Temperature-dependent photoluminescent and Raman studies on type-II CdS/ZnSe core/shell and CdS/CdZnS-ZnCdSe/ZnSe core/intermediate/shell nanoparticles

Due to promising applications in optoelectronic devices of type-II semiconducting nanocrystals, much effort has been made to study this material system. Many previous works investigated their UV-vis absorption and photoluminescent (PL) features at room temperature. However, no detailed study on temperature-dependent PL and Raman-scattering (RS) spectra has been carried out to learn about their optical properties. To further understand this problem, we prepared CdS, CdS/ZnSe core/shell (C/S), and CdS/CdZnS-ZnCdSe/ZnSe core/intermediate/shell (C/I/S) nanoparticles, and then studied their Raman, UV-vis absorption and PL properties. Spectroscopic studies at room temperature demonstrated the C/S and C/I/S samples belonging to the type-II structure. Detailed analyses of their PL spectra in a temperature range of 10-300 K indicated unusual changes in the spectral parameters, particularly at temperatures from about 160 to 250 K. Based on the results obtained from temperature-dependent RS studies, we suggest that such the unusual variations are predominantly due to compressive strain generated from the difference in the thermal expansion coefficients of the core and shell parts, and surroundings. We also point out that the intermediate layer formed between the core and shell plays an important role in reducing lattice strain, and can improve the PL quantum yield.