Entropic Ligand Mixing for Engineering 2D Layered Perovskite from Colloidal Monolayer Building Blocks

Layered 2D perovskites are solution-processed quantum-wells. Their effective band-gap is determined via the inorganic perovskite layer thickness and exciton quantum confinement effects. Alternatively, by changing the organic moieties, one can tune the dielectric constant and distance between the monolayers modifying the excitonic interactions. In colloidal perovskites, a dynamic equilibrium exists between the free organic moieties in the solution and the surface of the nanocrystal. Colloidal synthesis is used to make single monolayer L₂PbBr₄ platelets and assemble these into layered 2D stacks. In the experiment, L is an alkylamine surface ligand whose length (4-18 carbons) determines the interlayer distances between the quantum-wells. The dynamic equilibrium of ligand mixtures in solution and perovskite surfaces leads to optimal mixing of the molecules. During the self-assembly of monolayers, the distance between the inorganic layers is thus engineered. The interlayer distance is proportional to the average ligand mixture length. This results in controlled interactions between the 2D-excitons, enabling red-shifted absorption and emission and extended lifetimes for longer alkyl chains. Using entropic mixing of ligands for the engineering of 2D excitonic interactions is therefore demonstrated. Formation of layered 2D perovskites from colloidal building blocks allows intermixing of dissimilar materials opening possibilities for new heterostructures and junctions.