Photo-oxidative Degradation and Protection Mechanism of Black Phosphorus: Insights from Ultrafast Dynamics

The environmental instability and protection of black phosphorus (BP) is one of the most attractive hotspots in two-dimensional materials. The generation of superoxide is believed to be the key culprit, while the photogenerated electron dynamics is yet to be known. In this work, we carry out time domain ab initio nonadiabatic molecular dynamics to understand the photogenerated electron dynamics at the molecule/BP interface. It is found that oxygen can trap the photogenerated electrons of BP rapidly owing to strong electron-phonon (e-p) coupling and becomes an active superoxide under light illumination. A good protection layer, such as perylene diimide (PDI), has comparable capabilities of trapping photogenerated electrons to oxygen, which can efficiently prevent the formation of superoxide and thus suppress the degradation of BP. Moreover, PDI can enhance the separation of photogenerated electron-hole pairs of BP by prolonging the time of holding photogenerated electrons of BP. In contrast, 7,7,8,8-tetracyano- p-quinodimethane (TCNQ) has weak e-p coupling and a large band edge offset between trapping and donor states and thereby poor trapping ability for photogenerated electrons, leading to poor protection efficiency. This study provides the first in-depth understanding of the BP degradation and protection mechanism from excited-state dynamics and can be applicable to other 2D photo-oxidative degradation.