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.

NOTE: Simpler notations of &#8216;n = 1 BX&#8217; and &#8216;n = 2 ABX&#8217; 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
<ol>
	<li>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.
	<ol>
		<li>PbBr2 is not readily soluble in DMF at room temperature, keep the solution at 80 &#176;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.</li>
	</ol>
	</li>
	<li>Mix those individual precursor solutions in specific volumetric ratios for each target thickness and composition.
	<ol>
		<li>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.</li>
		<li>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.</li>
	</ol>
	</li>
</ol>
2. Synthesis of perovskite nanoplatelets via ligand-assisted reprecipitation method
<ol>
	<li>Inject 10 &#181;L 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 &#181;L. 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.</li>
	<li>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.</li>
</ol>
3. Characterization sample preparation and purification of colloidal perovskite nanoplatelet solution.
<ol>
	<li>Transmission electron microscopy (TEM) sample preparation.
	<ol>
		<li>Centrifuge the solution at 2050 x g for 10 min.</li>
		<li>Discard the supernatant.</li>
		<li>Redisperse the nanoplatelets in 1 mL of toluene.</li>
		<li>Drop 1 droplet on a TEM grid.</li>
		<li>Dry the sample under vacuum.</li>
	</ol>
	</li>
	<li>X-ray diffraction (XRD) sample preparation
	<ol>
		<li>Centrifuge the solution at 2050 x g for 10 min.</li>
		<li>Discard the supernatant.</li>
		<li>Redisperse the nanoplatelets in 30 &#181;L of toluene.</li>
		<li>Dropcast on a glass slide.</li>
		<li>Dry the sample under vacuum.</li>
	</ol>
	</li>
	<li>General purification
	<ol>
		<li>Centrifuge the solution at 2050 x g for 10 min.</li>
		<li>Discard the supernatant.</li>
		<li>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.</li>
	</ol>
	</li>
</ol>

The authors declare no competing financial interests.

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 &#8805; 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 &#8211; either for bromide or iodide nanoplatelets &#8211; 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 &#8211; 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.

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 &#8220;perovskite nanoplatelets&#8221;, 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.

