Highly Air-Stable N-Doped Two-Dimensional Violet Phosphorus with Atomically Flat Surfaces

Few-layer violet phosphorus (VP) shows excellent potential in optoelectronic applications due to its unique in-plane anisotropy and high mobility. However, the poor air stability of VP severely limits its practical applications. This article reports highly air-stable VP obtained by a two-step nitrogen plasma treatment where the nitrogen volume flow rate is controlled to coordinate physical etching and chemical doping. Specially, this plasma process can remove partial oxidations formed on the VP surface with barely etching to the intrinsic VP surface but efficiently incorporates nitrogen into VP, resulting in surface nitrogen-doped VP (N-VP) nanosheets with atomically smooth surfaces that exhibit excellent air stability. Atomic force microscopy images show that the N-VP nanosheet, nearing a monolayer thickness, maintained its surface morphology and flatness unchanged in ambient air for over 60 days. The improved stability of N-VP can be partly due to its atomically smooth surface, which reduces the number of active or oxidation sites. Further elucidation was made by density functional theory calculations, showing that this ultrastability may intrinsically be attributed to repairing P vacancies by N dopants. This research provides a feasible strategy for significantly enhancing the durability of VP.