Fabrication and Imaging of Monolayer Phosphorene with Preferred Edge Configurations via Graphene-Assisted Layer-by-Layer Thinning

Phosphorene, a monolayer of black phosphorus (BP), is an elemental 2D material with interesting physical properties including high charge carrier mobility and exotic in-plane anisotropic properties. To fundamentally understand its various physical properties, the atomic-scale structural investigation, various defect and preferred edge configurations for monolayer counterpart, is of critically importance. However, it has been challenging to perform imaging of monolayer phosphorene due to technical difficulty to prepare high-quality samples and damages induced during measurements via photons or high-energy electrons.
Here, we successfully fabricate monolayer phosphorene by a controlled thinning process inside a TEM and subsequently perform atomic-resolution imaging. Graphene protection suppresses the e-beam-induced damage to BP and one-side graphene lamination facilitates the layer-by-layer thinning of the samples, rendering monolayer and bilayer regions. We also observe crystalline edge predominantly aligned along zigzag and (101) terminations, which can be explained by edge kinetics under e-beam-induced sputtering process. Our study provides invaluable approach to manipulate the thickness and image beam-sensitive 2D materials.