Monolayer to Bulk Properties of Hexagonal Boron Nitride

Hexagonal boron nitride (h-BN) is widely used in two-dimensional electronics and serves as a host for single-photon emitters. We study the electronic structure of h-BN as a function of the number of layers and take into account different stacking configurations. Using first-principles calculations based on a hybrid functional, we find that the band gap of a single monolayer is direct, while for thicknesses above a monolayer, the band gap is indirect. By examining the positions of the band edges with respect to the vacuum level, we find this direct-to-indirect transition to be driven by a shift in the conduction-band minimum at the M point; this shift changes the band gap by 0.5 eV, going from a single monolayer to bulk. We analyze these results in terms of the orbital composition of the band edges at different high-symmetry points in the Brillouin zone.