Published papers

Initial products prepared via the surface immobilization of Ta-atom-encapsulated Si-16 cage (Ta@Si-16) nanoclusters on solid surfaces terminated with monolayer films of C-60 molecules were investigated using scanning tunneling microscopy (STM). The STM results indicated that marked aggregation and desorption of surface-immobilized Ta@Si-16 nanoclusters were not induced, even after thermal annealing at similar to 500 K, whereas the local vertical and lateral positions of the Ta@Si-16 nanoclusters with respect to neighboring adsorption sites in the C-60 film were modified. This local positional transition occurred on C-60 monolayer films weakly bonded (via van der Waals forces) to substrates such as highly oriented pyrolytic graphite (HOPG) but did not occur on C-60 monolayer films covalently bonded to substrates such as Si(111)7 x 7. These results indicated that the heterodimer consisting of a Ta@Si-16 nanocluster and a C-60 molecule, Ta@Si-16-C-60, was formed as an initial product via covalent bonding, which inhibited wide-range surface migration of the Ta@Si-16 nano clusters 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@Si-16 nanoclusters increased,, indicating that the barrier height of the precessional motion of the Ta@Si-16-C-60 heterodimer was decreased due to accumulation of the elastic strain energy generated in the C-60 films.

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.

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 Mn(n)Bz(n)(-). In stark contrast to the most widely studied vanadium-Bz sandwich clusters, it is found that Mn(n)Bz(n)(-) (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 Mn(18)Bz(18)(-) and its neutral state are computationally anticipated as an intriguing "cluster of Mn(1)Bz(1) clusters" in which the neutral Mn(18)Bz(18) has extremely high C-18h symmetry with an uncommon spin state of 2S + 1 = 55. The extensively delocalized electron environment of Mn(18)Bz(18) allows the simple Huckel model to reveal the strong intra-atomic exchange interactions within the Mn 3d electrons.

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.

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.

Time-resolved two-photon photoemission (TR-2PPE) spectroscopy is employed to probe the electronic states of a C-60 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 C-60 film: after hot excitons are nascently formed in C-60 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 C-60 film into the HOPG is evaluated to be 1.31 x 10(12) s(-1), suggesting a weak van der Waals interaction at the interface, where the photocarriers tentatively occupy the lowest unoccupied molecular orbital (LUMO) of C-60. 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 C-60 layers is 3.3 +/- 1.2 x 10(13)s(-1) .

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 S-0-T-1 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. Published by AIP Publishing.

A novel η9-coordinated double-decker sandwich complex of divalent europium is synthesized. The complex exhibits blue-green photoluminescence at 516 nm, which is significantly blue-shifted from those of other organoeuropium(II) sandwich complexes (∼630 nm). The blue-shift was quantitatively explained by the weakened electrostatic field of the expanded 10-π ring.

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.

The geometric and electronic properties of silicon-atom-doped aluminum clusters, AlnSim (n = 7-30, m = 0-2), were investigated experimentally. The size dependences of the ionization energy and electron affinity of AlnSim show that the stability of AlnSim is governed by the total number of valence electrons in the clusters, where Al and Si atoms behave as trivalent and tetravalent atoms, respectively. Together with theoretical calculations, it has been revealed that neutral Al10Si and Al12Si have a cage-like geometry with central Si atom encapsulation and closed electronic structures of superatomic orbitals (SAOs), and also that they both exhibit geometric robustness against reductive and oxidative changes as cage-like binary superatoms of Si@Al-10 and Si@Al-12. As well as the single-atom-doped binary superatoms, the effect of symmetry lowering was examined by doping a second Si atom toward the electron SAO closing of 2P SAO, forming Al11Si2. The corresponding anion and cation clusters keep their geometry of the neutral intact, and the ionization energy is low compared to others, showing that Al11Si2 is characterized to be, Si@Al11Si as an alkaline-like binary superatom. For Al21Si2, a face-sharing bi-icosahedral structure was identified to be the most stable as dimeric superatom clusters.

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.

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 S-0-T-1 and S-0-S(1)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 S-0, T-1, and S-1. 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.

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.

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.

© 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.

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.

© 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.

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.