Topochemical Synthesis and Optical Properties of Epitaxially Strained SrCo(O,F)_3-x Thin Films

Transition metal oxides (TMOs) are attractive in materials science for their structural versatility and property tunability. This tunability can be further expanded by incorporating a second anion, such as F or N, to alter the metal-anion bond ionicity or the metal charge density. In this work, we report on vapor-based fluorination of epitaxial SrCoO_2.5 films. The films are grown by oxygen-assisted molecular beam epitaxy (MBE) under conditions that result in the brownmillerite structure. Topochemical fluorination was conducted on the as-grown films at 200°C using a quartz tube furnace under Ar gas flow with poly(vinylidene fluoride) (PVDF) as the fluorine source. Fluorination reactions were performed on SrCoO_2.5 films synthesized on STO, LAO, LSAT and GSO substrates to understand the impact of substrate-induced strain on fluorine incorporation. Fluorine incorporation was confirmed via depth-dependent elemental analysis performed with X-ray photoelectron spectroscopy (XPS). Fluorine incorporation results in an expansion of the <i>c</i>-axis parameter as determined by X-ray diffraction. Optical absorption spectra, obtained through spectroscopic ellipsometry, reveals a blue-shift of the low-energy absorption edge by 0.1 - 0.3 eV depending on the substrate. This result is attributed to a widening of the band gap due to the increased ionicity of the Co-anion bonds.<br/><br/>This work was supported by the National Science Foundation under grant CMMI-2001888.