Scanning tunneling microscopy study of natural black arsenic

Black arsenic (BAs) has emerged to be an interesting elemental layered material after graphite and black phosphorus (BP), which can be exfoliated to yield high-quality two-dimensional flakes. Transport measurements have revealed its strong in-plane anisotropy, giant Stark effect, and strong spin-orbital coupling, all resulting from its special electronic properties. On the other hand, previous synthetic analysis indicated that the pure BAs phase is unstable and could only be stabilized by a substantial percentage of impurities. Thus, a microscopic study of its structural and electronic properties is important for understanding this material. Here, we use scanning tunneling microscopy and spectroscopy assisted with first-principles calculations to investigate natural BAs at the atomic level. A puckered lattice structure like that of BP is measured. A bandgap of $\ensuremath{\sim}0.29\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$ is detected with the Fermi level located close to the conduction band. The most common defects form parallel stripes along a specific crystal direction and are shown to be the possible origin of the $n\text{\ensuremath{-}}\mathrm{type}$ doping in the natural BAs. These results provide valuable information for further study of BAs.