Continuous Flow Synthesis of Lead Sulfide Quantum Dots for NIR/SWIR LED Applications

The development of new industrial applications in fields such as cell phones and automobiles depend on the ability to produce low-cost LEDs emitting in the NIR/SWIR range. A method for synthesizing large quantities of PbS/CdS core/shell quantum dots, known for their high photoluminescence, would enable this development.<br/>Since the first synthesis of PbS quantum dots (QDs) using the hot-injection method in 2003,[1] many efforts have been employed to develop monodisperse PbS quantum dots. Nowadays, the use of bis(trimethylsilyl) sulfide (TMS₂S) as sulfur precursor still remains very attractive due to the narrow size distribution giving rise to precise absorption/emission features in the NIR/SWIR region. However, scaling up this synthesis remains a challenge due to the difficulty of handling and the high toxicity of the H₂S gas released during synthesis using this sulfur precursor.<br/>Various sulfur precursors have been utilized to prepare PbS QDs, including elemental sulfur[2] and substituted thioureas[3]. The latter show a great potential as the reaction rate can be adjusted over several orders of magnitude by altering their substituents, which gives access to precisely controlled particle sizes in a wide range. Moreover, there are approximately 10⁴ different thioureas that can be synthesized in simple reactions using commercially available chemicals.<br/>To enhance the reproducibility of the syntheses and scale them up to larger quantities, continuous flow synthesis is an appealing alternative to widely used batch synthesis. Among other advantages, the strongly enhanced heat and mass transfer in small tubular reactors combined with controlled pressure can be cited. On the other hand, flow synthesis comes with several restrictions, which require in most cases an adaptation of the synthesis protocol. In particular, both the lead and sulfur precursors should be perfectly soluble at room temperature to avoid clogging, and the injected volumes should be balanced to keep consistent flow rates.<br/>In this study, we developed new synthesis conditions affording monodisperse PbS QDs via flow synthesis exhibiting identical optical properties as with classic batch synthesis, with excitonic peak to valley ratios &gt;3. Moreover, the excitonic peak position could be adjusted with the residence (=reaction) time, in contrast to batch reactions.<br/>The flow synthesis of highly luminescent PbS/CdS core/shell QDs was also achieved using ex-situ prepared cadmium oleate via the cation exchange method.<br/>Summarizing, the developed flow process gives access to grams of high-quality of PbS/CdS core/shell QDs in a couple of hours and in a highly reproducible manner, at wavelengths of particular interest for NIR QLEDs.<br/><br/>[1] M. a. Hines et G. d. Scholes, <i>Advanced Materials</i>, vol. 15, n^o 21, p. 18441849, 2003.<br/>[2] L. Cademartiri, J. Bertolotti, R. Sapienza, et D. S. Wiersma, <i>The Journal of Physical Chemistry B</i>, vol. 110, n^o 2, Art. n^o 2, 2006.<br/>[3] J. S. Owen, M. P. Campos, G. T. Cleveland, I. J.-L. Plante, et M. P. Hendricks, <i>Science</i>, vol. 348, n^o 6240, p. 12261230, 2015.