Controllable growth of wafer-scale monolayer transition metal dichalcogenides ternary alloys with tunable band gap.

The tuning of band gap is very important for the application of two-dimensional (2D) materials in optoelectronic devices. Alloying of 2D transition metal dichalcogenides (TMDCs) is an important way to tune the wide band gap. In this study, we report a multi-step vapor deposition method to grow monolayer TMDC ternary alloy films with wafer scale, including MoWS, MoWSeand MoSSe, which are accurately controllable in the elemental proportion (is from 0 to 1). The band gap of the three 2D ternary alloy materials are continuously tuned for the whole range of metal and chalcogen compositions. The metal compositions are controlled by the as-deposited thickness. Raman, photoluminescence, elemental maps and TEM show the high spatial homogeneity in the compositions and optical properties across the whole wafer. The band gap can be continuously tuned from 1.86 to 1.99 eV for MoWS, 1.56 to 1.65 eV for MoWSe, 1.56 to 1.86 eV for MoSSe. Electrical transport measurements indicate that MoWSand MoSSemonolayers show-type semiconductor behaviors, and the carrier types of MoWSecan be tuned as-type, bipolar and-type. Moreover, this control process can be easily generalized to other 2D alloy films, even to quaternary or multi-element alloy materials. Our study presents a promising route for the preparation of large-scale homogeneous monolayer TMDC alloys and the application for future functional devices.