Ultraviolet Raman scattering in ZnO nanowires: quasimode mixing and temperature effects

We use near-resonance Raman scattering to investigate zinc oxide (ZnO) nanowires grown by chemical vapor deposition on Si substrates. We discuss the role of quasimode mixing on the wavenumber of the longitudinal optical (LO) bands, and we perform Raman measurements with different excitation powers to investigate possible laser heating effects. We find that in the Raman spectra of as-deposited nanowires grown along the c-axis of wurtzite, the LO bands are located slightly below the E-1(LO) mode of bulk ZnO. We perform a calculation of the expected LO wavenumber in an ensemble of randomly oriented nanowires. Our analysis shows that light refraction, together with the orientation-dependent cross-section of the nanowires for the incoming light, counterbalances quasimode mixing effects in the as-grown product, giving rise to LO bands that are barely redshifted relative to the E-1(LO) mode. In the case of ZnO nanowires that have been mechanically removed (scratched) and subsequently deposited onto separate Si substrates, we observe clear laser-induced heating. Temperature effects account well for the Raman wavenumber shifts displayed by the LO bands in the Raman spectra of the scratched nanowires. Copyright (C) 2010 John Wiley & Sons, Ltd.