Two-Photon Absorption Photoluminescence Spectroscopy of Hierarchical ZnO nanostructures: Correlation between UV and defect emission

M.G. CAPELUTO; G. GRINBLAT; M. TIRADO; D. COMEDI; A.V. BRAGAS

Workshop; Workshop on Nanophotonics; 2012

In this work we present the study of two photon absorption photoluminescence (TPPL) spectra of nanostructures of ZnO with different morphologies. Hierarchical ZnO nanostructures (such as nanowires, nanocombs and nanohedgehogs) were grown by means of a vapor-phase transport technique in a tubular furnace. We measure the TPPL, illuminating the nanostructures with a focused Ti:Sa laser. The scattered light was collected in backscattering geometry and sent to a double grating Raman spectrometer (Jobin Yvon U1000) with liquid nitrogen cooled CCD camera detection. The UV region of the TPPL spectra contains various peaks which are dominated by free exciton emission and their phonon replicas. The specific patterns depend on the type of nanostructure and its orientation with respect to the incident polarization. For instance, for the nanocombs, Fabry-Perot modes originated from optical multiple reflections within the comb's teeth are present. The visible region shows a broad green band photoluminescence, which is dominated by defect-related emission. Although there is still controversy assigning the origin and location of the defects,our results support the idea that deep levels are associated to oxygen vacancies located at the surface of the nanostructures. We observe a remarkable correlation between the defects-related visible emission (Ivis) to the exciton photoluminescence (IUV) intensity ratio with the nanostructures specific surface areas.