Size dependence of surface optical mode and electron-phonon coupling in ZnO nanocombs

ZnO nanocombs with different sizes are synthesized by simple thermal evaporation methods. Scanning electron microscopy and transmission election microscopy testify the growth of single crystal ZnO nanocombs along [0 0 0 2] direction. The temperature-dependent Raman spectra show that the intensity of surface optical (SO) modes in ZnO nanocombs obviously increases with declining measure temperatures. With the decrease of diameters, the frequency of SO modes shows a blue shift due to the passivation of surface states. The resonant Raman scattering shows that the strength of electron-phonon coupling increases with decreasing size. Calculated on size-dependent electron-phonon interaction energy agrees well with measured values for a large size range. The origin of electron-phonon coupling in ZnO nanocombs is also discussed. (C) 2012 Elsevier B.V. All rights reserved.