Oxygen-Mediated Phase Evolution in Sputtered Cu-W-O: Insights into Surface Chemistry Variability

TL;DR

This study investigates how oxygen levels during sputtering influence phase formation, surface chemistry, and electronic structure in Cu-W-O thin films, revealing significant variability based on synthesis conditions.

Contribution

It provides detailed insights into the surface chemistry and electronic structure changes in Cu-W-O films caused by oxygen partial pressure variations during fabrication.

Findings

Low oxygen favors CuWO4 phase formation.

Higher oxygen levels produce mixed CuWO4 and Cu3WO6 phases.

Surface Cu shows binding energy shifts linked to initial-state effects.

Abstract

Thin films of Cu-W-O ternary compounds were fabricated via DC magnetron co-sputtering from Cu and W metallic targets under controlled oxygen partial pressures, followed by thermal annealing. Low-oxygen conditions favored the formation of a single CuWO4 phase, whereas higher oxygen levels produced a mixture of CuWO4 and Cu3WO6. Structural and optical properties were investigated by X-ray diffraction (XRD) and spectrophotometry, revealing phase coexistence and changes in preferential orientation depending on the deposition conditions. A detailed and carefully validated X-ray photoelectron spectroscopy (XPS) analysis provides insight into the surface chemical environment of Cu and W, indicating the presence of compositional inhomogeneities and surface-bulk differences associated with Cu migration and segregation. While the W 4f core levels remain remarkably stable across all tested oxygen partial pressures, a systematic shift is observed in the Cu 2p3/2 binding energy. Wagner plot analysis confirms that this displacement is dominated by initial-state effects, reflecting modifications of the Cu ground-state electronic structure and Cu-O-W hybridization rather than changes in final-state screening. Our findings demonstrate that sputtered Cu-W-O films, even when nominally identified as CuWO4, can exhibit substantially different structural and electronic states depending on synthesis conditions, highlighting the need for rigorous characterization to ensure reproducibility in ternary oxide research.