Giant band gap bowing in two dimensional Pb_1-xSn_xO alloys and Janus PbSnO monolayers

Two-dimensional monolayer alloys can be experimentally synthesized and the monolayer form of alpha-PbO and ultrathin SnO were recently synthesized. The band gap values of the materials can be tuned by varying the host atom composition by foreign atoms. Thus, desired band gap value can be obtained for various applications. In this study, we investigated electronic, mechanical and vibrational properties of energetically the most stable Pb1-xSnxO monolayer alloys and dynamically stable Janus PbSnO (J-PbSnO) monolayer using first-principles density functional theory calculations. We found that seven Pb1-xSnxO monolayer alloys can be synthesized by the exothermic reactions. However, to synthesize J-PbSnO monolayer, experimentally suitable environment conditions requires such as which is done to synthesize J-MoSSe monolayer. We found both with PBE and HSE06 functionals that PbO and Pb0.75Sn0.25 compounds have direct band gap values while the remaining compounds have indirect band gap values. In addition, the band gap values of Pb1-xSnxO monolayer alloys do not scale linearly with the changing of the composition and exhibit giant band gap bowing. This non-linearity is attributed to electronegativity and atomic radius differences between Pb and Sn atoms. In addition, we observed that in Pb1-xSnxO monolayer alloys localization of valence band maximum states are mostly distributed among Sn p(z) orbitals whereas the conduction band minimum states localized around Pb p(x) and p(y) orbitals by the increasing of Sn concentration. We believe that our theoretical predictions will guide the experimental realization of these Pb1-xSnxO monolayer alloys for various applications.