Motif-dependent electronic and optical properties of epitaxially stabilized Magnéli-like WOx thin films

Substoichiometric tungsten oxides (WOx) host multiple W valence states and local W–O coordination motifs, but their respective roles in determining electronic and optical behavior remain less understood. Here, we integrate scanning transmission electron microscopy, x-ray photoemission and absorption spectroscopies, spectroscopic ellipsometry, and density functional theory calculations to disentangle these effects in epitaxially stabilized WOx thin films with systematically varied oxygen deficiency. Lowering the oxygen partial pressure induces a structural evolution from a pseudocubic WO3 to W18O49-like Magnéli motifs, together with the gradual emergence of W5+ and W4+ states. Motif-resolved calculations show that reduced W sites are electronically inequivalent: W4+-rich edge-sharing environments are associated with localized in-gap states that enhance subgap optical absorption, whereas intermediate-valence W sites contribute to broader states near the Fermi level that are more favorable for electronic transport. In contrast, W6+-rich sites contribute primarily to high-energy O 2p-to-W 5d transitions characteristic of pseudocubic WO3 regions. Together, these results identify local W–O connectivity as a key determinant of electron localization in the WOx lattice, linking atomic-scale motifs to distinct electronic and optical properties.