Automated and scalable inline two-stage synthesis process for high quality colloidal quantum dots

By Michael Berger. Copyright © Nanowerk

longpredicte(Nanowerk Spotlight) Colloidal quantum dot (CQDnanocrystals are attractive materials for optoelectronics, sensing devices and  third generation photovoltaics, due to their low cost, tunable bandgap – i.e.  their optical absorption can be controlled by changing the size of the CQD  nanocrystal – and solution processability. This makes them attractive candidate  materials for cheap and scalable roll-to-roll printable device fabrication  technologies.


One key impediment that currently prevents CQDs from fulfilling  their tremendous promise is that all reports of high efficiency devices were  from CQDs synthesized using manual batch synthesis methods (in classical  reaction flasks).


Researchers have known that chemically producing nanocrystals  of controlled and narrow size-distributions requires stringent control over the  reaction conditions – e.g. temperature and reactant concentration – which is  only practical for small-scale reactions.


Such a synthesis is extremely  difficult to scale up, hence very costly to mass produce without severely  compromising quality.   The reason for this is that, just like rain droplets,  nanocrystals form sequentially by ‘nucleation’ and ‘growth’. Both these  phenomena are highly sensitive to temperature and reagent concentration.  Moreover, nucleation and growth must occur at substantially different  temperatures and, in fact, to obtain nanocrystals of uniform sizes, one must be  able to rapidly cool down the reaction from the nucleation temperature to the  growth temperature.


Hence, the quality of the product is contingent upon how  well and fast one can homogenize the reactor, both chemically and thermally.   Unfortunately, the only way to scale up batch reactors is by  increasing their volume, whereupon it becomes difficult to homogenize the  reactor and impractical to rapidly cool. The end result is nanocrystals of  low-quality and broad size distributions, which are not useful for fabricating  devices.

Some researchers have sought to circumvent this limitation by  conducting the reactions in narrow fluidic channels (less than a 1 mm in  diameter) while the reactants are continuously pumped through the channels, so  called ‘continuous-flow reactors’.


Conceptually, this scheme has several advantages. Narrow-width  channels afford uniform heating and mixing of the reaction, while the reaction  is scalable by simply increasing channel length and pump rate of the reagents.  This sort of scaling does not effect the quality of the product, because the  channel width, and hence the effective reaction volume, remains the same.  Despite these advantages, most attempts to use continuous-flow reactors in the  past have resulted in nanocrystals with a much lower quality than the batch  produced ones.


“We have analyzed the nucleation and growth of CQDs in  continuous-flow reactors and realized that, in order to achieve controllable  size and narrow size-distributions, one must employ two temperature stages in  the reactor: one for nucleation, and another for growth,” Osman Bakr, an  assistant professor in the Solar & Photovoltaics Engineering Research Center at King  Abdullah University of Science and Technology (KAUST), tells Nanowerk.

“By  separating these two crucial steps in the formation of the CQDs in time,  temperature, and space, we were able to obtain very high quality nanocrystals,  as good as the best batch synthesis, by a process that is low-cost,  mass-producible, and automated.”

Schematic of a conventional batch synthesis setup and a dual-stage continuous flow reactor setup



Schematic of (a) a conventional batch synthesis setup and (b) a  dual-stage continuous flow reactor setup with precursor A (Pb-oleate,  octadecene) and precursor B (bis(trimethylsilyl) sulfide in octadecene).  (Reprinted with permission from American Chemical Society)


Reporting their findings in ACS Nano (“Automated Synthesis of Photovoltaic-Quality Colloidal Quantum  Dots Using Separate Nucleation and Growth Stages”), Bakr and his team  demonstrated the quality of the CQDs produced by their method by using them to  make CQD-based solar cells that showed very high efficiencies.


“In this paper, we developed an automated, scalable, in-line  synthesis methodology of high-quality CQDs based on a flow-reactor with two  temperature-stages of narrow channel coils,” says Professor Ted Sargent from the  University of Toronto who, together with Bakr, led this work. “The flow-reactor  methodology not only enables easy scalability and cheap production, but also  affords rapid screening of parameters, automation, and low reagent consumption  during optimization. 

Moreover, the CQDs are as good in quality and device  performance as the best CQDs that are produced in the traditional batch  methodology.”   The team also developed a general theory for how one can use the  flow-reactors to finely tune the quality and size distribution of the CQDs, and  explained why previous attempts of using flow-reactors based on a  single-temperature-stage, as opposed to a dual-temperature-stage, necessarily  produce CQDs of low-quality and broad size distribution.


This work paves the way towards the large-scale and affordable  synthesis of high-quality CQD nanocrystals in tunable sizes, enabling  photovoltaics, light-emitting diodes, photodetectors, and biological tagging  technologies that take advantage of the nanoscale properties of those promising  materials.


“Over the last ten years we have seen tremendous advancements in  software and computer integration, in items that we use in our everyday lives,”  says Bakr. “Flow-reactors as a platform are ideally placed to take advantage of  this trend. Software that automates the routines of flow-reactors already  exists. In the near future, researchers will be able to run and monitor hundreds  of experiments to produce CQDs from home using a mobile app.


Moreover, because  flow-reactors contain very few moving parts, essentially just programmable  pumps, I expect that it will become an automated research platform that most  labs studying nanocrystals can afford.”   “Our work has shown that flow-reactors can produce nanocrystals  that are as good as the best batch produced reactions, with exquisite control  over reaction conditions,” he adds. “We believe that this will encourage the  nanomaterials community to take advantage of the enormous productivity gains in  R&D afforded by flow-reactors, which other chemical industries, such as  pharmaceuticals, are currently utilizing earnestly.”

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