We gratefully acknowledge support from the National Science Foundation under grant CHE-1552164 for development of the original synthesis and characterization methods presented in this manuscript. During preparation of this manuscript, we acknowledge the following agencies for support of student and postdoctoral salaries: Nayon Park (National Science Foundation, CHE-1552164), Madison Monahan (US Department of Energy, Office of Science, Office of Basic Energy Sciences, as part of the Energy Frontier Research Centers program: CSSAS--The Center for the Science of Synthesis Across Scales under Award Number DE-SC0019288), Andrew Ritchhart (National Science Foundation, CHE-1552164), Max R. Friedfeld (Washington Research Foundation).

CAUTION: Proper personal protective equipment should be worn at all times and the material safety data sheet (MSDS) should be read for each chemical prior to use. All steps should be done air-free, because exposing clusters to air and/or water will degrade clusters or prevent proper formation. Any point at which the reaction flask is open to air, N2 should be flowing vigorously to create a protective blanket over the reagents in the flask. All N2 used should be 99.9% or greater in purity.
1. Preparation of molecular precursors
<ol>
	<li>Purification of ligand precursor 
	NOTE: Myristic acid can be substituted by phenylacetic, oleic, or other long-chain carboxylic acids.
	<ol>
		<li>Place a 100 mL 3-neck flask, a stir bar, a reflux condenser, a glass stopper, a thermowell, a T-adapter, and a hose adapter into a 160 &#176;C oven overnight. 
		NOTE: Glassware can also be flame dried and placed in the oven for 1 h.</li>
		<li>Set up the flask, condenser, and thermowell on a Schlenk line while the glassware is still warm using high temperature vacuum grease to ensure glassware remains water-free. Close the final neck of the flask with a rubber septum.</li>
		<li>Place 2.65 g of myristic acid (HO2C14H27, 11.6 mmol) into the flask and flush with N2. Put the flask under vacuum for 2 h with mild stirring at 120 &#176;C to remove residual water from the acid.</li>
	</ol>
	</li>
	<li>Preparation of indium myristate (In(O2C14H27)3) 
	NOTE: Equivalent InP clusters can be prepared with trimethylindium or indium acetate.
	<ol>
		<li>Preparation via trimethylindium (InMe3)
		<ol>
			<li>Weigh out 0.512 g of InMe3 (3.2 mmol, anhydrous) and dissolve it in 10 mL of anhydrous toluene. Draw the solution into a syringe and seal with a rubber septum to keep the solution air-free during removal from the glovebox.</li>
			<li>Add 10 mL of anhydrous toluene to myristic acid from step 1.1 via a syringe and stir at room temperature until dissolved.</li>
			<li>Slowly add InMe3 from step 1.2.1.1 via a syringe (~2 drops/s) while stirring. This step should result in rapid gas formation visible to the eye. Allow to stir for 10 min to ensure complete reaction.</li>
		</ol>
		</li>
		<li>Preparation via indium acetate (In(OAc)3)
		<ol>
			<li>Weigh out 0.93 g of In(OAc)3 (3.2 mmol) and add to the flask containing myristic acid from step 1.1 under positive N2 flow.</li>
			<li>Evacuate the flask and heat the flask to 100 &#176;C while stirring. The mixture should melt. Let the solution off-gas acetic acid for 12 h (overnight) at 120 &#176;C.</li>
			<li>Refill the flask with N2 and add 20 mL of anhydrous toluene via a syringe through the rubber septum.</li>
		</ol>
		</li>
	</ol>
	</li>
</ol>
2. Synthesis of In37P20(O2CR)51
<ol>
	<li>Heat the reaction flask containing In(O2C14H27)3 solution to 110 &#176;C.</li>
	<li>Add 465 &#956;L of P(SiMe3)3 (1.6 mmol) to 10 mL of anhydrous toluene in the glovebox. Draw the solution into a syringe and insert the needle into a rubber stopper until the needle opening is completely covered. Be cautious when removing from the glovebox as P(SiMe3)3 is a pyrophoric liquid.</li>
	<li>Rapidly inject the P(SiMe3)3 solution into the hot In(O2C14H27)3 solution. The solution should turn yellow quickly after the addition of P(SiMe3)3. Monitor the reaction by taking 50 &#956;L aliquots of reaction solution into 3 mL of toluene for UV-Vis analysis. The reaction is finished when no further changes are seen in the aliquot spectra.</li>
	<li>Remove from heat to cool the flask and halt the reaction. 
