The Raman spectrum of ZnO nanocrystals is reported in Fig. 4, in the wavelength range from 150 to 1300 cm 1.
No high-order peaks were observed over 1300 cm. The Si signal coming from the substrate was used for the instrument calibration. The wavenumber position of the bands, the symmetry of the vibrations and the comparison with literature data are resumed in Table 1. The Raman spectrum of ZnO is consistent with the ones reported in literature for nanostructured ZnO [40–42]. We observed 4 out of 6 first order peaks (the one at 99 cm 1 is outside the investigation range due to instrumental limit, and the E1(TO) mode at 407 cm 1 can be hidden in the shoulder of Ehigh2mode). The Raman bands at 328 and 660 cm 1 are assigned to acoustic overtone A1 [40], and the ones at 1105 and 1152 cm 1 are assigned to second order overtone bands of E2 and to a combination of A1 and E1 Raman bands.