Optoelectronic properties of new direct bandgap polymorphs of single-layered Germanium sulfide

The single-layered Germanium sulfide exhibits interesting optoelectronic properties however the indirect bandgap hinders its practical applications for nanoscale optoelectronic and photovoltaic devices. Herein, we report two direct bandgap polymorphs of single-layered GeS (delta-GeS and epsilon-GeS) and explore their optoelectronic properties using the density functional theory based computational approaches. The indirect bandgap of the intrinsic GeS monolayer (alpha-GeS) calculated in present study amounts to 0.76 eV, whereas the direct bandgap energies for delta- and epsilon-derivatives of GeS have been recorded as 1.93 and 2.10 eV respectively. The optical spectra of these monolayers demonstrated a high degree of anisotropy and significantly different optical absorption, reflection, and refraction coefficients were seen in the x- and y-directions. They exhibited different plasmons energies along x- and y-directions which reveal the GeS monolayers as potential polarizers of electromagnetic radiations. Moreover, these monolayers demonstrated exceptionally large optical absorption spanning over a wide range of electromagnetic spectrum. The absorption coefficients were recorded typically larger along y-axis than x-axis. The designed direct bandgap monolayers are believed to overcome the major hurdles of single-layered GeS for nanoscale optoelectronic applications.