  Layered transition metal dichalcogenides (TMDs) offer many attractive
features for next-generation low-dimensional device geometries. Due to the
practical and fabrication challenges related to in situ methods, the atomistic
dynamics that give rise to realizable macroscopic device properties are often
unclear. In this study, in situ transmission electron microscopy techniques are
utilized in order to understand the structural dynamics at play, especially at
interfaces and defects, in the prototypical film of monolayer MoS2 under
electrical bias. Through our sample fabrication process, we clearly identify
the presence of mass transport in the presence of a lateral electric field. In
particular, we observe that the voids present at grain boundaries combine to
induce structural deformation. The electric field mediates a net vacancy flux
from the grain boundary interior to the exposed surface edge sites that leaves
molybdenum clusters in its wake. Following the initial biasing cycles, however,
the mass flow is largely diminished, and the resultant structure remains stable
over repeated biasing. We believe insights from this work can help explain
observations of non-uniform heating and preferential oxidation at grain
boundary sites in these materials.
