Equal-Bilayer\nMoSe<sub>2</sub> Grown by a Nucleation-Etching\nStrategy with High Carrier Mobility

Bilayer\ntransition metal chalcogenides (TMDs) have gradually attracted\na great deal of attention due to the higher density of states and\ncarrier mobility than monolayer TMDs. Controlling the uniformity of\nthe layer number is very crucial because it will intensively influence\nthe physical properties. However, it is difficult to synthesize equal-bilayer\n(EB) TMDs with two identical layers via a normal layer-by-layer strategy.\nMost reported bilayer TMDs are not uniform and such unequal bilayers\nwould introduce a sizable Schottky barrier, resulting in the low carrier\nmobility. Here, a nucleation-etching strategy is proposed to grow\nEB-MoSe<sub>2</sub> by chemical vapor deposition (CVD), which breaks\nthe limitations of normal layer-by-layer strategy. The second layer\nis preferentially formed beneath the first layer rather than above,\nand a different etching phenomenon is also observed, which occurs\nmore preferentially at the overlapping grain boundary sites on the\ntop layer. The obtained EB-MoSe<sub>2</sub> flakes are 3R-stack with\nhigh crystal quality. Furthermore, the contact between EB-MoSe<sub>2</sub> and metal electrodes is greatly improved, thereby EB-MoSe<sub>2</sub> transistors exhibit an order of magnitude higher carrier\nmobility (104 cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup>) than that of UEB-MoSe<sub>2</sub> transistors (12 cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup>). This value is also\nat a relatively high level compared with reported results. Our work\noffers a feasible strategy for the synthesis of EB-TMDs with high\ncarrier mobility, which is meaningful for developing high-performance\n2D optoelectronic devices.