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

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Summary

This work demonstrates facile room-temperature synthesis of colloidal quantum-confined lead halide perovskite nanoplatelets by ligand-assisted reprecipitation method. Synthesized nanoplatelets show spectrally narrow optical features and continuous spectral tunability throughout the visible range by varying the composition and thicknesses.

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Ha, S. K., Tisdale, W. A. Facile Synthesis of Colloidal Lead Halide Perovskite Nanoplatelets via Ligand-Assisted Reprecipitation. J. Vis. Exp. (152), e60114, doi:10.3791/60114 (2019).

Abstract

In this work, we demonstrate a facile method for colloidal lead halide perovskite nanoplatelet synthesis (Chemical formula: L2[ABX3]n-1BX4, L: butylammonium and octylammonium, A: methylammonium or formamidinium, B: lead, X: bromide and iodide, n: number of [BX6]4- octahedral layers in the direction of nanoplatelet thickness) via ligand-assisted reprecipitation. Individual perovskite precursor solutions are prepared by dissolving each nanoplatelet constituent salt in N,N-dimethylformamide (DMF), which is a polar organic solvent, and then mixing in specific ratios for targeted nanoplatelet thickness and composition. Once the mixed precursor solution is dropped into nonpolar toluene, the abrupt change in the solubility induces the instantaneous crystallization of nanoplatelets with surface-bound alkylammonium halide ligands providing colloidal stability. Photoluminescence and absorption spectra reveal emissive and strongly quantum-confined features. X-ray diffraction and transmission electron microscopy confirm the two-dimensional structure of the nanoplatelets. Furthermore, we demonstrate that the band gap of perovskite nanoplatelets can be continuously tuned in the visible range by varying the stoichiometry of the halide ion(s). Lastly, we demonstrate the flexibility of the ligand-assisted reprecipitation method by introducing multiple species as surface-capping ligands. This methodology represents a simple procedure for preparing dispersions of emissive 2D colloidal semiconductors.

Introduction

In the past decade, fabrication of lead halide perovskites solar cells1,2,3,4,5,6 has effectively highlighted the excellent properties of this semiconductor material, including long carrier diffusion lengths7,8,9,10, compositional tunability4,5,11 and low-cost synthesis12. In particular, the unique nature of defect tolerance13,14 makes lead halide perovskites fundamentally different from other semiconductors and thus highly promising for next-generation optoelectronic applications.

In addition to solar cells, lead halide perovskites have been shown to make excellent optoelectronic devices such as light-emitting diodes6,15,16,17,18,19,20,21,22, lasers23,24,25, and photodetectors26,27,28. Especially, when prepared in the form of colloidal nanocrystals18,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43, lead halide perovskites may exhibit strong quantum- and dielectric-confinement, large exciton binding energy44,45, and bright luminescence17,19 along with facile solution processability. Various reported geometries including quantum dots29,30,31,32, nanorods33,34 and nanoplatelets18,35,36,37,38,39,40,41,43 further demonstrate the shape tunability of lead halide perovskite nanocrystals.

Among those nanocrystals, colloidal two-dimensional (2D) lead halide perovskites, or “perovskite nanoplatelets”, are especially promising for light-emitting applications due to strong confinement of charge carriers, large exciton binding energy reaching up to hundreds of meV44, and spectrally narrow emission from thickness-pure ensembles of nanoplatelets39. Additionally, anisotropic emission reported for 2D perovskite nanocrystals46 and other 2D semiconductors47,48 highlights the potential of maximizing outcoupling efficiency from perovskite nanoplatelet-based light-emitting devices.

Here, we demonstrate a protocol for the simple, universal, room-temperature synthesis of colloidal lead halide perovskite nanoplatelets via a ligand-assisted reprecipitation technique36,38,49. Perovskite nanoplatelets incorporating iodide and/or bromide halide anions, methylammonium or formamidinium organic cations, and variable organic surface ligands are demonstrated. Procedures for controlling the absorption and emission energy and the thickness purity of the colloidal dispersion are discussed.

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Protocol

NOTE: Simpler notations of ‘n = 1 BX’ and ‘n = 2 ABX’ will be used from here instead of the complex chemical formula of L2BX4 and L2[ABX3]BX4, respectively. For better stability and optical properties of resulting perovskite nanoplatelets, it is recommended to complete the whole procedure under inert conditions49 (i.e., a nitrogen glovebox).

