Published papers

We have performed scanning tunneling microscopy (STM) and angle-resolved photoemission spectroscopy (ARPES) in Pb-deposited bilayer graphene (BLG) on the SiC(0001) substrate to investigate the dependence of the electronic structures on the Pb-deposition amount. We have observed that the Pb atoms form islands by STM and the π bands of the BLG shift toward the Fermi level by ARPES. This hole-doping-like energy shift is enhanced as the amount of Pb is increased, and we were able to tune the Dirac gap to the Fermi level by 4 ML deposition. Considering the band dispersion, we suggest that the hole-doping-like effect is related to the difference between the work functions of Pb islands and BLG/SiC; the work function of BLG/SiC is lower than that of Pb. Our results propose an easy way of band tuning for graphene with an appropriate selection of both the substrate and deposited material.

The mechanism for CO2 activation on the Ru(0001) surface, the dominant surface of catalytic Ru nanoparticles, was elucidated using operando ambient-pressure X-ray photoelectron spectroscopy in CO2/H2. At 300 K, the major reaction was CO2 dissociation, generating CO adsorbates at both atop and bridge sites (COt* and COb*, respectively). The appearance of such a coadsorbed phase of COt* and COb* was reproduced by density functional theory calculations. At 370–470 K, these CO* species were converted to elemental carbon. Because the reaction condition matched the temperature ranges of the CO2 methanation on the Ru-based catalysts, COt* and COb* coadsorbed specifically at the interface between Ru(0001) and CO2/H2 gases seem to be the critical factor for elucidating the reaction mechanism.

We present an enhanced method for synthesizing sheets of borophane. Despite the challenges associated with low efficiency, we discovered that incorporating hydrochloric acid into the ion-exchange reaction significantly improved the production yield from 20% to over 50%. After a thorough examination of the reaction, we gained insight into the underlying mechanisms and found that the use of hydrochloric acid provides two key benefits: accelerated production of borophene and isolation of high-purity products. This method has the potential to pave the way for the production of novel topological 2D materials with potential industrial applications.

Boron deposited on transition metal substrates has been actively investigated as a system that exhibits a wide range of surface structures. However, the specific case of nickel substrates remains largely unexplored. Nickel boride is used as a tribological material with excellent wear resistance and as a catalyst for hydrogenation, and its synthesis methods, structure, and functionality are being vigorously investigated. In this study, borophane, a two-dimensional material composed of hydrogen and boron, was heated to 300 °C in contact with a nickel substrate. We found that, compared with that annealed at 100 °C, annealed at 300 °C had a color that shifted toward longer wavelengths, and no longer dissolved in the synthesis solvent. However, x-ray diffraction and Fourier-transform infrared spectroscopy revealed no significant differences between the borophane samples annealed at 100 and 300 °C. X-ray absorption spectroscopy and x-ray photoelectron spectroscopy revealed that the surface oxide of nickel was reduced by borophane, resulting in the formation of nickel boride. Furthermore, thermal desorption spectroscopy showed that hydrogen desorbed from borophane upon contact with nickel, revealing that the borophane–nickel interface consisted of nickel boride and boron oxide layers. These findings provide insight into the surface science of boron on transition metal substrates and for the development of functional materials using these systems.

Hydrogen boride (HB) sheets are emerging as a promising two-dimensional (2D) boron material, with potential applications as unique electrodes, substrates, and hydrogen storage materials. The 2D layered structure of HB was successfully synthesized using an ion-exchange method. The chemical bonding and structure of the HB sheets were investigated using Fourier Transform Infrared (FT–IR) spectroscopy and Transmission Electron Microscopy (TEM), respectively. X-ray photoelectron spectroscopy (XPS) was employed to study the chemical states and transformation of the components before and after atomic hydrogen adsorption, thereby elucidating the atomic hydrogen adsorption process on HB sheets. Our results indicate that, upon atomic hydrogen adsorption onto the HB sheets, the B-H-B bonds were broken and converted into B-H bonds. This research highlights and demonstrates the changes in chemical states and component transformations of the boron element on the HB sheets’ surface before and after atomic hydrogen adsorption, thus providing a clearer understanding of the unique bonding and structural characteristics of the HB sheets.

We present an approach to selectively examine an asymmetric potential in the buried layer of solar cell devices by means of nonlinear x-ray spectroscopy. Detecting second harmonic generation signals while resonant to the SiO2 core level, we directly observe existence of the band bending effect in the SiO2 nanolayer, buried in the heterostructures of Al/LiF/SiO2/Si, TiO2/SiO2/Si, and Al 2 O 3 /SiO2/Si. The results demonstrate high sensitivity of the method to the asymmetric potential that determines performance of functional materials for photovoltaics or other optoelectronic devices.

