Infrared Study of Surface Phonon Modes in ZnO Small Crystals

Infrared transmission spectra of ZnO particles, whose diameters are much smaller than the wavelength of incident light, surrounded by various dielectric media with different dielectric constant have been measured in the photon energy region from 300 to 600 cm -1 . Four transmission peaks were found in the photon energy region between ω TO ( E ⊥ c : 406 cm -1 , E // c : 377 cm -1 ) and ω LO ( E ⊥ c : 589 cm -1 , E // c : 575 cm -1 ), which showed features specific to the surface phonon modes. By taking account of both the optical anisotropy and the actual shape of particles, these four peaks were identified to be surface phonon modes of spherical (F), cubic (c) or cylindrical (cy) particles; ω F ⊥ mode, ω c // mode, ω cy ⊥ mode and overlapped peak of ω c ⊥ and ω F // modes, where the symbols ⊥ and // stand for the correlation between the crystal axis and the electric field component of the incident light.