L-cysteine-induced photoluminescence enhancement of CdSe/ZnSe quantum dots in aqueous solution.

L-cysteine molecules dramatically enhance the photoluminescence of colloidal CdSe/ZnSe quantum dots (i.e., (CTAB/TOPO)QD). Based on our spectroscopic studies of temporal variations in QD quantum yields as well as the in situ infrared spectral features of QDs, we propose that adsorption and rearrangement of L-cysteine molecules at the QD-water interface induces the observed unusual enhancement of the photoluminescence quantum yield. Upon addition of L-cysteine to the (CTAB/TOPO)QD solution, the adsorption of L-cysteine to the (CTAB/TOPO)QD colloidal particles is driven by the formation of a kinetically favorable intermediate species, which is formed by the coordination of thiol groups to the QD surface Cd atoms. The above species then reacts further to form a thermodynamically stable QD species, which probably involves coordination of both the amine and thiol groups of L-cysteine on the QD surface. Additional comparison studies using (MPA)QD and other small ligands (i.e., L-alanine, L-serine, and MPA) confirmed our proposed mechanism of L-cysteine adsorption at the (CTAB/TOPO)QD-water interfaces. In addition to these adsorption structures, we also propose that the dramatic enhancement of QY observed in this study is probably induced by the rearrangement and structural organization of L-cysteine and CTAB molecules at the QD-water interface, which improves the homogeneity and self-organization of the interfacial molecules.