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Spatially separated ZnO nanopillars were grown on Pt-thin-film-coated Si substrates by electrochemical deposition in Zn(NO3)2 aqueous solution. The effects of growth parameters, such as Zn(NO3)2 concentration, cathodic potential, bath temperature, and growth time, were studied with respect to the morphological, structural, and photoluminescence properties of the ZnO nanopillars. The isolated nanopillars fabricated at a relatively low concentration of Zn(NO3)2 (∼5 × 10-4 M) and high temperature (90 °C) had the best structural and optical qualities. The separated ZnO nanopillars were found to exhibit a single-crystal wurtzite structure and to grow along the c axis perpendicularly to the substrate. The as-grown nanopillars had a moderate UV-to-visible emission ratio of ∼4.2, but this value could be substantially improved by annealing. Annealing at 300 °C in all three atmospheres studied (air, 5% H2/95% N2, and vacuum) resulted in a considerable improvement in the UV-to-visible emission ratio (16.7−28.6), although a considerable number of defects were created by annealing at temperatures above 500 °C.

Although upconversion phosphors have been widely used in nanomedicine, laser engineering, bioimaging, and solar cell technology, the upconversion luminescence mechanism of the phosphors has been fiercely debated. A comprehensive understanding of upconversion photophysics has been significantly impeded because the number of photons incorporated in the process in different competitive pathways could not be resolved. Few convincing results to estimate the contribution of each of the two-, three-, and four-photon channels of near-infrared (NIR) energy have been reported in yielding upconverted visible luminescence. In this study, we present the energy upconversion process occurring in NaYF:Yb,Er phosphors as a function of excitation frequency and power density. We investigated the upconversion mechanism of lanthanide phosphors by comparing UV/VIS one-photon excitation spectra and NIR multi-photon spectra. A detailed analysis of minor transitions in one-photon spectra and luminescence decay enables us to assign electronic origins of individual bands in multi-photon upconversion luminescence and provides characteristic transitions representing the corresponding upconversion channel. Furthermore, we estimated the quantitative contribution of multiple channels with respect to irradiation power and excitation energy.