  Black phosphorus (BP) has recently emerged as an alternative 2D semiconductor
owing to its fascinating electronic properties such as tunable bandgap and high
charge carrier mobility. The structural investigation of few-layer BP, such as
identification of layer thickness and atomic-scale edge structure, is of great
importance to fully understand its electronic and optical properties. Here we
report atomic-scale analysis of few-layered BP performed by aberration
corrected transmission electron microscopy (TEM). We establish the
layer-number-dependent atomic resolution imaging of few-layer BP via TEM
imaging and image simulations. The structural modification induced by the
electron beam leads to revelation of crystalline edge and formation of BP
nanoribbons. Atomic resolution imaging of BP clearly shows the reconstructed
zigzag (ZZ) edge structures, which is also corroborated by van der Waals first
principles calculations on the edge stability. Our study on the precise
identification of BP thickness and atomic-resolution imaging of edge structures
will lay the groundwork for investigation of few-layer BP, especially BP in
nanostructured forms.
