Atomic Structure and Dynamics of Defects and Grain Boundaries in 2D PdSe Monolayers.

We study the atomic structure and dynamics of defects and grain boundaries in monolayer PdSe using annular dark field scanning transmission electron microscopy. The PdSe monolayers are reproducibly created by thermally induced phase transformation of few-layered PdSe films in an heating holder in the TEM to promote Se loss. A variety of point vacancies, one-dimensional defects, grain boundaries (GBs), and defect ring complexes are directly observed in monolayer PdSe, which show a series of dynamics triggered by electron beam irradiation. High mobility of vacancies leads to self-healing of point vacancies by migration to the edge and subsequent edge etching under beam irradiation. Specific defects for PdSe are stabilized by the formation of Se-Se bonds, which can shift in a staggered way to buffer strain, forming a wave-like one-dimensional defect. Bond rotations are also observed and play an important role in defect and grain boundary dynamics in PdSe during vacancy production. The GBs form in a meandering pathway and migrate by a sequence of Se-Se bond rotations without large-scale vacancy formation. In the GB corners and tilted GBs, other highly symmetric vacancy defects also occur to adapt to the orientation change. These results give atomic level insights into the defects and grain boundaries in PdSe 2D monolayers.