	NOTE: The growth of InP clusters is optimal at a temperature range of 100-110 &#176;C. The reaction proceeds at lower temperatures, including room temperature, but goes very slowly. Higher temperatures result in evolution of quantum dots of varying sizes depending on the temperature. The reaction typically requires 20-60 min to go to completion, depending on the ligands used. Failure to run the reaction to completion could result in small, unstable clusters that will quickly decompose. Running the reaction past completion will not alter the cluster composition as long as the temperature is at or below 110 &#176;C.</li>
</ol>
3. Workup of In37P20(O2CR)51
NOTE: All solvents used in the purification steps are anhydrous and stored over 4 &#197; sieves in the N2-filled glovebox.
<ol>
	<li>Isolation of In37P20(O2CR)51
	<ol>
		<li>Remove the solvent from the cluster solution under reduced pressure on the Schlenk line.</li>
		<li>Seal the flask under N2 using the glass stopper and T-adapter from the oven. Fasten the flask and adapters with electrical tape and bring into the glovebox.</li>
	</ol>
	</li>
	<li>Purification via precipitation, centrifugation, and re-dissolution
	<ol>
		<li>Resuspend the clusters in minimal toluene (~1 mL) and centrifuge to remove solid impurities (7,197 x g, 10 min). Decant and keep the clear, yellow supernatant and discard any solids.</li>
		<li>Add 3 mL of acetonitrile (3:1, MeCN:tol) to the supernatant to precipitate the clusters (yellow precipitate) and centrifuge again under the same parameters. Discard the clear, colorless supernatant and resuspend the yellow solid pellet of clusters in minimal toluene.</li>
		<li>Repeat step 3.2.2 for a total of 5 cycles.</li>
	</ol>
	</li>
	<li>Purification via column
	<ol>
		<li>Take the clusters dissolved in minimal toluene (~0.5 mL) and apply them in a thin band to a freshly cleaned size-exclusion, liquid column (60 cm, 25 mm) packed with permeable gel beads (see Table of Materials) using toluene as the solvent.</li>
		<li>Allow the clusters to run through the column ensuring to keep the beads wet by adding fresh toluene as the liquid runs through the column. Collect all yellow fluid but make sure to stop collecting immediately after the yellow passes as excess ligand will come off after the cluster. Typically, the clusters take about 20 min to elute at room temperature. 
		NOTE: To confirm where the cluster region ends on the column, a laser pointer (405 nm) can be used to see where glowing resumes. The portion of the column containing clusters will not glow.</li>
		<li>Remove solvent under reduced pressure via a vacuum pump until a waxy solid is achieved. Store dry clusters under N2 for best stability. In a typical synthesis, 1.2 g of clusters should be isolated, representing a 90% yield.</li>
	</ol>
	</li>
</ol>
4. Synthesis of InP quantum dots using In37P20(O2CR)51 as a single source precursor
NOTE: Indium phosphide quantum dots can be synthesized from purified InP clusters using a heat-up or hot injection method.
<ol>
	<li>Heat-up method
	<ol>
		<li>Degas a 100 mL 3-neck round bottom flask equipped with a stir bar, a glass thermowell, a T-adapter, and a rubber septum. Assemble the glassware using high temperature vacuum grease.</li>
		<li>Dissolve 200 mg of purified InP clusters in 20 mL of anhydrous 1-octadecene. Inject the InP cluster solution into the reaction flask via a syringe under a positive N2 flow, followed by an additional 20 mL of anhydrous 1-octadecene. Briefly degas to ensure the reaction flask is air-free.</li>
		<li>Heat the solution to 300 &#176;C under a positive N2 flow while stirring. The growth of QDs can be monitored by UV-Vis spectroscopy with timed aliquots. The bright yellow color persists up to around 200 &#176;C and changes from yellow to bright orange to dark red-brown. The reaction is complete in 30-40 min.</li>
		<li>Cool down the reaction flask to room temperature by removing the heating mantle. The solution after cooling displays an optically clear red color. Remove 1-octadecene by vacuum distillation at 160 &#176;C. Replace the appropriate glassware as quickly as possible to limit air exposure.</li>
		<li>Dissolve InP QDs using minimal amount (&#60;5 mL) of anhydrous toluene in a N2-filled glovebox. Transfer the crude solution to a centrifuge tube. Add ~40 mL of anhydrous acetonitrile and centrifuge for purification (7,197 x g, 10 min).</li>
