Name: Facet-to-facet Linking of Shape-anisotropic Colloidal Cadmium Chalcogenide Nanostructures (Video) | JoVE Journal
Uploaded: 2017-08-10T14:00:00.000+00:00
Duration: 9 min 12 s
Description: Learn to link shape-anisotropic cadmium chalcogenide nanocrystals facet-to-facet via oriented attachment. This method enhances interparticle charge transport while maintaining solution processability for optoelectronic applications.

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5 minutes

This work was supported by JCO A*STAR Investigatorship grant (Project no. 1437C00135), A*STAR Science &amp; Engineering Research Council Public Sector Funding (Project no. 1421200076), and a JSPS-NUS Joint Research Projects grant (WBS R143-000-611-133).

1. Preparation of Precursor Stock Solutions
<ol>
	<li>TOP-Se stock solution.
	<ol>
		<li>In a nitrogen atmosphere glovebox, weigh out 11.84 g of selenium pellets into a 150-mL conical flask and place a magnetic stir bar in the flask.</li>
		<li>Add 100 mL of tri-n-octylphosphine (TOP) to the conical flask and seal the flask with a rubber septum.</li>
		<li>Stir the mixture overnight at 800 RPM. 
		NOTE: Once the selenium pellets have dissolved fully into the TOP, the TOP-Se stock solution (1.5 M) is ready to be used in further reactions.</li>
	</ol>
	</li>
	<li>TOP-S stock solution.
	<ol>
		<li>Weigh out 0.611 g of sulfur powder into a 20-mL single-necked round-bottom flask (RBF) and place a magnetic stir bar in the flask.</li>
		<li>Add 10 mL of TOP to the sulfur powder and seal the RBF with a rubber septum. Place it under a nitrogen atmosphere and stir at 800 RPM.</li>
		<li>Once the sulfur powder has dissolved and the solution is clear, use the TOP-S stock solution in further reactions.</li>
	</ol>
	</li>
</ol>
2. Synthesis of CdSe Quantum Dot (QD) Stock Solution
<ol>
	<li>Wurtzite CdSe QDs. 
	Note: See5.
	<ol>
		<li>Mix 9 g of tri-n-octylphosphine oxide (TOPO), 6 g of hexadecyl amine (HDA), and 0.25 mL of diisooctylphosphonic acid (DIPA) in a three-necked RBF. Add a magnetic stir bar to the mixture and insert a temperature probe into the RBF through a punctured rubber septum.
		<ol>
			<li>Mount a reflux condenser over the RBF and connect it to a Schlenk line via an adapter. Apply high vacuum grease at all glass-to-glass joints.</li>
		</ol>
		</li>
		<li>Degas the solution at 100 &#176;C for 2 h under constant stirring at 800 RPM.</li>
		<li>Separately, mix 0.320 g of cadmium acetylacetonate (Cd(acac)2), 0.570 g of hexadecanediol (HDDO), and 6 mL of 1-octadecene (ODE) in a single-necked RBF. Add a magnetic stir bar and degas the mixture under constant stirring over an oil bath for 2 h at 120 &#176;C and 800 RPM.</li>
		<li>After degassing for 2 h, increase the temperature of the three-necked RBF to 345 &#176;C. Concurrently, let the cadmium precursor mixture cool to room temperature and add 4 mL of TOP-Se stock solution to the single-necked RBF.</li>
		<li>Swiftly inject the entire cadmium and selenium precursor solution into the three-necked RBF once its temperature reaches 345 &#176;C. Remove the heating mantle immediately after injecting the precursor solution and allow the reaction mixture to cool to room temperature under ambient conditions.</li>
		<li>Add 15 mL of toluene to the crude reaction mixture and process the synthesized QDs by centrifuging for 3 min at 2,240 x g. Discard the precipitate and add an excess of methanol to the recovered supernatant. Centrifuge the mixture at 2,325 x g for 8 min and discard the supernatant.</li>
		<li>Add 5 mL of toluene to the precipitate to disperse the QDs. Carry out further processing cycles via the precipitation of the QDs using 30 mL of methanol, centrifuge the suspension at 2,240 x g for 3 min, discard the supernatant, and disperse the QDs in 5 mL of toluene.
		<ol>
			<li>Following two cycles of processing, disperse the QDs in a minimal amount (~2-3 mL) of toluene (henceforth referred to as the QD stock solution) for further use.</li>
		</ol>
		</li>
		<li>Determine the concentration of QDs in solution by taking 20 &#181;L of the stock solution and adding it to 3 mL of toluene. Measure the absorbance at 350 nm and calculate the concentration using the known molar absorptivity at 350 nm6. Scale the concentration accordingly to take into account the dilution of the stock solution. 
		NOTE: It is important to maintain vigorous stirring of the solution and to immediately remove the heating mantle after the swift injection of the precursor solution in step 2.1.5 to obtain highly size-monodisperse QDs.</li>
	</ol>
	</li>
	<li>Zinc-blende CdSe QDs. 
	Note: See7.
	<ol>
		<li>Mix 0.0384 g of cadmium oxide (CdO), 0.137 g of myristic acid (MA), and 5 mL of ODE in a three-necked RBF. Add a magnetic stir bar to the mixture and insert a temperature probe into the RBF through a punctured rubber septum. Mount a reflux condenser over the RBF and connect it to a Schlenk line via an adapter. Seal the remaining port with a rubber septum. Apply high vacuum grease at all glass-to-glass joints.</li>
