Solution Synthesis Approach to Colloidal Cesium Lead Halide Perovskite Nanoplatelets with Monolayer-Level Thickness Control

Abstract

We report a colloidal synthesis approach to CsPbBr3 nanoplatelets (NPLs). The nucleation and growth of the platelets, which takes place at room temperature, is triggered by the injection of acetone in a mixture of precursors that would remain unreactive otherwise. The low growth temperature enables the control of the plate thickness, which can be precisely tuned from 3 to 5 monolayers. The strong two-dimensional confinement of the carriers at such small vertical sizes is responsible for a narrow PL, strong excitonic absorption, and a blue shift of the optical band gap by more than 0.47 eV compared to that of bulk CsPbBr3. We also show that the composition of the NPLs can be varied all the way to CsPbBr3 or CsPbI3 by anion exchange, with preservation of the size and shape of the starting particles. The blue fluorescent CsPbCl3 NPLs represent a new member of the scarcely populated group of blue-emitting colloidal nanocrystals. The exciton dynamics were found to be independent of the extent of 2D confinement in these platelets, and this was supported by band structure calculations.

Figure 1

Structural analysis of CsPbBr₃ NPLs emitting at 460 nm. TEM images of CsPbBr₃ NPLs at (A) low concentrations and (B) high concentrations. HRTEM images of NPLs (C) in top view and (D) in stacks as well as their respective Fourier transform patterns (E,F). (G) STEM dark-field image of NPLs. (H) XRD pattern of CsPbBr₃ NPLs. Reference pattern of cubic CsPbBr₃, ICSD 29073. Scale bars correspond to 50 nm in A and B, 2 nm in C and D, and 20 nm in G.

Figure 2

Optical and structural properties of different thicknesses of CsPbBr₃ NPLs and the exchanged CsPbCl₃ and CsPbI₃ NPLs. (A–C) Three different thicknesses, with (A) 5 ML emitting at 2.70 eV, Stoke shift (Ss) = 0.11 eV, and fwhm = 0.11 eV; (B) 4 ML emitting at 2.76 eV, Ss = 0.12 eV, and fwhm = 0.17 eV; and (C) 3 ML emitting at 2.83 eV, Ss = 0.11 eV, and fwhm = 0.09 eV. (D,E) TEM and HRTEM image of 3 ML thick sample indicated a thickness of 1.8 nm. (F) Fine-tuning the PL of CsPbBr₃ NPLs with anion-exchange reactions to CsPbCl₃ and CsPbI₃. Scale bar corresponds to 50 nm in D, and 2 nm in E.

Figure 3

(A) Absorption and PL spectra of CsPbBr₃ as a thin film (see Figure SI13), cube-shaped nanocrystals, and NPLs of different thicknesses. (B) Employed cubic CsPbBr₃ 2D slabs and unit supercell. a–b are the periodic dimensions. (C) Comparison of the experimental (green line) and SR- and SOC-DFT calculated band gaps (black line and red line, respectively) as a function of the NPLs inverse squared dimension (1/r²).

Figure 4

(A) PL dynamics of cube-shaped nanocrystals (t = 4 ns) and NPLs (t = 3 ns). (B) Relative PLQY of 5 ML NPLs.