  Bilayer two-dimensional (2D) van der Waals (vdW) materials are attracting
increasing attention due to their predicted high quality electronic and optical
properties. Here we demonstrate dense, selective growth of WSe2 bilayer flakes
by chemical vapor deposition with the use of a 1:10 molar mixture of sodium
cholate and sodium chloride as the growth promoter to control the local
diffusion of W-containing species. A large fraction of the bilayer WSe2 flakes
showed a 0 and 60o twist between the two layers, while moire 15 and 30o-twist
angles were also observed. Well-defined monolayer-bilayer junctions were formed
in the as-grown bilayer WSe2 flakes, and these interfaces exhibited p-n diode
rectification and an ambipolar transport characteristic. This work provides an
efficient method for the layer-controlled growth of 2D materials, in
particular, 2D transition metal dichalcogenides and promotes their applications
in next-generation electronic and optoelectronic devices.
