The excitonic spectra of single-layer GeS and GeSe are predicted by ab initio GW and 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 $\mathrm{\Gamma }$ point, where the quasiparticle interactions are so strong that they shift the $\mathrm{\Gamma }$ exciton peak energy into the visible range and below the $\text{off-}\mathrm{\Gamma }$ 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 and 0.4 eV, respectively; but despite the strong binding, the calculated binding energies are in agreement with a Mott-Wannier model.

• Received 19 July 2016
• Revised 14 September 2016

DOI:https://doi.org/10.1103/PhysRevB.94.155428