<table><tbody><tr><td>&lt;strong&gt;Equipment&lt;/strong&gt;</td><td></td><td></td><td></td></tr><tr><td>365nm fiber-coupled LED</td><td>Thorlabs</td><td>M365FP1</td><td>Excitation source (Photoluminescence)</td></tr><tr><td>Avantes fiber-optic spectrometer</td><td>Avantes</td><td>AvaSpec-2048XL</td><td>Photoluminescence detector (Photoluminescence spectra)</td></tr><tr><td>Cary 5000</td><td>Agilent Technologies</td><td></td><td>UV-Vis spectrophotometer (Absorption spectra)</td></tr><tr><td>FEI Tecnai G2 Spirit Twin TEM</td><td>FEI Company</td><td></td><td>Transmission electron microscopy (TEM) operating at 120kV</td></tr><tr><td>PANalytical X&#039;Pert Pro MPD</td><td>Malvern Panalytical</td><td></td><td>X-ray diffraction (XRD) operating at 45 kV and 40 mA with a copper radiation source.</td></tr><tr><td>&lt;strong&gt;Materials&lt;/strong&gt;</td><td></td><td></td><td></td></tr><tr><td>n-butylammonium bromide (BABr)</td><td>GreatCell Solar</td><td>MS305000-50G</td><td></td></tr><tr><td>n-butylammonium chloride (BACl)</td><td>Fisher Scientific</td><td>B071025G</td><td>butylamine hydrochloride</td></tr><tr><td>n-butylammonium iodide (BAI)</td><td>Sigma-Aldrich</td><td>805874-25G</td><td></td></tr><tr><td>N,N-dimethylforamide (DMF)</td><td>Sigma-Aldrich</td><td>227056-1L</td><td>Anhydrous, 99.8%</td></tr><tr><td>n-dodecylammonium bromide (DDABr)</td><td>GreatCell Solar</td><td>MS300880-05</td><td></td></tr><tr><td>formamidinium bromide (FABr)</td><td>GreatCell Solar</td><td>MS350000-100G</td><td></td></tr><tr><td>formamidinium iodide (FAI)</td><td>GreatCell Solar</td><td>MS150000-100G</td><td></td></tr><tr><td>n-hexylammonium bromide (HABr)</td><td>GreatCell Solar</td><td>MS300860-05</td><td></td></tr><tr><td>lead bromide (PbBr2)</td><td>Sigma-Aldrich</td><td>398853-5G</td><td>.99.999%</td></tr><tr><td>lead chloride (PbCl2)</td><td>Sigma-Aldrich</td><td>268-690-5G</td><td>98%</td></tr><tr><td>lead iodide (PbI2) solution</td><td>Sigma-Aldrich</td><td>795550-10ML</td><td>0.55M in DMF</td></tr><tr><td>methylammonium bromide (MABr)</td><td>GreatCell Solar</td><td>MS301000-100G</td><td></td></tr><tr><td>methylammonium iodide (MAI)</td><td>GreatCell Solar</td><td>MS101000-100G</td><td></td></tr><tr><td>n-octylammonium bromide (OABr)</td><td>GreatCell Solar</td><td>MS305500-50G</td><td></td></tr><tr><td>n-octylammonium chloride (OACl)</td><td>Fisher Scientific</td><td>O04841G</td><td>octylamine hydrochloride</td></tr><tr><td>n-octylammonium iodide (OAI)</td><td>GreatCell Solar</td><td>MS105500-50G</td><td></td></tr><tr><td>iso-pentylammonium bromide (i-PABr)</td><td>GreatCell Solar</td><td>MS300710-05</td><td></td></tr><tr><td>toluene</td><td>Sigma-Aldrich</td><td>244511-1L</td><td>Anhydrous, 99.8%</td></tr></tbody></table>

facile synthesis of colloidal lead halide perovskite nanoplatelets via ligand-assisted reprecipitation

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.

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).
<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td></td>
			<td>MABr</td>
			<td>FABr</td>
			<td>PbBr2</td>
			<td>BABr</td>
			<td>OABr</td>
			<td>MAI</td>
			<td>FAI</td>
			<td>PbI2</td>
			<td>BAI</td>
			<td>OAI</td>
		</tr>
		<tr>
			<td>n=1 PbBr</td>
			<td>0</td>
			<td>0</td>
			<td>1</td>
			<td>1</td>
			<td>1</td>
			<td>0</td>
			<td>0</td>
			<td>0</td>
			<td>0</td>
			<td>0</td>
		</tr>
		<tr>
			<td>n=2 FAPbBr</td>
			<td>0</td>
			<td>1</td>
			<td>2</td>
			<td>5</td>
			<td>5</td>
			<td>0</td>
			<td>0</td>
			<td>0</td>
			<td>0</td>
			<td>0</td>
		</tr>
		<tr>
			<td>n=2 MAPbBr</td>
			<td>1</td>
			<td>0</td>
			<td>2</td>
			<td>5</td>
			<td>5</td>
			<td>0</td>
			<td>0</td>
			<td>0</td>
			<td>0</td>
			<td>0</td>
		</tr>
		<tr>
			<td>n=1 PbI</td>
			<td>0</td>
			<td>0</td>
			<td>0</td>
			<td>0</td>
			<td>0</td>
			<td>0</td>
			<td>0</td>
			<td>1</td>
			<td>1</td>
			<td>1</td>
		</tr>
		<tr>
			<td>n=2 FAPbI</td>
			<td>0</td>
			<td>0</td>
			<td>0</td>
			<td>0</td>
			<td>0</td>
			<td>0</td>
			<td>1</td>
			<td>2</td>
			<td>5</td>
			<td>5</td>
		</tr>
		<tr>
			<td>n=2 MAPbI</td>
			<td>0</td>
			<td>0</td>
			<td>0</td>
			<td>0</td>
			<td>0</td>
			<td>1</td>
			<td>0</td>
			<td>2</td>
			<td>5</td>
			<td>5</td>
		</tr>
	</tbody>
</table>
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.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/60114/60114fig01.jpg" /> 
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. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/60114/60114fig01large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/60114/60114fig02.jpg" /> 
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. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/60114/60114fig02large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" class="xfigimg" src="/files/ftp_upload/60114/60114fig03.jpg" /> 
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.
<img alt="Figure 4" class="xfigimg" src="/files/ftp_upload/60114/60114fig04.jpg" /> 
Figure 4. Transmission electron microscopy (TEM) images of perovskite nanoplatelets. 
Images show randomly-overlapping nanoplatelets. See also Figure S7. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/60114/60114fig04large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 5" class="xfigimg" src="/files/ftp_upload/60114/60114fig05.jpg" /> 
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. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/60114/60114fig05large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 6" class="xfigimg" src="/files/ftp_upload/60114/60114fig06.jpg" /> 
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. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/60114/60114fig06large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 7" class="xfigimg" src="/files/ftp_upload/60114/60114fig07.jpg" /> 
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. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/60114/60114fig07large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
Supplementary file. Supporting information. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/60114/supporting_info.pdf" target="_blank">Please click here to download this file.</a>