1. Preparation of perovskite nanoplatelet precursor solution

  1. Prepare ~1 mL of 0.2 M solutions of methylammonium bromide (MABr), formamidinium bromide (FABr), lead bromide (PbBr2), butylammonium bromide (BABr), octylammonium bromide (OABr), methylammonium iodide (MAI), formamidinium iodide (FAI), lead iodide (PbI2), butylammonium iodide (BAI), and octylammonium iodide (OAI) in N,N-dimethylformamide (DMF) either by dissolving each salt in DMF or by diluting commercially available solutions.
    1. PbBr2 is not readily soluble in DMF at room temperature, keep the solution at 80 °C for 10 min or longer for complete dissolution. Once dissolved, cool the solution back to room temperature before use.
      NOTE: Concentration of individual precursor solutions can be increased to synthesize more nanoplatelets, but the maximum concentration is usually limited by the solubilities of PbBr2 and PbI2 in DMF.
  2. Mix those individual precursor solutions in specific volumetric ratios for each target thickness and composition.
    1. To synthesize bromide-only or iodide-only nanoplatelets, see Table 1, which summarizes the volumetric ratios for n = 1 and n = 2 bromide and iodide nanoplatelets.
    2. To synthesize nanoplatelets with mixed halide compositions, combine bromide-only and iodide-only perovskite nanoplatelet precursor solutions of the same thickness at desired volumetric ratio for the target composition. For example, to make 30%-bromide-70%-iodide n = 2 perovskite nanoplatelets, mix the precursor solutions of n = 2 MAPbBr and n = 2 MAPbI at a 3:7 volumetric ratio.
      NOTE: Changing the organic cation does not significantly affect the optical transition energies13. Absorption and luminescence are primarily tuned by changing the halide composition or nanoplatelet thickness.

2. Synthesis of perovskite nanoplatelets via ligand-assisted reprecipitation method

  1. Inject 10 µL of mixed precursor solution into 10 mL of toluene under vigorous stirring. Nanoplatelets will instantaneously crystallize due to the abrupt change in the solubility.
    NOTE: The amount of mixed precursor solution injected into toluene can be increased up to ~100 µL. Total amount of injected precursor solution and injection speed do not seem to significantly affect perovskite nanoplatelet morphology (Figure S1). However, injection of too much DMF increases the polarity of the solution and reduces the crystallization.
  2. Leave the solution under stirring for 10 min until no further color change is observed from the solution to ensure complete crystallization of perovskite nanoplatelets.
    NOTE: Freshly synthesized perovskite nanoplatelets from freshly prepared precursor solutions usually show the best photoluminescence quantum yield and photostability49. And over time, nanoplatelets will slowly aggregate (Figure S2), deteriorating colloidal stability. Thus, it is recommended to use nanoplatelet solutions as soon as possible once synthesized.

3. Characterization sample preparation and purification of colloidal perovskite nanoplatelet solution.

  1. Transmission electron microscopy (TEM) sample preparation.
    1. Centrifuge the solution at 2050 x g for 10 min.
    2. Discard the supernatant.
    3. Redisperse the nanoplatelets in 1 mL of toluene.
    4. Drop 1 droplet on a TEM grid.
    5. Dry the sample under vacuum.
  2. X-ray diffraction (XRD) sample preparation
    1. Centrifuge the solution at 2050 x g for 10 min.
    2. Discard the supernatant.
    3. Redisperse the nanoplatelets in 30 µL of toluene.
    4. Dropcast on a glass slide.
    5. Dry the sample under vacuum.
  3. General purification
    1. Centrifuge the solution at 2050 x g for 10 min.
    2. Discard the supernatant.
    3. Redisperse the nanoplatelets in desired amount of solvent depending on the usage.
      NOTE: Depending on the usage of nanoplatelets, the volume of the redispersing solvent can be freely adjusted and other nonpolar organic solvents such as hexane, octane or chlorobenzene can be used instead of toluene.

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Representative Results

Schematic illustration of perovskite nanoplatelets and synthesis procedure gives an overview of the material and synthetic details (Figure 1). Pictures of colloidal perovskite nanoplatelet solutions under ambient light and UV (Figure 2), combined with photoluminescence and absorption spectra (Figure 3) further confirm the emissive and absorptive nature of nanoplatelets. TEM images (Figure 4) and XRD patterns (Figure 5) are used to estimate the lateral dimensions and stacking spacings of nanoplatelets, respectively, while also confirming the two-dimensional structure. Absorption spectra of perovskite nanoplatelet solutions with mixed halides demonstrate tunability of the bandgap (Figure 6). Insensitivity of the photoluminescence spectrum to the chemical identity of organic surface-capping ligands highlights the compositional flexibility of these materials (Figure 7).

MABr FABr PbBr2 BABr OABr MAI FAI PbI2 BAI OAI
n=1 PbBr 0 0 1 1 1 0 0 0 0 0
n=2 FAPbBr 0 1 2 5 5 0 0 0 0 0
n=2 MAPbBr 1 0 2 5 5 0 0 0 0 0
n=1 PbI 0 0 0 0 0 0 0 1 1 1
n=2 FAPbI 0 0 0 0 0 0 1 2 5 5
n=2 MAPbI 0 0 0 0 0 1 0 2 5 5

Table 1. Formulation guidelines for perovskite nanoplatelet precursor solutions.
Numbers in the table indicate the volumetric equivalents of each precursor solution (columns) that should be combined to achieve the targeted nanoplatelet (rows), according to the concentration specifications in the protocol text.

