Published papers

An unexplored material of copper boride has been realized recently in two-dimensional form at a (111) surface of the fcc copper crystal. Here, one-dimensional (1-D) boron growth was observed on the Cu(110) surface, as probed by atomically resolved scanning probe microscopy. The 1-D copper boride was composed of quasi-periodic atomic chains periodically aligned parallel to each other, as confirmed by Fourier transform analysis. The 1-D growth unexpectedly proceeded across surface steps in a self-assembled manner and extended over several 100 nm. The long-range formation of a 1-D quasi-periodic structure on a surface has been theoretically modeled as a 1-D quasi-crystal and the predicted conditions matched the structural parameters obtained by the experimental work here. The quasi-periodic 1-D copper boride system enabled a way to examine 1-D quasi-crystallinity on an actual material.

The electron deficiency of boron leads to rich polymorphism and a high degree of freedom in material design. This research focuses on the newly reported CuB/Cu(111) surface and elucidates its catalytic properties. To assess its catalytic activity, this research investigates elementary steps relevant to CO2 hydrogenation─specifically hydrogen, oxygen, and formate adsorption on this surface─by using a combination of first-principles calculations (OpenMX) and machine learning potentials (Matlantis). Our calculations verify five key points: (i) charge transfer occurs from copper atoms to boron chains, (ii) hydrogen atoms are observed to be adsorbed along the boron chain, (iii) hydrogen hopping between boron-tops shows three-center two-electron bonding in the transition state, (iv) surface oxidation can disrupt the boron chains, and (v) boron chains promote the adsorption of formate. In conclusion, the unique boron chains in CuB/Cu(111) can play a key role in the catalytic reactivity of this surface.

Second harmonic generation (SHG) is a unique non-linear optical effect that can be used to investigate chemical states of molecules at surfaces/interfaces. By the use of soft X-rays, SHG gains element selectivity through the inner-core excitation resonance. However, it is challenging to observe SHG signals separately from the second-order light generated from the undulator. Here, we report a new ellipsometry method for soft X-ray SHG to suppress the contribution of second-harmonic radiation from the light source. Through measurements of a GaAs(100) crystal, we demonstrate that pure SHG signals can be obtained for the horizontally polarized component. The present method is generally applicable regardless of the incident photon energy and hence the absorption edge of the targeted materials. If combined with optical filters blocking the second-harmonic radiation and equipped with soft X-ray phase shifters, the method allows one to obtain further information from SHG signals such as tensor components of second-order non-linear susceptibility.

The structure and chemical state of a boron chain, isoelectronic to polycumulene, were obtained with a combination of positron diffraction, photoelectron spectroscopy, and first-principles calculations. In the present work, an array of boron chains was found in a copper boride layer on a metal crystal substrate. The band structure of this material was determined to be electronically doped and hybridized by neighboring copper atoms. These boron atom chains exhibit a unique electrophilic character that is specific to the element. The properties of boron chains in the copper boride layer and the substrate effect are systematically investigated together.

A two-dimensional crystal of the copper boride was grown on the well-known Cu(111) surface by boron deposition. The atomic structure and the electronic state were analyzed by a concerting usage of positron diffraction and photoemission spectroscopy. The surface layer is composed of the B and Cu atomic chains that are alternately arranged. It has been known that the B and Cu phases are completely separated in three-dimensional (3D) materials. The present study intriguingly revealed that a compound is formed in two-dimension (2D) at the surface. In addition, the Cu(111) substrate was found to affect a relative height between the B and Cu chains. The 2D Cu-B compound on Cu(111) likely becomes an ideal B/Cu interface to develop surface science and boron chemistry.

We have investigated the structure of χ3-borophene on Ag(111), a monolayer material of boron atoms, via total-reflection high-energy positron diffraction (TRHEPD). By comparing the experimental rocking-curves with ones for several structures calculated by using dynamical diffraction theory, we confirmed that the χ3-borophene layer has a flat structure. The distance from the topmost layer of the metal crystal is 2.4 Å, which is consistent with results reported by X-ray standing wave-excited X-ray photoelectron spectroscopy. We also demonstrated that the in-plane structure of χ3-borophene is compatible with the theoretical predictions. These structural properties indicate that χ3-borophene belongs to a group of epitaxial monolayer sheets, such as graphene, which have weak interactions with the substrates.

The structure of an Al(111)3×3-Si surface was examined by combining data from positron diffraction and core-level photoemission spectroscopy. Analysis of the diffraction rocking curves indicated that the overlayer had a flat honeycomb lattice structure. Simulations of Si core-level spectra calculated via the first principles indicated that one of the Si atoms in the unit cell was replaced by an Al atom. The surface superstructure was thus a two-dimensional layer of Al-embedded silicene on Al(111).