Watch this Scientific Journal Video about Facile Synthesis of Colloidal Lead Halide Perovskite Nanoplatelets via Ligand-Assisted Reprecipitation at JoVE.com

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

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.

In this work, we demonstrate a facile method for colloidal lead halide perovskite nanoplatelet synthesis (Chemical formula: L2[ABX3]n-1BX4, L: ...

facile-synthesis-colloidal-lead-halide-perovskite-nanoplatelets-via

Nanyang Technological University

Research

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Chemistry

12.8K Views.  Massachusetts Institute of Technology. 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.

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

Harvesting Solar Energy by Means of Charge-Separating Nanocrystals and Their Solids

Conjoining different semiconductor materials in a single nano-composite provides synthetic means for the development of novel optoelectronic materials offering a superior control over the spatial distribution of charge carriers across material interfaces. As this study demonstrates, a combination of donor-acceptor nanocrystal (NC) domains in a single nanoparticle can lead to the realization of efficient photocatalytic1-5 materials, while a layered assembly of donor- and acceptor-like nanocrystals films gives rise to photovoltaic materials.
Initially the paper focuses on the synthesis of composite inorganic nanocrystals, comprising linearly stacked ZnSe, CdS, and Pt domains, which jointly promote photoinduced charge separation. These structures are used in aqueous solutions for the photocatalysis of water under solar radiation, resulting in the production of H2 gas. To enhance the photoinduced separation of charges, a nanorod morphology with a linear gradient originating from an intrinsic electric field is used5. The inter-domain energetics are then optimized to drive photogenerated electrons toward the Pt catalytic site while expelling the holes to the surface of ZnSe domains for sacrificial regeneration (via methanol). Here we show that the only efficient way to produce hydrogen is to use electron-donating ligands to passivate the surface states by tuning the energy level alignment at the semiconductor-ligand interface. Stable and efficient reduction of water is allowed by these ligands due to the fact that they fill vacancies in the valence band of the semiconductor domain, preventing energetic holes from degrading it. Specifically, we show that the energy of the hole is transferred to the ligand moiety, leaving the semiconductor domain functional. This enables us to return the entire nanocrystal-ligand system to a functional state, when the ligands are degraded, by simply adding fresh ligands to the system4.
To promote a photovoltaic charge separation, we use a composite two-layer solid of PbS and TiO2 films. In this configuration, photoinduced electrons are injected into TiO2 and are subsequently picked up by an FTO electrode, while holes are channeled to a Au electrode via PbS layer6. To develop the latter we introduce a Semiconductor Matrix Encapsulated Nanocrystal Arrays (SMENA) strategy, which allows bonding PbS NCs into the surrounding matrix of CdS semiconductor. As a result, fabricated solids exhibit excellent thermal stability, attributed to the heteroepitaxial structure of nanocrystal-matrix interfaces, and show compelling light-harvesting performance in prototype solar cells7.

