High crystal quality and purity Cu2O by consecutive in-situ annealing and thermal oxidation of Cu under H2 and O2 at elevated temperatures

Abstract

Cu₂O has been obtained via the thermal oxidation of Cu foils with a maximum thickness of 120 μm under Ar and O₂ at 1020°C for 30 min. Care was taken to suppress the oxidation of Cu during the temperature ramp by using H₂ prior to admitting O₂ at 1020°C. We suggest that the ruby red Cu₂O crystals are obtained in this way by strain-driven grain growth and consist of ∼ 500 μm² grains that have a cubic crystal structure. More importantly we find that the crystal quality and purity of the Cu₂O is improved significantly by pre-annealing the Cu under Ar and H₂ at 1000°C and 10 mbar for up to 180 min. The Cu₂O exhibited photoluminescence at 6.5 K with a maximum at 2.01 eV corresponding to the direct energy gap in very good agreement with density functional theory calculations of the electronic band structure. No emission due to recombination of bound excitons to copper vacancies V_Cu⁺¹ and single V_O⁺¹ or double charged V_O⁺² oxygen vacancies was observed consistent with a high crystal quality and purity.