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 Mo1-x W x S2, Mo1-x W x Se2 and MoS2x Se2(1-x), which are accurately controllable in the elemental proportion (x 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 Mo1-x W x S2, 1.56 to 1.65 eV for Mo1-x W x Se2, 1.56 to 1.86 eV for MoS2x Se2(1-x). Electrical transport measurements indicate that Mo1-x W x S2 and MoS2x Se2(1-x) monolayers show n-type semiconductor behaviors, and the carrier types of Mo1-x W x Se2 can be tuned as n-type, bipolar and p-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.