Excitons in few-layer hexagonal boron nitride: Davydov splitting and surface localization

Abstract

Hexagonal boron nitride (hBN) has been attracting great attention because of its strong excitonic effects. Taking into account few-layer systems, we investigate theoretically the effects of the number of layers on quasiparticle energies, absorption spectra, and excitonic states, placing particular focus on the Davydov splitting of the lowest bound excitons. We describe how the inter-layer interaction as well as the variation in electronic screening as a function of layer number N affects the electronic and optical properties. Using both ab initio simulations and a tight-binding model for an effective Hamiltonian describing the excitons, we characterize in detail the symmetry of the excitonic wavefunctions and the selection rules for their coupling to incoming light. We show that for N > 2, one can distinguish between surface excitons that are mostly localized on the outer layers and inner excitons, leading to an asymmetry in the energy separation between split excitonic states. In particular, the bound surface excitons lie lower in energy than their inner counterparts. Additionally, this enables us to show how the layer thickness affects the shape of the absorption spectrum.

Giant exciton-phonon coupling and zero-point renormalization in hexagonal monolayer boron nitride

“…This is in stark difference to the case with excitons at q→0 in bulk-hBN. At such condition, the doubly degenerate dark exciton house themselves in lower energy compared to the doubly degenerate bright excitons [10,12]. This degeneracy between bright and dark is removed at finite q where the later possess a very small oscillator strength [11].…”

Exciton-Phonon Coupling in the Ultraviolet Absorption and Emission Spectra of Bulk Hexagonal Boron Nitride

“…This procedure is used to correct the usual quasiparticle gap underestimation given by the G 0 W 0 approximation [25] in BN systems. [26] We obtain a direct quasiparticle gap of 6.46 eV, and an indirect one of 5.96 eV ( Fig. 1a).…”

“…In the following we will approximate the momentum difference of the indirect band gap as q = |K − M|. A discussion on this approximation with respect to the exact positions T 1 and T 2 [27] [28], we obtain two doubly degenerate excitonic states at q = 0 which form a Davydov pair [29,30] due to interlayer interaction [26]: The first exciton at 5.70 eV has E 2g symmetry and is thus dark. The second exciton at 5.75 eV (gray vertical line in Fig.…”

“…(More on the group theory can be found in section VI of the SI and in Ref. 26. Note that we are only dealing with light polarized parallel to the BN planes which dominates the optical spectra.…”

Two-photon absorption in two-dimensional materials: The case of hexagonal boron nitride

“…[28][29][30] ) on the electronic and optical properties of both the bulk and the single layer. These studies, supported by theoretical investigations, [31][32][33][34][35][36] have shown that h-BN is an indirect band gap insulator whose optical absorption spectrum is dominated by strongly bound excitons. Then, the electronic structures of the h-BN monolayer and of the bulk structure are fairly well known-which is convenient for the tight-binding model.…”