Characterization of ZnO nanoparticles by resonant Raman scattering and cathodoluminescence spectroscopies

We have measured the Raman and cathodoluminescence (CL) spectra of ZnO nanoparticles with different crystallite sizes. The Raman spectra show that the frequency of the E-2 (high) phonon in these nanoparticles, which is initially high, decreases with the crystallite size, while its linewidth increases. This result is explained by the size effect. The CL spectra show that the relative intensity ratio of the CL peak at around 500-600 nm to the band-edge CL peak at approximately 360 nm drastically increases as the crystallite size decreases. We use the surface recombination model to explain this result. According to this model, the relative intensity ratio increases because of an increase in the concentration of deep levels on the surface of the crystallites and/or an increase in the free-carrier concentration. (C) 2008 American Institute of Physics.