		<li>Degas the solution at 90 &#176;C under constant stirring at 800 RPM.</li>
		<li>Separately, degas a mixture of 0.05 mL of oleic acid (OA), 0.5 mL of oleylamine (OM), and 2 mL of ODE in a single-necked RBF under constant stirring at 75 &#176;C and 800 RPM.</li>
		<li>Increase the temperature of the cadmium precursor solution to 250 &#176;C and maintain it for 15 min. Observe a color change in the solution, from dark brown to colorless, indicating the formation of cadmium myristate.</li>
		<li>Cool the reaction mixture to 90 &#176;C and add 12 mL of ODE. Degas the mixture again for 1 h.</li>
		<li>Let the reaction mixture cool to room temperature. Add 0.012 g of selenium powder and degas the mixture for 20 min.</li>
		<li>Increase the temperature of the reaction mixture to 240 &#176;C under N2. Observe a color change, from colorless to yellow at 150 &#176;C and then to orange-red upon reaching 240 &#176;C, signifying the formation of zb-CdSe QDs.</li>
		<li>Grow the QDs for 5 min at 240 &#176;C and then cool the mixture to room temperature. Add the degassed mixture of OM, OA, and ODE dropwise to the reaction solution. Leave the solution to cool to room temperature under ambient conditions.</li>
		<li>Process the synthesized QDs by adding acetone to the growth solution until the total volume is about 50 mL. Centrifuge the resulting suspension at 2,240 x g for 3 min. Subsequently, discard the supernatant.</li>
		<li>Add 5 mL of toluene to the precipitate to disperse the QDs. Carry out further processing cycles: precipitate the QDs using 30 mL of methanol, centrifuge the suspension at 2,240 x g for 3 min, discard the supernatant, and disperse the QDs in 5 mL of toluene.
		<ol>
			<li>Following two cycles of processing, disperse the QDs in a minimal amount (~2-3 mL) of toluene (henceforth referred to as the QD stock solution) for further use.</li>
		</ol>
		</li>
		<li>Determine the concentration of CdSe QDs in the stock solution by taking 20 &#181;L of the processed nanoparticles and adding it to 3 mL of toluene. Measure the absorbance at 350 nm and calculate the concentration using the known molar absorptivity at 350 nm6. Scale the concentration accordingly to take into account the dilution of the stock solution. 
		NOTE: For the successful synthesis of monodisperse QDs, it is critical to ensure that all the selenium powder is added to the reaction mixture and that none of it is left stuck to the neck and sidewalls of the RBF.</li>
	</ol>
	</li>
</ol>
3. Synthesis of CdSe-seeded CdS NRs
Note: See8.
<ol>
	<li>Mix 3 g of TOPO, 0.90 g of CdO, 0.80 g of hexylphosphonic acid (HPA), and 0.29 g of octadecylphosphonic acid (ODPA) in a 50-mL three-necked RBF. Add a magnetic stir bar to the mixture and insert a temperature probe through a punctured rubber septum.</li>
	<li>Mount a reflux condenser over the RBF and connect it to a Schlenk line via an adapter. Seal the remaining port with a rubber septum. Apply high vacuum grease at all glass-to-glass joints. Heat the RBF to 150 &#176;C and place it under a vacuum for 1.5 h to degas, with stirring at 800 RPM.</li>
	<li>Place 1.8 mL of the TOP-S stock solution into a 10-mL single-necked RBF and seal it with a rubber septum. Add 80 nmol of wurtzite CdSe QDs in toluene to the TOP-S solution and subsequently remove the toluene under vacuum at 70 &#176;C. Allow the solution to degas under stirring at 800 RPM for a further 30 min.</li>
	<li>Place the RBF containing the Cd precursor under nitrogen and raise the temperature to 350 &#176;C. At 320 &#176;C, add 1.8 mL of TOP to the RBF through the rubber septum.</li>
	<li>At 350 &#176;C, draw the TOP-S solution containing the w-CdSe QDs into a syringe and rapidly inject it into the RBF containing the Cd precursor. Allow the solution to stir at 800 RPM for an additional 6 min to allow for the growth of the NRs. Subsequently, remove the heating mantle and cool the solution to room temperature under ambient conditions.</li>
	<li>To process the solution of NRs, add 2 mL of toluene to the growth solution and place the entire mixture into a 50-mL centrifuge tube. Add 30 mL of methanol. Subsequently, centrifuge the resulting suspension at 2,240 x g for 3 min and then discard the supernatant.</li>
	<li>Add 5 mL of toluene to the precipitate to disperse the NRs. Carry out further processing cycles: precipitate the NRs using 30 mL of methanol, centrifuge the suspension at 2,240 x g for 3 min, discard the supernatant, and disperse the NRs in 5 mL of toluene.
	<ol>
		<li>Following 2-3 cycles of processing, disperse the NRs in 5 mL of toluene (henceforth referred to as the NR stock solution) for further use.</li>
	</ol>
