, was found to be much shorter than that of decanethiol (C10-) SAM, 11 ps. By forming the C10-SAM beforehand, the IPS lifetime for FcC11-&C10-SAM was lengthened to similar to 9 ps; this can enable improvement in molecular ordering and substantial formation of C10 domains. (C) 2013 Elsevier B. V. All rights reserved.

DOI： 10.1016/j.cplett.2013.01.053

掲載種別：研究論文（学術雑誌）  

We present a nondestructive characterization method for buried hetero-interfaces for organic/organic and metal/organic systems using hard x-ray photoelectron spectroscopy (HAXPES) which can probe electronic states at depths deeper than similar to 10 nm. A significant interface-derived signal showing a strong chemical interaction is observed for Au deposited onto a C-60 film, while there is no such additional feature for copper phthalocyanine deposited onto a C-60 film reflecting the weak interaction between the molecules in the latter case. A depth analysis with HAXPES reveals that a Au-C-60 intermixed layer with a thickness of 5.1 nm is formed at the interface. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4768940]

DOI： 10.1063/1.4768940

掲載種別：研究論文（学術雑誌）  

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

DOI： 10.1021/nn302405r

掲載種別：研究論文（学術雑誌）  

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

DOI： 10.1021/jp302545r

掲載種別：研究論文（学術雑誌）  

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

DOI： 10.1021/jz3002579

掲載種別：研究論文（学術雑誌）  

We present a nondestructive characterization method for buried hetero-interfaces for organic/organic and metal/organic systems using hard x-ray photoelectron spectroscopy (HAXPES) which can probe electronic states at depths deeper than similar to 10 nm. A significant interface-derived signal showing a strong chemical interaction is observed for Au deposited onto a C-60 film, while there is no such additional feature for copper phthalocyanine deposited onto a C-60 film reflecting the weak interaction between the molecules in the latter case. A depth analysis with HAXPES reveals that a Au-C-60 intermixed layer with a thickness of 5.1 nm is formed at the interface. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4768940]

DOI： 10.1063/1.4768940

掲載種別：研究論文（学術雑誌）  

DOI： 10.1103/PhysRevB.85.205431

掲載種別：研究論文（学術雑誌）  

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

DOI： 10.1021/nn302405r

掲載種別：研究論文（学術雑誌）  

DOI： 10.1021/jp205922q

掲載種別：研究論文（学術雑誌）  

Adsorbed structures of naphthalene on Cu(111) have been studied using low temperature scanning tunneling microscope (LT-STM) and low energy electron diffraction (LEED). Starting from single molecules, three kinds of long-range ordered superstructures, (5 root 3 x 5 root 3)R30 degrees, (2 root 3 x 3)rect-1C(10)H(8), and (-4 1 1 -4) are observed depending on the molecular concentrations and the substrate temperatures during molecular adsorption. One of the self-assembled ordered phases with a (5 root 3 x 5 root 3) R30 degrees periodicity is chiral in adsorption-induced arrangement though a single naphthalene molecule itself has no inherent chirality In STM images, isolated single molecules appear as depressions whereas the molecules are seen as protrusions in self-assembled layers. Coverage dependent two-photon photoemission (2PPE) spectra show that the adsorption-induced occupied states is formed at around Cu 3d bands, and this results in the enhanced tunneling of occupied state images in assembled layers