		<li>Pour off the supernatant and re-dissolve the precipitate in approximately 5 mL of anhydrous toluene. Repeat the purification steps for a total of 3 cycles. Store the purified product dissolved in anhydrous toluene.</li>
	</ol>
	</li>
	<li>Hot-injection method
	<ol>
		<li>Degas a 100 mL 3-neck round bottom flask equipped with a thermowell, a Schlenk adapter, and a rubber septum. Assemble the glassware using high temperature vacuum grease.</li>
		<li>Inject 35 mL of anhydrous 1-octadecene into the reaction flask. Heat the solvent to 300 &#176;C under inert gas while stirring.</li>
		<li>Dissolve 200 mg of purified InP clusters in 5 mL of anhydrous 1-octadecene and inject the cluster solution into the reaction flask. The reaction is complete in 15-20 min.</li>
		<li>Cool down the solution to room temperature by removing the heating mantle. Distill off 1-octadecene and purify InP QDs as described in steps 4.1.5 to 4.1.6.</li>
	</ol>
	</li>
</ol>
5. Characterization of In37P20(O2CR)51 and InP quantum dots
<ol>
	<li>Transmission electron microscopy (TEM)
	<ol>
		<li>Prepare a dilute solution of cluster (~5 mg) or quantum dots in pentane:toluene (1:1, 2 mL total). Solution color should be barely visible.</li>
		<li>Pick up a lacey carbon TEM grid (400 mesh, ultra-fine) with tweezers and place off the edge of a surface, making sure the grid is not touching anything besides the tweezers.</li>
		<li>Drop-cast one large drop onto the grid and let it dry completely with no dabbing away solution (~20 min). Place the grid under vacuum and let further dry overnight.</li>
		<li>Image the sample with TEM. Typical image conditions include spot size 5,200 kV electron beam, objective aperture completely open, and magnification of 350,000X.</li>
		<li>In the native image format (.dm3), measure the particle size using the straight-line tool to create a line across the imaged particle (see Table of Materials for software). The command tool finder will give a length in nanometers corresponding to the drawn line, when the line is actively being dragged or clicked on and measure the particle diameter.</li>
	</ol>
	</li>
	<li>Nuclear magnetic resonance (NMR) 
	NOTE: 1H NMR needs as little as 20 mg of clusters, but 31P NMR requires at least 40 mg of clusters to resolve the cluster region.
	<ol>
		<li>Prepare cluster solution (40 mg) in deuterated benzene (C6D6, ~0.7 mL) in the glovebox and transfer the solution into an oven-dried J-young NMR tube. Seal the tube under N2 and remove from the glovebox for measurements.</li>
		<li>Collect 1H NMR spectra on a 300 MHz or greater instrument. Common parameters include 2 dummy scans, 6 scans, and 30 s delay time.</li>
		<li>Collect 31P NMR spectra on a 500 MHz or greater instrument. Use a phosphoric acid standard to calibrate the instrument prior to use. Common parameters include an offset of -100 ppm, sweep width of 500 ppm, and 256 scans repeated 40 times (about 14 h of total run time to ensure a strong enough signal).</li>
	</ol>
	</li>
	<li>X-ray Diffraction (XRD)
	<ol>
		<li>Prepare a highly concentrated solution of clusters or quantum dots by either dissolving dried material in minimal toluene (&#60; 1 mL) or removing toluene from a stock solution.</li>
		<li>Drop-cast onto an oven-dried Si wafer and let dry for ~30 min. For consistency of drop size, use a digital micropipette with the setting at 3~5 &#181;L. Repeat 2-3 times until a film of clusters or quantum dots has sufficiently set.</li>
		<li>Collect XRD data from 10 to 70&#176; with 5.5&#176; intervals. Set each interval to 30 s acquisition time for a quick run or 240 s acquisition for higher resolution.</li>
	</ol>
	</li>
	<li>Photoluminescence (PL)
	<ol>
		<li>Transfer the solution for PL analysis into a 1 x 1 cm quartz fluorescence spectrophotometer cell.</li>
		<li>Set the excitation at 450 nm and the monochromator slit widths for both entrance and exit slits at 3 nm. Set the integration time at 0.1 s/nm or 1 s/nm for higher resolution.</li>
	</ol>
	</li>
</ol>

<ol>
