Crystal Structure, Morphology, and Surface Termination of Cyan-Emissive, Six-Monolayers-Thick CsPbBr₃ Nanoplatelets from X-ray Total Scattering

Abstract

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.

Bandgap determination from individual orthorhombic thin cesium lead bromide nanosheets by electron energy-loss spectroscopy

“…Figure 2 shows an ADF-STEM image from an edge of a thin NS. It is clear that the last row of atoms corresponds to Cs-Br planes, in agreement with the surface termination reported by Bertolotti et al 17 These correspond to (110) Cs-Br planes in the Pbmn structure. The termination has to be related to the preferential passivation of the ligands to Cs rather than Pb and is consistent with the results from the statistical-based quantification along the [001] projections.…”

“…These data are always above the curve predicted by DFT for the nanosheets, suggesting a different behavior in the case of quantum dots (3D confinement) compared to the nanosheets (1D confinement). The data point from orthorhombic nanoplatelets (NPs) from Bertolotti et al 17 () is also shown in Figure 3b. It agrees well with the present nanosheets behavior, and suggests this observation is related to the anisotropy of the nanoplatelets, with one dimension out of the three being confined as in our NS.…”

“…15 On the other hand, the orthorhombic structure (according to the Pbnm description) 16 was confirmed in the case of small nanoplatelets by X-ray total scattering experiments. 17 Platelets as thin as monolayers have been recently identified from optical data analysis, as a product of high-throughput robotic synthesis inside a mixture of other species. 18 In summary, different possible phases were reported for nanostructures of CsPbBr 3 with reduced dimensions.…”

Surface Termination of CsPbBr3 Perovskite Quantum Dots Determined by Solid-State NMR Spectroscopy

“…68 Kovalenko and co-workers have recently used wide angle total X-ray scattering and TEM to determine that colloidal CsPbBr3 nanoplatelets, different from the cuboidal morphology studied here, are likely CsBr terminated with Cs vacancies. 69 Also, models with alkylammonium ligands substituting into surface Cs sites have previously been considered as one of the most likely surface terminations for CsPbBr3 QDs. 23 In comparison, the Cs-terminated surface without substitution of ammonium into the Cs position gives an adequate fit of the 133 Cs dipolar dephasing curve, however, the sub-surface Pb sites are very distant and essentially no 207 Pb dipolar dephasing is predicted, in contradiction with the experiment (Figure 4B and Figure S14).…”

Structure, Morphology, and Faceting of TiO2 Photocatalysts by the Debye Scattering Equation Method. The P25 and P90 Cases of Study

“…The DSE method, enabling calculation of the WAXTS trace of any kind of nanomaterials, was used by us, and others, to derive statistically sound ensemble properties. These include size and shape average values and variances, aspect ratio, faceting, and structural defectiveness, for a large variety of either pure crystalline nanophases or complex hybrids of technologically relevant classes of materials (semiconductors, oxides, perovskites, bioceramics, and metals), including TiO 2 nanoparticles prepared by sol-gel synthesis [17][18][19][20][21][22][23][24][25][26].…”