Layer dependence of excitonic absorption peaks in few layer hexagonal BN*

The dependence of optical spectra on the number of layers in 2D materials is crucial for their characterization. While Raman spectra can be intuitively interpreted by the splitting of phonon modes in few-layer systems, much less is known about the splitting of excitonic peaks in absorption spectra.

We discuss the 2D prototype system of hexagonal BN (hBN), a wide bandgap insulator with strong excitonic effects and UV luminescence properties. We investigate the optical properties of few layer hBN, focusing on the Davydov splitting of the lowest bound exciton, which dominates the absorption. We use ab initio methods and a model mapping the Bethe-Salpeter equation onto an effective tight-binding Hamiltonian with few parameters. We thus obtain a precise characterization of the symmetries of the excitonic states and their optical selection rules.

We discuss the splitting of excitonic states in systems of N layers, and construct a simple model for the absorption spectrum as a function of N. We show that for N>2, one can distinguish between surface and inner excitons. The former are found to be lower in energy and can be optically active.