  Two-dimensional (2D) layered tungsten diselenides (WSe2) material has
recently drawn a lot of attention due to its unique optoelectronic properties
and ambipolar transport behavior. However, direct chemical vapor deposition
(CVD) synthesis of 2D WSe2 is not as straightforward as other 2D materials due
to the low reactivity between reactants in WSe2 synthesis. In addition, the
growth mechanism of WSe2 in such CVD process remains unclear. Here we report
the observation of a screw-dislocation-driven (SDD) spiral growth of 2D WSe2
flakes and pyramid-like structures using a sulfur-assisted CVD method.
Few-layer and pyramid-like WSe2 flakes instead of monolayer were synthesized by
introducing a small amount of sulfur as a reducer to help the selenization of
WO3, which is the precursor of tungsten. Clear observations of steps, helical
fringes, and herring-bone contours under atomic force microscope
characterization reveal the existence of screw dislocations in the as-grown
WSe2. The generation and propagation mechanisms of screw dislocations during
the growth of WSe2 were discussed. Back-gated field-effect transistors were
made on these 2D WSe2 materials, which show on/off current ratios of 106 and
mobility up to 44 cm2/Vs.