Figure 1
Figure 1. Perovskite nanoplatelet structure and synthesis procedure.
(a) Illustration of perovskite unit cell and nanoplatelet structure. (b) Schematic illustration of colloidal perovskite nanoplatelet synthesis. Reprinted (adapted) with permission from Ref. 48. Copyright 2019 American Chemical Society. Please click here to view a larger version of this figure.

Figure 2
Figure 2. Colloidal perovskite nanoplatelet solutions illuminated by UV light.
Emission from the nanoplatelets can be clearly seen along the beam path. Reprinted (adapted) with permission from Ref. 48. Copyright 2019 American Chemical Society. Please click here to view a larger version of this figure.

Figure 3
Figure 3. Photoluminescence and absorption spectra of colloidal perovskite nanoplatelet solutions.
Bandgap of the nanoplatelets can be tuned with thickness and composition. Longpass filter (Cut-on wavelength: 400 nm) was used to filter out excitation UV light before photoluminescence spectrum collection and it could have slightly altered n = 1 lead bromide nanoplatelet emission spectrum.

Figure 4
Figure 4. Transmission electron microscopy (TEM) images of perovskite nanoplatelets.
Images show randomly-overlapping nanoplatelets. See also Figure S7. Please click here to view a larger version of this figure.

Figure 5
Figure 5. X-ray diffraction (XRD) patterns and d-spacings of perovskite nanoplatelets.
XRD patterns are dominated by nanoplatelet stacking peaks which confirm the two-dimensional nature of the nanoplatelets and their face-to-face self-assembly in dropcasted films. Please click here to view a larger version of this figure.

Figure 6
Figure 6. Absorption spectra of colloidal perovskite nanoplatelet solutions with mixed halides.
Continuous shift of first excitonic absorption features shows bandgap tunability with halide composition. Please click here to view a larger version of this figure.

Figure 7
Figure 7. Photoluminescence spectra of n = 1 PbBr and n = 2 MAPbBr nanoplatelets synthesized with different ligand species.
The reprecipitation method can be easily extended to other ligand chemistries. See also Table S2 for formulation guidelines. Longpass filter (Cut-on wavelength: 400 nm) was used to filter out excitation UV light before photoluminescence spectrum collection and it could have slightly altered n = 1 lead bromide nanoplatelet emission spectrum. Please click here to view a larger version of this figure.

Supplementary file. Supporting information. Please click here to download this file.

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Discussion

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 reports37,38,50,51, 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 reported36,52. 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 nanocrystals29,32,56.

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Disclosures

The authors declare no competing financial interests.

Acknowledgments

This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences (BES) under award number DE-SC0019345. Seung Kyun Ha was partially supported by the Kwanjeong Education Foundation Overseas Doctoral Program Scholarship. This work made use of the MRSEC Shared Experimental Facilities at MIT, supported by the National Science Foundation under award number DMR-08-19762. We thank Eric Powers for assistance with proofing and editing.

Materials

Name Company Catalog Number Comments
Equipment
365nm fiber-coupled LED Thorlabs M365FP1 Excitation source (Photoluminescence)
Avantes fiber-optic spectrometer Avantes AvaSpec-2048XL Photoluminescence detector (Photoluminescence spectra)
Cary 5000 Agilent Technologies UV-Vis spectrophotometer (Absorption spectra)
FEI Tecnai G2 Spirit Twin TEM FEI Company Transmission electron microscopy (TEM) operating at 120kV
PANalytical X'Pert Pro MPD Malvern Panalytical X-ray diffraction (XRD) operating at 45 kV and 40 mA with a copper radiation source.
Materials
n-butylammonium bromide (BABr) GreatCell Solar MS305000-50G
n-butylammonium chloride (BACl) Fisher Scientific B071025G butylamine hydrochloride
n-butylammonium iodide (BAI) Sigma-Aldrich 805874-25G
N,N-dimethylforamide (DMF) Sigma-Aldrich 227056-1L Anhydrous, 99.8%
n-dodecylammonium bromide (DDABr) GreatCell Solar MS300880-05
formamidinium bromide (FABr) GreatCell Solar MS350000-100G
formamidinium iodide (FAI) GreatCell Solar MS150000-100G
n-hexylammonium bromide (HABr) GreatCell Solar MS300860-05
lead bromide (PbBr2) Sigma-Aldrich 398853-5G .99.999%
lead chloride (PbCl2) Sigma-Aldrich 268-690-5G 98%
lead iodide (PbI2) solution Sigma-Aldrich 795550-10ML 0.55M in DMF
methylammonium bromide (MABr) GreatCell Solar MS301000-100G
methylammonium iodide (MAI) GreatCell Solar MS101000-100G
n-octylammonium bromide (OABr) GreatCell Solar MS305500-50G
n-octylammonium chloride (OACl) Fisher Scientific O04841G octylamine hydrochloride
n-octylammonium iodide (OAI) GreatCell Solar MS105500-50G
iso-pentylammonium bromide (i-PABr) GreatCell Solar MS300710-05
toluene Sigma-Aldrich 244511-1L Anhydrous, 99.8%

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