	</li>
	<li>Prepare the sample for TEM analysis.5
	<ol>
		<li>Place a drop of the NR solution onto a 300-mesh copper grid covered with a continuous carbon film for electron microscopy analysis.</li>
		<li>Remove the excess solution with an adsorbent paper and dry the sample at room temperature.</li>
	</ol>
	</li>
	<li>Perform TEM imaging (and analysis) to obtain the dimensions of the NR so that the average volume of the NR and the number of moles of CdS per NR can be determined.
	<ol>
		<li>Determine the concentration of CdSe-seeded CdS NRs in the stock solution by taking 20 &#181;L of the processed NRs and adding it to 3 mL of toluene. Measure the absorbance at 350 nm and calculate the concentration of NRs using the known molar absorptivity at that wavelength (assumed to be dominated by CdS)9. Scale the concentration accordingly to account for the dilution of the stock solution. 
		NOTE: For a typical synthesis, the yield of NRs is nearly 100%, with very few byproducts such as branched structures.</li>
	</ol>
	</li>
</ol>
4. Synthesis of CdSe-seeded CdSe NRs
Note: See8.
<ol>
	<li>Mix 1.035 g of CdO, 0.1657 g of HPA, 0.1543 g of n-tetradecylphosphonic acid (TDPA), and 3 g of TOPO in a three-necked RBF. Add a magnetic stir bar to the mixture and insert a temperature probe into the RBF through a punctured rubber septum. Mount a reflux condenser over the RBF and connect it to a Schlenk line via an adapter. Seal the remaining port with a rubber septum. Apply high vacuum grease at all glass-to-glass joints.</li>
	<li>Degas the solution at 150 &#176;C for 1.5 h under constant stirring at 800 RPM.</li>
	<li>Separately, mix 2 mL of the TOPSe stock solution and 10 nmol of w-CdSe QDs (dispersed in toluene) in a single-necked RBF and degas under constant stirring over an oil bath at 800 RPM and 90 &#176;C until all the toluene is removed.</li>
	<li>After degassing, increase the temperature of the cadmium precursor mixture to 340 &#176;C. As the temperature increases, observe a color change in the solution, from dark brown to colorless, indicating the formation of the cadmium-alkyl phosphonic acid complex.</li>
	<li>As the temperature of cadmium precursor reaches 340 &#176;C, swiftly inject 1.8 mL of TOP into the mixture and allow the temperature to recover to 340 &#176;C.</li>
	<li>Upon reaching 340 &#176;C, swiftly inject 1.8 mL of the TOP-Se/CdSe QD mixture into the three-necked RBF, whereupon the temperature will drop to 320 &#176;C. Upon recovery to 340 &#176;C, maintain the reaction solution at this temperature for ~3 min before removing the heating mantle and allowing the solution to cool to room temperature under ambient conditions.</li>
	<li>After the reaction solution has cooled to room temperature, add 2 mL of toluene and transfer the entire solution to a 50-mL centrifuge tube. Top up the centrifuge tube with methanol (99%) to 50 mL and centrifuge the resulting suspension at 2,240 x g for 3 min. Subsequently, discard the supernatant.</li>
	<li>Add 5 mL of toluene to the precipitate to disperse the NRs. Carry out further processing cycles: precipitate the NRs using 30 mL of methanol (99%), centrifuge the suspension at 2,240 x g for 3 min, discard the supernatant, and disperse the NRs in 5 mL of toluene.
	<ol>
		<li>Following 2-3 cycles of processing, disperse the NRs in 5 mL of toluene (henceforth referred to as the NR stock solution) for further use.</li>
	</ol>
	</li>
	<li>Place a drop of NR solution onto a 300-mesh copper grid covered with a continuous carbon film for electron microscopy analysis. Remove the excess solution with an adsorbent paper and dry the sample at room temperature.</li>
	<li>Perform TEM imaging and analysis to obtain the dimensions of the NRs so that the average volume of the NR and the number of moles of CdSe per NR can be determined.
	<ol>
		<li>Determine the concentration of NRs in solution by taking 20 &#181;L of the stock solution and adding it to 3 mL of toluene. Measure the absorbance at 350 nm and calculate the concentration using the known molar absorptivity at that wavelength6. Scale the concentration accordingly to take into account the dilution of the stock solution. 
		NOTE: For a typical synthesis, the yield of NRs is close to 100%, with very few byproducts such as branched structures.</li>
	</ol>
	</li>
</ol>
5. Synthesis of CdSe-seeded CdSe TPs
Note: See10.
<ol>
	<li>Mix 0.43 g of CdO, 3.8 mL of OA, 2.2 mL ODE, and 0.3 mL of TOP in a three-necked RBF. Add a magnetic stir bar to the mixture and insert a temperature probe into the RBF through a punctured rubber septum. Mount a reflux condenser over the RBF and connect it to a Schlenk line via an adapter. Seal the remaining port with a rubber septum. Apply high vacuum grease at all glass-to-glass joints.</li>
