Microfluidic reactors for the synthesis of nanocrystals

Abstract

Several microfluidic reactors were designed and applied to the synthesis of colloidal semiconductor nanocrystals (NCs). Initially, a simple single-phase capillary reactor was used for the synthesis of CdSe NCs. Precursors were delivered into a section of the capillary maintained at high temperature where they decomposed and reacted to form NCs. Monodisperse, bright CdSe NC samples were prepared over a significant range of average sizes. The excellent stability and reproducibility of the continuous flow system was also demonstrated. However, a limitation of the single-phase flow approach was that slow mixing and large residence time distributions can be detrimental to the overall quality (eg. size distribution) of the NC samples produced. These limitations were overcome by designing and fabricating in silicon a gas-liquid segmented flow microreactor. In contrast to the single-phase flow, recirculation within the liquid segments provides a mechanism to exchange fluid elements located near the channel walls with those in the center. This recirculation has the dual of effect of reducing axial dispersion and greatly improving the mixing efficiency - factors which have a strong influence on the ultimate size and size distribution of NCs produced.

(cont.) Compared to single-phase operation, preparation of CdSe NCs in segmented flow resulted in superior reactor throughput and narrower size distributions. Finally, the segmented flow method was extended in a microreactor designed for the synthesis of more complicated NC architectures. The design incorporated multiple inlet channels, which allowed for continuous injection of multiple precursor streams. This reactor was used to synthesize several core/shell NC structures - CdSe/ZnS, CdSe/ZnSe, and CdSe/CdxZnl-xSe.