  Herein, we report the reversible structural transition of single-layer VSe2
grown on Au(111) through alternating thermal annealing and Se replenishment.
Using scanning tunneling microscopy (STM) and angle-resolved photoemission
spectroscopy (ARPES), we demonstrate the epitaxial growth of high-quality VSe2
on Au(111) with the octahedral (1T) structure and the Se-vacancy-induced
transformation of VSe2 from the metallic moir\'e (1T) phase to the
semiconducting (2H) phase. With convincing agreement between the experimental
results and DFT calculations, the nanostructure near the grain boundary in the
defective intermediate phase is confirmed, as well as the reaction pathway with
Se gradually depleting at elevated temperatures. Importantly, it is revealed
that the density of the linear Se defects plays a crucial role in the formation
of the 2H domain phase due to the increment of the in-plane lattice parameter
after Se desorption and the better thermal stability of the 2H phase compared
to the 1T phase. The proper control of the density of Se atoms in the topmost
Se layer of VSe2 could feasibly manipulate the ratio between the 1T phase and
the 2H phase in the steak-shaped domain, which is regarded as a good platform
for 2D homojunctions in nanoelectronics.
