Defect Passivation and Photoluminescence Enhancement of Monolayer MoS 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 MoS monolayers through a sodium halide-assisted CVD method. Our results show that the addition of sodium halides into MoO precursors leads to the rapid growth of highly crystalline MoS monolayers. Moreover, the overall photoluminescence (PL) intensity of MoS 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 MoS monolayers are slightly smaller than those of pristine MoS 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 MoS and other TMD monolayers.