Abstract
The encapsulation of colloidal lead halide perovskite nanocrystals within silica (SiO2) is one of the strategies to protect them from polar solvents and other external factors. Here, we demonstrate the overcoating of CsPbBr3 perovskite nanocrystals with silica by exploiting the anhydride-induced transformation of Cs4PbBr6 nanocrystals. CsPbBr3@SiO2 core–shell nanocrystals are obtained after (i) a reaction between colloidal Cs4PbBr6 nanocrystals and maleic anhydride in toluene that yields CsPbBr3 nanocrystals and maleamic acid and (ii) a silica-shell growth around CsPbBr3 nanocrystals via hydrolysis of added alkoxysilanes. The reaction between Cs4PbBr6 nanocrystals and maleic anhydride is necessary to promote shell formation from alkoxysilanes, as demonstrated in control experiments. The best samples of as-prepared CsPbBr3@SiO2 nanocrystals consist of ∼10 nm single-crystal CsPbBr3 cores surrounded by ∼5–7 nm amorphous silica shell. Despite their core–shell structure, such nanostructures are poor emitters and degrade within minutes of exposure to ethanol. The photoluminescence intensity of the core–shell nanocrystals is improved by the treatment with a solution of PbBr2 and ligands, and their stability in ethanol is extended to several days after applying an additional silica growth step. Overall, the investigated approach outlines a strategy for making colloidal core–shell nanocrystals utilizing the transformative chemistry of metal halides and reveals interesting insights regarding the conditions required for CsPbBr3@SiO2 nanocrystal formation.
Highlights
Lead halide perovskites (LHPs) are emerging semiconductor materials that promise to revolutionize optoelectronic devices such as photodetectors, solar concentrators, scintillators, solar cells, and light-emitting diodes (LEDs).[1−7] Colloidal nanocrystals (NCs) of LHPs capped with long-chain organic ligands attracted the attention of many research groups due to their straightforward synthesis, strong light absorption, and bright and tunable photoluminescence (PL).[8−11] Despite their outstanding optoelectronic properties, LHP NCs suffer from several issues that constitute a hurdle for practical applications
Article densation that drives Cs4PbBr6 → CsPbBr3 conversion and hydrolysis of alkoxysilanes could be seen as an alternative approach to colloidal atomic layer deposition developed to deposit alumina onto perovskite NCs.[63−65] The synthetic strategy presented here allows for the preparation of core− shell NCs and demonstrates the potential of transformative chemistry of metal halides for the fabrication of LHP-based nanoheterostructures
It would be interesting to extend this approach to NCs of other metal halides, e.g., Cs4PbI6 and Cs4PbCl6, as well as to other metal oxides that can be grown by wet chemistry approaches, e.g., TiO2, ZrO2, Al2O3, and SnO2
Summary
Lead halide perovskites (LHPs) are emerging semiconductor materials that promise to revolutionize optoelectronic devices such as photodetectors, solar concentrators, scintillators, solar cells, and light-emitting diodes (LEDs).[1−7] Colloidal nanocrystals (NCs) of LHPs capped with long-chain organic ligands attracted the attention of many research groups due to their straightforward synthesis, strong light absorption, and bright and tunable photoluminescence (PL).[8−11] Despite their outstanding optoelectronic properties, LHP NCs suffer from several issues that constitute a hurdle for practical applications. NC stock solution, 100 μL of toluene, 20 μL of OLAM, and 1 mL of MANH stock solution were combined in this order in a 4 mL vial and kept under stirring (∼500 rpm) on a stir plate for 15 min, after which the NCs were isolated. If the green-emissive sample turned orange-/red-emissive (substitution of Br− with I−),[50] the shell overgrowth was deemed incomplete, and 1 μL of water was added to the reaction mixture to catalyze TEOS hydrolysis.[48] Once the halide exchange in the aliquot was suppressed (after 24−48 h for TEOS, 12−24 h for TMOS, depending on the batch), the CsPbBr3@SiO2@ SiO2 NCs were collected by precipitation with ethyl acetate (250 μL, followed by centrifugation at 4000 rpm for 5 min) and redispersion of the precipitate in 250 μL of toluene. The full details of the NMR experiments are described in the Supporting Information
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