Defect emissions in ZnO nanostructures

Abstract

Zinc oxide (ZnO) is of great interest in photonic applications due to its wide bandgap (3.37 eV) and high exciton binding energy (60 meV). In the photoluminescence (PL) spectrum of ZnO, typically one U V band-edge emission peak and one or more peaks at the visible spectral range due to defect emission are observed. The PL emission of ZnO is commonly green, but other colors like yellow and orange are also reported. Out of the different visible peaks, the origin of the green one is the most controversial. The most commonly cited explanation for it is the transition between a singly oxidized oxygen vacancy and a photoexcited hole [K. Vanheusden, C. H. Seager, W. L. Warren, D. R. Tallant, and J. A. Voigt, Appl. Phys. Lett. 68, 403 (1996).]. However, this hypothesis is established on ZnO phosphors but not on nanostructured samples. In this work, several ZnO nanostructures (nanorods, nanoneedles, nanoshells and tetrapod nanorods) were synthesized by thermal evaporation and chemical methods. The obtained nanostructures were examined by scanning electron microscopy (SEM), X-ray diffraction (XRD), photoluminescence (PL), and electron paramagnetic resonance spectroscopy (EPR). It was found that fabrication methods significantly affect the defect emissions of the nanostructures. For different fabrication conditions, defect emissions in the green, yellow, and orange spectral ranges were observed. No correlation was found between the deep levels responsible for the visible emission and the EPR signal. Origins of the different defect emissions are discussed.