In Situ TEM Imaging of Novel Edge Reconstruction in Bilayer Phosphorene

Atomic-scale characterization of two-dimensional (2D) materials and their heterostructure is indispensable to understanding their physical, electrical, and chemical properties. In particular, atomic-scale configurations and morphology of 2D crystals’ edges strongly influence properties including electronic, transport, and optical properties. Transmission electron microscopy (TEM) has been employed to observe the atomic-scale edge structure of various 2D materials. Moreover, via in-situ TEM capabilities, the controlled edge formation under various environments and stimuli has been achieved. On the other hand, atomic-scale TEM imaging of crystalline edges of phosphorene has been quite challenging compared to other 2D crystals. The main obstacle is the sample’s vulnerability to ambient exposure and the characterization process.<br/><br/>Here, we report the atomic scale TEM observation of ultra-stable self-passivated phosphorene edges in bilayer phosphorene. We use graphene as a supporting layer for phosphorene to suppress the electron-beam-induced radiolysis effect during TEM measurement. We prepare phosphorene/graphene vertical heterostructure samples on an in-situ heating chip by dry-transfer technique. Using in situ heating, the prepared phosphorene/graphene samples suffer from little residues and adsorbates. At elevated temperatures, the layer-by-layer etching of black phosphorus has been observed, which leads to the formation of monolayer and bilayer phosphorene. While etching of monolayer phosphorene under e-beam is pronounced, we observe that the edge of bilayer phosphorene with zigzag edge configuration shows high stability under the electron beam.<br/><br/>To investigate the detailed edge configuration of stable bilayer ZZ phosphorene edge, we perform TEM imaging at various defocus values as well as tilting configurations. Based on first-principles calculations and TEM image simulations, we confirm that the bilayer ZZ edge shows the reconstruction with the interlayer self-passivating covalent bonds. Our theoretical calculations also confirm the low formation energy of the observed edge configuration compared to other possible configurations. Our study demonstrates that atomic-scale reconstruction of phosphorus can be harnessed to fabricate stable phosphorene nanostructures with precisely controlled thickness and edge configuration.