Strongly bound Mott-Wannier Excitons in GeS and GeSe monolayers

TL;DR

This study predicts strongly bound excitons in monolayer GeS and GeSe using ab initio calculations, revealing their energy levels, binding energies, and optical properties relevant for optoelectronic applications.

Contribution

It provides the first detailed ab initio analysis of excitonic spectra in GeS and GeSe monolayers, highlighting their strong binding and Mott-Wannier nature.

Findings

GeS has a band gap of 2.85 eV; GeSe has 1.70 eV.

Exciton binding energies are 1.05 eV for GeS and 0.4 eV for GeSe.

Excitons are strongly bound and primarily excited along the zigzag direction.

Abstract

The excitonic spectra of single layer GeS and GeSe are predicted by ab initio GW-Bethe Salpeter equation calculations. G 0 W 0 calculations for the band structures find a fundamental band gap of 2.85 eV for GeS and 1.70 eV for GeSe monolayer. However, excitons are tightly bound, specially in GeS at the {\Gamma} point, where the quasi-particle interactions are so strong that they shift the {\Gamma} exciton peak energy into the visible range and below the off-{\Gamma} exciton peak. The lowest energy excitons in both materials are excited by light along the zigzag direction and have exciton binding energies of 1.05 eV and 0.4 eV, respectively, but despite the strong binding, the calculated binding energies are in agreement with a Mott-Wannier model.