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Method Article

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

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DOI:

10.3791/60114

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October 1st, 2019

 , 
1Department of Chemical Engineering, Massachusetts Institute of Technology

In This Article

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.

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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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 pat...

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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 i...

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Disclosures

The authors declare no competing financial interests.

Acknowledgements

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.

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Materials

List of materials used in this article
NameCompanyCatalog NumberComments
Equipment
365nm fiber-coupled LEDThorlabsM365FP1Excitation source (Photoluminescence)
Avantes fiber-optic spectrometerAvantesAvaSpec-2048XLPhotoluminescence detector (Photoluminescence spectra)
Cary 5000Agilent TechnologiesUV-Vis spectrophotometer (Absorption spectra)
FEI Tecnai G2 Spirit Twin TEMFEI CompanyTransmission electron microscopy (TEM) operating at 120kV
PANalytical X'Pert Pro MPDMalvern PanalyticalX-ray diffraction (XRD) operating at 45 kV and 40 mA with a copper radiation source.
Materials
n-butylammonium bromide (BABr)GreatCell SolarMS305000-50G
n-butylammonium chloride (BACl)Fisher ScientificB071025Gbutylamine hydrochloride
n-butylammonium iodide (BAI)Sigma-Aldrich805874-25G
N,N-dimethylforamide (DMF)Sigma-Aldrich227056-1LAnhydrous, 99.8%
n-dodecylammonium bromide (DDABr)GreatCell SolarMS300880-05
formamidinium bromide (FABr)GreatCell SolarMS350000-100G
formamidinium iodide (FAI)GreatCell SolarMS150000-100G
n-hexylammonium bromide (HABr)GreatCell SolarMS300860-05
lead bromide (PbBr2)Sigma-Aldrich398853-5G.99.999%
lead chloride (PbCl2)Sigma-Aldrich268-690-5G98%
lead iodide (PbI2) solutionSigma-Aldrich795550-10ML0.55M in DMF
methylammonium bromide (MABr)GreatCell SolarMS301000-100G
methylammonium iodide (MAI)GreatCell SolarMS101000-100G
n-octylammonium bromide (OABr)GreatCell SolarMS305500-50G
n-octylammonium chloride (OACl)Fisher ScientificO04841Goctylamine hydrochloride
n-octylammonium iodide (OAI)GreatCell SolarMS105500-50G
iso-pentylammonium bromide (i-PABr)GreatCell SolarMS300710-05
tolueneSigma-Aldrich244511-1LAnhydrous, 99.8%

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Colloidal Perovskite NanoplateletsLead Halide PerovskitesBand Gap TuningPhotoluminescence AbsorptionTransmission Electron MicroscopyX ray DiffractionSolution CentrifugationSurface Capping LigandsHalide Ion Stoichiometry
Facile Synthesis of Colloidal Lead Halide Perovskite Nanoplatelets via Ligand-Assisted Reprecipitation
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