Interpretation of the Phonon Frequency Shifts in ZnO Quantum Dots
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J13.21.1
Interpretation of the Phonon Frequency Shifts in ZnO Quantum Dots Khan A. Alim, Vladimir A. Fonoberov, and Alexander A. Balandin Nano-Device Laboratory Department of Electrical Engineering University of California – Riverside Riverside, California 92521 Web-site: http://ndl.ee.ucr.edu/ ABSTRACT Nanostructures made of zinc oxide (ZnO), a wide-bandgap semiconductor, have recently attracted attention due to their proposed applications in low-voltage and shortwavelength (368 nm) electro-optical devices, transparent ultraviolet (UV) protection films, gas sensors, and varistors. Raman spectroscopy presents a powerful tool for identifying specific materials in complex structures and for extracting useful information on properties of nanoscale objects. At the same time the origin of Raman peak deviation from the bulk values is not always well understood for new material systems. There are three main mechanisms that can induce phonon shifts in the free-standing undoped ZnO nanostructures: (i) phonon confinement by the quantum dot boundaries; (ii) phonon localization on defects and (iii) the laser-induced heating in nanostructure ensembles. Here, we report results of the combined non-resonant and resonant Raman scattering studies of an ensemble of ZnO quantum dots with diameter 20 nm. Based on our experimental data, we have been able to identify the origin of the observed phonon frequency shifts. It has been found that the ultraviolet laser heating of the ensemble induces a large red shift of the phonon frequencies. It is calculated that the observed red shift of 14 cm-1 corresponds to the local temperature of the quantum dot ensemble of about 700o C. INTRODUCTION Zinc oxide (ZnO) presents interesting material system because of 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 [1-2]. Raman spectroscopy is a non-destructive characterization method of choice for many recent studies of vibrational properties of ZnO nanostructures. The origin of the frequency shift in ZnO nanostructures is still under debate [3]. In this paper, we present details of the experimental study, which indicates that the large red shift (up to 14 cm-1 ) in nanocrystals with the diameter of 20 nm is related to local heating rather than to phonon confinement. The experimental results are in excellent agreement with the theory of the optical phonons in wurtzite nanocrystals developed by Fonoberov and Balandin [4-6].
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EXPERIMENTAL DETAILS The samples used for the study are 20 nm powder type ZnO QDs and bulk ZnO sample. The bulk ZnO crystal has wurtzite structure and dimensions 5 × 5 × 0.5 mm3 with the a-plane (1120) facet. The investigated ZnO QDs have been produced by the wet chemistry method. The chemical purity of the nanocrystal sample is 99.5%. A Renishaw micro-Raman spectrometer RM 2000 with visible (488 nm) and UV (325 nm) excitation lasers was employed to measure the non-
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