Highly Luminescent and Stable Halide Perovskite Nanocrystals by Interfacial Defect Passivation and Amphiphilic Ligand Capping

Halide vacancies cause lattice degradation and nonradiativelossesin halide perovskites. In this study, we strategically fill bromidevacancies in CsPbBr3 perovskite nanocrystals with NaBr,KBr, or CsBr at the organic-aqueous interface for hydrophobicligand-capped nanocrystals or in a polar solvent (2-propanol) foramphiphilic ligand-capped nanocrystals. Energy-dispersive X-ray spectra,powder X-ray diffraction data, and scanning transmission electronmicroscopy images help us confirm vacancy filling and the structuresof samples. The bromide salts increase the photoluminescence quantumyield (98 & PLUSMN; 2%) of CsPbBr3 by decreasing the nonradiativedecay rate. Single-particle studies show the quantum yield increaseoriginates from the poorly luminescent nanocrystals becoming highlyluminescent after filling vacancies. Furthermore, we tune the opticalband gap (ultraviolet-visible-near-infrared) of thehydrophobic ligand-capped nanocrystals by halide exchange at the toluene-waterinterface using saturated NaCl or NaI solutions, which completes inabout 60 min under continuous mixing. In contrast, the amphiphilicligand accelerates the halide exchange in 2-propanol, suggesting ambipolarfunctional groups speed up the ion-exchange reaction. The bromidevacancy-filled or halide-exchanged samples in a toluene-waterbiphasic solvent show higher stability than amphiphilic ligand-cappedsamples in 2-propanol. This strategy of defect passivation, ion exchange,and ligand chemistry to improve quantum yields and tune band gapsof halide perovskite nanocrystals can be promising for designing stableand water-soluble perovskite samples for solar cells, light-emittingdiodes, photodetectors, and photocatalysts.