	<li>Degas the solution at 90 &#176;C for 1 h under constant stirring at 800 RPM and then increase the temperature to 265 &#176;C. As the temperature reaches 265 &#176;C, observe the color of the solution changing, from dark brown to colorless, indicating the formation of the cadmium oleate complex.</li>
	<li>Reduce the temperature to 50 &#176;C and add 1.7 mL of 2 M TOPSe (separately prepared by sonicating, at 37 kHz and 320 W, a mixture of 0.316 g of Se powder in 2 mL of TOP in an 8-mL glass vial) and 0.017 g of hexadecyltrimethylammonium bromide (CTAB).</li>
	<li>Separately, mix 7 mL of ODE, 0.025 g of CTAB, 0.5 mL of TOP, 0.75 mL of OA, and 100 nmol of zinc-blende CdSe QDs (dispersed in toluene) in a three-necked RBF. Add a magnetic stir bar to the mixture and insert a temperature probe into the RBF through a punctured rubber septum.
	<ol>
		<li>Mount a condenser over the RBF and connect it to the Schlenk line via an adapter. Seal the remaining port with a rubber septum. Apply high vacuum grease at all glass-to-glass joints.</li>
	</ol>
	</li>
	<li>Degas the mixture prepared in step 5.4 at 90 &#176;C and 800 RPM under constant stirring for ~45 min until all the toluene is removed.</li>
	<li>Increase the temperature of the reaction mixture containing the zinc-blende CdSe QDs to 260 &#176;C. When the temperature reaches 260 &#176;C, add 8 mL of the cadmium precursor solution to the mixture containing zinc-blende CdSe QDs at an injection rate of 0.25 mL/min using a syringe pump. After the addition is completed, allow the solution to cool to room temperature under ambient conditions.</li>
	<li>After the solution has cooled to room temperature, transfer it to a 50-mL centrifuge tube. Add 40 mL of acetone (99%) to precipitate the TPs and centrifuge the suspension at 1,340 x g for 10 min. Subsequently, discard the supernatant.</li>
	<li>Add 5 mL of toluene to the precipitate of TPs to disperse them. Carry out further processing cycles: precipitate the TPs using 30 mL of methanol (99%), centrifuge the suspension at 2,240 x g for 3 min, discard the supernatant, and disperse the TPs in 5 mL of toluene.
	<ol>
		<li>Following 2-3 cycles of processing, disperse the TPs in 5 mL of toluene (henceforth referred to as the TP stock solution) for further use.</li>
	</ol>
	</li>
	<li>Place a drop of the TP solution onto a 300-mesh copper grid covered with a continuous carbon film for electron microscopy analysis. Remove the excess solution with adsorbent paper and dry the sample at room temperature.</li>
	<li>Perform TEM imaging and analysis to obtain the dimensions of the TPs so that the average volume of the TP and the number of moles of CdSe per TP can be determined.
	<ol>
		<li>Determine the concentration of QDs in solution by taking 20 &#181;L of the stock solution and adding it to 3 mL of toluene.</li>
		<li>Measure the absorbance at 350 nm and calculate the concentration using the known molar absorptivity at that wavelength6. Scale the concentration accordingly to account for the dilution of the stock solution. 
		Note: For a typical synthesis, the yield for the TPs is ~80%, with ~20% being bipods and tripods.</li>
	</ol>
	</li>
</ol>
6. Facet Activation and the Linking of Nanostructures
<ol>
	<li>Preparation of a dodecylamine (DDA) stock solution.
	<ol>
		<li>Prepare a DDA stock solution by adding 0.140 g of DDA to 5 mL of ethanol. Sonicate the solution at 37 kHz and 320 W for ~5 min to ensure that the DDA is fully dissolved.</li>
	</ol>
	</li>
	<li>Cation exchange and linking.
	<ol>
		<li>Prepare a 1-mL solution of NC (NR or TP) at the appropriate concentration (see Table 1). Add 6 mg of ODPA to the 1 mL of NC solution and sonicate for 10 min at 37 kHz and 320 W.</li>
		<li>Separately, mix 1 mL of DDA stock solution and 1 mL of Ag+ solution at the appropriate concentration (see Table 1) in a vial. Add a magnetic stir bar and stir the solution vigorously at 800 RPM.</li>
		<li>While stirring, add 1 mL of the NC solution to the vial and allow the reaction to proceed for the corresponding amount of time, as listed in Table 1.</li>
		<li>At the end of the reaction, stop the stirring and allow the solution to phase-separate. Extract and remove the bottom aqueous layer. Add 5 mL of methanol to the organic layer, to precipitate out the NCs. Centrifuge the vial for 3 min at 2,240 x g.</li>
		<li>After centrifugation, discard the supernatant and add 1 mL of toluene to re-disperse the product for further characterization. 
		NOTE: The cation exchange reaction occurs at different rates for different materials. Table 1 summarizes the set of conditions under which the synthesis of linked nanostructures of other materials and morphologies are synthesized. The volume of NC solution (1 mL), aqueous Ag+ solution (1 mL), and ethanol-DDA solution (1 mL) and the amount of ODPA (6 mg) added are kept the same, as mentioned above, for each set of reactions.</li>
	</ol>
	</li>
</ol>

