Optimization of synthetic conditions for the preparation of core-shell structures of CdTe/ZnSe QDs in water

Quantum Dots (QDs) are nanocrystals of semiconductor materials that, when entered into a quantum confinement regime, feature unique optical properties. Because they are nanometric structures (2-10 nm), they have high density of dangling bonds on their surface, causing surface defects that can compromise their fluorescent properties. The superficial modification of QDs with layers from another semiconductor is an alternative to reduce these surface defects. In the present study, CdTe QDs were synthesized in an aqueous system, and subjected to different coating approaches with the ZnSe semiconductor, aiming for a core-shell heterostructure. Two QDs with different stabilizers were used: mercaptusuccinic acid (MSA) and cysteamine (CYA). Some experimental parameters were evaluated to coat these QDs with ZnSe (by controlled precipitation in a colloidal aqueous medium), such as the molar ratio of precursors, pH, reaction temperature, and the volume/frequency addition of precursors to the reaction medium. The absorption and emission spectra of all systems were analyzed before and after their modification. In general, it was observed that for CdTe-MSA, it was seen that the coating at room temperature (~ 30 ° C), at pH ~ 10.5 and with the addition of the precursors divided into five portions, was the best synthesis condition for the best response sought. It was obtained a photoluminescence improvement greater than 300%, with small spectral change. However, for CdTe-CYA QDs the emission enhancement was much lower, around 55%. Thus, this study reinforces the importance of studying the experimental conditions to control the surface modification process of QDs for an optimized emission.