A chemical understanding of the anomalously large bandgap of alpha-PbO

Current intuition suggests that the band gap of binary semiconductors decreases when descending the Periodic Table. Surprisingly, α-PbO has an anomalously large bandgap of 2.4 eV compared to α-SnO at 0.7 eV. This oxide system is novel and intriguing, considering that α-SnO is the only ambipolar dopable semiconductor. While the properties of these oxides can easily be computed in DFT, there is no satisfying physical explanation for the PbO band gap increase. We use Maximally-Localized Wannier Functions to decompose the complex electronic structure into a tight-binding description, and show how the interplay of simple tight-binding parameters--such as orbital on-site energies and offsite hopping parameters --emerge into major qualitative features of the bandstructure like band position and width. Given the orbital radii and interatomic distance of GeO, SnO, PbO, our conceptual model properly rationalizes the anomalously large band gap of PbO. Briefly, the 6s orbitals of Pb exhibit Lanthanide contraction, which shifts the conduction band up and narrows its width, enlarging the band gap. Our conceptual and reductionist interpretation of the band structure may be generalizable, which would enable the intuitive navigation of other anomalous trends in semiconductor electronic structure.