Direct Exchange of Re and Mo atoms in monolayer MoS₂

Two-dimensional materials provide opportunities to directly observe atomic-scale defect dynamics. The direct exchange between a substitutional impurity and a neighboring host atom has been discussed in the literature, but the energy barrier for such a process is generally believed to be too large. No atomic-scale observation of direct exchange event has been reported. Here we use scanning transmission electron microscopy to observe substitutional Re impurities in monolayer MoS₂ undergo direct exchanges with neighboring Mo atoms. Density functional theory calculations find that the energy barrier for direct exchange is too large for either thermal or beam-induced exchange. Microscopy further reveals the presence of an ever-changing number of S vacancies, but the calculated energy barrier remains too large to account for the jumps. The density functional theory calculations further find that a Re impurity and surrounding S vacancies introduce an ever-changing set of localized levels in the energy gap. We propose that these levels mediate an “explosive” recombination-enhanced migration via multiple electron-hole recombination events.