Facile Synthesis of Colloidal Lead Halide Perovskite Nanoplatelets via Ligand-Assisted Reprecipitation

The product of this synthesis is colloidal lead halide nanoplatelets capped by alkylammonium halide surface ligands (Figure 1a). Figure 1b demonstrates the synthetic procedure of colloidal perovskite nanoplatelets via ligand-assisted reprecipitation. To summarize, constituent precursor salts were dissolved in a polar solvent DMF in specific ratios for desired thickness and composition, and then injected into toluene, which is nonpolar. Due to the abrupt change in solubility, colloidal perovskite nanoplatelets started to crystallize instantaneously. When preparing the mixed precursor solution, ratios between constituent precursors primarily determined the thickness of resulting nanoplatelets (Figure S3), and the presence of excess ligands in precursor solution was crucial to ensure the thickness homogeneity of the product (Figure S4). In general, any polar solvent can be used to dissolve perovskite precursor salts while any nonpolar solvent can be used to disperse colloidal nanoplatelets. However, miscibility of those nonpolar and polar solvents is crucial for homogeneous synthesis of colloidal perovskite nanoplatelets, and thus we chose DMF and toluene. Also, it is important to have nonpolar solvent in large excess to the added polar solvent for the crystallization of perovskite nanoplatelets to occur. Adding too much polar solvent increases the polarity of the resulting solvent mixture (i.e. DMF + toluene), which can dissolve the nanoplatelets. Chloride- and cesium-incorporating nanopatelets can also be synthesized by this approach (Figure S5), though the chloride-containing nanoplatelets are nonemissive and the cesium-based nanoplatelets suffer from inferior stability and thickness homogeneity relative to the methylammonium-based nanoplatelets when synthesized via this method³⁸. Finally, we note that only the n = 1 and n = 2 members have been synthesized with good thickness homogeneity by this method; attempts at making thicker (n ≥ 3) nanoplatelets typically yield mixed-thickness dispersions (Figure S6).

Figure 2 shows the images of as-synthesized colloidal perovskite nanoplatelet solutions illuminated by UV light, where the emission of the nanoplatelets can be clearly seen along the beam path. Figure 3 shows the normalized photoluminescence (PL) and absorption spectra of colloidal perovskite nanoplatelet solutions, which are consistent with previous reports^37,38,50,51, demonstrating the tunability of perovskite nanoplatelets with thickness and constituent species. For all nanoplatelets, strong excitonic features in the absorption spectra and significant blue-shift of the spectra compared to bulk perovskites³⁵ were observed due to strong quantum- and dielectric-confinement. Changing the organic cation from methylammonium to formamidinium did not significantly affect the band gap – either for bromide or iodide nanoplatelets – in agreement with understanding of the valence electronic structure in lead halide perovskites¹³. Table S1 summarizes the photoluminescence quantum yields (PLQYs) of those colloidal perovskite nanoplatelet solutions.

The two-dimensional structure of the perovskite nanoplatelets was confirmed by TEM and XRD. In Figure 4, TEM images show partially overlapping two-dimensional perovskite nanoplatelets, with individual lateral dimensions ranging from a few hundred nanometers to a micrometer. The image contrast and random configuration of nanoplatelets on the TEM grid suggests that they are dispersed in solution as individual sheets – rather than stacked lamellar crystals. Small, dark spherical dots appeared upon electron beam irradiation as observed in Figure 4, and they are believed to be metallic Pb as previously reported^36,52. Due to the large lateral dimensions of perovskite nanoplatelets, they preferentially lay flat on top of each other when cast into a film, and periodic stacking peaks dominated the XRD pattern as shown in Figure 5. Considering that the lattice constant for the cubic perovskite unit cell is ~ 0.6 nm⁵³, it can be deduced that the organic ligand layer is 1 nm thick in stacked nanoplatelet films regardless of the nanoplatelet species³⁸.

The absorption and emission resonance could be continuously tuned by varying the halide composition. Figure 6 shows the normalized absorption spectra of colloidal n = 1 PbX and n = 2 MAPbX nanoplatelet solutions with varying ratios of bromide and iodide. Clear excitonic absorption peaks indicate strong confinement of carriers in nanoplatelets, and continuous shift of those peaks with halide composition demonstrates band gap tunability through halide composition variation (Figure S8). However, photoluminescence spectra of mixed-halide nanoplatelets exhibit broad or multiple features (Figure S9), which is possibly due to photoinduced halide segregation.⁵⁴

The ligand-assisted reprecipitation method is particularly amenable to changing the identity of the long-chain capping ligand, as shown in Figure 7. This opens up the possibility of tuning the nature of the surface-bound organic species for the optimized performance of a specific device or application⁵⁵. We note, however, that the ratios between individual precursors may require slight adjustment when employing new ligand species for the best thickness homogeneity of the resulting system (Figure S10 and Table S2).

In conclusion, we have demonstrated a simple, versatile method for synthesizing colloidal lead halide perovskite nanoplatelets of varying composition (Figure S11). The ligand-assisted reprecipitation approach is potentially amenable to high-throughput synthesis and further data-driven analysis. Thickness-, composition- and ligand-tunability can be achieved without any major modifications in the synthetic protocols. Moving forward, it would be desirable to further increase the photoluminescence efficiency to levels commensurate with other perovskite nanocrystals^29,32,56.