Tuning the postfocused size of colloidal nanocrystals by the reaction rate: from theory to application

We show that adjusting the reaction rate in a hot injection synthesis is a viable strategy to tune the diameter of colloidal nanocrystals at the end of the size distribution focusing, i.e., the postfocused diameter. The approach is introduced by synthesis simulations, which describe nucleation and growth of colloidal nanocrystals from a solute or monomer that is formed in situ out of the injected precursors. These simulations indicate that the postfocused diameter is reached at almost full yield and that it can be adjusted by the rate of monomer formation. We implement this size-tuning strategy using a particular CdSe quantum dot synthesis that shows excellent agreement with the model synthesis. After demonstrating that the reaction rate depends in first order on the Cd and Se precursor concentration, the proposed strategy of size control is explored by varying the precursor concentration. This enables the synthesis of colloidal nanocrystals with a predefined size at almost full yield and sharp size distributions. In addition, we demonstrate that the same tuning strategy applies to the synthesis of CdS quantum dots. This result is highly relevant especially in the context of reaction upscaling and automation. Moreover, the results obtained challenge the traditional interpretation of the hot injection synthesis, in particular the link between hot injection, burst nucleation, and sharp size distributions.