Continuous flow synthesis of PbS/CdS quantum dots using substituted thioureas

To enhance the reproducibility and scale up the synthesis of colloidal quantum dots (QDs), continuous flow synthesis is an appealing alternative to the widely used batch synthesis. Amongst other advantages, the strongly enhanced heat and mass transfer in small tubular reactors combined with controlled pressure can be cited. Nonetheless, the widespread use of this technique is hampered by special requirements such as the absence of solid or gaseous products and the room-temperature solubility of precursors. Therefore, the transfer of established reaction conditions from batch to flow is not straightforward and in most reported works the optical properties of the obtained QDs lag behind those prepared in batch reactions. This is also the case for PbS-based QDs, which are established near infrared (NIR) absorbers/emitters. Here we identified experimental conditions giving access to high-quality PbS core and PbS/CdS core/shell QDs obtained in an automated, easily scalable continuous flow synthesis. In particular, substituted thioureas have been selected as the sulfur source and ex-situ synthesized lead and cadmium oleate as the metal precursors, and appropriate solvent mixtures have been identified for each precursor. Highly luminescent PbS/CdS QDs emitting at the target wavelengths 940 and 1130 nm of special interest for NIR light-emitting diodes have been prepared, exhibiting a photoluminescence quantum yield up to 91%.