Electronic and optical properties of the monolayer group-IV monochalcogenides MX ( M=Ge,Sn ; X

By using density-functional theory and many-body perturbation theory based first-principles calculations, we have systematically investigated the electronic and optical properties of monolayer group-IV monochalcogenides $MX$ ($M=\mathrm{Ge},\phantom{\rule{4pt}{0ex}}\mathrm{Sn}$; $X=\mathrm{S},\phantom{\rule{4pt}{0ex}}\mathrm{Se},\phantom{\rule{4pt}{0ex}}\mathrm{Te}$). All $MX$ monolayers are predicted to be indirect gap semiconductors, except the GeSe monolayer, which has a direct gap of 1.66 eV. The carrier mobilities of $MX$ monolayers are estimated to be on the order of ${10}^{3}$ to ${10}^{5}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{V}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$, which is comparable to, and in some cases higher than, that of phosphorene using a phonon-limited scattering model. Moreover, the optical spectra of $MX$ monolayers obtained from GW-Bethe-Salpeter equation calculations are highly orientation dependent, especially for the GeS monolayer, suggesting their potential application as a linear polarizing filter. Our results reveal that the GeSe monolayer is an attractive candidate for optoelectronic applications as it is a semiconductor with a direct band gap, a relatively high carrier mobility, and an onset optical absorption energy in the visible light range. Finally, based on an effective-mass model with nonlocal Coulomb interaction included, we find that the excitonic effects of the GeSe monolayer can be effectively tuned by the presence of dielectric substrates. Our studies provide an improved understanding of electronic, optical, and excitonic properties of group-IV monochalcogenides monolayers and might shed light on their potential electronic and optoelectronic applications.