  Two-dimensional (2D) transition metal dichalcogenides MX2 (M = Mo, W, X = S,
Se, Te) attracts enormous research interests in recent years. Its 2H phase
possesses an indirect to direct bandgap transition in 2D limit, and thus shows
great application potentials in optoelectronic devices [1]. The 1T' crystalline
phase transition can drive the monolayer MX2 to be a 2D topological insulator.
Here we realized the molecular beam epitaxial (MBE) growth of both the 1T' and
2H phase monolayer WSe2 on bilayer graphene (BLG) substrate. The crystalline
structures of these two phases were characterized using scanning tunneling
microscopy. The monolayer 1T'-WSe2 was found to be metastable, and can
transform into 2H phase under post-annealing procedure. The phase transition
temperature of 1T'-WSe2 grown on BLG is lower than that of 1T' phase grown on
2H-WSe2 layers. This thermo-driven crystalline phase transition makes the
monolayer WSe2 to be an ideal platform for the controlling of topological phase
transitions in 2D materials family.
