Published papers

Elemental substitution and doping validate the optimization of chemical and physical properties of functional materials, and the composition ratio of the substituting atoms generally determines their properties by changing their geometric and electronic structures. For atomically precise nanoclusters (NCs) consisting of countable atom aggregates, the composition can be controlled accurately to provide an ideal model to study the heteroatom substitution effects. Since aluminum (Al) and boron (B) both belong to group 13 in the periodic table, the effect of B atom substitution on Aln NCs can be investigated while maintaining the total number of valence electrons in AlnBm NCs. In this study, oxidative reactivities of small Al NCs with B atom substitution are studied for AlnBm NCs (m = 1, n = 6-14 and m = 2, n = 11) supported on organic surfaces by using X-ray photoelectron spectroscopy and oxygen molecule (O2) exposure measurements. Before completing the endohedral B@Al12- superatomic NC, one B atom substitution in Al NCs (AlnB) enhances oxidative reactivities 3-20 times compared to those of Aln+1, particularly for n ≤ 11. When one Al atom of Al12B is further substituted by a B atom to form Al11B2, the reactivity drastically increases (6.6 × 102 times), showing that the B atom substitution makes the NC chemically active or inactive geometrically depending on the exohedral or endohedral site for the B atom in the Al NC. In addition, density functional theory calculations show that the electronegative B atom contributes to forming a locally positive Al site to facilitate O2 adsorption except in Al12B, in which the B atom is geometrically shielded by the surface of the Al12 cage in B@Al12

Transition metal atom (M)-encapsulating silicon cage nanoclusters (M@Si16) exhibit a superatomic nature, depending on the central M atom owing to the number of valence electrons and charge state on organic substrates. Since M@Si16 superatom featuring group 4 and 5 transition metal atoms exhibit rare-gas-like and alkali-like characteristics, respectively, group 6 transition metal atoms are expected to show alkaline earth-like behavior. In this study, M@Si16, comprising a central atom from group 6 (MVI = Cr, Mo, and W) were deposited on C60 substrates, and their electronic and chemical stabilities were investigated in terms of their charge state and chemical reactivity against oxygen exposures. In comparison to alkali-like Ta@Si16, the extent of charge transfer to the C60 substrate is approximately doubled, while the oxidative reactivity is subdued for MVI@Si16 on C60, especially for W@Si16. The results show that a divalent state of MVI@Si162+ appears on the C60 substrate, which is consistently calculated to be a symmetrical cage structure of W@Si162+ in C3v, revealing insights into the “periodic law” of M@Si16 superatoms pertaining to the characteristics of alkaline earth metals.

Understanding molecular film growth on substrates and the ultrafast electron dynamics at their interface is crucial for advancing next-generation organic electronics. We have focused on studying the ultrafast photoexcited electron dynamics in nanoscale organic crystals of an aromatic molecule, pentacene, on a two-dimensional material of graphite substrate. Through the use of time-resolved two-photon photoelectron emission microscopy (2P-PEEM), we have visualized the ultrafast lateral evolution of photoexcited electrons. By resonantly tuning the incident photon to excite pentacene molecules, polarization-dependent 2P-PEEM has revealed that pentacene nanocrystals (sub- to several μm) on the substrate exhibit a preferential orientation, in which a molecular π-orbital contacts the substrate in a “lying flat” orientation, facilitating electron transfer to the substrate. The time-resolved 2P-PEEM captures the motion of excited electrons in a femto- to pico-second timescale, clearly imaging the ultrafast charge transfer and lateral expansion two-dimensionally on the graphite substrate. Moreover, we found that the lying-flat molecular orientation of pentacene nanocrystals is transformable into a “standing-up” one through gentle heating up to 50 °C. These experimental insights using time-resolved 2P-PEEM will be highly valuable in enhancing the photofunctionalities of organic electronic devices by controlled molecular deposition.

Superatoms (SAs) with specific compositions have the potential to significantly advance the field of nanomaterials science, leading to next-generation nanoscale functionalities. In this study, we fabricated assembled layers with tantalum metal-atom encapsulating silicon cage (Ta@Si16) SAs on an organic C60 substrate through deposition, and we characterized their electronic and optical properties by photoelectron spectroscopy and microscopy. The alkaline nature of Ta@Si16 SAs reveals their electronic behaviors, such as charge transfer and electromagnetic near-field sensing, through two-photon photoemission (2PPE) spectroscopy and microscopy with a femtosecond laser. The evolution of the work function for Ta@Si16 SAs on C60, observed by 2PPE spectroscopy, demonstrates charge transfer complexation between the topmost C60 layer and the first Ta@Si16 layer, consistent with the electron-donating alkaline characteristics of Ta@Si16 SAs. Specifically, a small amount of Ta@Si16 SA deposition leads to a dramatic increase in 2PPE intensity, attributable to electromagnetic near-field enhancements, suggesting applications as sensitizers for nonlinear imaging in photoemission microscopy. For the assembled Ta@Si16 SA layers, a plasmonic response of hνp = 17.9 eV is spectroscopically identified, including their valence and conduction band structures, and the plasmonic energetics are discussed in the context of metal doping in bulk silicon.

