(159d) Miniature CSTRs for the Synthesis of InP-Based Semiconductor Nanocrystals

Quantum dots (QDs) or semiconductor nanocrystals have attracted significant attention due to their size-tunable photophysical properties.¹ The traditional and well-studied QD systems are Cd-based and Pb-based nanostructures, which are restricted in highly regulated applications.² Among the heavy-metal-free semiconductor nanocrystals, III-V QDs stand out as the most promising alternatives for the development of efficient optoelectronic devices.² Currently, highly pyrophoric and toxic phosphorous precursors are used to synthesize InP QDs,³ thus restricting access to scalable synthesis and a broader application of the material. To address these issues, the use of the commercially available reagent tris(diethylamino)phosphine has been proposed.^{4, 5} However, the scalable production of InP-based nanocrystals is limited since the synthesis of these nanostructures is typically performed in laboratory flask-reactors with inherent limitations regarding spatial and temporal variation of parameters inside the reaction mixture and product irreproducibility.⁶ Continuous flow or segmented flow systems can overcome the above-mentioned limitations; however, they are limited to reactions with rapid reaction kinetics and at the same time multi-dosing of precursors remains a challenging process.⁶ In contrast, a CSTR cascade configuration offers greater flexibility regarding multiple addition of reagents and reactions with slower reaction kinetics (reaction times in the range of minutes to hours).⁷ In addition, the homogeneous concentration and temperature profiles realized by strong agitation in each chamber can result in nearly ideal CSTRs in series RTD profiles and accurate predictability of reaction conversions. Such approach is beneficial for the introduction of new reagents in a controlled manner avoiding secondary nucleation, which would degrade sample quality, and yielding QDs with controllable compositions.

Herein, we use a series of micro-sized CSTRs for the multistep and high-temperature colloidal synthesis of various types of semiconductor nanocrystals, while focusing on the continuous formation of InP-based core-shell QDs. Temperature and stirring rate of precursors are independently controlled in each μCSTR. Residence time distribution (RTD) measurements have been performed to assess the mixing properties of the fabricated CSTRs. In all experimental conditions, the measured and ideal RTD profiles have shown high consistency. We have also demonstrated the applicability of the synthetic platform on the formation of binary QDs with product characteristics comparable to well-established batch and microfluidic techniques. Most importantly, we have re-engineered the continuous formation of InP-based core/shell QDs using a synthetic protocol involving a safer phosphine precursor. In this protocol, the μCSTR cascade configuration can handle reactions between 20 minutes and 3 hours, which are almost impossible to be employed in tube-based continuous flow reactors. Finally, we demonstrate the efficiency of the platform by tuning the optical properties of the particles by altering the reaction time, temperature, molar ratios of precursors and shell thickness. With this approach, we aim to deliver a safer, cost-efficient, and scalable continuous process for the formation of indium-based QDs for their subsequent use in optoelectronic applications.

References

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