Formation of Layer-Structured Black Phosphorus Nanocrystals during High-Speed Rotation of Two-Dimensional Amorphous Ultrathin Films

Sonication and centrifugation of two-dimensional nanosheets have been widely used to produce various layer-structured nanocrystals (LSNCs), but the formation mechanism is not yet clear. A general understanding is that the formation of LSNCs is due to the splitting of crystalline sheets under sonication/centrifugation. However, this has not been supported by experimental evidence. Here we experimentally show that high-speed rotation of amorphous black phosphorus ultrathin film can lead to the regulated formation of massive LSNCs confined in the spatial region of the original film. The probable sizes of these LSNCs are several nanometers depending on the rotation speed. Their volumes show a clear log-normal distribution having a line-width increase with rotation speed. This phenomenon can be explained based on the two-dimensional continuity and momentum equations. Our findings provide insight into formation and mechanisms of ultrathin amorphous films and LSNCs under sonication and centrifugation generally used.