  To ensure the practical application of atomically thin transition metal
dichalcogenides, it is essential to characterize their structural stability
under external stimuli such as electric fields and currents. Using vacancy
monolayer islands on TiSe2 surfaces as a model system, for the first time we
have observed a shape evolution and growth from triangular to hexagonal driven
by scanning tunneling microscopy (STM) electrical stressing. The size of
islands shows linear growth with a rate of (3.00 +- 0.05) x 10-3 nm/s, when the
STM scanning parameters are held fixed at Vs = 1.0 V and I = 1.8 nA. We further
quantified how the growth rate is related to the tunneling current magnitude.
Our simulations of monolayer island evolution using phase-field modeling are in
good agreement with our experimental observations, and point towards
preferential edge atom dissociation under STM scanning driving the observed
growth. The results could be potentially important for device applications of
ultrathin transition metal dichalcogenides and related 2D materials subject to
electrical stressing under device operating conditions.