	<li>Shirasaki, Y., Supran, G. J., Bawendi, M. G., Bulovi&#263;, V. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Emergence+of+Colloidal+Quantum-Dot+Light-Emitting+Technologies.">Emergence of Colloidal Quantum-Dot Light-Emitting Technologies.</a> Nature Photonics. 7, 13-23 (2013).</li><li>Shea-Rohwer, L. E., Martin, J. E., Cai, X., Kelley, D. F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Red-Emitting+Quantum+Dots+for+Solid-State+Lighting.">Red-Emitting Quantum Dots for Solid-State Lighting.</a> ECS Journal of Solid State Science and Technology. 2, R3112-R3118 (2013).</li><li>Kazlas, P. T., Linton, J. R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Quantum+Dot-Based+Light+Sheets+Useful+for+Solid-State+Lighting.">Quantum Dot-Based Light Sheets Useful for Solid-State Lighting.</a> , (2010).</li><li>Kairdolf, B. A., Smith, A. M., Stokes, T. H., Wang, M. D., Young, A. N., Nie, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Semiconductor+Quantum+Dots+for+Bioimaging+and+Biodiagnostic+Applications.">Semiconductor Quantum Dots for Bioimaging and Biodiagnostic Applications.</a> Annual Review of Analytical Chemistry. 6, 143-162 (2013).</li><li>Petryayeva, E., Algar, W. R., Medintz, I. L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Quantum+Dots+in+Bioanalysis:+A+Review+of+Applications+across+Various+Platforms+for+Fluorescence+Spectroscopy+and+Imaging.">Quantum Dots in Bioanalysis: A Review of Applications across Various Platforms for Fluorescence Spectroscopy and Imaging.</a> Applied Spectroscopy. 67, 215-252 (2013).</li><li>Sambur, J. B., Chen, P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Approaches+to+Single-Nanoparticle+Catalysis.">Approaches to Single-Nanoparticle Catalysis.</a> Annual Review of Physical Chemistry. 65, 395-422 (2014).</li><li>Xia, Y., Yang, H., Campbell, C. T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Nanoparticles+for+Catalysis.">Nanoparticles for Catalysis.</a> Accounts of Chemical Research. 46, 1671-1672 (2013).</li><li>Lewis, N. S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Toward+Cost-Effective+Solar+Energy+Use.">Toward Cost-Effective Solar Energy Use.</a> Science. 315, 798-801 (2007).</li><li>Semonin, O. E., Luther, J. M., Beard, M. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Quantum+Dots+for+Next-Generation+Photovoltaics.">Quantum Dots for Next-Generation Photovoltaics.</a> Materials Today. 15, 508-515 (2012).</li><li>Carey, G. H., Abdelhady, A. L., Ning, Z., Thon, S. M., Bakr, O. M., Sargent, E. H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Colloidal+Quantum+Dot+Solar+Cells.">Colloidal Quantum Dot Solar Cells.</a> Chemical Reviews. 115, 12732-12763 (2015).</li><li>Colegrove, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Quantum+Dot+Technology+Supply+Chain+and+Market+Forecast.">Quantum Dot Technology Supply Chain and Market Forecast.</a> Display Research. , (2018).</li><li>Pu, Y., Cai, F., Wang, D., Wang, J. X., Chen, J. F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Colloidal+Synthesis+of+Semiconductor+Quantum+Dots+toward+Large-Scale+Production:+A+Review.">Colloidal Synthesis of Semiconductor Quantum Dots toward Large-Scale Production: A Review.</a> Industrial Engineering Chemical Research. 57, 1790-1802 (2018).</li><li>Gary, D. C., Terban, M. W., Billinge, S. J. L., Cossairt, B. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Two-Step+Nucleation+and+Growth+of+InP+Quantum+Dots+via+Magic-Sized+Cluster+Intermediates.">Two-Step Nucleation and Growth of InP Quantum Dots via Magic-Sized Cluster Intermediates.</a> Chemistry of Materials. 27, 1432-1441 (2015).</li><li>Cossairt, B. M., Owen, J. S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=CdSe+Clusters:+At+the+Interface+of+Small+Molecules+and+Quantum+Dots.">CdSe Clusters: At the Interface of Small Molecules and Quantum Dots.</a> Chemistry of Materials. 23, 3114-3119 (2011).</li><li>Zhang, J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Evolution+of+Self-Assembled+ZnTe+Magic-Sized+Nanoclusters.">Evolution of Self-Assembled ZnTe Magic-Sized Nanoclusters.</a> Journal of the American Chemical Society. 137, 742-749 (2015).</li><li>Zhu, T., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Two-Step+Nucleation+of+CdS+Magic-Size+Nanocluster+MSC-311.">Two-Step Nucleation of CdS Magic-Size Nanocluster MSC-311.</a> Chemisty of Materials. 29, 5727-5735 (2017).</li><li>Gary, D. C., Flowers, S. E., Kaminsky, W., Petrone, A., Li, X., Cossairt, B. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Single-Crystal+and+Electronic+Structure+of+a+1.3+Nm+Indium+Phosphide+Nanocluster.">Single-Crystal and Electronic Structure of a 1.3 Nm Indium Phosphide Nanocluster.</a> Journal of the American Chemical Society. 138, 1510-1513 (2016).</li><li>Janke, E. M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Origin+of+Broad+Emission+Spectra+in+InP+Quantum+Dots:+Contributions+from+Structural+and+Electronic+Disorder.">Origin of Broad Emission Spectra in InP Quantum Dots: Contributions from Structural and Electronic Disorder.