Interpretation of the Phonon Frequency Shifts in ZnO Quantum Dots

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 corresponds to the local temperature of the quantum dot ensemble of about 700C. 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) 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]. Mater. Res. Soc. Symp. Proc. Vol. 872 © 2005 Materials Research Society J13.21.1