Equal-Bilayer MoSe₂ Grown by a Nucleation-Etching Strategy with High Carrier Mobility

Bilayer transition metal chalcogenides (TMDs) have gradually attracted a great deal of attention due to the higher density of states and carrier mobility than monolayer TMDs. Controlling the uniformity of the layer number is very crucial because it will intensively influence the physical properties. However, it is difficult to synthesize equal-bilayer (EB) TMDs with two identical layers via a normal layer-by-layer strategy. Most reported bilayer TMDs are not uniform and such unequal bilayers would introduce a sizable Schottky barrier, resulting in the low carrier mobility. Here, a nucleation-etching strategy is proposed to grow EB-MoSe₂ by chemical vapor deposition (CVD), which breaks the limitations of normal layer-by-layer strategy. The second layer is preferentially formed beneath the first layer rather than above, and a different etching phenomenon is also observed, which occurs more preferentially at the overlapping grain boundary sites on the top layer. The obtained EB-MoSe₂ flakes are 3R-stack with high crystal quality. Furthermore, the contact between EB-MoSe₂ and metal electrodes is greatly improved, thereby EB-MoSe₂ transistors exhibit an order of magnitude higher carrier mobility (104 cm² V^-1 s^-1) than that of UEB-MoSe₂ transistors (12 cm² V^-1 s^-1). This value is also at a relatively high level compared with reported results. Our work offers a feasible strategy for the synthesis of EB-TMDs with high carrier mobility, which is meaningful for developing high-performance 2D optoelectronic devices.