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)/ZnSe_N, 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)/ZnSe_N samples (with d = 3 and 6 nm and N = 2) at 300 °C for various times t _an = 10–600 min, we created an intermediate layer composed of Zn_1−xCd_xSe and Cd_1−xZn_xS 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)/ZnSe_N 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. (paper)