Investigation on type-II Cu ₂ S–CdS core/shell nanocrystals: synthesis and characterization

Type-II Cu2S–CdS core/shell (including Cu2S/CdS and CdS/Cu2S) semiconductor nanocrystals (NCs), which can produce efficient spatial separation of electrons and holes between the core and shell, were successfully synthesized. The results from absorption spectra, photoluminescence (PL) spectra, X-ray photoelectron spectra (XPS), X-ray diffraction (XRD), transmission electron microscopy (TEM), and PL lifetime measurements all confirmed the formation of the type-II core/shell NCs. The emission wavelength of the type-II Cu2S/CdS core/shell NCs can be readily tuned between 515 nm and 760 nm through two methods: core size control and shell thickness control. The highest PL quantum yield that the Cu2S/CdS core/shell NCs reached was ∼12% by controlling the shell thickness, while the core Cu2S nanocrystals had no PL at all. The PL decay data revealed that Cu2S/CdS NCs had a dual exponential characteristic and the PL emission with the longest lifetime (more than 1400 ns) occupied more than 85%. After overcoating ZnS, a Cu2S/CdS/ZnS core/shell1/shell2 structure was formed, which resulted in a blue-shift of the emission wavelength. Using the synthesis strategy of Cu2S/CdS, CdS/Cu2S type-II NCs also produced tunable emissions in the visible range.