Controllable, Wide-Ranging n-Doping and p-Doping of Monolayer Group 6 Transition-Metal Disulfides and Diselenides

Developing processes to controllably dope transition-metal dichalcogenides (TMDs) is critical for optical and electrical applications. Here, molecular reductants and oxidants are introduced onto monolayer TMDs, specifically MoS2, WS2, MoSe2, and WSe2. Doping is achieved by exposing the TMD surface to solutions of pentamethylrhodocene dimer as the reductant (n-dopant) and "Magic Blue," [N(C6H4-p-Br)(3)]SbCl6, as the oxidant (p-dopant). Current-voltage characteristics of field-effect transistors show that, regardless of their initial transport behavior, all four TMDs can be used in either p- or n-channel devices when appropriately doped. The extent of doping can be controlled by varying the concentration of dopant solutions and treatment time, and, in some cases, both nondegenerate and degenerate regimes are accessible. For all four TMD materials, the photoluminescence intensity; for all four materials the PL intensity is enhanced with p-doping but reduced with n-doping. Raman and X-ray photoelectron spectroscopy (XPS) also provide insight into the underlying physical mechanism by which the molecular dopants react with the monolayer. Estimates of changes of carrier density from electrical, PL, and XPS results are compared. Overall a simple and effective route to tailor the electrical and optical properties of TMDs is demonstrated.