<ol><li>Figuerola, A., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((End-to-End+Assembly+of+Shape-Controlled+Nanocrystals+via+a+Nanowelding+Approach+Mediated+by+Gold+Domains%5BTitle%5D)+AND+%22Adv.+Mat%22%5BJournal%5D)">End-to-End Assembly of Shape-Controlled Nanocrystals via a Nanowelding Approach Mediated by Gold Domains.</a> Adv. Mat. 21, 550-554 (2009).</li>
<li>Tang, Z., Kotov, N. A., Giersig, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Spontaneous+Organization+of+Single+CdTe+Nanoparticles+into+Luminescent+Nanowires%5BTitle%5D)+AND+%22Science%22%5BJournal%5D)">Spontaneous Organization of Single CdTe Nanoparticles into Luminescent Nanowires.</a> Science. 297, 237-240 (2002).</li>
<li>Kim, D., Kim, W. D., Kang, M. S., Kim, S. -H., Lee, D. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Self-Organization+of+Nanorods+into+Ultra-Long+Range+Two-Dimensional+Monolayer+End-to-End+Network%5BTitle%5D)+AND+%22Nano+Lett%22%5BJournal%5D)">Self-Organization of Nanorods into Ultra-Long Range Two-Dimensional Monolayer End-to-End Network.</a> Nano Lett. 15, 714-720 (2015).</li>
<li>Schliehe, C., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Ultrathin+PbS+Sheets+by+Two-Dimensional+Oriented+Attachment%5BTitle%5D)+AND+%22Science%22%5BJournal%5D)">Ultrathin PbS Sheets by Two-Dimensional Oriented Attachment.</a> Science. 329, 550-553 (2010).</li>
<li>Chakrabortty, S., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Facet+to+facet+Linking+of+Shape+Anisotropic+Inorganic+Nanocrystals+with+Site+Specific+and+Stoichiometric+Control%5BTitle%5D)+AND+%22Nano+Lett%22%5BJournal%5D)">Facet to facet Linking of Shape Anisotropic Inorganic Nanocrystals with Site Specific and Stoichiometric Control.</a> Nano Lett. 16, 6431-6436 (2016).</li>
<li>Leatherdale, C. A., Woo, W. K., Mikulec, F. V., Bawendi, M. G. <a target="_blank" href="http://pubs.acs.org/doi/abs/10.1021/jp025698c">On the Absorption Cross Section of CdSe Nanocrystal Quantum Dots.</a> J Phys Chem B. 106, 7619-7622 (2002).</li>
<li>Dabbousi, B. O., et al. <a target="_blank" href="http://pubs.acs.org/doi/abs/10.1021/jp971091y">(CdSe)ZnS Core-Shell Quantum Dots: Synthesis and Characterization of a Size Series of Highly Luminescent Nanocrystallites.</a> J Phys Chem B. 101, 9463-9475 (1997).</li>
<li>Carbone, L., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Synthesis+and+Micrometer-Scale+Assembly+of+Coilloidal+CdSe%2FCdS+Nanorods+Prepared+by+Seeded+Grwoth+Approach%5BTitle%5D)+AND+%22Nano+Lett%22%5BJournal%5D)">Synthesis and Micrometer-Scale Assembly of Coilloidal CdSe/CdS Nanorods Prepared by Seeded Grwoth Approach.</a> Nano Lett. 7, 2942-2950 (2007).</li>
<li>Shaviv, E., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Absorption+properties+of+Metal-Semiconductor+Hybrid+Nanoparticles%5BTitle%5D)+AND+%22ACS+Nano%22%5BJournal%5D)">Absorption properties of Metal-Semiconductor Hybrid Nanoparticles.</a> ACS Nano. 5, 4712-4719 (2011).</li>
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</ol>