</a> Journal of the American Chemical Society. 140, 15791-15803 (2018).</li><li>Stein, J. L., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Probing+Surface+Defects+of+InP+Quantum+Dots+Using+Phosphorus+K&#945;+and+K&#946;+X-Ray+Emission+Spectroscopy.">Probing Surface Defects of InP Quantum Dots Using Phosphorus K&#945; and K&#946; X-Ray Emission Spectroscopy.</a> Chemistry Materials. 30, 6377-6388 (2018).</li><li>Giansante, C., Infante, I. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Surface+Traps+in+Colloidal+Quantum+Dots:+A+Combined+Experimental+and+Theoretical+Perspective.">Surface Traps in Colloidal Quantum Dots: A Combined Experimental and Theoretical Perspective.</a> Journal of Physical Chemistry Letters. 8, 5209-5215 (2017).</li><li>Tessier, M. D., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Interfacial+Oxidation+and+Photoluminescence+of+InP-Based+Core/Shell+Quantum+Dots.">Interfacial Oxidation and Photoluminescence of InP-Based Core/Shell Quantum Dots.</a> Chemistry of Materials. 30, 6877-6883 (2018).</li><li>Brown, R. P., Gallagher, M. J., Fairbrother, D. H., Rosenzweig, Z. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Synthesis+and+Degradation+of+Cadmium-Free+InP+and+InPZn/ZnS+Quantum+Dots+in+Solution.">Synthesis and Degradation of Cadmium-Free InP and InPZn/ZnS Quantum Dots in Solution.</a> Langmuir. 34, 13924-13934 (2018).</li><li>Brodu, A., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Exciton+Fine+Structure+and+Lattice+Dynamics+in+InP/ZnSe+Core/Shell+Quantum+Dots.">Exciton Fine Structure and Lattice Dynamics in InP/ZnSe Core/Shell Quantum Dots.</a> ACS Photonics. 5, 3353-3362 (2018).</li><li>Yang, J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Chemical+Synthesis,+Doping,+and+Transformation+of+Magic-Sized+Semiconductor+Alloy+Nanoclusters.">Chemical Synthesis, Doping, and Transformation of Magic-Sized Semiconductor Alloy Nanoclusters.</a> Journal of the American Chemical Society. 139, 6761-6770 (2017).</li><li>Xie, L., Harris, D. K., Bawendi, M. G., Jensen, K. F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Effect+of+Trace+Water+on+the+Growth+of+Indium+Phosphide+Quantum+Dots.">Effect of Trace Water on the Growth of Indium Phosphide Quantum Dots.</a> Chemistry of Materials. 27 (14), 5058-5063 (2015).</li><li>Gary, D. C., Petrone, A., Li, X., Cossairt, B. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Investigating+the+Role+of+Amine+in+InP+Nanocrystal+Synthesis:+Destabilizing+Cluster+Intermediates+by+Z-Type+Ligand+Displacement.">Investigating the Role of Amine in InP Nanocrystal Synthesis: Destabilizing Cluster Intermediates by Z-Type Ligand Displacement.</a> Chemical Communications. 53 (1), 161-164 (2017).</li><li>De Roo, J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Probing+Solvent-Ligand+Interactions+in+Colloidal+Nanocrystals+by+the+NMR+Line+Broadening.">Probing Solvent-Ligand Interactions in Colloidal Nanocrystals by the NMR Line Broadening.</a> Chemistry of Materials. 30 (15), 5485-5492 (2018).</li><li>Kirkwood, N., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Finding+and+Fixing+Traps+in+II-VI+and+III-V+Colloidal+Quantum+Dots:+The+Importance+of+Z-Type+Ligand+Passivation.">Finding and Fixing Traps in II-VI and III-V Colloidal Quantum Dots: The Importance of Z-Type Ligand Passivation.</a> Journal of the American Chemical Society. 140 (46), 15712-15723 (2018).</li><li>Ritchhart, A., Cossairt, B. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Templated+Growth+of+InP+Nanocrystals+with+a+Polytwistane+Structure.">Templated Growth of InP Nanocrystals with a Polytwistane Structure.</a> Angewandte Chemie International Edition. 57 (7), 1908-1912 (2018).</li><li>Friedfeld, M. R., Johnson, D. A., Cossairt, B. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Conversion+of+InP+Clusters+to+Quantum+Dots.">Conversion of InP Clusters to Quantum Dots.</a> Inorganic Chemistry. 58 (1), 803-810 (2019).</li><li>Gary, D. C., Cossairt, B. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Role+of+Acid+in+Precursor+Conversion+During+InP+Quantum+Dot+Synthesis.">Role of Acid in Precursor Conversion During InP Quantum Dot Synthesis.</a> Chemistry of Materials. 25 (12), 2463-2469 (2013).</li><li>Becker, G., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Inorganic Synthesis. 27, 243-249 (1990).</li></ol>

The authors have nothing to disclose.

