Defect Passivation and Photoluminescence Enhancement of Monolayer MoS₂ Crystals through Sodium Halide-Assisted Chemical Vapor Deposition Growth

Recent success in the chemical vapor deposition (CVD) growth of atomically thin transition metal dichalcogenide (TMD) crystals opens up prospects for exploiting these materials in nanoelectronic and optoelectronic devices. However, CVD-grown TMDs often suffer from weak crystal quality because of the formation of defects during the growth, which makes a large impact on their electrical and optical properties. Here, we report a facile synthesis of high-quality MoS2 monolayers through a sodium halide-assisted CVD method. Our results show that the addition of sodium halides into MoO3 precursors leads to the rapid growth of highly crystalline MoS2 monolayers. Moreover, the overall photoluminescence (PL) intensity of MoS2 monolayers can be greatly enhanced by up to 2 orders of magnitude. The PL enhancement originates from that the deep trap states induced by sulfur vacancies are passivated by the substitution doping of halogen atoms, which promotes the emission of excitons and trions. Density functional theory calculations indicate that the band gaps of halogen-doped MoS2 monolayers are slightly smaller than those of pristine MoS2 monolayer, which is responsible for the small red shift of PL peaks (∼30 meV). These findings provide a new route toward engineering electronic and optical properties of MoS2 and other TMD monolayers.