Nanostructures, such as quantum dots, nanocrystals and nanowires, made of wurtzite ZnO have recently attracted
attention due to their proposed applications in optoelectronic devices. Raman spectroscopy has been widely used to
study the optical phonon spectrum modification in ZnO nanostructures as compared to bulk crystals. Understanding the
phonon spectrum change in wurtzite nanostructures is important because the optical phonons affect the light emission
and absorption. The interpretation of the phonon peaks in the Raman spectrum from ZnO nanostructures continues to be
the subject of debates. Here we present a comparative study of micro-Raman spectra from ZnO quantum dots,
nanocrystals and nanowires. Several possible mechanisms for the peak position shifts, i.e., optical phonon confinement,
phonon localization on defects and laser-induced heating, are discussed in details. We show that the shifts of ~2 cm-1 in
non-Resonant spectra are likely due to the optical phonon confinement in ZnO quantum dots with the average diameter
of 4 nm. The small shifts in the non-Resonant spectra from ZnO nanowires with the diameter ~20 nm - 50 nm can be
attributed to either defects or large size dispersion, which results in a substantial contribution from nanowires with
smaller diameters. The large red-shifts of ~10 cm-1 in the resonant Raman spectrum from nanocrystals were proved to be
due to local laser heating.
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