Raman analysis of CdSe/CdS core-shell quantum dots with different CdS shell thickness

We report the first- order and high- order Raman scattering from core - shell CdSe/ CdS nanocrystals and investigate the evolution of the longitudinal mode of CdSe ( LO1) and CdS ( LO2), and the surface mode of CdSe ( SO1) and CdS ( SO2) with increasing shell thickness ( 0 - 5.5 ML ( monolayer)). We find that the shift of the LO2 peak from 268 to 291 cm(-1) agrees well with the theoretical values based on the phonon confinement model. The variation of the dielectric environment of the CdSe core with increasing CdS shell thickness is modified according to the shift of SO1 from 198 to 185 cm(-1). The SO2 modes at 267 and 275 cm(-1), corresponding to the shell thickness 3.5 and 5.5 ML in CdSe/ CdS nanocrystals, respectively, are also obtained for the first time in our experiment. Moreover, they agree well with the theoretical values of the dielectric corresponding function model. Besides, a new Raman peak at 482 cm(-1) is observed, and it remains at that value with shell growth, the new peak is supposed to be caused by the alloying at the core - shell interface. Therefore, Raman spectroscopy can be used to determine the shell thickness and other surface and interface parameters of CdSe/ CdS core - shell nanocrystals.