The synthesis of InP magic-sized clusters and their conversion into quantum dots follow straightforward procedures that have been shown to consistently produce high quality samples. The ability to synthesize and isolate InP clusters as an intermediate has distinct advantages in terms of subjecting these nanostructures to modifications that can be well-characterized and consequently be incorporated in the final QDs. The atomically precise nature of the clusters and the high reproducibility provide a platform for innovative studies in surface modifications, defects, and alloying of the InP systems and open doors to a wide range of applications such as in displays, solid-state lighting, catalysis, and photovoltaics.
In the synthesis of InP clusters, it is critical that all reagents are of high purity and thoroughly dried, as the success of the synthesis is contingent upon water- and air-free experimental conditions and purity of the precursors for uniform growth in high yields. Additionally, it is recommended that sufficient precautions are taken when handling P(SiMe3)3, which is light-sensitive and pyrophoric. This reagent should be stored in a light-, air-, and water-free environment and caution should be taken to prevent air and water exposure before and during the reaction. For efficient growth of the clusters, the temperature range should be 100-110 &#176;C; at room temperature, the growth is extremely slow, and a higher temperature will result in conversion into quantum dots of varying sizes depending on the temperature. The presented protocol is also highly scalable and versatile, allowing synthetic control and modifications through a breadth of parameters. The myristic acid used as the ligands for InP clusters and subsequent QDs can be replaced by phenylacetic acid, oleic acid, or other short and long-chain carboxylic acids. Post-synthetic addition of P(SiMe3)3 to solutions of InP clusters that have slightly perturbed absorption features (red-shifted and/or broadened) has been observed to result in a size focusing effect where the consumption of excess indium myristate results in a ~3 nm blueshift in the absorption spectra29.
The purification method of the clusters has been empirically optimized in our lab to avoid oxidation and to isolate the highest possible yields. The choice of acetonitrile as the antisolvent and its volume ratio with toluene fulfill these goals. Finally, the clusters are resuspended in minimal amount of toluene and centrifuged to remove any solid impurities that may have resulted during synthesis. Removing toluene from the final solution gives a yellow paste that can be stored for at least 36 months under air- and water-free conditions. It should also be noted in regard to preparing NMR samples for characterization of the purified product that the precise chemical shifts for the 11 distinct resonances in 31P NMR spectrum vary depending on the identity of the indium precursors. Furthermore, insufficient purification and variation in cluster concentration can result in line broadening. In order to obtain a clean spectrum with sharp features, it is suggested that at least 40 mg of the cluster is dissolved in a minimal amount of anhydrous C6D6 (~0.7 mL).
Similarly, the synthesis of InP QDs via clusters must be performed under water- and air-free conditions. Previous studies have shown that the presence of water in indium precursors and the addition of trace amounts of water or hydroxide lead to significant changes in the growth of InP QDs and the surface chemistry of the final product25. When running the reaction at a different scale than described in the protocol, it should be noted that for the hot-inject method, the cluster solution for injection should be sufficiently concentrated and the volume should be smaller compared to the heated solvent in the flask. This is to minimize the abrupt decrease in temperature as the reaction temperature profile plays a nontrivial role in the synthesis. Detailed work on the conversion mechanism of InP clusters to QDs has been recently reported where the effects of the addition of different precursors (i.e., carboxylic acid, indium carboxylate), temperatures, and concentration have been explored30. Through these studies, it has been revealed that thermolysis temperatures &#62; 220 &#176;C are required for obtaining high yields of optimal quality QDs. The purification of InP QDs follows similar logic and process as mentioned above for the clusters, except that the storage of purified QDs is recommended in solution with a solvent such as toluene. In solid form, the QDs have been observed to form aggregates over time, preventing homogeneous colloidal dispersion. One final note regarding the protocol is that removing 1-octadecene by vacuum distillation after the synthesis of InP QDs rather than by only precipitation-redissolution is a recommended first step of QD purification. This is to limit the volume of solvent required in the workup and because the residual ODE may interdigitate with the long-chain carboxylate ligand shell, causing difficulties with sample preparation for characterization and subsequent use.