We do not have anything to disclose.

The linking technique described in this work allows for shape-anisotropic cadmium chalcogenide nanoparticles that can undergo cation exchange with Ag+ to be joined, facet-to-facet, into assemblies such as linear chains or branched networks. Failure to form well-dispersed, extensive assemblies of facet-to-facet linked nanoparticles is often because of two reasons: (i) the ODPA is not dispersed well in the NR-containing solution, which can be addressed by sonicating the mixture for the prescribed amount of time ...

The directed assembly of colloidal semiconductor NCs offers a synthetic pathway to the fabrication of nanostructures whose physicochemical properties are either the collective sum of or radically different from their individual NC building blocks1,2,3,4. Among the various approaches to nanoparticle assembly, the method of oriented attachment - in which NCs are essentially fused with each other - stands out as one that allows for interparticle electronic coupling. However, conventional oriented attachment typically requires the delicate balancing of particle dipole-, ligand- and solvent-based interactions that are generally difficult to execute and make applicable to different NC systems.
We have recently developed a wet-chemical method of covalently joining shape-anisotropic cadmium chalcogenide NCs by introducing a reactive inorganic intermediate through a site-selective nucleation process. The particles are subsequently linked by the spontaneous fusion of the reactive inorganic intermediate domains5. Although the technique is still based on an oriented attachment mechanism, there is much less need to consider weak interparticle interactions, thus allowing for more flexibility and control. The linking of shape-anisotropic cadmium chalcogenide NCs is carried out by first converting their tip facets to silver chalcogenide via a partial cation exchange process (in solution); this is followed by the selective removal of ligands passivating the surface. The NCs then come together via the fusion of the exposed silver chalcogenide facets, resulting in assemblies of cadmium chalcogenide NCs that are linked end-to-end.
In this protocol, we demonstrate that the linking technique can be applied to a variety of shape-anisotropic cadmium chalcogenide NCs (i.e., CdSe-seeded CdS NRs and CdSe-seeded CdSe NRs or TPs), yielding long linear NR chains or highly branched TP networks. These results suggest that the technique can be extended to a wide variety of NC shapes and metal chalcogenides amenable to silver cation exchange.

<table><tbody><tr><td>Cadmium oxide (CdO), 99.5%</td><td>Sigma Aldrich</td><td></td><td>Highly toxic</td></tr><tr><td>Tri-n-octylphosphine oxide (TOPO), 90 % and 99%</td><td>Sigma Aldrich</td><td></td><td>Technical and analytical grade</td></tr><tr><td>Cadmium acetylacetonate (Cd(acac)&lt;sub&gt;2&lt;/sub&gt;), 99.9%</td><td>Sigma Aldrich</td><td></td><td>Highly toxic</td></tr><tr><td>Hexadecanediol (HDDO), 90%</td><td>Sigma Aldrich</td><td></td><td>Technical grade</td></tr><tr><td>1-octadecene (ODE), 90%</td><td>Sigma Aldrich</td><td></td><td>Technical grade</td></tr><tr><td>Dodecylamine (DDA), 98%</td><td>Sigma Aldrich</td><td></td><td>Toxic</td></tr><tr><td>Cadmium nitrate tetrahydrate ((CdNO&lt;sub&gt;3&lt;/sub&gt;)&lt;sub&gt;2&lt;/sub&gt;.4H&lt;sub&gt;2&lt;/sub&gt;O), 98%</td><td>Sigma Aldrich</td><td></td><td>Highly toxic</td></tr><tr><td>Myristic acid (MA), 99%</td><td>Sigma Aldrich</td><td></td><td>Analytical grade</td></tr><tr><td>Octyl phosphonic acid (OPA), 97%</td><td>Sigma Aldrich</td><td></td><td>Analytical grade</td></tr><tr><td>Oleylamine (Oly), 70%</td><td>Sigma Aldrich</td><td></td><td>Technical grade</td></tr><tr><td>Hexadecyltrimethylammonium bromide (CTAB), 95%</td><td>Sigma Aldrich</td><td></td><td>Toxic</td></tr><tr><td>Selenium pellets (Se, 5 mm), 99.99%</td><td>Sigma Aldrich</td><td></td><td>Analytical grade</td></tr><tr><td>Hexadecylamine (HDA), 90%</td><td>Alfa Aesar</td><td></td><td>Technical grade, toxic</td></tr><tr><td>n-tetradecylphosphonic acid (TDPA), 98%&amp;nbsp;</td><td>Alfa Aesar</td><td></td><td>Analytical grade</td></tr><tr><td>Silver nitrate (AgNO&lt;sub&gt;3&lt;/sub&gt;), 99.9%</td><td>Alfa Aesar</td><td></td><td>Analytical grade</td></tr><tr><td>Oleic acid (OA), 90%</td><td>Alfa Aesar</td><td></td><td>Technical grade</td></tr><tr><td>Tri-n-octylphosphine (TOP), 97%</td><td>Strem</td><td></td><td>Analytical grade, toxic, air sensitive</td></tr><tr><td>n-hexylphosphonic acid (HPA), 97%</td><td>Strem</td><td></td><td>Analytical grade</td></tr><tr><td>n-octadecylphosphonic acid (ODPA), 97%</td><td>Strem</td><td></td><td>Analytical grade</td></tr><tr><td>Tellurium powder (Te), 99.9%</td><td>Strem</td><td></td><td>Air sensitive</td></tr><tr><td>Tri-n-butylphosphine (TBP), 99%</td><td>Strem</td><td></td><td>Analytical grade, highly toxic, air sensitive</td></tr><tr><td>Diisooctylphosphonic acid (DIPA), 90%</td><td>Fluka</td><td></td><td>Technical grade, toxic</td></tr></tbody></table>

