Photoluminescence and electron paramagnetic resonance of ZnO tetrapod structure

ZnO tetrapod nanostructures have been prepared by the evaporation of Zn in air (no flow), dry and humid argon flow, and dry and humid nitrogen flow. Their properties have been investigated using scanning electron microscopy (SEM), X-ray diffraction (XRD), photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopies (at different temperatures), and electron paramagnetic resonance (EPR) spectroscopy at -160degreesC and room temperature. It is found that the fabrication conditions significantly influence the EPR and PL spectra obtained. While a g = 1.96 EPR signal is present in some of the samples, green PL emission can be observed from all the samples. Therefore, the green emission in our samples does not originate from the commonly assumed transition between a singly charged oxygen vacancy and a photoexcited hole [K. Vanheusden, C. H. Seager, W. L. Warren, D. R. Tallant, J. A. Voigt, Appl. Phys. Lett. 1996, 68, 403]. However, the green emission can be suppressed by coating the nanostructures with a surfactant for all fabrication conditions, which indicates that this emission originates from surface defects.