Publication date: 15th May 2025
Metal halide perovskite quantum dots (QDs) are bright narrowband emitters that offer outstanding optoelectronic properties based on size manipulation [1,2]. Despite this, size control has relied on empirical selection of binary acid-base pairs (e.g., trioctylphosphine oxide (TOPO) and oleic acid (OA)) to solubilise precursor metal halide salts (PbX2) alongside unsystematic reaction conditions due to sub-second crystallization rates restricting the evaluation of precursor transformation kinetics thus evolution of QD size (dispersity) [3,4,5]. To address limited access to early-time kinetics and precursor chemistry, a bespoke helical microfluidic platform is developed to spectroscopically (in-situ) monitor the growth evolution of crystallites across three orders of magnitude, from milliseconds to minutes, of synthesis time using dynamic flow ramping [6]. We find that crystallisation proceeds through dissociative ligand interchange in which abstraction of Br anions from PbX2(TOPO)n complexes, via deprotonation of OA, allows the sub-second evolution of PbB species; a predecessor to monomer formation. Through kinetic analysis of preceding steps, modest activation energies (5-15 kJ/mol) are extracted elucidating their fast formation under the availability of excess OA, [TOPO]:[OA] < 1, otherwise restricted through the generation of hydrogen bonded adducts, [TOPO]:[OA] > 1. Moreover, we showcase a simultaneous fast equilibrium between PbX2 and TOPO whereby the degree of coordination (n = 1-4) is found to destabilise Pb-X bonds whilst concurrently strengthening Pb-O restricting TOPO removal hindering QD growth as supported by quantum chemical calculations and 207Pb NMR. In turn, early-stage (300-1000 ms) growth of highly confined quasi-spherical QDs (~2 nm) are accelerated by excess addition of OA whilst increased [TOPO]:[PbX2] depresses growth. Beyond the role in precursor conversion, OA further facilities late-stage (> 5 s) surface reconstruction (ripening) in weakly confined (~8 nm) crystallites. Correlating time-resolved bulk compositional analysis with time-resolved photoluminescence reveals healing of Cs-vacancies yielding a twofold increase in PLQY, 20% to 50%. Overall, our study exacerbates the kinetic role of solubilising ligands in synthesising crystallising bright and monodispersed QDs of the desired size.
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