facet-to-facet linking of shape-anisotropic colloidal cadmium chalcogenide nanostructures

Facet-to-facet Linking of Shape-anisotropic Colloidal Cadmium Chalcogenide Nanostructures

Here, we describe a protocol that allows for shape-anisotropic cadmium chalcogenide nanocrystals (NCs), such as nanorods (NRs) and tetrapods (TPs), to be covalently and site-specifically linked via their end facets, resulting in polymer-like linear or branched chains. The linking procedure begins with a cation-exchange process in which the end facets of the cadmium chalcogenide NCs are first converted to silver chalcogenide. This is followed by the selective removal of ligands at their surface. This results in cadmium chalcogenide NCs with highly reactive silver chalcogenide end facets that spontaneously fuse upon contact with each other, thereby establishing an interparticle facet-to-facet attachment. Through the judicious choice of precursor concentrations, an extensive network of linked NCs can be produced. Structural characterization of the linked NCs is carried out via low- and high-resolution transmission electron microscopy (TEM), as well as energy-dispersive X-ray spectroscopy, which confirm the presence of silver chalcogenide domains between chains of cadmium chalcogenide NCs.

Using CdSe-seeded CdS NRs as a model system, as illustrated in Figure 1(a), we demonstrated that we can use a partial Ag+ exchange process to specifically transform the facets at the NR tips to Ag2S. The Ag2S facets are capped by DDA, which reacts with ODPA via an acid-base reaction to form an insoluble salt5. This causes the DDA ligands to be removed from the Ag2S facets, ca...

A protocol detailing how shape-anisotropic colloidal cadmium chalcogenide nanocrystals can be covalently linked via their end facets is presented here.

Here, we describe a protocol that allows for shape-anisotropic cadmium chalcogenide nanocrystals (NCs), such as nanorods (NRs) and tetrapods (TPs), to be ...

