Lead halide perovskites are promising materials for a range of applications owing to their unique crystal structure and optoelectronic properties. Understanding the relationship between the atomic/mesostructures and the associated properties of perovskite materials is crucial to their application performances. Herein, the detailed pressure processing of CsPbBr perovskite nanocube superlattices (NC-SLs) is reported for the first time. By using in situ synchrotron-based small/wide angle X-ray scattering and photoluminescence (PL) probes, the NC-SL structural transformations are correlated at both atomic and mesoscale levels with the band-gap evolution through a pressure cycle of 0 ↔ 17.5 GPa. After the pressurization, the individual CsPbBr NCs fuse into 2D nanoplatelets (NPLs) with a uniform thickness. The pressure-synthesized perovskite NPLs exhibit a single cubic crystal structure, a 1.6-fold enhanced photoluminescence quantum yield, and a longer emission lifetime than the starting NCs. This study demonstrates that pressure processing can serve as a novel approach for the rapid conversion of lead halide perovskites into structures with enhanced properties.

The assembly of uniform nanocrystal building blocks into well ordered superstructures is a fundamental strategy for the generation of meso- and macroscale metamaterials with emergent nanoscopic functionalities. The packing of spherical nanocrystals, which frequently adopt dense, face-centred-cubic or hexagonal-close-packed arrangements at thermodynamic equilibrium, has been much more widely studied than that of non-spherical, polyhedral nanocrystals, despite the fact that the latter have intriguing anisotropic properties resulting from the shapes of the building blocks. Here we report the packing of truncated tetrahedral quantum dot nanocrystals into three distinct superstructures-one-dimensional chiral tetrahelices, two-dimensional quasicrystal-approximant superlattices and three-dimensional cluster-based body-centred-cubic single supercrystals-by controlling the assembly conditions. Using techniques in real and reciprocal spaces, we successfully characterized the superstructures from their nanocrystal translational orderings down to the atomic-orientation alignments of individual quantum dots. Our packing models showed that formation of the nanocrystal superstructures is dominated by the selective facet-to-facet contact induced by the anisotropic patchiness of the tetrahedra. This study provides information about the packing of non-spherical nanocrystals into complex superstructures, and may enhance the potential of self-assembled nanocrystal metamaterials in practical applications.

This article describes the use of gel permeation chromatography (GPC) as a means to separate natively capped colloidal CdSe and CdSe/Cd x Zn 1−x S quantum dots (QDs) from small-molecule impurities in hydrophobic solvents. A range of analysis techniques, including 1 H NMR, diffusion-ordered NMR analysis (DOSY), and thermogravimetric analysis (TGA) have been used to compare the nature and quantities of ligands adsorbed on the QDs after GPC and after alternative purification methods. We show that the GPC purified samples display lower ligand-to-QD ratio (135 oleate substituents per nanocrystal for CdSe QDs with lowest-energy absorption peak at 534 nm) than what we can achieve by the multiple precipitation/redissolution method, and the GPC purified samples are stable at both room temperature and high temperature (180−200 °C for CdSe QDs). The achievement of an efficient and highly reproducible method for the preparation of clean QD samples allowed us to test whether impurities that reside in samples prepared by standard purification methods have a significant effect on further surface modification reactions. We found that the reactivity of CdSe QDs toward precursors for CdS shell growth was profoundly affected by the presence of excess ligands in impure QD samples prepared by multiple precipitations and that the removal of excess ligands and impurities significantly improved the speed and reliability by which water-soluble CdSe/Cd x Zn 1−x S QDs could be prepared by ligand exchange with cysteine. GPC purification provides a preparative-scale, consistent, size-based purification of QDs without perturbing the solvent environment and as such could serve as the basis for advanced syntheses and enable detailed measurements of QD surface chemical properties.