Optical Properties of Epitaxial Sr-Ti-O Compounds from First Principles

SrTiO$_{3}$ is a representative of the property-rich perovskite family, and a material whose ability to convert solar photons to H$_{2}$ fuel would be more efficient if its wide optical bandgap (3.25 eV) better matched the solar spectrum. The Sr- and Ti-based Ruddlesden-Popper (RP) phases, Sr$_{n+1}$Ti$_{n}$O$_{3n+1}$, are structural modifications of SrTiO$_{3}$ with potentially useful electronic properties. While bulk growth is limited to $n<4$ and $n=\infty$ (SrTiO$_{3}$), thin films of larger finite $n$ structures have been grown epitaxially. In optical experiments, bandgaps of these films decrease monotonically with increasing $n$.\footnote{C.-H. Lee et al., to be published.} In density functional theory (DFT), however, the $n=\infty$ gap is larger than those for finite $n>3$. This disagreement could stem from limitations in both experiment and theory. We explore this issue in depth using DFT, many-body perturbation theory, and tight-binding techniques.