Electronic Self-Passivation of Single Vacancy in Black Phosphorus via Ionization

We report that monoelemental black phosphorus presents a new electronic self-passivation scheme of single vacancy (SV). By means of low-temperature scanning tunneling microscopy and noncontact atomic force microscopy, we demonstrate that the local reconstruction and ionization of SV into negatively charged SV??? leads to the passivation of dangling bonds and, thus, the quenching of in-gap states, which can be achieved by mild thermal annealing or STM tip manipulation. SV exhibits a strong and symmetric Friedel oscillation (FO) pattern, while SV??? shows an asymmetric FO pattern with local perturbation amplitude reduced by one order of magnitude and a faster decay rate. The enhanced passivation by forming SV??? can be attributed to its weak dipolelike perturbation, consistent with density-functional theory numerical calculations. Therefore, self-passivated SV??? is electrically benign and acts as a much weaker scattering center, which may hold the key to further enhance the charge mobility of black phosphorus and its analogs.