Probing Early Time Crystallisation Kinetics and Photophysics of Colloidal Metal Halide Quantum Dots—A Time-Resolved Microfluidic Flow Study

Metal halide perovskite quantum dots (QDs) are bright narrowband emitters that offer outstanding photonic properties through the manipulation of their size. Despite this, size control has historically relied upon an iterative methodology (e.g., ligand selection) that lacks true kinetic insight into the transformation rates of precursors due to limitations of fast sub-second crystallisation times. More specifically, acid-base ligand pairs, such as oleic acid (OA) and trioctylphosphine oxide (TOPO), have been shown to play a pivotal role in modulating size and dispersity; yet no studies have examined the kinetic effects of the acid base equilibrium upon the crystallisation kinetics, limiting mechanistic understanding of their synthesis. To gain insight into their formation, a bespoke helical microfluidic platform is developed to spectroscopically monitor, in situ, the nucleation and growth of crystallites across three orders of magnitude, from milliseconds to minutes, of synthesis time. By coupling time-resolved spectroscopic data with ³¹P and ¹H NMR, we find that increased addition of free OA enhances the formation of hydrogen bonded adducts (TOPO-OA) which accelerate consumption of halide salts lending to the formation of highly confined, ~2 nm, monodispersed quasi-spherical QDs at early times, 300 ms. Primary growth of crystallites was observed to proceed through a discrete epitaxial, monolayer by monolayer, addition which was found to terminate about a reaction time of 2 s yielding quasi-spherical QDs. At late times, 2-5 s, crystallites observed facet-limited surface reconstruction evolving into weakly confined (~ 8 nm) cubic nanocrystals. By coupling bulk compositional analysis with time-resolved photoluminescence and absorption, we demonstrate secondary growth is consequence of healing of Cs-vacancies resulting in a twofold increase in PLQY (20% to 50%) at much later synthesis times, 10 s. Overall, our study highlights the dual importance of the TOPO:OA equilibrium in crystallising highly uniform bright QDs.