Raman spectroscopy study of the wurtzite-zinc blende phase transition of bare CdSe nanoparticles

Structural and electronic properties characterization results show that the crystallographic structure of CdSe films, deposited by chemical bath synthesis, is controlled by the bath growth temperature. The synthesis parameters employed produced a set of nanostructured CdSe films on glass substrates with controlled crystal structure. The effect of bath temperature (T_b) on CdSe films was studied in the 0 ≤ T_b ≤ 80 °C range. The average crystal diameter (AD) of the films lies within the 7 ≤ AD ≤ 12 nm interval, where AD depends on the selected T_b. X-ray diffractograms (XRD) shown that at low T_b values the formation of the hexagonal wurtzite (WZ) is promoted while at the other extreme the cubic zinc-blende (ZB) crystalline structure dominates. It is observed that the WZ → ZB transition occurs at the critical temperature T_bc ~ 40 °C. The AD in each films for CdSe-NP's was obtained from XRD analysis employing the Scherrer-Debye formula. The values of the lattice interplanar spacing (IS), determined from XRD analysis, as function of T_b increases continuously except at temperatures around T_bc where a local minimum is observed_. The presence of stress acting on CdSe NP's is identified by correlating the IS values with the crystalline structure: compression occurs for 0 ≤ T_b ≤ 40 °C, and tension for 50 ≤ T_b ≤ 80 °C. The band gap energy, obtained from optical absorption spectra, decreases monotonically but a local minimum is observed at T_bc = 40 °C. Results from Raman spectroscopy show that the CdSe Raman LO-mode hardens for T_bc as consequence of the WZ ↔ ZB structural transition.