Supercritical Continuous‐Microflow Synthesis of Narrow Size Distribution Quantum Dots

Colloidal semiconductor quantum dots (QDs) have been used in biological imaging, electroluminescent devices, and lasers due to their size-tunable optical properties and chemical stability. Most of these applications require highly crystalline samples with narrow size distributions, which are often difficult to achieve in a single-step batch process with its often poor control of reaction conditions. Synthesis of QDs in microfluidic devices offers several advantages over conventional macroscale chemical processes, including enhancement of mass and heat transfer, reproducibility, potential for sensor integration for in situ reaction monitoring, rapid screening of parameters, and low reagent consumption during optimization. Previous studies have realized the continuous synthesis of CdSe QDs at atmospheric pressure using single-phase laminar flow microreactor designs. One difficulty of these synthetic procedures is the requirement for solvents that can both dissolve the precursors at ambient conditions and also remain liquid over the entire operatingtemperature range (25 8C to 350 8C), significantly limiting the set of solvents, ligands, and precursors that are compatible with continuous flow systems. Furthermore, the solvents that are available are typically very viscous (500mPa s< h< 1500mPa s), leading to slow mixing, broad residence-time distributions (RTD), and as a consequence, broad QD-size distributions (typically >10%). Segmentation of the reacting phase with an immiscible phase can overcome such limitations by narrowing the RTD and improving reactant mixing. Application of segmented flow for continuous synthesis of narrowly distributed CdSe QDs has been previously demonstrated for liquid–gas, and liquid–liquid segmented flows. However, even with flow segmentation, limitations on the number of compatible chemistries and the limited number of available high-boiling-point solvents have been major obsta-