(328d) Microfluidic Studies of Colloidal Atomic Later Deposition

Colloidal atomic layer deposition (cALD) is a versatile technique for the solution-phase, room temperature, formation of complex heterostructure nanoparticles.^[1] With cALD, a multi-composition shell can be grown from a starting nanocrystal layer-by-layer through a sequence of independent, self-limited half cycles that include growth reagent injection of often immiscible precursors and nanocrystal solution washing. Despite the limited number of existing studies, the versatility of this method has allowed for the formation of a diverse set of complex nanostructures including Au/CdS, Ag/AlO_X, CeO₂/AlO_X, SiO₂/AlO_X, SiO₂/AlPO₄, CdSe/CdS, CdSe/ZnS, PbS/CdS, HgSe/CdSe, HgSe/CdS, HgSe/CdSe, and CsPbX₃/AlO_X.^[2] However, a detailed study of this powerful synthetic route remains challenging due to a variety of factors. Relative to well-developed gas phase ALD processes, it is difficult to quantify shell growth in cALD with online characterization techniques due to the sensitivity of nanoparticle electronic and optical properties to a wide variety of factors, such as surface defects and ligand coverage. Beyond the difficulty of qualifying half reaction cycles as self-limiting, there are factors that make achieving cALD cycles challenging, including the formation of secondary reactive species and the complexing of stabilizing surface ligands with excess reactants. Therefore, cALD processes require controlled reagent delivery and precise reaction times and washing protocols to perform consistently. These drawbacks are compounded by the substantial labor requirement for the conduction of cALD experiments. Conducting five, full cALD cycles alone can require 45 separate reagent injections and phase removals in series, which conducted manually are difficult to perform consistently. Combined with a large parameter space, the labor-intensive requirements of conducting cALD experiments manually make complete cALD space exploration and optimization impractical through traditional experimental means. In response, we present a reconfigurable microfluidic platform for accelerated fundamental and applied studies of cALD.^[3]

Using the shelling of cadmium sulfide (CdS) onto cadmium selenide (CdSe) quantum dots (QDs) as a case study, we developed a single droplet flow chemistry platform to automatically investigate the full cALD cycles in sequence, using only a 10 mL droplet. The developed platform is fully modular in design and contains a reagent addition, an oscillatory reactor, online spectral monitoring, and in-line phase separation module. Using this reactor, reagent addition sequences can be varied, and phases separated and purified, without reactor reconfiguration or user intervention. A typical full reaction cycle includes the addition of oleylamine in toluene, followed by the addition and removal of sodium sulfide in formamide, washing of the reaction phase droplet with formamide, addition and removal of cadmium acetate in formamide, then additional formamide washing. Following each reagent addition, the biphasic droplet is oscillated in the reactor module by alternating between forward and reverse flow of the inert gas carrier phase. Reaction progress is monitored through a UV-Vis absorption and photoluminescence flow cell positioned at the end of the reactor coupled with automated spectra feature extraction algorithms, which are used to monitor the first absorption peak position, emission peak position and intensity, linewidth, and droplet phase lengths.

We utilized the single-droplet flow chemistry platform for the rapid mapping of the of the cALD experimental space and monitored the excitonic peak shift and kinetics during each cALD half cycle. With the optimized conditions established, 9 full cALD cycles, consisting of 63 consecutive reagent injections and phase separations, were conducted on a single droplet and monitored continuously over more than 500 optical spectra. The layer-by-layer addition of CdS in these 9 cycles resulted in a first absorption peak shift of 70 nm without losing any of the optical features of the starting QDs.

The modular flow chemistry platform presented in this work offers facile access to high-dimensional reaction spaces that are otherwise highly challenging to optimize and explore. Further implementation of the designed single-droplet system could help to unlock more complex heterostructure synthesis routes through high efficiency screening across a wide range of nanoparticles. Furthermore, this reactor design has direct applications in many dynamic multistage and multiphase reaction processes throughout nanoscience and chemistry.

References

[1] J. Am. Chem. Soc. 2012, 134, 45, 18585–18590

[2] ACS Materials Lett. 2020, 2, 9, 1182–1202

[3] Adv. Mater. 2021, 33, 2004495