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Micro-Raman investigation



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JOURNAL OF APPLIED PHYSICS 97 124313 2005 Micro Raman investigation of optical phonons in ZnO nanocrystals Khan A Alim Vladimir A Fonoberov Manu Shamsa and Alexander A Balandina Nano Device Laboratory Department of Electrical Engineering University of California Riverside Riverside California 92521 Received 28 February 2005 accepted 5 May 2005 published online 27 June 2005 We have measured nonresonant and resonant Raman scattering spectra from ZnO nanocrystals with an average diameter of 20 nm Based on our experimental data and comparison with the recently developed theory we show that the observed shifts of the polar optical phonon peaks in the resonant Raman spectra are not related to the spatial phonon confinement The very weak dispersion of the polar optical phonons in ZnO nanocrystals does not lead to any noticeable redshift of the phonon peaks for 20 nm nanocrystals The observed phonon shifts have been attributed to the local heating effects We have demonstrated that even the low power ultraviolet laser excitation required for the resonant Raman spectroscopy can lead to the strong local heating of ZnO nanocrystals The latter causes significant up to 14 cm 1 redshift of the optical phonon peaks compared to their position in bulk crystals Nonresonant Raman excitation does not produce noticeable local heating The obtained results can be used for identification of the phonons in the Raman spectra of ZnO nanostructures 2005 American Institute of Physics DOI 10 1063 1 1944222 I INTRODUCTION Zinc oxide ZnO presents interesting material system for investigation due to its wide band gap of 3 37 eV and some intriguing optical properties A prominent feature of ZnO is its large exciton binding energy 60 meV at room temperature which results in extreme stability of excitons 1 Recently nanostructures made of ZnO have attracted significant attention due to their proposed applications in the lowvoltage and short wavelength 368 nm electro optical devices transparent



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