Backscattered Raman spectra were measured using a Horiba HR800 micro-Raman spectrometer at room temperature. Linearly polarized laser radiation with a wavelength of 488 nm was used for excitation. Typical Raman spectra are presented in Fig. . Peaks corresponding to single-phonon modes (A 1 (TO) at 380 cm -1 , E 1 (TO) at 408 cm -1 , E 2 high at 437 cm -1 ) are clearly visible in the spectra .