Temperature Effects on Defect-Induced Luminescence in ZnO Nanocrystals

ZnO nanocrystals, synthesized via a mild precipitation method assisted by a deep eutectic solvent, exhibit an intense orange luminescence, whose intensity is strongly affected by the temperature. The luminescence quantum yield of the powder reaches nearly 100% at 77 K, decreases to 50% at 180 K, and drops to 10% at room temperature, without any modification of the spectral profile; this behaviour of the orange emission, attributed to oxygen-related defects, makes the material a promising candidate for cryogenic temperature sensing. Steady-state and time-dependent luminescence studies as a function of temperature indicate that the dynamics of the emitting defects are linked to the 1LO phonon modes of the nanocrystals, and a relaxation energy barrier of ca. 8 kJ mol−1 has been determined. The detailed structural and morphological characterization of the nanostructured powder by X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), High-Resolution Transmission Electron Microscopy (HR-TEM), and Raman spectroscopy, enables a better understanding of the nature and properties of ZnO orange luminescence.