A general strategy for the development of charge-separating semiconductor nanocrystal composites deployable for solar energy production is presented. We show that assembly of donor-acceptor nanocrystal domains in a single nanoparticle geometry gives rise to a photocatalytic function, while bulk-heterojunctions of donor-acceptor nanocrystal films can be used for photovoltaic energy conversion.

Conjoining different semiconductor materials in a single nano-composite provides synthetic means for the development of novel optoelectronic materials ...

Engineering

A Technique to Functionalize and Self-assemble Macroscopic Nanoparticle-ligand Monolayer Films onto Template-free Substrates

This protocol describes a self-assembly technique to create macroscopic monolayer films composed of ligand-coated nanoparticles1,2. The simple, robust and scalable technique efficiently functionalizes metallic nanoparticles with thiol-ligands in a miscible water/organic solvent mixture allowing for rapid grafting of thiol groups onto the gold nanoparticle surface. The hydrophobic ligands on the nanoparticles then quickly phase separate the nanoparticles from the aqueous based suspension and confine them to the air-fluid interface. This drives the ligand-capped nanoparticles to form monolayer domains at the air-fluid interface.&#160; The use of water-miscible organic solvents is important as it enables the transport of the nanoparticles from the interface onto template-free substrates.&#160; The flow is mediated by a surface tension gradient3,4 and creates macroscopic, high-density, monolayer nanoparticle-ligand films.&#160; This self-assembly technique may be generalized to include the use of particles of different compositions, size, and shape and may lead to an efficient assembly method to produce low-cost, macroscopic, high-density, monolayer nanoparticle films for wide-spread applications.

A simple, robust and scalable technique to functionalize and self-assemble macroscopic nanoparticle-ligand monolayer films onto template-free substrates is described in this protocol.

This protocol describes a self-assembly technique to create macroscopic monolayer films composed of ligand-coated nanoparticles1,2. The simple, robust and ...

Fabrication of Fully Solution Processed Inorganic Nanocrystal Photovoltaic Devices

We demonstrate a method for the preparation of fully solution processed inorganic solar cells from a spin and spray coating deposition of nanocrystal inks. For the photoactive absorber layer, colloidal CdTe and CdSe nanocrystals (3-5 nm) are synthesized using an inert hot injection technique and cleaned with precipitations to remove excess starting reagents. Similarly, gold nanocrystals (3-5 nm) are synthesized under ambient conditions and dissolved in organic solvents. In addition, precursor solutions for transparent conductive indium tin oxide (ITO) films are prepared from solutions of indium and tin salts paired with a reactive oxidizer. Layer-by-layer, these solutions are deposited onto a glass substrate following annealing (200-400 &#176;C) to build the nanocrystal solar cell (glass/ITO/CdSe/CdTe/Au). Pre-annealing ligand exchange is required for CdSe and CdTe nanocrystals where films are dipped in NH4Cl:methanol to replace long-chain native ligands with small inorganic Cl- anions. NH4Cl(s) was found to act as a catalyst for the sintering reaction (as a non-toxic alternative to the conventional CdCl2(s) treatment) leading to grain growth (136&#177;39 nm) during heating. The thickness and roughness of the prepared films are characterized with SEM and optical profilometry. FTIR is used to determine the degree of ligand exchange prior to sintering, and XRD is used to verify the crystallinity and phase of each material. UV/Vis spectra show high visible light transmission through the ITO layer and a red shift in the absorbance of the cadmium chalcogenide nanocrystals after thermal annealing. Current-voltage curves of completed devices are measured under simulated one sun illumination. Small differences in deposition techniques and reagents employed during ligand exchange have been shown to have a profound influence on the device properties. Here, we examine the effects of chemical (sintering and ligand exchange agents) and physical treatments (solution concentration, spray-pressure, annealing time and annealing temperature) on photovoltaic device performance.

