Atomic-Scale Friction of Black and Violet Phosphorus Crystals: Implications for Phosphorus-Based Devices and Lubricants

Black phosphorus (BP) has received much attention as a two-dimensional layered solid lubricant in reducing friction and protecting against wear. Violet phosphorus (VP) is another stable allotrope of phosphorus with unique layered structures. However, the friction properties of VP have not been explored. Herein, we present a comprehensive study of the atomic-scale friction of BP and VP by friction force microscopy. The friction properties of VP were characterized for the first time. Atomic-scale stick–slip friction measurements along the lattice orientations of BP and VP clearly revealed the correlation between friction anisotropy and crystallographic structures. Relative to the nitrogen atmosphere, the friction behavior of BP and VP in water was also investigated. It was found that the friction coefficient was significantly increased in water, indicating that water was not a good medium for phosphorus achieving superlubricity. The results in this study not only provide in-depth insights into the fundamental friction properties of phosphorus but also pave the crucial pathways toward such applications as lubricants in micro/nanoelectromechanical systems and a phosphorus-based superlubric generator with high efficiency and ultralong life.