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Two-dimensional (2D) layered tungsten diselenides (WSe2) material hasrecently drawn a lot of attention due to its unique optoelectronic propertiesand ambipolar transport behavior. However, direct chemical vapor deposition(CVD) synthesis of 2D WSe2 is not as straightforward as other 2D materials dueto the low reactivity between reactants in WSe2 synthesis. In addition, thegrowth mechanism of WSe2 in such CVD process remains unclear. Here we reportthe observation of a screw-dislocation-driven (SDD) spiral growth of 2D WSe2flakes and pyramid-like structures using a sulfur-assisted CVD method.Few-layer and pyramid-like WSe2 flakes instead of monolayer were synthesized byintroducing a small amount of sulfur as a reducer to help the selenization ofWO3, which is the precursor of tungsten. Clear observations of steps, helicalfringes, and herring-bone contours under atomic force microscopecharacterization reveal the existence of screw dislocations in the as-grownWSe2. The generation and propagation mechanisms of screw dislocations duringthe growth of WSe2 were discussed. Back-gated field-effect transistors weremade on these 2D WSe2 materials, which show on/off current ratios of 106 andmobility up to 44 cm2/Vs.

Screw-Dislocation-Driven Growth of Two-Dimensional Few-Layer and Pyramid-Like WSe2 by Sulfur-Assisted Chemical Vapor Deposition