Proton Beam Modifications on 2D TMDs—Optically Active Chalcogen Vacancies and Robust N-type Doping

Defect engineering of atomically thin semiconducting crystals is an attractive route to developing single photon sources and valleytronic devices. For these applications, defects with well-defined optical characteristics need to be generated in a precisely controlled manner. However, defect-induced optical features are often complicated by the presence of multiple defect species, hindering identification of their structural origin. Here, we report systematic generation of optically active atomic defects in monolayer MoS₂, WS₂, MoSe₂, and WSe₂ <i>via</i> proton beam irradiation. Defect-induced emissions are found to occur 100~200 meV below the neutral exciton peak, showing typical characteristics of localized excitons such as saturation at high excitation rate and long lifetime. Using scanning transmission electron microscopy, we show that freshly created chalcogen vacancies are responsible for the localized exciton emission. Density functional theory and GW-BSE calculations reveal that the observed emission can be attributed to transitions involving defect levels of chalcogen vacancy and the valence band edge state.<br/>Besides the optically active property, the defects generated by proton irradiation also induce a robust n-type doping effect in 2D TMDs. For example, we demonstrated a reliable and long-time air stable n-type doping scheme with WSe₂ by proton irradiation. The irradiated WSe₂ remains an n-type semiconductor even after it is exposed to ambient conditions for a year. Localized ion irradiation with a focused beam can directly pattern on the sample to make high performance homogenous p-n junction diodes.<br/>Controllable proton irradiation provides both optically active and N-type doping effects, which is also compatible with current CMOS processes, thus would be an ideal platform for future complementary high-performance electronics and optoelectronics applications.