Highly anisotropic colloidal CsPbBr3 nanoplatelets (NPLs) represent an appealing class of colloidal quantum wells with enhanced light emissivity. Strong quantum confinement imposed by the small platelet thickness and atomic flatness gives rise to enhanced oscillator strength, higher exciton binding energy, and narrow emission linewidth. While discrete thicknesses manifest themselves in discrete bandgap energies, fine-tuning of the emission energy can be achieved by compositional modulations. Here we address one of the most debated aspects of perovskite nanoplatelets: their crystal structure. Starting with the direct imaging by high-resolution electron microscopy (providing a clue on the pseudocubic faceting of the NPLs), we focus the study on X-ray total scattering techniques, based on the Debye scattering equation (DSE) approach, to obtain better atomistic insight. The nanoplatelets are six-monolayers thick and exhibit an orthorhombic structure. A thorough structure–morphology characterization unveils a specific orientation of the axial and equatorial bromides of the PbBr6 octahedra versus the NPLs thickness; we found that {010} and {101} planes of the orthorhombic CsPbBr3 lattice (Pnma space group) correspond to the six facets of the NPL, with basal planes being of {101} type. The NPLs undergo a lattice relaxation in comparison to cuboidal CsPbBr3 NCs; the major deformation is observed in the axial direction, which suggests a structural origin of the higher compliance along the b axis. The DSE-based analysis also supports a CsBr surface termination model, with half Cs sites and a half (or slightly more) Br sites vacant.

Colloidal nanocrystals (NCs) of fully inorganic lead halide perovskites (LHPs) are the latest generation of quantum-dot materials with compelling optical characteristics. Favored by the nearly regular cubic shape of NCs, they readily self-organize into highly ordered supercrystals (SCs) that have been stimulating in-depth investigation of uncommon collective properties at the fundamental level. Herein, we explore LHP NC SCs formed in colloidal suspensions and as solid materials, without any prior rigorous NC size-selection process. The spontaneous self-organization manifests itself in synchrotron wide-angle X-ray total scattering (WAXTS) measurements of randomly oriented ensembles of SCs by virtue of the substructure in the 100 C peak (in cubic notation). By developing atomistic models of SCs, we find an evidence that, in spite of the initial NCs size polydispersity (up to 20%), SCs of CsPbBr 3 and CsPbBr 1.5 I 1.5 NCs form through a size segregation process. The SCs exhibit high spatial structural coherence, that is, crystallographic alignment between NCs, along two and three directions, a degree of orientational order (the SCs mosaicity) below 2°, and coherent sizes on the order of tens of nanometers. In situ coupled photoluminescence (PL) and WAXTS experiments evidence pronounced electronic coupling between NCs in SCs, seen as an ca. 50−70 meV red-shifted emission in dried samples compared to the corresponding dilute solution of NCs.

The long search for nontoxic alternatives to lead halide perovskites (LHPs) has shown that some compelling properties of LHPs, such as low effective masses of carriers, can only be attained in their closest Sn­(II) and Ge­(II) analogues, despite their tendency toward oxidation. Judicious choice of chemistry allowed formamidinium tin iodide (FASnI3) to reach a power conversion efficiency of 14.81% in photovoltaic devices. This progress motivated us to develop a synthesis of colloidal FASnI3 NCs with a concentration of Sn­(IV) reduced to an insignificant level and to probe their intrinsic structural and optical properties. Intrinsic FASnI3 NCs exhibit unusually low absorption coefficients of 4 × 103 cm–1 at the first excitonic transition, a 190 meV increase of the band gap as compared to the bulk material, and a lack of excitonic resonances. These features are attributed to a highly disordered lattice, distinct from the bulk FASnI3 as supported by structural characterizations and first-principles calculations.

Characterization of functional nanocrystalline materials in terms of quantitative determination of size, size dispersion, type, and extension of exposed facets still remains a challenging task. This is particularly the case of anisotropically shaped nanocrystals (NCs) like the TiO2 photocatalysts. Here, commercially available P25 and P90 titania nanopowders have been characterized by wide-angle X-ray total scattering techniques. Synchrotron data were modelled by the reciprocal space-based Debye scattering equation (DSE) method using atomistic models of NC populations (simultaneously carrying atomic and nanoscale structural features) for both anatase and rutile phases. Statistically robust descriptors are provided of size, morphology, and {101} vs. {001} facet area of truncated tetragonal bipyramids for anatase, jointly to polymorph quantification. The effects of using the proper NC shape on the X-ray diffraction pattern are analyzed in depth through DSE simulations by considering variable bipyramid aspect ratios (resulting in different {101} vs. {001} surface) and relative dispersion in a bivariate manner. We demonstrate that using prismatic NCs having equal volume and aspect ratio as bipyramids provides reasonably accurate sizes and {101} and {001} surface areas of the parent morphology.