Tunable luminescent emission characterization of type-I and type-II systems in CdS-ZnSe core-shell nanoparticles: Raman and photoluminescence study

We used wet chemical methods to synthesize core-shell nanocrystalline samples CdS(d)/ZnSeN, where d = 3-6 nm and N = 1-5 are the size of CdS cores and the number of monolayers grown on the cores, respectively. By annealing typical CdS(d)/ZnSeN samples (with d = 3 and 6 nm and N = 2) at 300 degrees C for various times t(an) = 10-600 min, we created an intermediate layer composed of Zn1-xCdxSe and Cd1-xZnxS alloys with various thicknesses. The formation of core-shell structures and intermediate layers was monitored by Raman scattering and UV-vis absorption spectrometers. Careful photoluminescence studies revealed that the as-prepared CdS(d)/ZnSeN samples with d - 5 nm and N - 2-4, and the annealed samples CdS(3 nm)/ZnSe2 with t(an) <= 60 min and CdS(6 nm)/ZnSe2 with t(an) <= 180 min, show the emission characteristics of type-II systems. Meanwhile, the other samples show the emission characteristics of type-I systems. These results prove that the partial separation of photoexcited carriers between the core and shell is dependent strongly on the engineered core-shell nanostructures, meaning the sizes of the core, shell, and intermediate layers. With the tunable luminescence properties, CdS-ZnSe-based core-shell materials are considered as promising candidates for multiple-exciton generation and single-photon sources.