Bandgap engineering of few-layered MoS with low concentrations of S vacancies.

Band-gap engineering of molybdenum disulfide (MoS) by introducing vacancies is of particular interest owing to the potential optoelectronic applications. In this work, systematic density functional theory (DFT) calculations were carried out for few-layered 3R-MoS with different concentrations of S vacancies. All results revealed that the defect energy levels introduced on both sides of the Fermi level formed an intermediate band in the band gap. Both the edges of the intrinsic and intermediate bands of the structures with the same type of vacancies were generally closer to the Fermi level, and the gaps decreased as the number of layers increased from 2 to 4. The preferentially formed S vacancies at the top layer and the transition of defect types from point to line led to similar indirect band gaps for 2- and 4-layered 3R-MoS with a low bulk concentration (around 5%) of S vacancies. This is different from most reported results about transition metal dichalcogenide (TMD) materials that the indirect band gap decreases as the number of layers increases and the low concentrations of vacancies show negligible influence on the band gap value.