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

Raman spectroscopic investigations of phonons of different symmetries in anatase TiO 2 nanocrystals synthesized by the sol−gel method are carried out. Out of six Raman active phonons, the line 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 1g 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.