Wide-Angle X-ray Diffraction Evidence of Structural Coherence in CsPbBr₃ Nanocrystal Superlattices

Abstract

Films made of colloidal CsPbBr 3 nanocrystals packed in isolated or densely-packed superlattices display a remarkably high degree of structural coherence. The structural coherence is revealed by the presence of satellite peaks accompanying Bragg reflections in wide-angle X-ray diffraction experiments in parallel-beam reflection geometry. The satellite peaks, also called “superlattice reflections”, arise from the interference of X-rays diffracted by the atomic planes of the orthorhombic perovskite lattice. The interference is due to the precise spatial periodicity of the nanocrystals separated by organic ligands in the superlattice. The presence of satellite peaks is a fingerprint of the high crystallinity and long-range order of nanocrystals, comparable to those of multilayer superlattices prepared by physical methods. The angular separation between satellite peaks is highly sensitive to changes in the superlattice periodicity. These characteristics of the satellite peaks are exploited to track the superlattice compression under vacuum, as well as to observe the superlattice growth in situ from colloidal solutions by slow solvent evaporation.

Multilayer Diffraction Reveals That Colloidal Superlattices Approach the Structural Perfection of Single Crystals

“…solvent). 21 However, the superlattices in this work have been kept under vacuum prior to the thermal annealing, suggesting that the melting of surface capping molecules (i.e. oleylammonium bromide and cesium oleate, which are solid at room temperature) may also contribute to the mesostructure contraction via the interdigitation of hydrocarbon tails.…”

“…This is likely due to the combination of factors such as ligand interdigitation, loss of impurities, and increased nanocrystal lattice symmetry at higher temperatures. 21,[25][26][27] Other factors might coexist, like the intriguing possibility of superlattice defects annihilation (e.g. crowdions-voidions pair).…”

“…CsPbBr3 nanocrystal superlattices were grown following our previously published method from tetrachloroethylene or toluene dispersions of CsPbBr3 nanocrystals. 21 To synthetize the starting CsPbBr3 nanocrystals, 74 mg (0.2 mmol) of PbBr2 were dissolved in 5 ml of ODE together with 500 μL of OLAM and 50 μL of OA at 120° under N2 atmosphere, continuous stirring and inside a 20 mL glass vial. After the solubilization was completed, the vial was heated up to 170°C, removed from the hotplate and let cool in air down to the desired injection temperature (usually 163°C).…”

“…[18][19][20] In fact, the origin of this structured profile was first recognized in out-of-plane diffraction patterns of CsPbBr3 superlattices. 21 All nanocrystals within a superlattice diffract X-rays at the same Bragg angles, acting as orientation-selective light sources. The superlattice periodicity introduces a phase modulation on the diffracted X-rays, causing an additional interference which in turn produces the fine structure.…”

Perovskite Nano‐Heterojunctions: Synthesis, Structures, Properties, Challenges, and Prospects

“…In this case, small‐angle XRD or wide‐angle XRD operating in parallel‐beam reflection geometry can be used to reveal the structural coherence of the superlattice. [ 242 ] All these techniques provide strong support for the characterization of perovskite heterojunction interfaces.…”