Nearly Monodisperse Insulator Cs4PbX6 (X = Cl, Br, I) Nanocrystals, Their Mixed Halide Compositions, and Their Transformation into CsPbX3 Nanocrystals

Abstract

We have developed a colloidal synthesis of nearly monodisperse nanocrystals of pure Cs₄PbX₆ (X = Cl, Br, I) and their mixed halide compositions with sizes ranging from 9 to 37 nm. The optical absorption spectra of these nanocrystals display a sharp, high energy peak due to transitions between states localized in individual PbX₆^4- octahedra. These spectral features are insensitive to the size of the particles and in agreement with the features of the corresponding bulk materials. Samples with mixed halide composition exhibit absorption bands that are intermediate in spectral position between those of the pure halide compounds. Furthermore, the absorption bands of intermediate compositions broaden due to the different possible combinations of halide coordination around the Pb²⁺ ions. Both observations are supportive of the fact that the [PbX₆]^4- octahedra are electronically decoupled in these systems. Because of the large band gap of Cs₄PbX₆ (>3.2 eV), no excitonic emission in the visible range was observed. The Cs₄PbBr₆ nanocrystals can be converted into green fluorescent CsPbBr₃ nanocrystals by their reaction with an excess of PbBr₂ with preservation of size and size distributions. The insertion of PbX₂ into Cs₄PbX₆ provides a means of accessing CsPbX₃ nanocrystals in a wide variety of sizes, shapes, and compositions, an important aspect for the development of precisely tuned perovskite nanocrystal inks.

Keywords: Cs4PbX6; halide perovskites; nanocrystals; synthesis; transformations.

Figure 1

Structural models of (a) 3D cubic CsPbX₃ and (b) 0D hexagonal Cs₄PbX₆ perovskites with X = Cl, Br, I.

Figure 2

TEM images of (a) 9.2, (b) 14.7, (c) 27.2, and (d) 37.0 nm Cs₄PbBr₆ NCs. (e) HRTEM image of a Cs₄PbBr₆ NC. (f) XRD patterns of different sized Cs₄PbBr₆ NCs matching with the Cs₄PbBr₆ reference pattern (98-002-5124). (g) Optical absorption spectra of different sized Cs₄PbBr₆ NCs, characterized by a strong absorption band at 314 nm, independent of the NC size. (h) DFT density of states calculation of Cs₄PbBr₆ confirming the large band gap of 3.99 eV. (i) Comparison of pure Cs₄PbBr₆ NC film, pure CsPbBr₃ NC film and a Cs₄PbBr₆ NC film intentionally doped with 2% CsPbBr₃ NCs. The latter sample has a PL that is typical of CsPbBr₃, but its diffraction pattern can be indexed with the Cs₄PbBr₆ phase alone. Scale bars are 100 nm in all TEM images and 2 nm in the HRTEM image.

Figure 3

TEM images of (a) Cs₄PbCl₆ and (b) Cs₄PbI₆ NCs with HRTEM images in their respective insets (c,d). (e) XRD patterns of pure Cs₄PbX₆ NCs showing evident shifts toward larger unit cells when going from Cl to Br and then to I. (f) Absorption spectra of pure Cs₄PbX₆ NCs. Scale bars are 100 nm in all TEM images and 2 nm in all the HRTEM images.

Figure 4

TEM images of NCs obtained after interparticle anion exchange of Cs₄PbBr₆ NCs with (a) Cs₄PbCl₆ and (b) Cs₄PbI₆. Scale bars are 100 nm in both images. (c) XRD patterns confirm intermediate crystal phase after exchange. (d) Optical absorption spectra recorded on pristine and mixed NCs with intermediate, broadened absorptions. (e) Sketch of the PbX₆ model used for the calculations with point charges at the cubic sites. (f) Simulated absorption spectra of charge-compensated PbI₆, PbBr₆, and intermediate compositions. The calculated TDDFT excitation energies and oscillator strengths have been convoluted by Gaussian functions with a σ = 0.075 eV.

Figure 5

(a) Reaction of Cs₄PbBr₆ with PbBr₂, forming CsPbBr₃. (b) Phase transformation of the hexagonal Cs₄PbBr₆ phase to the cubic CsPbBr₃ phase after insertion of additional PbBr₂. (c–e) TEM images of different-sized Cs₄PbBr₆ NCs before and after their transformation into CsPbBr₃, indicating the preservation of size but transformation of the hexagonal shape to a cubic shape. (f) Optical absorption and PL of pristine Cs₄PbBr₆ NCs and of the same sample after the insertion reaction, highlighting the large shift of the band gap, the disappearance of the strong Cs₄PbBr₆ absorption feature and the appearance of a strong green PL after the transformation. (g) XRD patterns before and after the insertion reaction, confirming the rearrangement of the crystal lattice from hexagonal Cs₄PbBr₆ to cubic CsPbBr₃. Scale bars are 100 nm in all TEM images. XRD references patterns are 98-010-9295 for CsPbBr₃ and 98-002-5124 for Cs₄PbBr₆.