This protocol describes the synthesis and solution deposition of inorganic nanocrystals layer by layer to produce thin film electronics on non-conductive surfaces. Solvent-stabilized inks can produce complete photovoltaic devices on glass substrates via spin and spray coating following post-deposition ligand exchange and sintering.

We demonstrate a method for the preparation of fully solution processed inorganic solar cells from a spin and spray coating deposition of nanocrystal ...

Influence of Hybrid Perovskite Fabrication Methods on Film Formation, Electronic Structure, and Solar Cell Performance

Hybrid organic/inorganic halide perovskites have lately been a topic of great interest in the field of solar cell applications, with the potential to achieve device efficiencies exceeding other thin film device technologies. Yet, large variations in device efficiency and basic physical properties are reported. This is due to unintentional variations during film processing, which have not been sufficiently investigated so far. We therefore conducted an extensive study of the morphology and electronic structure of a large number of CH3NH3PbI3 perovskite where we show how the preparation method as well as the mixing ratio of educts methylammonium iodide and lead(II) iodide impact properties like film formation, crystal structure, density of states, energy levels, and ultimately the solar cell performance.

We present an extensive study on the effects of different fabrication methods for organic/inorganic perovskite thin films by comparing crystal structures, density of states, energy levels, and ultimately the solar cell performance.

Hybrid organic/inorganic halide perovskites have lately been a topic of great interest in the field of solar cell applications, with the potential to ...

Low Pressure Vapor-assisted Solution Process for Tunable Band Gap Pinhole-free Methylammonium Lead Halide Perovskite Films

Organo-lead halide perovskites have recently attracted great interest for potential applications in thin-film photovoltaics and optoelectronics. Herein, we present a protocol for the fabrication of this material via the low-pressure vapor assisted solution process (LP-VASP) method, which yields ~19% power conversion efficiency in planar heterojunction perovskite solar cells. First, we report the synthesis of methylammonium iodide (CH3NH3I) and methylammonium bromide (CH3NH3Br) from methylamine and the corresponding halide acid (HI or HBr). Then, we describe the fabrication of pinhole-free, continuous methylammonium-lead halide perovskite (CH3NH3PbX3 with X = I, Br, Cl and their mixture) films with the LP-VASP. This process is based on two steps: i) spin-coating of a homogenous layer of lead halide precursor onto a substrate, and ii) conversion of this layer to CH3NH3PbI3-xBrx by exposing the substrate to vapors of a mixture of CH3NH3I and CH3NH3Br at reduced pressure and 120 &#176;C. Through slow diffusion of the methylammonium halide vapor into the lead halide precursor, we achieve slow and controlled growth of a continuous, pinhole-free perovskite film. The LP-VASP allows synthetic access to the full halide composition space in CH3NH3PbI3-xBrx with 0&#160;&#8804; x&#160;&#8804; 3. Depending on the composition of the vapor phase, the bandgap can be tuned between 1.6 eV&#160;&#8804; Eg&#160;&#8804; 2.3 eV. In addition, by varying the composition of the halide precursor and of the vapor phase, we can also obtain CH3NH3PbI3-xClx. Films obtained from the LP-VASP are reproducible, phase pure as confirmed by X-ray diffraction measurements, and show high photoluminescence quantum yield. The process does not require the use of a glovebox.

Here, we present a protocol for the synthesis of CH3NH3I and CH3NH3Br precursors and the subsequent formation of pinhole-free, continuous CH3NH3PbI3-xBrx thin films for the application in high efficiency solar cells and other optoelectronic devices.

Organo-lead halide perovskites have recently attracted great interest for potential applications in thin-film photovoltaics and optoelectronics. Herein, ...

