  Black arsenic (BAs) is a van der Waals layered material with a puckered
honeycomb structure and has received increased interest due to its anisotropic
properties and promising performance in devices. Here, crystalline structure,
thickness-dependent dielectric responses, and ambient stability of BAs
nanosheets are investigated using STEM imaging and spectroscopy.
Atomic-resolution HAADF-STEM images directly visualize the three-dimensional
structure and evaluate the degree of anisotropy. STEM-EELS is used to measure
the dielectric response of BAs as a function of the number of layers. Finally,
BAs degradation under different ambient environments is studied highlighting
high sensitivity to moisture in the air.
