Tuning the Optical Properties of Cesium Lead Halide Perovskite Nanocrystals by Anion Exchange Reactions

Abstract

We demonstrate that, via controlled anion exchange reactions using a range of different halide precursors, we can finely tune the chemical composition and the optical properties of presynthesized colloidal cesium lead halide perovskite nanocrystals (NCs), from green emitting CsPbBr3 to bright emitters in any other region of the visible spectrum, and back, by displacement of Cl(-) or I(-) ions and reinsertion of Br(-) ions. This approach gives access to perovskite semiconductor NCs with both structural and optical qualities comparable to those of directly synthesized NCs. We also show that anion exchange is a dynamic process that takes place in solution between NCs. Therefore, by mixing solutions containing perovskite NCs emitting in different spectral ranges (due to different halide compositions) their mutual fast exchange dynamics leads to homogenization in their composition, resulting in NCs emitting in a narrow spectral region that is intermediate between those of the parent nanoparticles.

Scheme 1. Overview of the Different Routes and Precursors for the Anion Exchange Reactions on CsPbX₃ (X = Cl, Br, I) NCs Reported in This Work

Figure 1

(A) PL of the exchanged NCs obtained from CsPbBr₃ by adding: (from top to bottom) OLAM-X, ODA-X, TBA-Cl and PbX₂ (X = I, Cl). (B) PL of the CsPbBr₃ NCs obtained starting from CsPbCl₃ and CsPbI₃ NCs using (from top to bottom) ODA-Br, TBA-Br and PbBr₂.

Figure 2

(A) PL spectra of the CsPb(Br:X)₃ (X = Cl, I) NCs prepared by anion exchange from CsPbBr₃ NCs. (B) PL calibration curves: a targeted emission energy could be obtained by adding a precise amount of halide precursor to a crude solution of CsPbBr₃ NCs. The curves are reported as a function of the molar ratio between the added halide (or exchange halide) and the Br amount in the starting NCs. (C) PLQY recorded on the exchanged NCs (dots) as well as on the directly synthesized NCs (stars).

Figure 3

(A–C) TEM images of pristine CsPbBr₃ NCs (B) and of fully exchanged CsPbCl₃ (A) and CsPbI₃ NCs (C), indicating overall size and size preservation upon anion exchange. Scale bars correspond to 50 nm. (D) Zoom of the XRD patterns of pristine CsPbBr₃ NCs (middle pattern in green), of the anion exchanged CsPbCl₃ (top, violet), CsPbI₃ (bottom, red) and of two intermediate Cl^– (middle-top, light blue) and I^– (middle-bottom, yellow) exchanged samples. Full XRD patterns are reported in Figure S9.

Figure 4

(A) Sketch of interparticle anion exchange. (B) XRD patterns of the pristine CsPbCl₃, CsPbBr₃ and CsPbCl₃ NCs and of the samples after mixing. (C) Optical absorption and PL spectra of various CsPb(Br:Cl)₃ and CsPb(Br:I)₃ NCs prepared via interparticle exchange. (D) and (E) TEM images of CsPbBr₃:CsPbCl₃ 1:1 (8.5 ± 1.2 nm) and CsPbBr₃:CsPbCl₃ 1:1 (8.9 ± 1.4 nm), respectively. Scale bars correspond to 50 nm.