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Milad Abolhasani1

1, NC State University, Raleigh, North Carolina, United States

Since the discovery of colloidal metal halide perovskite quantum dots (QDs) three years ago, [1] they have rapidly grown to become one of the most promising classes of nanomaterials for applications in low-cost and highly efficient optoelectronic devices. Anion exchange reactions of the highly luminescent perovskite QDs provide a facile post-synthetic route for tuning of the absorption/emission bandgap of these exciting nanocrystals. The post-synthetic anion exchange reactions allow precise bandgap tuning of perovskite QDs tailored for the desired application. Synthesis, screening, and optimization of colloidal QDs are conventionally conducted using the time- and material-intensive flask-based approaches. Process optimization is therefore limited by the sampling rate, off-line analysis time, and batch reactor/reaction process control. Batch reactors suffer from mixing and heat transfer inefficiencies that degrade the resulting physicochemical properties of the QDs and worsen with the reaction scale (i.e., polydispersed nanocrystals after scale-up).
Our group has recently developed a modular intelligent flow reactor integrated with a translational in situ spectral monitoring probe for continuous synthesis and systematic studies of the colloidal synthesis and anion-exchange reactions of perovskite QDs. [2] Utilizing the developed flow synthesis platform, we have demonstrated, for the first time, a mixing-controlled growth kinetics of cesium lead tribromide perovskite nanocrystals.[2]
The intelligent flow synthesis platform consists of modular heating units equipped with a unique in-situ translational flow cell (UV-Vis absorption and fluorescence spectroscopy). The translational movement of the spectral monitoring probe along the tubular reactor decouples the effect of early timescale mixing of QD precursors from the residence time (i.e., growth time) along the microreactor. Automated sampling along the continuous flow reactor enables rapid photoluminescence and absorption spectra sampling across 68 ports (i.e., reaction times) spanning residence times ranging four orders of magnitude – from 100 ms to 17 min. Varying the average droplet velocity moving in the flow reactor tunes the degree of QD precursor mixing within droplets, resulting in perovskite nanocrystals with different optical properties.
The developed flow synthesis approach enables rapid discovery, screening, and optimization of perovskite QDs with desired optoelectronic properties via high-throughput screening (>10,000 experimental conditions per day) of the accessible synthesis parameter space. In addition, the modularity of the developed QD synthesis platform enables direct scale up using a numbered-up strategy (i.e. parallel flow reactors) for large-scale continuous nanomanufacturing of high-quality perovskite QDs.

[1] Protesescu et al., Nano Lett. 15, 3692–3696 (2015).
[2] Epps et al., Lab Chip 17,4040–4047 (2017).

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