Effects of Opposite Atoms on Electronic Structure and Optical Absorption of Two‐Dimensional Hexagonal Boron Nitride

The first‐principles many‐body GW and Bethe–Salpeter equation (BSE) calculations on the 2D hexagonal boron nitride (2D‐hBN) to explore the effects of opposite atoms on the electronic structure and linear one‐photon absorption (OPA) are performed. Five AA‐ and AB‐stacked bilayer and eight AAB‐stacked trilayer structures are considered. The AAB‐stacked trilayer hBN (TL‐BN) structures are constructed by mixing the AA‐ and AB‐stacked bilayer hBN (BL‐BN). It is shown that the GW approximation gives rise to different types (i.e., indirect or direct) of fundamental bandgaps from the independent particle approximation for all structures except those dominated by the B–B opposite. The stacking modes dominated by the B–B opposite have a direct fundamental bandgap in both approximations. The OPA spectra are calculated by solving the BSE combined with the GW quasi‐particle correction. The binding energy and Davydov splitting of excitons of TL‐BN strongly depend on the opposite atoms and are related to the role of the stacking BL‐BN substructure. Finally, taking the six‐layer and below AB‐stacked structures as examples, it is shown that the B–B opposite unit is helpful in constructing the turbostratic‐phase‐like stacking structures with a direct fundamental bandgap which are more suitable for optoelectronic applications. Using the advanced GW+BSE calculations, a new structure–property relationship in 2D hexagonal boron nitride (2D‐hBN) is shown and that a consecutive B–B opposite unit is essential for 2D‐hBN to form a direct bandgap is demonstrated.