Structure of χ-Borophene Studied by Total-Reflection High-Energy Positron Diffraction (TRHEPD).

We have investigated the structure of χ-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 χ-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 χ-borophene is compatible with the theoretical predictions. These structural properties indicate that χ-borophene belongs to a group of epitaxial monolayer sheets, such as graphene, which have weak interactions with the substrates.