Substrate-DependentOptical Blue-Shift upon F Incorporationin Oxyfluoride SrCo(O,F)_3–<i>x</i> Films

Heteroanionic oxides are attractive for their structural versatility and property tunability. In this work, we report on optoelectronic properties in epitaxial oxyfluoride SrCo(O,F)_3–<i>x</i> films synthesized on multiple (001)-oriented perovskite substrates. Topochemical fluorination was conducted on the as-grown SrCoO_2.5 films at 200 °C using vapor from poly(vinylidene fluoride) (PVDF) as the fluoride source, producing films with a nominal SrCoO_2.2F_0.6 composition. Uniform fluoride insertion in each film was confirmed via depth-dependent elemental analysis, performed with X-ray photoelectron spectroscopy (XPS). Fluoride incorporation results in an expansion of the <i>c</i>-axis parameter, an increase in resistivity, and a blue-shift of the optical absorption edge by 0.18–0.5 eV. The magnitude of the band gap increase is strongly dependent on the in-plane lattice parameter of the substrate with larger blue-shifts observed in films grown on substrates with smaller lattice parameters, a trend that mirrors the larger resistivity enhancements present in the compressively strained oxyfluoride films. These results are attributed to the interplay between epitaxial strain and fluoride lattice site occupation, suggesting strain-dependent control of anionic arrangements is a promising route for engineering optoelectronic properties in heteroanionic films.