In-line electronic and structural characterization of reactively sputtered Cu-Co-Mn black spinel oxides

Cu-Co-Mn spinel oxide thin films are deposited by reactive magnetron sputtering. The x-ray diffraction (XRD) spectrum suggests a nanocrystalline spinel film when sputtered at ≈465 °C. In-line scanning tunneling microscopy confirms nanometric sized grains in the order of 5–10 nm, which then form larger agglomerations of 30–70 nm as observed by scanning electron microscopy. The pristine sample surfaces are characterized by in-line UV photoelectron spectroscopy (UPS) and x-ray photoelectron spectroscopy (XPS). From UPS He I and He II spectra, a valence band edge of 0.38 eV, compatible with a bandgap of ≈0.8 eV, is determined. XPS infers Cu2+, Co3+, and Mn3+ as dominant, thus, fitting well the general spinel formula A2+B23+O42−. The elemental quantification based on XPS core-level peak integration indicates some Cu enrichment and a secondary CuO phase formation at the spinel surface that is congruent with the XRD results where peaks associated with tenorite CuO are also identified. The partially filled 3d bands make the Cu-Co-Mn oxides a promising candidate for selective solar absorbers. The measured spectral reflectance of CuCoMnOx//SiO2 double layer yields a solar absorptance of 0.8 and thermal emittance of 0.05. The coating durability and thermal stability in the air have been confirmed by accelerated aging tests at 270 °C for a duration of 600 h.