The design of materials with intriguing electronic properties is crucial for advancing nanoscale technologies, where precise control over atomic structure and electronic behavior is essential. Metal-encapsulating silicon cage superatoms (SAs) provide a new paradigm for molecular-scale material design, allowing fine-tuning of both structure and electronic characteristics. The formation of superatoms mimicking halogens, noble gases, and alkali metals has been well-studied, particularly with M@Si16, where early transition metals from groups 3 to 5 stabilize within a Si16 cage, achieving a 68-electron configuration. For late transition metals with excess electrons, a Si15 cage offers enhanced stability by fulfilling the 68-electron rule with one fewer Si atom. This research synthesizes Si15 cage-SAs with rhenium (Re) from group 7 and iridium (Ir) from group 9 on p-type and n-type organic substrates. The stability of Re@Si15 and Ir@Si15 is evaluated via oxidative reactivity with X-ray photoelectron spectroscopy and theoretical calculations, including osmium (Os) from group 8. Re@Si15-, Os@Si150, and Ir@Si15+ exhibit superatomic behaviors similar to halogens, noble gases, and alkali metals due to the 68-electron shell closure. Among them, Re@Si15- on p-type organic substrates shows superior electronic and geometric properties. These findings advance our understanding of M@Sin systems for transition metals, addressing longstanding questions about their properties at n = 15 and 16.

Surface modification with uniform nanostructures is a promising way to fabricate functionalized surface, and the synthetic electrode functionalization can be performed by depositing atomically precise nanoclusters with a specific number of atoms and compositions. During atomic nucleation and growth in gas phase, atomic aggregates exhibit novel electronic properties, mimicking atoms at a specific number of atoms and composition, which is called superatom (SA). Tantalum-encapsulating Si16 cage nanoclusters (Ta@Si16) and aluminum 13-mer (Al13) are promising SAs for designing their assembled materials with tunable properties, where one-electron-excessive Ta@Si16 and one-electron-deficient Al13 exhibit alkali- and halogen-like characteristics, respectively, owing to the electron shell closing. In this study, with an intense nanocluster ion source, Ta@Si16+ and Al13− were generated during the formation of TaSin + cations and Aln − anions, where Ta@Si16+ was remarkably generated compared with the neighboring species. SA deposition provided uniformly decorated surfaces exhibiting nanofunctional properties superior to those obtained by conventional atom deposition. With mass spectrometry, Ta@Si16+ and Al13− SAs were size-selectively soft-landed on the substrate surfaces predeposited with n-type fullerene (C60) and p-type hexa-tert-butyl-hexa-peri-hexabenzocoronene (HB-HBC, C66H66). The electronic states of Ta@Si16 and Al13 SAs immobilized on the organic substrates were characterized by X-ray photoelectron spectroscopy and oxidative reactivities under O2 exposure.

Ptn subnanoclusters (n = 3–9) on a carbon substrate exhibit 1.6–2.2 times higher activity than the standard Pt/C catalysts. EXAFS experiments and DFT calculations show plausible structures and energetics for reaction intermediates in the processes.

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 (M-V = V, Nb, and Ta) encapsulating silicon (Si) cage nanoclusters (M-V@Si-16), 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 M-V@Si-16 assembled films revealed that the electrical conduction mechanism is not band transport but hopping transport with Efros-Shklovskii variable range hopping for all central M-V 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 M-V and is several times the radius of M-V@Si-16 (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.

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.

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.

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.

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.

Single-size platinum Pt6 sub-nanoclusters exhibit superior mass-specific and surface-specific activities for the oxygen reduction reaction.

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.

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.

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)10for M = group 4 elements in neutral ground state. The most important issue for M@Si16BCSs 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 Si16cage; the replacement of M yields a series of M@Si16BCSs, 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@Si16have not yet been isolated.In this Account, we have focused on recent advances in synthesis and characterizations of M@Si16BCSs (M = Ti and Ta). A series of M@Si16BCSs (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 C60fullerene 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.

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.

We have employed a two-photon photoelectron emission microscopy (2P-PEEM) to observe the photocarrier electron dynamics in an organic thin film of fullerene (C-60) formed on a highly oriented pyrolytic graphite with a spatial resolution of ca. 135 nm. In this approach, photocarrier electrons in C-60 single-layer islands generated by the first pump photon are detected by the second probe photon. These spectromicroscopic observations conducted over a 100 x 100 nm(2) region of C-60 islands consistently reproduced the macroscopic two-photon photoemission spectrum of fully covered C-60 monolayer film, where the energy of photocarrier electron in the islands was +0.9 eV relative to the Fermi level. Time-resolved 2P-PEEM revealed that the photocarrier electron decayed from the monolayered C-60 islands into the substrate with a time constant of 470 +/- 30 fs. Published by AIP Publishing.

We applied hard x-ray photoelectron spectroscopy (HAXPES) to a sample under ambient pressure conditions using an environmental cell with an approximately 24 nm-thick SiNx membrane window. As a model chemical substance, europium (II) iodide (EuI2) sealed in the cell with argon gas was investigated with HAXPES to identify the chemical species present inside the cell. The optical and morphological properties of the sample within the cell were measured with optical and fluorescent microscopy, scanning electron microscopy, cathodoluminescence, and energy dispersive x-ray spectrometry. We confirmed the effectiveness of the gas barrier properties of the cell with the SiNx window and demonstrated its applicability to various other optical and electron measurements as well as HAXPES.