Recombination Dynamics in Thin-film Photovoltaic Materials via Time-resolved Microwave Conductivity

A method for investigating recombination dynamics of photo-induced charge carriers in thin film semiconductors, specifically in photovoltaic materials such as organo-lead halide perovskites is presented. The perovskite film thickness and absorption coefficient are initially characterized by profilometry and UV-VIS absorption spectroscopy. Calibration of both laser power and cavity sensitivity is described in detail. A protocol for performing Flash-photolysis Time Resolved Microwave Conductivity (TRMC) experiments, a non-contact method of determining the conductivity of a material, is presented. A process for identifying the real and imaginary components of the complex conductivity by performing TRMC as a function of microwave frequency is given. Charge carrier dynamics are determined under different excitation regimes (including both power and wavelength). Techniques for distinguishing between direct and trap-mediated decay processes are presented and discussed. Results are modelled and interpreted with reference to a general kinetic model of photoinduced charge carriers in a semiconductor. The techniques described are applicable to a wide range of optoelectronic materials, including organic and inorganic photovoltaic materials, nanoparticles, and conducting/semiconducting thin films.

A Time Resolved Microwave Conductivity technique for investigating direct and trap-mediated recombination dynamics and determining carrier mobilities of thin film semiconductors is presented here.

A method for investigating recombination dynamics of photo-induced charge carriers in thin film semiconductors, specifically in photovoltaic materials ...

Monovalent Cation Doping of CH3NH3PbI3 for Efficient Perovskite Solar Cells

Here, we demonstrate the incorporation of monovalent cation additives into CH3NH3PbI3 perovskite in order to adjust the optical, excitonic, and electrical properties. The possibility of doping was investigated by adding monovalent cation halides with similar ionic radii to Pb2+, including Cu+, Na+, and Ag+. A shift in the Fermi level and a remarkable decrease of sub-bandgap optical absorption, along with a lower energetic disorder in the perovskite, was achieved. An order-of-magnitude enhancement in the bulk hole mobility and a significant reduction of transport activation energy within an additive-based perovskite device was attained. The confluence of the aforementioned improved properties in the presence of these cations led to an enhancement in the photovoltaic parameters of the perovskite solar cell. An increase of 70 mV in open circuit voltage for AgI and a 2 mA/cm2 improvement in photocurrent density for NaI- and CuBr-based solar cells were achieved compared to the pristine device. Our work paves the way for further improvements in the optoelectronic quality of CH3NH3PbI3 perovskite and subsequent devices. It highlights a new avenue for investigations on the role of dopant impurities in crystallization and controls the electronic defect density in perovskite structures.

Monovalent Cation Doping of CH3NH3PbI3 for Efficient Perovskite Solar Cells

Here, we present a protocol to adjust the properties of solution-processed CH3NH3PbI3 through the incorporation of monovalent cation additives in order to achieve highly efficient perovskite solar cells.

Here, we demonstrate the incorporation of monovalent cation additives into CH3NH3PbI3 perovskite in order to adjust the optical, excitonic, and electrical ...

Liquid-cell Transmission Electron Microscopy for Tracking Self-assembly of Nanoparticles

Drying a nanoparticle dispersion is a versatile way to create self-assembled structures of nanoparticles, but the mechanism of this process is not fully understood. We have traced the trajectories of individual nanoparticles using liquid-cell transmission electron microscopy (TEM) to investigate the mechanism of the assembly process. Herein, we present the protocols used for liquid-cell TEM studies of the self-assembly mechanism. First, we introduce the detailed synthetic protocols used to produce uniformly sized platinum and lead selenide nanoparticles. Next, we present the microfabrication processes used to produce liquid cells with silicon nitride or silicon windows and then describe the loading and imaging procedures of the liquid-cell TEM technique. Several notes are included to provide helpful tips for the entire process, including how to manage the fragile cell windows. The individual motions of nanoparticles tracked by liquid-cell TEM revealed that changes in the solvent boundaries caused by evaporation affected the self-assembly process of nanoparticles. The solvent boundaries drove nanoparticles to primarily form amorphous aggregates, followed by flattening of the aggregates to produce a 2-dimensional (2D) self-assembled structure. These behaviors are also observed for different nanoparticle types and different liquid-cell compositions.

