Facile synthesis of composition-gradient Cd1-xZnxS quantum dots by cation exchange for controlled optical properties

Three different types of water-soluble Cd1-xZnxS quantum dots (QDs) having nearly identical sizes and compositions have been synthesized via simple and low-cost methods to understand the effect of internal structures on the optical properties of QDs. Compared with the other two types, CdS@ZnS core-shell Cd1-xZnxS QDs and alloy Cd1-xZnxS QDs, composition-gradient CdS@ZnS core-shell Cd1-xZnxS (G-Cd1-xZnxS) QDs, which have been prepared by exchanging the Cd2+ ions of CdS QDs partially with Zn2+ ions conserving the shapes and sizes, have shown the longest lifetime and the highest quantum yield of photoluminescence due to the smallest nonradiative and the largest radiative decay constants of photogenerated charge carriers. The composition-gradient ZnS shells, which passivate the CdS cores optimally alleviating the lattice strain caused by the lattice mismatches between the CdS cores and the ZnS shells, have been considered to be the main reason for the enhanced optical properties of G-Cd1-xZnxS QDs. Among our prepared G-Cd1-xZnxS QDs, the quantum yield and the lifetime of photoluminescence are the highest (22%) and the longest (290 ns), respectively, due to the smallest nonradiative decay constant when about half of the Cd2+ ions in CdS QDs are replaced by Zn2+ ions with composition gradients from their surfaces, suggesting that internal structures play an important role in the relaxation dynamics of photoexcited charge carriers.