Structure and Surface Passivation of Ultrathin Cesium Lead Halide Nanoplatelets Revealed by Multilayer Diffraction

Abstract

The research on two-dimensional colloidal semiconductors has received a boost from the emergence of ultrathin lead halide perovskite nanoplatelets. While the optical properties of these materials have been widely investigated, their accurate structural and compositional characterization is still challenging. Here, we exploited the natural tendency of the platelets to stack into highly ordered films, which can be treated as single crystals made of alternating layers of organic ligands and inorganic nanoplatelets, to investigate their structure by multilayer diffraction. Using X-ray diffraction alone, this method allowed us to reveal the structure of ∼12 Å thick Cs-Pb-Br perovskite and ∼25 Å thick Cs-Pb-I-Cl Ruddlesden-Popper nanoplatelets by precisely measuring their thickness, stoichiometry, surface passivation type and coverage, as well as deviations from the crystal structures of the corresponding bulk materials. It is noteworthy that a single, readily available experimental technique, coupled with proper modeling, provides access to such detailed structural and compositional information.

Keywords: Ruddlesden−Popper; X-ray; lead halide perovskite; multilayer diffraction; nanoplatelet; structure; surface.

Figure 1

Experimental and simulated multilayer diffraction patterns. The slow solvent evaporation drives the self-assembly of (a) colloidal nanoplatelets of uniform thickness into (b) highly oriented iridescent thin films on top of a silicon wafer (1 cm × 1 cm). In such films, the electron-dense platelets and the comparatively electron-light ligands alternate along the vertical direction. These are multilayered systems that modulate the intensity of the diffracted X-rays yielding patterns into characteristic periodic sharp fringes. Such experimental patterns (c, black solid line “Raw Data”) can be conveniently exploited to refine the structure of the nanoplatelets by comparing them with a simulation once the Lorentz Polarization Absorption correction has been applied (c, red solid line “LPA-corrected data”). Indeed, the ordered stacking into a multilayer allows the reduction of the 3D structure of randomly oriented nanoplatelets (d) to a 1D representation of the film along its z-axis (e). Such representation is used to compute the nanoplatelet structure factor F_NPL(q) through eq 2 and then to convolute its square modulus |F_NPL(q)|² (f, black dashed line) with the fringes arising from the nanometer-scale periodicity of the multilayer (f, vertical blue lines). This produces a simulation of the whole multilayer diffractogram (f, red solid line) that can be matched with the experimental data to refine the input 1D representation of the nanoplatelet structure.

Figure 2

Structural refinement of Cs–Pb–Br nanoplatelets. (a) Absorption (ABS) and photoluminescence (PL) spectra of two [PbBr₆]^4– octahedra thick Cs–Pb–Br nanoplatelets. Inset: a photograph of a colloidal suspension of nanoplatelets under ambient illumination (left) and UV light (right), the latter showing the characteristic bright blue emission. Preliminary simulations testing the hypotheses of (b) a PbBr₂ surface termination and (c) a CsBr surface termination. A visual examination reveals that these surface terminations do not capture accurately the intensity of the peaks in the experimental patterns. (d) Best-fit of the diffraction pattern obtained by refining the structural parameters of the Br^–/R-NH₃⁺ termination model, including the occupancies, and the vertical coordinates of atoms in the platelet structure. The residual signal at q ∼ 15 Å^–1 is not a multilayer fringe, but a peak from a fraction of misaligned platelets ({110} in the pseudocubic notation for CsPbBr₃). The inset illustrates the nanoplatelet structure in scale, highlighting its expansion along the vertical direction, the partially occupied surface layers and the tilted [PbBr₆]^4– octahedra. Color legend for atoms: Cs⁺ = light-blue; Br^– = brown; Pb²⁺ = dark gray (octahedra); N = violet; C = black; H = white.

Figure 3

Cs–Pb–I–Cl Ruddlesden–Popper nanoplatelets. (a) Absorption spectrum of Cs–Pb–I–Cl nanoplatelets. The spectral position of the excitonic peak matches that reported in the past for nanocrystals of the same material (400 nm/3.10 eV),^, and is only weakly shifted from that of bulk Cs₂PbI₂Cl₂ (408 nm/3.04 eV). (b,c) TEM images and SAED diffraction pattern of Cs₂PbI₂Cl₂. The SAED pattern provides information about the structure of the platelets along the horizontal direction and is compatible with what is expected for oriented Cs₂PbI₂Cl₂ crystals. (d) Multilayer Diffraction fit of the Cs–Pb–I–Cl nanoplatelets pattern. The starting model for the fit was a slice of the published bulk structure (ICSD-6337), I^–/R-NH₃⁺ terminated, with a thickness of three [PbI₂Cl₄]^4– octahedra. In the refined structure, the Cs⁺ and I^– ions are found to converge into one common layer of ions as the platelet expands slightly along the vertical direction (Pb–Pb distance +1.4%). The residual nonfitted signals correspond to the {100} and {200} Bragg peaks of the CsPbCl₃ impurity and to some signals from the substrate. Color legend for atoms: Cs⁺ = light-blue; Cl^– = green; I^– = purple; Pb²⁺ = dark gray (octahedra); N = violet; C = black; H = white.