A copper boride phase found on Cu(110) exhibits dimensional crossover from 1D to 2D structures. The discovered surface phase, 7Hebrew letter bet1'-B/Cu(110), is composed of alternating two-atom and three-atom rows that are separated by trenches. The narrower rows behave as a 1D template for atomic boron chains, and the wider rows provide a new dimensional degree of freedom that facilitates the generation of 2D clusters with chirality. The 7Hebrew letter bet1' phase is nonperiodic in the chain direction and exhibits intermediate boron coverage, between that of the quasiperiodic 1D and periodic 2D phases of copper boride on Cu(110), demonstrating a unique feature of phase transitions at the surface.

The discovery of fullerene following the synthesis of graphene marked a paradigm shift in chemistry. Here, we report the discovery of biycycloborane, arising from the synthesis of borophane (hydrogen boride). Uniquely, this synthesis method involves a decomposition mechanism rather than traditional atom-by-atom assembly, marking an unique approach to constructing complex borane structures. The mass spectrometry unveiled that the stable molecule has a mass of 178 in atomic mass unit with a stoichiometry of B14H26. Optical spectra and simulations further evidenced its bicyclic structure, featuring fulvene-like heptagons or octagons. This borane molecule, analogous to cyclic hydrocarbons, adopts a unit configuration with a three-center two-electron (3c-2e) bonding, akin to diborane. The B14H26 molecule has been historically anticipated as a distant descendant of the dodecahedron borane, but it was born from the hydrogen boride sheet with a non-symmorphic symmetry. The discovery of biycycloborane expands the frontiers of boron chemistry, promising advancements in boron-based nanomaterials and beyond.

We studied electronic states of borophene in a crystal of rare-earth aluminum/chromium boride and the hydrogenated sheet, borophane, successfully prepared by ion exchange and liquid exfoliation of the crystal. The layer has a boron network with five-membered and seven-membered rings. Electronic structures of all the boron-derived layers are found to be gapless by soft x-ray absorption and emission spectroscopy measurements at the B K-shell absorption edge and density functional theory calculations. The present results support the existence of a Dirac nodal loop at the Fermi level in the borophane layer with five- and seven-membered rings, predicted recently by topological calculations.

We have performed a comprehensive electronic characterization of the needle-like single crystal YCrB4 metal boride, prepared by cleavage, characterized using angle-resolved photoemission spectroscopy with a microfocused beam. Our findings reveal the three-dimensional (3D) band structure of YCrB4, with band dispersions closely aligning with the first-principles calculations, confirming it is a topologically trivial insulator with a narrow band gap. Furthermore, a new metallic surface band was observed after cleavage, likely due to a space charge layer of the electron accumulation type. This study highlights the pivotal role of surface phenomena in shaping the electronic structure of YCrB4 and provides insights into the interplay between bulk and surface states, offering a path for exploring its potential applications in surface-sensitive technologies.

We systematically investigated electronic evolutions of nonsymmorphic borophene with chemical environments that were realized by the ion exchange method. Electronic structures can be characterized by the topological Z2 invariant. Spectroscopic experiments and DFT calculations unveiled that a sheet of hydrogenated borophene (borophane) is the Dirac nodal loop semimetal (Z2=-1), while a layered crystal of YCrB4is an insulator (Z2=1). The results demonstrate the electronic topological transition by replacement of the counter atoms on the nonsymmorphic borophene layer.

We present a series of procedures that prepared and analyzed chromium borides (Cr-B) microcrystals using a combination of micro-focused ion and quantum beam techniques. The chemical state analyses were carried out using micro-photoemission spectroscopy and structural analyses were performed using micro-reflection high-energy electron diffraction and single-crystal X-ray diffraction. This approach successfully identified the existence of CrB2 and Cr2B3 phases in a Cr-B microcrystal and also provided detailed chemical state evaluation of Cr2B3 phase. Our integration of these techniques enabled to distinguish and evaluate micro-scale phases within complex materials, identifying different phases or unexpected secondary products of microcrystal.