DOI： 10.1021/jp1045194

掲載種別：研究論文（学術雑誌）  

DOI： 10.1103/PhysRevB.81.115426

掲載種別：研究論文（学術雑誌）  

DOI： 10.1103/PhysRevB.80.113310

掲載種別：記事・総説・解説・論説等（学術雑誌）  

J-GLOBAL

掲載種別：記事・総説・解説・論説等（学術雑誌）  

J-GLOBAL

掲載種別：記事・総説・解説・論説等（学術雑誌）  

J-GLOBAL

掲載種別：記事・総説・解説・論説等（学術雑誌）  

J-GLOBAL

掲載種別：記事・総説・解説・論説等（学術雑誌）  

J-GLOBAL

掲載種別：記事・総説・解説・論説等（学術雑誌）  

J-GLOBAL

掲載種別：記事・総説・解説・論説等（学術雑誌）  

J-GLOBAL

掲載種別：記事・総説・解説・論説等（学術雑誌）  

J-GLOBAL

掲載種別：記事・総説・解説・論説等（学術雑誌）   共著区分：単著  

<p>Understanding of the electronic structure and photoexcited state dynamics at well-prepared layered functional materials on substrates is essentially important in order to precisely design and control the future electronic or optical nanodevices. I have been so far engaged in this research field experimentally from the view point of molecular science, where the electronic states and dynamics at nanoscale functional films fabricated with organic molecules and/or nanocluster superatoms are investigated by probing photoelectrons, combining with a femtosecond light source and with a nanocluster deposition system. In this account, firstly, I show the electronic structures and photoexcited state dynamics at two-dimensional (2D) molecular monolayer systems of alkanethiolate self-assembled monolayers by two-photon photoemission spectroscopy which clarifies novel ultrafast phenomena characteristic to the 2D assembly of the functional molecules. Secondly, I present the visualization of local photoexcited states in the organic films by changing the probe system into two-photon photoelectron emission microscopy. Finally, the electronic states and chemical properties of nanocluster superatoms as a new class of functional nanomaterials are explained, which are non-destructively landed onto the substrates.</p>

DOI： 10.3175/molsci.13.A0105

CiNii Article

掲載種別：記事・総説・解説・論説等（学術雑誌）  

<p>Understanding of the electronic structure and photoexcited state dynamics at well-prepared layered functional materials on substrates is essentially important in order to precisely design and control the future electronic or optical nanodevices. I have been so far engaged in this research field experimentally from the view point of molecular science, where the electronic states and dynamics at nanoscale functional films fabricated with organic molecules and/or nanocluster superatoms are investigated by probing photoelectrons, combining with a femtosecond light source and with a nanocluster deposition system. In this account, firstly, I show the electronic structures and photoexcited state dynamics at two-dimensional (2D) molecular monolayer systems of alkanethiolate self-assembled monolayers by two-photon photoemission spectroscopy which clarifies novel ultrafast phenomena characteristic to the 2D assembly of the functional molecules. Secondly, I present the visualization of local photoexcited states in the organic films by changing the probe system into two-photon photoelectron emission microscopy. Finally, the electronic states and chemical properties of nanocluster superatoms as a new class of functional nanomaterials are explained, which are non-destructively landed onto the substrates.</p>

DOI： 10.3175/molsci.13.A0105

CiNii Article

J-GLOBAL

掲載種別：記事・総説・解説・論説等（学術雑誌）  

J-GLOBAL

掲載種別：記事・総説・解説・論説等（学術雑誌）  

The electronic structures of alkanethiolate self-assembled monolayers (SAMs) associated with their molecular-scale geometry have been evaluated by combining two types of scanning tunneling spectroscopy (STS): current-voltage (I/V) and distance-voltage (z/V) spectroscopy. In the STS spectra, in addition to electronic states originating from alkyl-chain and SAM/Au interface, an image potential state (IPS) formed in a vacuum gap is clearly observed. The result is well consistent with our two-photon photoemission spectroscopy (2PPES) measurements. Furthermore, by using time- and angle-resolved 2PPES, we found that the electron excited into IPS is highly separated from the Au substrate and has an extremely long lifetime of the order of 100 ps owing to the excellent insulating property of SAM. The lifetime can be prolonged by increasing the length of alkyl chain and by lowering the substrate temperature, which indicates that the degree of electron separation can be precisely controlled by designing the molecular layer.

DOI： 10.1380/jsssj.35.432

CiNii Article

掲載種別：記事・総説・解説・論説等（学術雑誌）  

CiNii Article

掲載種別：記事・総説・解説・論説等（学術雑誌）  

DOI： 10.1117/12.679894

会議種別：ポスター発表  

会議種別：公開講演，セミナー，チュートリアル，講習，講義等  

会議種別：シンポジウム・ワークショップ パネル（指名）  

会議種別：口頭発表（招待・特別）  

会議種別：ポスター発表  

会議種別：ポスター発表  

会議種別：ポスター発表  

会議種別：ポスター発表  

会議種別：ポスター発表  

会議種別：口頭発表（招待・特別）  

開催地：オンライン  

会議種別：ポスター発表  

開催地：大阪大学  

会議種別：ポスター発表  

開催地：大阪大学  

会議種別：ポスター発表  

開催地：大阪大学  

会議種別：口頭発表（一般）  

開催地：分子科学研究所（愛知県）  

会議種別：口頭発表（一般）  

開催地：分子科学研究所（愛知県）  

会議種別：ポスター発表  

開催地：分子科学研究所（愛知県）  

会議種別：ポスター発表  

開催地：分子科学研究所（愛知県）  

会議種別：口頭発表（招待・特別）  

開催地：Tirana, Albania  

開催地：京都大学  

会議種別：ポスター発表  

開催地：慶應義塾大学（神奈川県）  

会議種別：ポスター発表  

開催地：慶應義塾大学（神奈川県）  

会議種別：口頭発表（一般）  

開催地：慶應義塾大学（神奈川県）