Study of the layer-dependent properties of MoS2 nanosheets with different crystal structures by DFT calculations

As a typical representative of layered transition metal dichalcogenides, MoS2 nanosheets have been widely studied in both experimental and theoretical works, and have a wide range of applications in the fields of nanotechnology and microelectronics. Based on hybrid functional within density functional theory calculations, the microstructure, electronic structures, and optical properties of MoS2 nanosheets with different crystal phases are investigated. Based on the analysis of phonon dispersion, it can be determined that the 2H-MoS2 monolayer is thermodynamically stable. In the case of 2H-MoS2 nanosheets, the band gap monotonically decreases from 2.219 to 1.441 eV by the exponential form, as the layer number increases from 1 to infinite. Moreover, the 2H-MoS2 monolayer is a direct band gap semiconductor. On the other hand, the 3R-MoS2 monolayer still exhibits metallic characteristics, while the 1T'-MoS2 monolayer has a very narrow band gap. The main features of the electronic structure of MoS2 nanosheets are contributed by the intra-layer interaction, and the inter-layer interaction only induces slight perturbation. But the latter has an important influence on the electronic structure of MoS2 ultrathin nanosheets, especially the monolayer. Furthermore, some fitting equations about optical properties are also provided, which not only contribute to better understanding of the variation of the electronic structure with respect to change in the layer number, but also provide a convenient method to determine the layer number of MoS2 nanosheets in practice. These results indicate that MoS2 nanosheets may serve as promising candidates for photoelectric applications.