Red-to-Ultraviolet Emission Tuning of Two-Dimensional Gallium Sulfide/Selenide

Graphene-like two-dimensional (2D) nanostructures have attracted significant attention because of their unique quantum confinement effect at the 2D limit. Multi layer nanosheets of GaS GaSe alloy are found to have a band gap (E-g) of 2.0-2.5 eV that linearly tunes the emission in red-to-green. However, the epitaxial growth of monolayers produces a drastic increase in this E-g to 3.3-3.4 eV, which blue-shifts the emission to the UV region. First-principles calculations predict that the E-g of these GaS and GaSe monolayers should be 3.325 and 3.001 eV, respectively. As the number of layers is increased to three, both the direct/indirect E-g decrease significantly; the indirect E-g approaches that of the multilayers. Oxygen adsorption can cause the direct/indirect E-g of GaS to converge, resulting in monolayers with a strong emission. This wide E-g tuning over the visible-to-UV range could provide an insight for the realization of full-colored flexible and transparent light emitters and displays.