Raman scattering of polar modes of ZnO crystallites

One of the key issues of phonon dynamics of nano- and micrometer-scale crystals is the identification of the observed Raman modes. Due to the tilted orientation of small crystallites, the usual Raman selection rules pertaining to the symmetry axes no longer hold, and mixed-symmetry modes need to be considered in order to explain the polar phonon properties of the crystallites. The Raman modes of ZnO crystallites of the wurtzite structure were investigated via micro-Raman scattering. The nonpolar E-2 mode was the predominant mode in the spectra for out-of-resonant conditions. In resonance the crystallites exhibited a predominant mode at similar to 580 cm(-1), intermediate to the frequencies of the A(1)(LO) and the E-1(LO) modes of a reference ZnO single crystal at 568 and 586 cm(-1), respectively. Our analysis indicates that the observed frequency of the crystallite ensemble can be explained in terms of Loudon's model of a quasimode behavior that is due to a preferential orientation of a crystallite ensemble. Additionally, model calculation of the quasi-LO frequency of totally random ensemble is presented. (c) 2005 American Institute of Physics.