  We report that mono-elemental black phosphorus presents a new electronic
self-passivation scheme of single vacancy (SV). By means of low-temperature
scanning tunneling microscopy and bond-resolved non-contact atomic force
microscopy, we demonstrate that the local reconstruction and ionization of SV
into negatively charged $\mathrm{SV}^-$ 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 $\mathrm{SV}^-$ 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 $\mathrm{SV}^-$ can be
attributed to its weak dipole-like perturbation, consistent with
density-functional theory and numerical calculations. Therefore,
self-passivated $\mathrm{SV}^-$ is electronically benign and acts as a much
weaker scattering center, which may hold the key to further enhance the charge
mobility of BP and its analogs.
