Raman Line Shapes of Optical Phonons of Different Symmetries in Anatase TiO2 Nanocrystals
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Abstract
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 line shapes of two Eg modes and one B1g 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 B1g mode is found to be of opposite nature than those in the 144 and 639 cm−1 Eg modes. This arises due to the negative dispersion of the 397 cm−1 B1g 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 Eg mode and largest for the 397 cm−1 B1g 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.
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