Raman Line Shapes of Optical Phonons of Different Symmetries in Anatase TiO2 Nanocrystals

Raman spectroscopic investigations of phonons of different symmetries in anatase TiO2 nanocrystals synthesized by the sol-gel method are carried out. Out of six Raman active phonons, the fine shapes of two E-g modes and one B-1g mode have been analyzed quantitatively to distinguish between the contributions of laser-induced local heating, phonon confinement effects, and defects to the line broadening. The line shape asymmetry arising from confinement of optical phonons in the 397 cm(-1) B mode is found to be of opposite nature than those in the 144 and 639 cm(-1) E-g modes. This arises due to the negative dispersion of the 397 cm(-1) B-1g phonon dispersion curve. The measured spectra show larger broadening than those predicted by a phonon confinement model. The excess broadening, attributed to intrinsic defects, is found to be least for the 144 cm(-1) E-g mode and largest for the 397 cm(-1) B-1g mode. In addition, finite laser power of the excitation wavelength is found to raise the temperature of the nanocrystals, with heating being maximum for the smallest size nanocrystals.