Over the past decade, perovskite quantum dots (PQDs) have garnered considerable attention in the field of optoelectronics. Merely semiconductor nanocrystals (NC) which exhibit quantum confinement due to their diminishing size (~5 nm), such characteristics give rise to desirable properties which have seen their use extend from photovoltaics (PV) to light-emitting diodes (LEDs). Often, such properties are strongly tied to NC size and shape, offering a facile route via synthesis in modifying electronic properties. Despite this apparent simplicity, governing control over size and shape is no easy feat in design given the absence of well characterised formation kinetics which shroud synthetic pathways. By attempting to harness the understanding of crystallisation kinetics, that is the speed and pathway the reaction advances, we extend the performance of optoelectronic devices.


Synthesis of colloidal PQDs is frequently carried out by combining appropriate molecular precursors in the presence of ligands with an appropriate solvent. Upon sufficient heating, two key steps follow. First is nucleation, this can be viewed as the formation of scaffolding seed crystals which self-assemble as a consequence of precursor molecules clustering together. Thermodynamically driven, this step occurs at very fast timescales (milliseconds) and is responsible for dictating number of nuclei and final size distribution of NCs. The uncapturable, truly intrinsic, nature of the system during this step is first hurdle in gaining kinetic insights due to its velocity. Moreover, this is not aided by the fact that perovskite NCs exhibit ionic bonding, which compared to covalent systems, showcase accelerated crystallisation kinetics.
Continued supply of precursor clusters, monomers, towards nuclei sets the second stage of formation termed growth. Here, multiple factors dictate the final size and shape of crystallites; one of which is time. With increasing duration, monomers in solution become depleted as they are consumed by growing crystallites eventually reaching a state where no further monomers are available. As a result, due to thermodynamic limitations, smaller crystallites begin to dissolve and resupply monomers to larger crystallites causing the skew in the size distribution of as synthesised NCs. Making a distinction as to when this coarsening process begins is difficult due to the absence of sensitivity of current methods in identification.


During my PhD, I will be attempting to probe both the nucleation and growth steps of perovskite NCs using a kinetic platform. Consisting of a helical microfluidic reactor and in-situ spectroscopic measurement, this platform takes advantage of fast diffusive mixing allowing for intrinsic measurement of kinetics avoiding heterogeneity of batch mediated investigations. Through this, we hope to shed light on the kinetic and thermodynamic aspects associated with NC formation from nucleation to growth.

Figure is taken from: Narayan Pradhan ACS Physical Chemistry Au 2022 2 (4), 268-276 DOI: 10.1021/acsphyschemau.2c00001

Tariq Hussein

NanoDTC PhD Student, c2022