Optical phonon confinement in ZnO nanorods and nanotubes

Abstract

The structure and morphology of ZnO nanorods and nanotubes synthesized by hydrothermal method were studied by TEM, XRD and Raman spectroscopy. The Raman scattering spectra at room temperature were excited by He-Ne laser with wavelength 633 nm. The perfect wurtzite structures of ZnO nanorods and nanotubes were verified by the intense E 2 (high) mode. The strong red shifts of phonon frequencies, broadening of the FWHM of Raman peaks and asymmetry of line shapes were attributed to phonon confinement effect. An optical phonon confinement model was used for calculating the theoretical line shapes, asymmetric broadening and shift of the optical phonons. The results were in good agreement with the experimental values. Besides the fundamental phonon modes, overtone and combination modes were also observed because of the anharmonicity effect. From this study, it was found that the phonon confinement effect was more obvious in ZnO nanotubes as compared to nanorods.