Accelerated Synthesis of Colloidal Quantum Dots in Multi-Stage Microfluidic Reactors

Since their discovery more than three decades ago, quantum dots (QDs), have been used as a high-performing solution-processed optical/optoelectronic material in a multitude of applications across energy, chemical, and biomedical industries. The success of QDs in devices has been driven by their intriguing optical and optoelectronic properties that can be precisely tuned via controlling their morphology, size, and chemical composition. However, recent environmental and health restrictions have prioritized the synthesis of potentially benign QD alternatives (<i>e.g.</i>, indium phosphide, InP) to QDs containing carcinogenic elements. Specifically, core-shell InP/ZnSe/ZnS heteronanostructure is considered a promising non-toxic material candidate that could reach desirable optical properties (high quantum yield and narrow emission linewidth) similar to the cadmium-based QDs. Despite the potential of InP-based QDs for next-generation photonic devices, their high-dimensional design space because of the multi-stage synthesis nature makes it extremely challenging for manual batch reactors to comprehensively explore. An attractive platform for the accelerated synthesis space exploration and intensified manufacturing of emerging colloidal QDs are continuous flow strategies, due to their facile integration with in-situ characterization techniques, intensified heat and mass transfer rates, high material efficiency, and superior reproducibility when compared to traditional batch synthesis techniques. However, matching the QD quality obtained in multi-stage batch reactors and microfluidic reactors remains a challenge. Due to the significant difference in the microreaction environments of batch and flow reactors, solely “translating” the synthetic process parameters and protocol such as temperature profiles as well as rate and sequence of precursor addition and residence times in continuous multi-stage flow reactors is not sufficient to achieve high performing QDs.<br/>In this work, we present a modular library of 2D and 3D flow reactors to achieve maximum flexibility and reconfigurability for the controlled and intensified synthesis of InP QDs. We demonstrate the significance of both time- and temperature-to-distance transformations achieved through the reconfiguration of our flow reactor modules to optimize the multi-stage QD synthetic route. Thus, achieving high-quality InP QDs in continuous flow reactors. We report a systematic study of the multi-stage flow synthesis of InP QDs, utilizing <i>in-situ</i> absorption spectroscopy <i>via</i> both hot-injection and heat-up synthesis methods followed by the analysis and optimization of reaction cycling for sequential core growth. Furthermore, the resulting continuous flow synthesis protocol is up to an order of magnitude shorter than previously reported batch protocols. Further adoption of the developed modular flow chemistry strategies in this work could provide access to the unexplored design and synthesis space regions of the other colloidal QDs beyond InP QDs.