We have demonstrated the synthesis and characterization of atomically-precise InP magic-size clusters, In37P20(O2CR)51, and their use as single source precursors for the synthesis of InP quantum dots using both heat-up and hot-injection methods. The reported synthesis of InP clusters is versatile and can be generalized to a wide range of alkyl carboxylate ligands. The synthesis of the InP QDs from the clusters provides a highly reproducible method for the synthesis of these challenging nanostructures with high quality in terms of size distribution and crystallinity. Opportunities abound for further elaboration of this method through post-synthetic modification of the clusters themselves and for engineering the cluster to quantum dot conversion strategy. Because of this, we believe these methods are useful and potentially technologically meaningful for the synthesis of InP and related emissive materials for display and lighting applications.

An erratum was issued for: <a href="https://www.jove.com/video/59425/synthesis-in37p20-o2cr-51-clusters-their-conversion-to-inp-quantum">Synthesis of In37P20(O2CR)51 Clusters and Their Conversion to InP Quantum Dots</a>.&#160; The Representative Results&#160;were&#160;updated.
The fifth sentence in the second paragraph of the Representative Results was updated from:
The 31P NMR spectrum of myristate-capped InP clusters shows 11 distinct peaks (2 P atoms on the C2 axis that each give a unique peak and the remaining 18 P each have a symmetry equivalent, resulting in an additional 9 peaks) ranging from -256 to -311 ppm (Figure 1d)26.
to:
The 31P NMR spectrum of myristate-capped InP clusters shows 11 distinct peaks (2 P atoms on the C2 axis that each give a unique peak and the remaining 18 P each have a symmetry equivalent, resulting in an additional 9 peaks) ranging from -187&#160;to -242&#160;ppm (Figure 1d)26.&#160;
Figure 1 in the Representative Results was updated from:
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/59425/59425fig1.jpg" />
to:
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/59425/59425fig1v2.jpg" />

An erratum was issued for: <a href="https://www.jove.com/video/59425/synthesis-in37p20-o2cr-51-clusters-their-conversion-to-inp-quantum">Synthesis of In37P20(O2CR)51 Clusters and Their Conversion to InP Quantum Dots</a>.&#160; The Representative Results&#160;were&#160;updated.

Erratum: Synthesis of In37P20(O2CR)51 Clusters and Their Conversion to InP Quantum Dots

Colloidal semiconductor quantum dots have seen an acceleration in synthetic development over the previous three decades owing to their potential in a variety of optoelectronic applications including displays1, solid-state lighting2,3, biological imaging4,5, catalysis6,7, and photovoltaics8,9,10. Given their recent commercial success in the area of wide-color gamut displays, the quantum dot market is expected to exceed 16 billion dollars by 202811. A significant shift in material focus from the II-VI (and IV-VI) to the III-V family has occurred in the last several years as the search for less toxic, Cd and Pb-free alternatives for use in highly distributed electronics applications has begun. Indium phosphide in particular has been identified as a leading drop-in replacement for CdSe12. It has become apparent, however, that optimization of InP-based quantum dots is more difficult and does not always benefit from the same methods used for the more well-established chalcogenide materials. This is primarily because the nucleation and growth profile of InP nanoparticles follows a non-classical, two-step mechanism13. This mechanism is invoked due to the intermediacy of locally stable, atomically precise intermediates known as &#8220;magic-sized&#8221; clusters14,15,16. In particular, In37P20(O2CR)51 has been identified as one key, isolable intermediate in the synthesis of InP from P(SiMe3)3, indium carboxylate, and carboxylic acid17.
The presence of this intermediate on the reaction coordinate has many tangible effects on the growth of InP nanostructures. The existence of cluster intermediates themselves invalidates classical concepts of nucleation and growth based on the La Mer model and means that optimizing reaction conditions such as concentration, temperature, and precursor may not achieve sufficiently uniform ensemble properties. Rather, it has been shown that the use of the InP cluster as a single-source precursor results in highly monodisperse quantum dots with narrow optical features13. Recent literature has suggested that monodispersity, however, is not the only factor limiting InP&#8217;s parity with other optoelectronic materials18. Surface defects, oxidation, and alloying are critical factors still under intense research that will require significant innovation for optimized InP architectures19,20,21,22,23,24. The atomically precise nature of clusters, such as In37P20(O2CR)51, makes them ideal platforms for probing the consequences of many post-synthetic surface modifications. Normally, ensemble inhomogeneity of nanoparticles makes determining surface and compositional effects difficult, but because the cluster of InP is known to be atomically precise, both compositionally and crystallographically, it is an ideal model system.
