Publication date: 15th May 2025
Metal halide perovskite quantum dots (QDs) are bright narrowband emitters that offer outstanding optoelectronic properties based on size manipulation. Despite this, size control has relied upon empirical selection of binary acid-base pairs (e.g., trioctylphosphine oxide (TOPO) and oleic acid (OA)) used to solubilise precursor metal halide salts (PbBr2) alongside unsystematic reaction conditions due to sub-second crystallisation rates restricting the evaluation of precursor transformation kinetics thus evolution of QD size (dispersity). 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. We find that crystallisation proceeds through a first order dissociative ligand exchange by which abstraction of Br anions from PbBr2(TOPO)n complexes, via deprotonation of OA followed by TOPO displacement, allows the sub second evolution of highly reactive PbBr-3 species; a predecessor to monomer formation. Moreover, we observe a simultaneous binding equilibrium between PbBr2 and TOPO where the degree of coordination (n = 1-4) is found to destabilise Pb-Br bonds whilst concurrently strengthening Pb-O restricting TOPO removal as supported by quantum chemical calculations and 207Pb NMR. In turn, early (~300 ms) growth kinetics of highly confined quasi-spherical QDs (~2 nm) are accelerated by excess addition of OA whilst suppressed upon increasing TOPO:PbBr2. Beyond precursor kinetics, we additionally note the role of OA in late time (> 5 s) surface reconstruction (ripening) of weakly confined crystallites. By coupling bulk compositional analysis with time-resolved photoluminescence, we showcase secondary growth is consequence of healing of Cs-vacancies yielding a twofold increase in PLQY (20% to 50%) at later (5-20 s) reaction times. Overall, our study highlights the dual importance of solubilising ligands in synthesising crystallising bright and monodispersed QDs of the desired size.
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