Resonant-Raman scattering of ZnO crystallites: the quasi nature of the LO mode

The Raman modes of ZnO crystallites of the wurtzite structure were investigated via micro-Raman scattering at resonant and out-of-resonant conditions. The E-2 mode was the predominant mode in the spectra for out-of-resonant conditions. For the resonant conditions one mode at the spectral range of the LO's of ZnO single crystal was the predominant mode: its frequency was found to be at similar to 580 cm(-1). The A(1)(LO) and the E-1(LO) modes of a reference ZnO single crystal were found to be 568 cm(-1) and 586 cm(-1) respectively. Two possible mechanisms were considered that may explain the mode frequency of the crystallites: it can be regarded as a confined E-1(LO) mode or as a quasi-LO mode. The UV-photoluminescence of the crystallites was found to have the same energy as that of the single crystal similar to 3.3 eV, indicating the lack of size-confinement of the electronic states in the crystallites, and inter alia that of the phonon states. Our analysis indicated that the observed frequency can be explained in terms of Loudon's model of a quasi-mode behavior which is due to the crystallites tilt.