(662c) Spanning the Atomic to the Agglomerate Dimensions in Colloidal Dispersions of 2D Lead Halide Perovskites

This talk presents a systematic study of the colloidal microstructure of 2D perovskite nanocrystal dispersions at multiple length scales to identify and control their dynamic evolution. Hybrid organic-inorganic perovskites (HOIPs) are promising semiconductors with long carrier lifetimes, synthetic tunability, remarkable defect tolerance and simple processing. Recently, 2D analogues have been made by adding insulating cations (e.g. butylammonium) to separate the perovskite structure into sheets. These 2D materials can be formed either in thin films or as dispersed nanoplatelets of only a few PbX₆ octahedra thick. The number of PbX₆ octahedra confined between insulating layers (i.e. the ‘n-value’) can be tuned by varying precursor stoichiometry, enabling discrete tunability of the bandgap through dielectric and quantum confinement. However, it is challenging to control the polydispersity of crystallites with different confinement dimensions. Monodisperse colloids of single (n=1) and double (n=2) layered 2D nanocrystals can be grown by antisolvent precipitation, but for n > 2 the nanocrystal thickness is a distribution of n-values. Moreover, this distribution can change in both average n-value and variance with the choice of insulating molecules, particle concentration, dispersion aging or deposition onto a substrate. Using optical absorption and emission of dispersions, along with XRD and TEM of deposited films, we find that the selection of insulating molecules controls the distribution of n-values and agglomeration behavior. A mixture of shorter alkylammonium cations (e.g. butylammonium) and longer, bulkier alkylammonium cations is found to systematically narrow the distribution of n-values in 2D perovskite dispersions, even for n > 2. We present a combined transmission wide-angle x-ray scattering (WAXS) and small angle neutron scattering (SANS) study to demonstrate the role of insulating cation size, solubility and motility on the atomic structure and self-assembly of dispersed 2D perovskites. This study provides a novel framework to discuss the length-scales of growth for confined 2D perovskite colloids, and suggests new avenues to control phase, morphology and dispersion properties for solution-processed optoelectronics.