Raman- and IR-Active Phonons in CdSe/CdS Core/Shell Nanocrystals in the Presence of Interface Alloying and Strain

Semiconductor core-shell nanocrystals (NCs) have greatly improved luminescent properties including better resistance to photobleaching and ligand exchange. It was suggested that compound alloying at the core/shell interface could play an important role in obtaining bright and stable NCs. Here, we investigate the interface composition and strain evolution in spherical and dot-in-plate CdSe/CdS nanocrystals with shell thickness ranging from 1 to 3 nm, using a combination of Raman and infrared spectroscopy. A slower rate of strain accumulation in the core is observed for dot-in-plate nanocrystals and is linked to the anisotropic shape of the plate-like shell. We resolved the respective contributions of the core, shell, and alloyed interface and observed a drastic change of the shell-related Raman feature with the appearance of the bulk-like optical phonon in the shell thicker than nm. The average composition of the alloy interface is estimated using the frequencies of the alloy modes. Because of the high crystallinity of the samples, up to fourth-order optical phonon processes are observed and analyzed. This work confirms the presence of an alloyed interface in core/shell CdSe/CdS structures of different geometries and establishes a precise roadmap for its quantitative analysis using vibrational spectroscopy.