Lithium halide monolayer sheets: First-principles many-body calculations

Abstract

The geometric, electronic and optical properties of two-dimensional lithium halide (LiF, LiCl, LiBr) monolayers are systematically explored by using ab initio density functional theory, the HSE06 functional, the GWA method and Bethe–Salpeter equation calculations. The stabilities of these structures have been further evaluated by cohesive energy, phonon spectra, mechanical stability analysis and ab initio molecular dynamics. The structural results exhibit that the buckled configurations of these monolayer systems are stable; therefore, it is possible to synthesize these structures in experiments. The electronic properties were analyzed by HSE06 functional and quasi-particle many-body perturbation theory (MBPT) via GW approach. It is found that all these nanostructures are indirect band gap insulators. The HSE06 and G0W0 gave remarkably larger band gaps than the PBE functional. The optical properties and excitonic effects of these materials are investigated in independent-particle, independent-quasiparticle and including excitonic effects (BSE). The formation of first exciton peaks at 8.48, 7.7, and 6.92 eV with large binding energy of 2.92, 2.13 and 1.70 eV was observed for LiF, LiCl and LiBr monolayers, respectively. The strong Coulomb interaction between electrons and holes in these compounds leads to the large binding energy and small exciton Bohr radius. The interesting stability, electronic and optical properties of these monolayers are potentially useful for future electronic and optoelectronic device design such as VUV transmitter and X-ray monochromator plates.