One-Step Formation of Plasmonic Cu Nanodomains in p-Type Cu₂O Matrix Films for Enhanced Photoconversion of n-ZnO/p-Cu₂O Heterojunctions

Plasmonic Cu nanoparticles were in situ grown into a Cu₂O semiconductor matrix by using reactive magnetron sputtering and adjusting the amount of oxygen available during the synthesis in order to prevent the oxidation of part of the copper atoms landing on the film surface. Varying only the oxygen flow rate (OFR) and using a single Cu target, it was possible to observe the evolution in the simultaneous formation of metallic Cu and Cu₂O phases for oxygen-poor conditions. Such formation is accompanied by the development of the surface plasmon band (SPB) corresponding to Cu, as evidenced by UV-vis spectrophotometry and spectroscopic ellipsometry. The bandgap values of the elaborated composites containing embedded Cu plasmonic nanodomains were lower than the bandgap of single-phase Cu₂O films, likely due to the higher defect density associated with the nanocrystalline nature of films promoted by the presence of metallic Cu. The resistivity of the thin films increased with more oxidative deposition conditions and was associated with an increase in Cu₂O/Cu ratio and smaller and more isolated Cu particles, as evidenced by high-resolution transmission electron microscopy and X-ray diffraction. Photoconversion devices based on the studied nanocomposites were characterized by I-V and spectral photocurrent measurements, showing an increase in the photocurrent density under light illumination as a consequence of the plasmonic particle excitation leading to hot carrier injection in the nearby ZnO and Cu₂O semiconductors.