A method to restore the intrinsic dielectric functions of 2D materials in periodic calculations.

Previous calculations of the dielectric and optical properties of 2D materials often overlooked or circumvented the influence of vacuum spacing introduced in periodic calculations, which gave rise to mispredictions of the intrinsic properties of 2D materials or merely qualitative results. We first elucidated the relationship between the vacuum spacing and the dielectric and optical properties of 2D materials in periodic calculations, and then formulated an effective method to accurately predict the dielectric and optical properties of 2D materials by restoring the intrinsic dielectric functions of 2D materials independent of the additional vacuum spacing. As examples, the intrinsic dielectric and optical properties of ultrathin hexagonal boron nitride (h-BN) and molybdenum sulphide (MoS) from a monolayer to a pentalayer, including dielectric functions, optical absorption coefficients, refraction indexes, reflectivities, extinction coefficients, and energy loss functions, have been calculated by our method. Our calculations reveal that the out-of-plane optical dielectric constants, static refraction indexes, and static reflectivities of 2D h-BN and MoS increase as the number of layers increases, while the in-plane counterparts remain unchanged. The excitonic frequency-dependent optical properties of h-BN and MoS from a monolayer to bulk are also calculated by solving the Bethe-Salpeter equation and they show strong anisotropy. The present method shows better agreement with the experimental results compared to previous calculations and demonstrates enormous potential to investigate the dielectric and optical properties of other 2D materials extensively and quantitatively.