Controllable Doping Characteristics for WS_xSe_y Monolayers Based on the Tunable S/Se Ratio

Transition metal dichalcogenides (TMDs) have attracted much attention because of their unique characteristics and potential applications in electronic devices. Recent reports have successfully demonstrated the growth of 2-dimensional MoS_xSe_y, Mo_xW_yS₂, Mo_xW_ySe₂, and WS_xSe_y monolayers that exhibit tunable band gap energies. However, few works have examined the doping behavior of those 2D monolayers. This study synthesizes WS_xSe_y monolayers using the CVD process, in which different heating temperatures are applied to sulfur powders to control the ratio of S to Se in WS_xSe_y. Increasing the Se component in WS_xSe_y monolayers produced an apparent electronic state transformation from p-type to n-type, recorded through energy band diagrams. Simultaneously, p-type characteristics gradually became clear as the S component was enhanced in WS_xSe_y monolayers. In addition, Raman spectra showed a red shift of the WS₂-related peaks, indicating n-doping behavior in the WS_xSe_y monolayers. In contrast, with the increase of the sulfur component, the blue shift of the WSe₂-related peaks in the Raman spectra involved the p-doping behavior of WS_xSe_y monolayers. In addition, the optical band gap of the as-grown WS_xSe_y monolayers from 1.97 eV to 1.61 eV is precisely tunable via the different chalcogenide heating temperatures. The results regarding the doping characteristics of WS_xSe_y monolayers provide more options in electronic and optical design.