Huge excitonic effects in layered hexagonal boron nitride

Area of Science:

• Materials Science
• Condensed Matter Physics
• Computational Chemistry

Background:

• Hexagonal boron nitride (h-BN) is a layered material with unique electronic and optical properties.
• Previous studies have debated the nature of its energy band gap and excitonic behavior.
• Accurate theoretical modeling is crucial for understanding h-BN's potential applications.

Purpose of the Study:

• To accurately determine the energy band gap nature of bulk hexagonal boron nitride.
• To compute and analyze the optical spectrum and excitonic properties of h-BN.
• To provide a theoretical basis for experimental observations and clarify discrepancies in the literature.

Main Methods:

• All-electron GW approximation for calculating the energy band gap.
• Bethe-Salpeter equation for solving the electron-hole two-particle Green function.
• Analysis of excitonic structures and binding energies.

Main Results:

• The energy band gap of bulk h-BN is confirmed to be indirect.
• Computed in-plane polarized optical spectrum shows excellent agreement with experimental data.
• A strong anisotropy is observed between in-plane and out-of-plane polarized spectra.
• Low-lying excitons are identified as tightly confined, Frenkel-class excitons within layers.
• Calculated exciton binding energy is significantly larger than previously reported values.

Conclusions:

• The indirect nature of the h-BN band gap is established.
• The theoretical model accurately reproduces experimental optical spectra, highlighting strong anisotropy.
• Frenkel-type excitons confined within layers dominate the optical properties.
• The findings challenge previous models that assumed a direct band gap for h-BN.