▎ 摘　　要

The transition-metal dichalcogenide (TMDC) in the family of MX2 (M = Mo, W; X = S, Se) and the graphene (Gr) monolayer are an atomically thin semiconductor and a semimetal, respectively. The monolayer MX2 has been discovered as a new class of semiconductors for electronics and optoelectronics applications. Because of the hexagonal lattice structure of both materials, MX2 and Gr are often combined with each other to generate van der Waals heterostructures. Here, the MX2/Gr heterostructures are investigated theoretically based on density functional theory (DFT). The electronic structure and the optical properties of four different MX2/Gr heterostructures are computed. We systematically compare these MX2/Gr heterostructures for their complex permittivity, absorption coefficient, reflectivity and refractive index.