Realizing Indirect-to-Direct Band Gap Transition in Few-Layer Two-Dimensional MX₂ (M = Mo, W; X = S, Se)

In the applications of two-dimensional (2D) transition metal dichalcogenides (TMDs) for solar cell and optoelectronic devices, two challenging issues remain: (1) the direct-to-indirect band gap transition from single layer to a few layers and (2) the absence of an effective and robust doping procedure. In this study, we explore the feasibility to realize indirect-to-direct band gap transition and control the Fermi level by intercalating few-layer TMDs with embedded metals. Specifically, utilizing density functional theory calculations, we examine the electronic properties of few-layer MX2 (M = Mo, W; X = S, Se) intercalated with metals (Zn, Sn, Mg and Ga). Our calculation results reveal that (1) Ga intercalation can realize an indirect-to-direct band gap transition in few-layer TMDs, and as a result, the absorption efficiency is increased by two orders compared with that of pristine MX2; and (2) intercalated Ga acts as an n-type shallow donor, which markedly increases the charge density and electrical conductivity. Therefore, Ga intercalation may provide a potential practical route for manipulating few-layer TMDs for high performance solar and optoelectronic devices.