Preparation and Characterization of Gradient‐Index Copper‐Oxide Films

A copper oxide film with a gradual oxidation profile is produced by deposition and oxidation of Cu layers on glass at stepwise decreasing temperatures (650–450 K) and 25 mbar O ₂ pressure. The oxidation process is monitored in situ by recording the integral and wavelength‐dependent optical transmission and ex situ by scanning transmission electron microscopy (STEM). While two oxidation steps are revealed above 550 K, Cu → Cu ₂ O resulting in a rising transmission and Cu ₂ O → CuO causing the transmission to decrease, only the Cu → Cu ₂ O conversion takes place at 450 K. The Cu oxidation state can thus be controlled by the reaction temperature and a graded bandgap oxide can be produced. The film configuration is determined by modeling the measured transmission response with the transfer‐matrix and the Bruggeman formalisms. The fitting reveals a layered oxide composition, evolving from Cu ₂ O at the surface to CuO at the interface, in good agreement with the STEM results. Our study thus demonstrates that the optical properties of CuO _x films can be tuned via the oxidation parameters, opening a route to fabricate graded bandgap materials for photovoltaics and gradient‐refractive‐index (GRIN) optics.