Theoretically predicted materials are often synthesized in low yields, and unexpected relationships are often encountered between the target materials and byproducts. Recently, two-dimensional boron materials proposed on the basis of model simulations and first principles calculations and possessing abundant atomic structures have attracted considerable interest. Borophane or the hydrogen boride (HB) sheet has been predicted to be the Dirac nodal semimetal when it has a boron network of nonsymmorphic symmetry. Upgrading the standard method, we fabricated freestanding HB sheets possessing either an apparent Fermi edge, reduced spectral weight, or a Fermi-level energy gap, as confirmed by using microbeam photoemission spectroscopy. The gapless electronic structures were correlated with terminal B-H bonds at the sheet edges, indicating the electronic modification of the porous structure as directly microscopically observed. The gapped or insulating sheet was fabricated via oxidation. This research provides methods for regulating the structural morphology and electronic states of HB sheets during synthesis.

We report a homotopic analysis of quantum states in two-dimensional (2D) polymorphs. Two typical models, namely the honeycomb-type and Shastry-Sutherland-type models, are continuously connected through a deformable herringbone lattice model having a nonsymmorphic group symmetry. The evolution of Dirac fermions is characterized within the phase space spanned by the deformation parameters. Our tight-binding models, based on single orbitals for each site with and without spin-orbit coupling (SOC), reveal how the SOC convolutes, the manner being important in facilitating the search and design of 2D topological insulators focused on the homotopic feature of symmetry-protected properties independent of specific materials.

Millimeter-scale growth of YCrB4 crystals was achieved by postannealing, followed by conventional arc melting. The bulk crystal was semiconducting, whereas the surface was metallic, as observed by photoemission spectroscopy. First-principles calculations consistently reproduced the metallicity in a model of the boron-terminated surface of the YCrB4 crystal. These results support further investigations of different states on the surface of metal boride bulk materials.

Growths of novel two-dimensional materials have recently attracted interests to develop chemistry and physics at surfaces. By the boron deposition on Cu(110), we discovered a 1D anisotropic ordered phase, 3×1, as a pattern of electron diffraction. The electronic analysis by photoelectron spectroscopy unveiled that the surface superstructure is a layer of the Cu boride with a component of the negatively charged boron atoms. This surface boron atoms likely form the chain structure that is isoelectronic to carbon polymers, such as bumulene that was recently reported on Cu(111). Considering the two-fold rotation symmetry of Cu(110) substrate, the novel surface phase likely becomes an ideal system to examine 1D boron materials.

Second harmonic generation (SHG) spectroscopy in the visible and infrared regions has been a useful tool to selectively probe electronic properties at surfaces and interfaces. By examining variation of SHG under a magnetic field, one can also evaluate magnetic properties at the surfaces/interfaces. When multiple elements are involved in SHG, however, it is difficult to separate their contributions. In order to meet the demand of studying increasingly complex magnetic multilayer materials, element selectivity is desired. Here, using an Fe-based multilayer sample with broken inversion symmetry, we present observation of magnetization-induced SHG in the soft x-ray regime around the Fe M-shell absorption edge. Significant variation of SHG signal was captured depending on the direction of the magnetic moment, assuring sensitivity of the measurement likely enhanced by the Fe M-edge inner-shell resonance. The present methodology paves the way for element specific studies of magnetic properties at buried interfaces.

Polymorphs of boron provide various types of atomic structures that lead to the emergence of physical phenomena and become playgrounds to examine electronic states. There are lithium-boron compounds that embed quasi-one-dimensional (quasi-1D) boron incommensurately in the hexagonal lithium sublattices. Based on first-principles calculations, k groups, and topological invariants, we unveiled that the quasi-1D structures show double π bonding orbitals, unavoidable band crossings, and nodal lines around the Fermi level. The origin of this phenomenon is attributed to the interaction between B-B chains through sublattice Li atoms and to the nonsymmorphic crystal symmetry.

We demonstrate a series of procedures to prepare a clean surface of micro-sized graphite, mined from a bulk flake and securely affixed onto a macroscopic Si plate by focused ion beam scanning electron microscope. Analyses of structure and electronic (chemical) states were made using micro-beam X-ray photoelectron spectroscopy and angle-resolved photoemission spectroscopy. At the surface of the micro graphite, the band dispersion from a single-domain structure was observed. The proposed methodology showcases its capability to produce clean and high-quality micro samples suitable for surface-sensitive analyses. This technique paves the way to investigate surfaces of unexplored microcrystals embedded in complex materials.

We have extensively searched for a cyclic hydrogenated boron molecule that has a three-center two-electron bond at the center. Using first-principles calculations, we discovered a stable molecule of 2:4:6:8:-2H-1,5:1,5-μH-B8H10 and propose its existence. This molecule can be regarded as a building block for sheets of topological hydrogen boride (borophane), which was recently theoretically proposed and experimentally discovered. The electronic structure of the cyclic hydrogenated boron molecule is discussed in comparison with that of cyclic hydrogenated carbon molecules.