Artificial Stacking Dependences of Band Structures and Second-Harmonic Generations in Bilayer Hexagonal Boron Nitride

Abstract

Stacking order critically influences the optoelectronic properties of 2D van der Waals materials. Here, first-principles calculations were performed to study the geometries, band structures, and second-harmonic generation (SHG) of hexagonal boron nitride (h-BN) bilayers constructed by the relative shifts and rotations between h-BN layers. Our results indicate that the stability, interlayer coupling, and band structures of h-BN bilayers are sensitive to the stacking orders. For interlayer sliding, the direction and size of lateral displacement obviously affect the band gap and components at the band edge. By contrast, the band structure of twisted h-BN bilayers is highly angle-dependent, and when the sum of twist angles in two moiré superlattices is 60°, they have similar band structures due to identical band folding. As for the second-order susceptibility, interlayer sliding tends to enhance the SHG intensity compared to that of the original AA stacking. When the incident angle of the pump light deviates from the normal direction of the h-BN bilayer, the change in lattice symmetry induced by interlayer sliding results in distinct variations in SHG patterns, thereby facilitating identification of the corresponding structures through polarization-resolved SHG spectroscopy. For twisted configurations, as the rotation angle increases from 0 to 60°, the evolution of SHG intensity departs significantly from the coherent superposition model due to the strong exciton effects in h-BN bilayers. Although the interlayer rotation preserves the SHG polarization image, the experimental measurement of relative SHG intensity enables the determination of the rotation angle, which allows for distinguishing structures of twisted h-BN bilayers.