Room-temperature near-band-edge photoluminescence of ZnO is composed of contributions from free-exciton recombination and its longitudinal-optical phonon replica. By tracking the photoluminescence of ZnO nanowires from 4K up to room temperature, the authors show that the relative contributions of these emission lines show a strong variation for samples grown under different conditions. The varying coupling strengths of the excitons and phonons thus lead to a significant shift of the energy position of the room-temperature photoluminescence. They verify that this is not caused by laser heating or stress/strain but is most probably related to crystalline imperfections in the surface region.
We report on low temperature photoluminescence studies of ZnO nanowires embedded in different polymers. Comparing the spectra of as-grown and embedded ZnO nanowires, we find a decrease of the deep-level emission and an increase of the near band-edge emission after the embedding process. The near band-edge emission of the embedded ZnO nanowires is dominated by a surface exciton band. The observed effects are independent of the selected polymer. The decrease of the deep-level emission scales with the balling abilities of the different polymers. We propose a model to explain the spectral changes.
Single crystal ZnO wurtzite nanowires grown along the
c-axis with diameters down to 4 nm were synthesized by a catalytic vapor transport
technique. Photoluminescence spectra of these wires indicate a blue shift of the free exciton
by 19 meV due to confinement. This result was obtained by analyzing the line shape of the
blue-shifted LO phonon replica of the free exciton. In addition, a surface-related excitonic
luminescence feature centered at 3.366 eV was observed with a strongly elevated thermal
activation energy.
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