Tweaking the Electronic and Optical Properties of α-MoO3 by Sulphur and Selenium Doping - a Density Functional Theory Study

First-principles calculations were carried out to understand how anionic isovalent-atom doping affects the electronic structures and optical properties of alpha-MoO3. The effects of the sulphur and selenium doping at the three unique oxygen sites (O-t, O-a, and O-t of alpha-MoO3 were examined. We found that the valence p orbitals of Sulphur/Selenium dopant atoms give rise to impurity bands above the valence band maximum in the band structure of alpha-MoO3. The number of impurity bands in the doped material depends on the specific doping sites and the local chemical environment of the dopants in MoO3. The impurity bands give rise to the enhanced optical absorptions of the 5-and Se-doped MoO3 in the visible and infrared regions. At low local doping concentration, the effects of the dopant sites on the electronic structure of the material are additive, so increasing the doping concentration will enhance the optical absorption properties of the material in the visible and infrared regions. Further increasing the doping concentration will result in a larger gap between the maximum edge of impurity bands and the conduction band minimum, and will undermine the optical absorption in the visible and infrared region. Such effects are caused by the local geometry change at the high local doping concentration with the dopants displaced from the original O sites, so the resulting impurity bands are no long the superpositions of the impurity bands of each individual on-site dopant atom. Switching from 5-doping to Se-doping decreases the gap between the maximum edge of the impurity bands and conduction band minimum, and leads to the optical absorption edge red-shifting further into the visible and infrared regions.