Defect-related multicolour emissions in ZnO smoke: from violet, over green to yellow

Abstract

The nature of defects in ZnO smoke was studied at different stages of the material’s history by combining photoluminescence (PL) and electron paramagnetic resonance (EPR) spectroscopy. In contrast to studies previously reported on ZnO nanopowders, high vacuum conditions (P < 10⁻⁵ mbar) have been applied during sample storage, handling and spectroscopic investigations. Two pairs of violet-PL/EPR signals (2.88 eV/ g = 1.956 and 2.80 eV/ g = 1.960) were observed in the as-synthesized ZnO powder and attributed to surface (dominant) and bulk zinc interstitials (Zn_i⁺). Upon annealing in O₂-poor conditions, green-PL emission (2.41 eV) and EPR signal at g = 2.002 develop along with EPR signals specific of superoxide radicals (O₂⁻). In the absence of any external O₂ supply, the oxygen necessary for the creation of a notable amount of O₂⁻ is provided by the lattice of ZnO smoke, so that the green emission and its EPR counterpart are unambiguously assigned to singly charged oxygen vacancies (V_O⁺). Annealing at high P_O2 results in a broad PL emission (∼2.07 eV) without an EPR counterpart. This yellow emission was assigned to peroxide-like surface species (O₂²⁻). Overall, this study shows that the visible emissions in ZnO smoke nanopowders can range from violet, over green to yellow as a function of sample history and that the corresponding PL/EPR fingerprints can serve as guidelines for the recognition of defects in other ZnO types.