Two-Dimensional Layered Gallium Selenide: Preparation, Properties, and Applications

Gallium selenide (GaSe) is a layered III-VI semiconductor. It consists of covalently bonded stacks of four atomic layers in the sequence of Se-Ga-Ga-Se to form the tetra layers that are held together by a weak interaction of the van der Waals (vdW) force. The layered structure suggests the possibility of existence of two-dimensional (2D) GaSe like its pioneer graphene. Besides micro-mechanical exfoliation, 2D GaSe sheets can be obtained by various methods of vapor-phase mass transport, vdW epitaxy, molecular beam epitaxy, and pulse laser deposition. The fabricated 2D GaSe flakes have a tunable indirect band gap which is little lower than their direct counterpart. For monolayer, the experimental value of mobility is about 0.6 cm2 V-1 s-1 according to the transport properties of fieldeffect transistors (FETs). As expected, the 2D GaSe flakes exhibit layer-dependent nonlinear optical properties. The fabricated GaSe layers can enable the design of electronic and optoelectronic devices to realize functional applications of FETs and photodetectors. In this chapter, we focus on the scientific progress of 2D layered GaSe crystals to date, including various synthesis methods, characterization techniques, and electrical and optical properties as well as electronic and optoelectronic applications.