Preparation and formation process of high efficient and stable CsPbBr<sub>3</sub>-Cs<sub>4</sub>PbBr<sub>6</sub> nanocrystals with mixed phase

Abstract

CsPbBr<sub>3</sub>-Cs<sub>4</sub>PbBr<sub>6</sub> dual-phase nanocrystals are prepared by adding the mixture ligand of oleylamine and tetradecyl-phosphonic acid (OLA-TDPA) to CsPbBr<sub>3</sub> perovskite nanocrystals through ligand post-treatment. The structure, the morphology, optical property and the stability of CsPbBr<sub>3</sub>-Cs<sub>4</sub>PbBr<sub>6</sub> dual-phase nanocrystals are characterized by X-ray diffraction, transmission electron microscopy (high-resolution TEM), UV-vis spectrophotometer, fluorescence spectrophotometer, and transient fluorescence spectrophotometer. The as-obtained nanocrystals have a high photoluminescence quantum yield of 78% and long fluorescence lifetime of 476 ns when prepared at the optimal molar ratio of CsPbBr<sub>3</sub>, TDPA and OLA (1∶1∶15). Moreover, the nanocrystal is quite stable at room temperature for at least 25 days, and has a good thermal stability in five heating-cooling cycles at temperature in a range between 293 K and 328 K. The formation of dual-phase nanocrystals go through two stages of surface passivation/dissolution and recrystallization to generate CsPbBr<sub>3</sub>-Cs<sub>4</sub>PbBr<sub>6</sub> nanocrystals. In the first stage (<i>t</i> ≤ 1 h), the m OLA-TDPA mixing ligand can form (RNH<sub>3</sub>)<sub>2</sub>PO<sub>3</sub> X type ligand and exchanges with [RNH<sub>3</sub>]<sup>+</sup>-[RCOO]<sup>–</sup> at the surface of CsPbBr<sub>3</sub> nanocrystals, which can effectively passivate surface defects by strong interaction with Pb<sup>2+</sup> and high ligand content at surface, thus improving the quantum yield and fluorescence life of CsPbBr<sub>3</sub> nanocrystals with spherical shape. In the second stage, with the increase of reaction time, PbBr<sub>2</sub> partially dissolves from the surface of CsPbBr<sub>3</sub> nanocrystals, then some CsPbBr<sub>3</sub> nanocrystals transform into lead-depleted Cs<sub>4</sub>PbBr<sub>6</sub> nanocrystals with hexagonal phase, thus improving the stability of nanocrystals. This work has a certain reference value for promoting the applications of high efficient and stable perovskite nanocrystals.