Here we introduce experimental protocols for the real-time observation of a self-assembly process using liquid-cell transmission electron microscopy.

Drying a nanoparticle dispersion is a versatile way to create self-assembled structures of nanoparticles, but the mechanism of this process is not fully ...

A Modular Microfluidic Technology for Systematic Studies of Colloidal Semiconductor Nanocrystals

Colloidal semiconductor nanocrystals, known as quantum dots (QDs), are a rapidly growing class of materials in commercial electronics, such as light emitting diodes (LEDs) and photovoltaics (PVs). Among this material group, inorganic/organic perovskites have demonstrated significant improvement and potential towards high-efficiency, low-cost PV fabrication due to their high charge carrier mobilities and lifetimes. Despite the opportunities for perovskite QDs in large-scale PV and LED applications, the lack of fundamental and comprehensive understanding of their growth pathways has inhibited their adaptation within continuous nanomanufacturing strategies. Traditional flask-based screening approaches are generally expensive, labor-intensive, and imprecise for effectively characterizing the broad parameter space and synthesis variety relevant to colloidal QD reactions. In this work, a fully autonomous microfluidic platform is developed to systematically study the large parameter space associated with the colloidal synthesis of nanocrystals in a continuous flow format. Through the application of a novel translating three-port flow cell and modular reactor extension units, the system may rapidly collect fluorescence and absorption spectra across reactor lengths ranging 3 - 196 cm. The adjustable reactor length not only decouples the residence time from the velocity-dependent mass transfer, it also substantially improves the sampling rates and chemical consumption due to the characterization of 40 unique spectra within a single equilibrated system. Sample rates may reach up to 30,000 unique spectra per day, and the conditions cover 4 orders of magnitude in residence times ranging 100 ms - 17 min. Further applications of this system would substantially improve the rate and precision of the material discovery and screening in future studies. Detailed within this report are the system materials and assembly protocols with a general description of the automated sampling software and offline data processing.

Detailed herein are the operation and assembly protocols of a modular microfluidic screening platform for the systematic characterization of colloidal semiconductor nanocrystal syntheses. Through fully adjustable system arrangements, highly efficient spectra collection may be carried out across 4 orders of magnitude reaction time scales within a mass transfer-controlled sampling space.

Colloidal semiconductor nanocrystals, known as quantum dots (QDs), are a rapidly growing class of materials in commercial electronics, such as light ...

Molten-Salt Synthesis of Complex Metal Oxide Nanoparticles

The development of feasible synthesis methods is critical for the successful exploration of novel properties and potential applications of nanomaterials. Here, we introduce the molten-salt synthesis (MSS) method for making metal oxide nanomaterials. Advantages over other methods include its simplicity, greenness, reliability, scalability, and generalizability. Using pyrochlore lanthanum hafnium oxide (La2Hf2O7) as a representative, we describe the MSS protocol for the successful synthesis of complex metal oxide nanoparticles (NPs). Furthermore, this method has the unique ability to produce NPs with different material features by changing various synthesis parameters such as pH, temperature, duration, and post-annealing. By fine-tuning these parameters, we are able to synthesize highly uniform, non-agglomerated, and highly crystalline NPs. As a specific example, we vary the particle size of the La2Hf2O7&#160;NPs by changing the concentration of the ammonium hydroxide solution used in the MSS process, which allows us to further explore the effect of particle size on various properties. It is expected that the MSS method will become a more popular synthesis method for nanomaterials and more widely employed in the nanoscience and nanotechnology community in the upcoming years.

Here, we demonstrate a unique, relatively low-temperature, molten-salt synthesis method for preparing uniform complex metal oxide lanthanum hafnate nanoparticles.

The development of feasible synthesis methods is critical for the successful exploration of novel properties and potential applications of nanomaterials. ...