Microcarrier-Assisted Inorganic Shelling of Lead Halide Perovskite Nanocrystals

Abstract

The conventional strategy of synthetic colloidal chemistry for bright and stable quantum dots has been the production of epitaxially matched core/shell heterostructures to mitigate the presence of deep trap states. This mindset has been shown to be incompatible with lead halide perovskite nanocrystals (LHP NCs) due to their dynamic surface and low melting point. Nevertheless, enhancements to their chemical stability are still in great demand for the deployment of LHP NCs in light-emitting devices. Rather than contend with their attributes, we propose a method in which we can utilize their dynamic, ionic lattice and uniquely defect-tolerant band structure to prepare non-epitaxial salt-shelled heterostructures that are able to stabilize these materials against their environment, while maintaining their excellent optical properties and increasing scattering to improve out-coupling efficiency. To do so, anchored LHP NCs are first synthesized through the heterogeneous nucleation of LHPs onto the surface of microcrystalline carriers, such as alkali halides. This first step stabilizes the LHP NCs against further merging, and this allows them to be coated with an additional inorganic shell through the surface-mediated reaction of amphiphilic Na and Br precursors in apolar media. These inorganically shelled NC@carrier composites offer significantly improved chemical stability toward polar organic solvents, such as γ-butyrolactone, acetonitrile, N-methylpyrrolidone, and trimethylamine, demonstrate high thermal stability with photoluminescence intensity reversibly dropping by no more than 40% at temperatures up to 120 °C, and improve compatibility with various UV-curable resins. This mindset for LHP NCs creates opportunities for their successful integration into next-generation light-emitting devices.

Keywords: core/shell; lead halide perovskite; luminescence; nanocrystals; stability.

Scheme 1. Encapsulation of LHP NCs into Inorganic Microcrystals

In the first step (Ia–c), LHP NCs heterogeneously nucleate and grow on the surface of the microcrystalline carriers (NaX, KX, RbX, Cs₄PbX₆, (Cs_xRb_1–x)₄PbX₆, MgX₂, CaX₂, SrX₂, BaX₂, and ZnX₂ where X = Cl, Br, I or their mixtures.) In Ic, the crossed-arrows indicate that inter-NC merging is strongly diminished. The obtained NCCs are then shelled with NaBr in the second step of the synthesis through the direct reaction of amphiphilic Na and Br precursors in an apolar solvent (IIa,b). The entire process yields the final, shelled polycrystalline NCC/NaBr particles.

Figure 1

(a) Visual appearance of FAPbBr₃@NaBr NCC powder under UV light and (b) a confocal microscopy image. (c) FAPbBr₃@NaBr NCC powder under UV light with an optical microscope. STEM images of CsPbBr₃@RbBr NCC powder in (d) secondary electron mode and (e) HAADF mode. (f) A zoomed-in view of several NCCs in HAADF-mode.

Figure 2

Optical properties of unshelled CsPbX₃ NCCs. (a) PL tunability of CsPbBr₃@KBr NCCs by variation of KBr:CsPbBr₃ molar ratio; the average size of LHP NCs progressively decreases with increasing carrier:LHP ratio. (b) PL QY dependence on carrier:LHP ratio for the CsPbBr₃@KBr system. The highlighted areas correspond to different regimes: orange, coexistence of LHP NCs and bulkier LHP microcrystals that reabsorb emitted light; green, the optimal range, where LHPs are mainly present in the form of large NCs (weak confinement regime or without quantum confinement); and blue, LHP NCs are small and exhibit medium or strong quantum confinement. (c) Time-resolved PL spectra of CsPbBr₃@KBr NCCs with various KBr:CsPbBr₃ ratios; the pumping intensity 10 nJ/cm². (d) A photograph depicting various NCCs dispersed in a variety of containers such as plastic balls, glass dishes, and capillaries to demonstrate the visual tunability of their emission color through the size and composition of LHP NCs.

Figure 3

Shelled CsPbBr₃@KBr/NaBr microcrystals: (a) suspension in toluene under UV light and (b) optical and (c) confocal microscopy. (d) Comparison of PL spectrum of initial CsPbBr₃@KBr NCC (blue curve) and shelled NCC/NaBr (green curve). (e) Time-resolved PL of CsPbBr₃@KBr NCC and shelled CsPbBr₃@KBr/NaBr in comparison with colloidal CsPbBr₃ NCs.

Figure 4

(a) PL dependence on temperature for shelled CsPbBr₃@KBr/NaBr. The dashed line shows PL spectrum of bulk CsPbBr₃ at 15 K. Inset shows PL fwhm dependence on temperature for smaller CsPbBr₃@KBr/NaBr (PL maximum at 520 nm at room temperature, green symbols) and very large CsPbBr₃@Cs₄PbBr₆ (PL maximum at 525 nm at room temperature, blue symbols). (b) Time-resolved PL spectra of shelled CsPbBr₃@KBr/NaBr microcrystals at various temperatures.

Figure 5

Stability of colloidal CsPbBr₃ NCs and shelled CsPbBr₃@KBr/NaBr microcrystals against select polar solvents. The decreased PL QY (a) is ascribed to the degree of merging of LHP NCs in each case. The weight loss for materials treated with the corresponding solvent is shown in (b). In all cases, the total amount of LHP was kept constant at 0.1 mg. For colloidal NCs, only a small amount of the polar solvent (9 vol %) was added to the toluene solution. For NCC/NaBr, the powders were sonicated for 5 min in 200 μL of the pure, polar solvent. A 100% weight loss in the case of colloidal NCs indicates the complete dissolution of the LHP NCs treated with NMP, GBL, TMU, and TEA.

Figure 6

Stability of NaBr-shelled LHP NCCs. (a) Scheme of the samples used for reliability tests: NaBr-shelled LHP NCCs are embedded into a polymer encapsulant. The polymer encapsulant is prepared either by drying the polymer solution (b) or by UV-curing a mixture of monomers (c). PL intensity (d) and peak position (e) change during heating to 120 °C (closed symbols) followed by cooling to room temperature (opened symbols) for shelled FAPbBr₃ NCC/NaBr (green) and colloidal FAPbBr₃ NCs (red curve). (f) PL QY drops after 190 h in three different tests of shelled CsPbBr₃ NCC/NaBr in comparison with colloidal CsPbBr₃ NCs: (i) initial sample, (ii) NCs embedded in acrylate film, (iii) reliability test at 95% RH and 50 °C, (iv) thermal stability test at 80 °C, and (v) thermal stability test at 120 °C.