The mechanism of interactions between CdS/CTS QDs and Cu 2+ ions has been investigated using XRD (Figure ), FT-IR (Figure ), XPS (Figure , Figure ), TEM, and HR-TEM images (Figure ), STEM and EDX analysis/mapping (Figure ). XRD pattern of CdS/CTS QDs after interaction with Cu 2+ ions shows the prominent diffraction peaks of pure CdS/CTS QDs (Figure ). Furthermore, extra peaks at Bragg's diffraction angles were also observed corresponding to the diffraction peaks of CuS (JCPDS no. 79-2321, Figure ). This observation indicates that Cu 2+ replaces Cd 2+ in CdS/CTS QDs (i.e., formation of Cu_CdS/CTS) or coordinated to the functional groups of CTS. There is no dramatic change in the particle size of CdS/CTS after interaction with Cu 2+ ions. TEM and HR-TEM images show aggregation of quantum dots with particle sizes less than 10 nm (Figure ).