Intrinsically Substitutional Carbon Doping in CVD-Grown Monolayer MoS₂ and the Band Structure Modulation

Heterogeneous atom doping has been proven as an efficient route to tune the physical and chemical properties of semiconductors, represented by the technically mature boron and phosphorus doping in bulk silicon. In the two-dimensional (2D) transitional-metal dichalcogenides semiconductors, substitutional doping dominates compared with the interstitial sites doping due to the ultrathin nature of 2D materials. However, unintentional doping can also obscure the structure–property relationship and cause the deviations from the ideally optical/electrical performances. Here, substitutional carbon doping into the monolayer molybdenum disulfide (MoS₂) lattice during the normal chemical vapor deposition (CVD) synthesis process is discovered through a thorough analysis of the intermediate and final reaction products. The carbon originates from the relatively low-purity molybdenum precursor, which can be completely eliminated when a high-purity molybdenum precursor is utilized. The carbon-doped monolayer MoS₂ exhibits mid-gap states brought by the Mo-d, S-p, and C-p orbital hybridization and gradually reduced band gaps as the doping concentration increases. As a result, the suppressed photoluminescence (PL) intensity and red shift PL position are observed. The finding is fundamental for understanding the unintentional carbon doping process in CVD growth of 2D semiconductors and identifies a source for the inconsistent PL performances in the CVD-derived samples.