Origin of Proton-Beam-Induced Subgap Emission in MoSe₂ Monolayers

Deterministic creation of quantum emitters in 2D materials is an important challenge in exploring their potential for quantum photonic devices. One of the promising approaches exploits ion beam irradiation for controlled defect engineering. However, the types of defects generated and their corresponding optical properties are not well understood. In this study, we investigate the electronic properties of defect states in monolayer MoSe₂ created by proton beam irradiation. Proton beam irradiation results in emergence of narrow subgap emission peaks, which are spectrally and thermally stable (up to 160 K). We probe the electronic origin of these peaks by studying the effect of electrostatic doping and their valley selectivity. We find that these peaks exhibit modest degree of circular dichroism, which suggests that the transitions involve valley-polarized holes, in accordance with the recent theoretical predictions for selenium-vacancy-induced optical transitions in MoSe₂. Our scanning transmission electron microscopy (STEM) imaging reveals that proton beam irradiation results in increase in selenium vacancies but no changes in metal vacancy concentration, further corroborating the role of unpassivated selenium vacancies in the observed emission.