Synthesis of Lead Halide Perovskite Nanocrystals for Studying Their Optical, Magnetic and Charge Transfer Properties

Abstract

Lead halide perovskite (LHP) nanocrystal (NC) has emerged as one of the most promising materials in the last decade as the sources of photons and charges. This class of materials is naturally defect tolerant that leads to its high photoluminescence quantum yield without the need of extra shelling. The ease of solution process of LHP NC makes it even more attractive for a broad range of applications. For both fundamental studies and practical applications of LHP NCs, control over the quantum confinement, size distribution and dimensionality of the material is of great importance as they determine the properties of the material. The general strategy to synthesize semiconductor NCs with desired size and shape is to separate nucleation and growth processes and control their kinetics. However, the growth of LHP NCs is too fast to be controlled by quenching the crystal formation reaction in practice.

A new synthetic approach based on controlling the thermodynamic equilibrium of LHP NCs formation has demonstrated its capability to prepare highly uniform, strongly quantum confined LHP NCs in desired dimensionality. More specifically, the size of LHP NCs is determined by the chemical equilibrium of halide ions between NC lattice and their surrounding solution. The highly labile lattice of LHP NC and the size dependent halide concentration of the NC enable the control of size based on thermodynamic equilibrium. When halide is highly concentrated in the surrounding solution of the NCs, the thermodynamic equilibrium overrules the NC growth kinetics and only smaller NCs can exist due to the higher halide concentration in these NCs. LHP quantum dots (QDs) were synthesized first with this strategy in a hot injection to yield uniform NCs in the strongly quantum confinement regime. These QDs form superlattices easily due to their size uniformity. At low temperature where halide diffusion is slower, thermodynamic equilibrium could be combined with the anisotropic growth kinetics to prepare LHP nanowires, nanoribbons and nanoplatelets.

Mn2+ doped LHP NCs has been of interest from the many optical and magnetic properties that introduced by dopants. Quantum confinement is important for these properties because that dopant-exciton interaction increases with the strength of quantum confinement. Thermodynamically controlled synthesis has made uniform and strongly quantum confined LHP NCs available. Further modification of the hot-injection procedure and post-synthetic doping have been pursued to generate Mn2+ doped quantum confined LHP NCs. Excitons under the effect of magnetic dopants was then studied in Mn-doped CsPbI3 QDs to show the acceleration of dark exciton by the magnetization of dopants.

Additionally, LHP NCs as sources of charges have been studied. Charge transfer from LHP NCs is naturally more efficient than other semiconductor NCs for being defect tolerant. The unique light-induced anion exchange of LHP NCs with solvent molecules such as dihalomethane further allows the balance of redox reactions when charges are extracted. The facile anion exchange of LHP NCs is an important driving force for the electron extraction thus protects the oxidation reaction product from being consumed by charge recombination. This shows the great potential of LHP NCs as the source of charges to perform photo-redox reactions over the previous semiconductor NC systems.