Overcoming the thermodynamic barrier of alloying 2D transition metal dichalcogenides

Abstract

Alloying of two-dimensional (2D) transition metal dichalcogenides (TMDs) enables modulation of their energy structure and bandgap, and thus, is crucially important for their integration into optical and electrooptical devices. However, achieving high yield and efficient alloying is often challenging due to the thermodynamic preference of one of the components over the other. Here, we develop a process for growing Mo1−xWxS2 alloys. While common chemical vapor deposition (CVD)-based growing techniques give limited alloying due to the thermodynamic dominance of MoS2 over WS2, we present here an efficient means to achieve high-yield alloying via a two-step CVD process. In this process, first, we grew monolayer WS2 flakes on a substrate, followed by the nucleation-diffusion second step. In this step, MoS2 nucleates mostly on the edges of the WS2, followed by diffusion of the Mo atoms into the flakes to achieve Mo1−xWxS2 alloys. Indeed, we obtained a large amount of alloy growth with different atomic compositions (i.e., various values of x). We employed several analysis methods, including Raman, photoluminescence, x-ray photoelectron spectroscopy, and electron dispersive spectroscopy, to characterize the properties of the grown alloys. These analyses demonstrate the transition from WS2- (x→1) to MoS2-like (x→0) behaviors, through alloy behavior (0<x<1), which combines the properties of both materials and presents a homogenous distribution of the diffused atoms. We also showed that controlling the diffusion time can efficiently determine the average atomic composition of the alloys. This work, therefore, overcomes the inherent thermodynamic barrier that limits the large-scale synthesis of TMD alloys, and thus, it paves the path toward their integration into advanced optoelectronic tunable bandgap applications.