Atomic Structure and Dynamics of Defects and Grain Boundaries in 2D Pd₂Se₃ Monolayers

We study the atomic structure and dynamics of defects and grain boundaries in monolayer Pd₂Se₃ using annular dark field scanning transmission electron microscopy. The Pd₂Se₃ monolayers are reproducibly created by thermally induced phase transformation of few-layered PdSe₂ films in an in situ 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 Pd₂Se₃, 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 Pd₂Se₃ 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 Pd₂Se₃ 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 Pd₂Se₃ 2D monolayers.