The overall goal of this synthetic protocol is to link colloidal shape-anisotropic semiconductor nanocrystals, facet-to-facet via an oriented attachment process. While this method can potentially address key issues in the field of colloidal nanoparticle based optoelectronic, such as the need to improve interparticle charge transport while preserving solution process ability. The main advantage of this technique is that the facet-to-facet linking process can be applied to semi conductor nanoparticles of various shapes and sizes.We first had the idea for this method when we observed signs of oriented attachment at the carrying out exchange reactions in shape anisotropic semiconductor nanocrystals under certain reactions and conditions. Demonstrating the procedure will be Xuanwei Ong and Shashank Gupta, grad students from my laboratory. First, add TOPO, cadmium oxide, hexylphosphonic acid and octadecylphosphonic acid to a 15 milliliter, three neck round bottom flask.After adding a magnetic stir bar to the mixture, insert a temperature probe through a punctured rubber septum and seal one of the cords of the flask with the septum. Mount the reflux condenser on the round bottom flask and connect the via an adapter, then, seal the remaining port with a rubber septum. After applying high vacuum grease on all glass elastic joints, heat the stirring mixture to 150 degree celsius and place it under a vacuum for one and a half hours to degas.Next, add 1.8 milliliters of a previously prepared TLP sulfide stock solution to a 10 millimeter single neck round bottom flask and seal it with a rubber septum. At 80 nanomoles of a previously prepared wurtzite cadmium selenide quantum dots telluwin solution to the flask, and subsequently remove the telluwin under the vacuum at 70 degree celsius. Allow the solution to degas under stirring at 800 RPM for an additional 30 minutes.Place the round bottom flask containing the cadmium precursor under nitrogen and increase the temperature to 350 degree celsius. At 320 degree celsius, add 1.8 milliliters of TLP to the round bottom flask through the rubber septum. At 350 degree celsius, draw the TLP sulfide solution containing the quantum dots, into a syringe and rapidly inject it into the round bottom flask containing the cadmium precursor.Allow the solution to stir at 800 RPM for an additional six minutes, to allow for growth of the nanorods. Subsequently, remove the heating mantle and cool the solution to room temperature under ambient conditions. To process the solution of nanorods, add two milliliters of telluwin to the growth solution and transfer the entire mixture to a 50 milliliter centrifuge tube.Add 30 milliliters of methanol to the mixture. Then, centrifuge the resulting suspension at 2, 240 x G for three minutes. After discarding the supernatant, add five milliliters of telluwin to the precipitate to disperse the nanorods.Following two to three cycles of the previous processing steps, disperse the nanorods in five millimeters of telluwin for further use. Next, place a drop of the nanorod solution onto a 300 mesh copper grid, covered with a continuous carbon film for electron microscopy analysis. Remove the excess solution with an adsorbent paper and allow the sample to dry at room temperature.Now, determine the concentration of cadmium selenide seeded cadmium sulfide nanorods in the stock solution, by adding 20 microliters of the processed nanorods to three milliliters of telluwin. Measure the absorbents of 350 nanometers and calculate the concentration of nanorods, using the known molar absorptivity at that wavelength. Prepare a dodecylamine stock solution, by adding 0.14 grams of dodecylamine to five milliliters of ethanol.Sonicate the solution at 37 kilohertz and 320 watts for approximately five minutes, to ensure folded solution. Following this, prepare a one milliliter solution of nanocrystals at the appropriate concentration. Add six milligrams of octadecylphosphonic acid to the nanocrystal solution and sonicate for 10 minutes at 37 kilohertz and 320 watts.Manually agitate the solution mixture while sonicating as it is critical to completely dissolve octadecylphosphonic acid in the nanocrystal solution. In a separate vial, mix one milliliter of silver nitride solution at the appropriate concentration and one milliliter of dodecylamine stock solution. Add a magnetic stir bar and stir the solution vigorously at 800 RPM, whilst stirring, add one milliliter of the nanocrystal solution to the vial and allow the reaction to proceed for the appropriate amount of time, according to the text protocol.At the end of the reaction, stop the stirring and allow the solution to phase separate. Then, remove the bottom aqueous layer. Add five milliliters of methanol to the organic layer to precipitate out the nanocrystals.Then, centrifuge the mixture at 2, 240 x G for three minutes. After discarding the supernatant, add one milliliter of telluwin to re-disperse the product for further characterization. Using cadmium selenide seeded cadmium sulfide nanorod as a model system, a partial silver ion exchange process to transform the facets of the nanorod tips to silver sulfide tips was demonstrated.After reacting with octadecylphosphonic acid, dodecylamine ligands desorbed from the surface and the facets fuse together and form linked nanorod chains. Hybrid solution, TEM analysis on the joint regions shows silver sulfide domains and amputextile contacts with two nanorods. A first fourier transform analysis reveals two lattice constants that can be ascribed to the 001 facets of silver sulfide and cadmium sulfide.EDX analysis on the linkage region shows the presence of silver and the absence of cadmium. The yield and statistical nature of the linking process can be visualized via a histogram that shows the number of rods linked within a nanorod chain. Without octadecylphosphonic acid, no linking is observed and the histogram shows a large proportion of single unlinked nanorods.With low silver ion concentration, only short chains were obtained. The linking statistics were the short chains feature of substantial proportion of dimers, followed by monomers. The silver mediated linking process can be extended to cadmium selenide seeded cadmium selenide nanorods and tetrapods.Under the appropriate conditions, similar chained networks of these nanorods and tetrapods can be achieved. Once mastered, this technique can be done in three hours if performed properly. While attempting this procedure, it's important to remember to ensure that the purity of the reagents used is the same as those listed in our protocol as impurities can significantly affect synthetic outcomes.Following this procedure, other methods like, dynamic light scattering can be performed to answer additional questions like, what are the hydrodynamic sizes of the linked nanoparticles in solution. After watching this video, you should have a good understanding of how to link anisotropic semiconductor nanocrystals via a cathartic exchange based approach. Don't forget that working with cadmium based nanocrystals can be extremely hazardous and precautions such as, wearing proper personal protective equipment should always be taken while performing this procedure.

facet-to-facet-linking-shape-anisotropic-colloidal-cadmium

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Learn to link shape-anisotropic cadmium chalcogenide nanocrystals facet-to-facet via oriented attachment. This method enhances interparticle charge transport while maintaining solution processability for optoelectronic applications.

Facet-to-facet Linking of Shape-anisotropic Colloidal Cadmium Chalcogenide Nanostructures

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