Hybrid DFT computation of the band structures in bilayer (a) and bulk (b) GaS. For comparison, the DFT calculations for three- and four-layer GaS are presented in Supplementary Fig. 7. (c) Tauc plot used for determining the electronic band transition with simplified electronic band diagram (inset). Red lines are the valence and conduction band edges, black is the Fermi level and blue is the location of the deep trap band. (d) PESA demonstrating the valence band maximum (VBM) energy is −5.15 eV. (e) XPS valance analysis revealing an energy difference of 1.8 eV between the VBM and Fermi level. (f) PL decay with fitted exponential (red) and mean excitonic decay (blue) time indicated.