The synthesis of the In37P20(O2CR)51 cluster is no more difficult than the synthesis of more widely used nanoparticles such as CdSe, PbS, or ZnO. It requires only standard glassware, widely available chemicals, and basic knowledge of air-free Schlenk and glovebox techniques. The procedure itself can be done on the gram scale and with yields in excess of 90%. As we will show, the successful synthesis of InP cluster is not &#8220;magic&#8221; but rather an exercise in fundamentals. Pure reagents, dry glassware, proper air-free techniques, and attention to detail are all that is required to access this atomically precise nanocluster. Moreover, we also elaborate on ideal methods for its conversion to highly crystalline InP quantum dots with narrow size distributions.

<table><tbody><tr><td>Acetonitrile, anhydrous, 99.8%</td><td>Sigma Aldrich</td><td>271004</td><td>Dried over 4&amp;Aring; sieves</td></tr><tr><td>Adapter, Airfree, 14/20 Joint, 0 - 4mm Chem-Cap (T-adapter)</td><td>Chemglass Life Sciences LLC</td><td>AF-0501-01</td><td></td></tr><tr><td>Adapter, Inlet, 14/20 Inner Joint</td><td>Chemglass Life Sciences LLC</td><td>CG-1014-14</td><td></td></tr><tr><td>Bio-Beads S-X1, 200-400 mesh</td><td>Bio-Rad Laboratories</td><td>152-2150</td><td></td></tr><tr><td>Cary 5000 UV-Vis-NIR</td><td>Agilent</td><td></td><td></td></tr><tr><td>Column, Chromatography, 24/40 Outer Joint, 3/4in ID X 10in E.L., 2mm Stpk</td><td>Chemglass Life Sciences LLC</td><td>CG-1188-06</td><td></td></tr><tr><td>Condenser, Liebig, 185mm,&lt;br/&gt; 14/20 Top Outer, 14/20 Lower Inner, 110mm Jacket Length</td><td>Chemglass Life Sciences LLC</td><td>CG-1218-A-20</td><td></td></tr><tr><td>Distilling heads, short paths, jacketed</td><td>Chemglass Life Sciences LLC</td><td>CG-1240</td><td></td></tr><tr><td>Eppendorf&amp;nbsp;Microcentrifuge 5430</td><td>Fisher Chemical</td><td>05-100-177</td><td></td></tr><tr><td>Falcon 15mL Conical Centrifuge Tubes</td><td>Fisher Chemical</td><td>14-959-49B</td><td></td></tr><tr><td>Flask, Round Bottom, 50mL, Heavy Wall, 14/20 - 14/20, 3-Neck, Angled 20&amp;deg;</td><td>Chemglass Life Sciences LLC</td><td>CG-1524-A-05</td><td></td></tr><tr><td>ImageJ</td><td>Developed at National Institutes of Health&amp;nbsp;and the Laboratory for Optical and Computational Instrumentation</td><td></td><td>Open source Java image processing program</td></tr><tr><td>Indium acetate, 99.99%</td><td>Sigma Aldrich</td><td>510270</td><td></td></tr><tr><td>Myristic acid, 99%</td><td>Sigma Aldrich</td><td>M3128</td><td></td></tr><tr><td>Temperature controller</td><td>Fisher Chemical</td><td>50 401 831</td><td></td></tr><tr><td>Thermometers, non-mercury, 10/18</td><td>Chemglass Life Sciences LLC</td><td>CG-3508-N</td><td></td></tr><tr><td>Thermowell, 14/20 Inner Jt, 1/2&quot; OD above the Jt, 6mm OD Round Bottomed Tube below the Jt, for 25ml RBF</td><td>Chemglass Life Sciences LLC</td><td>UW-1205-171JS</td><td>Custom ordered</td></tr><tr><td>Toluene, anhydrous, 99.8%</td><td>Sigma Aldrich</td><td>244511</td><td>Dried over 4&amp;Aring; sieves</td></tr><tr><td>Trimethylindium, 98%</td><td>Strem</td><td>49-2010</td><td>Heat sensitive, moisture sensitive</td></tr><tr><td>Tris(trimethylsilyl)phosphine</td><td>Ref #31, 32</td><td></td><td>Pyrophoric</td></tr><tr><td>Ultrathin Carbon Film on Lacey Carbon Support Film, 400 mesh, Copper</td><td>Ted Pella Inc.</td><td>1824</td><td></td></tr><tr><td>Vacuum gauge 1-STA 115VAC 60Hz</td><td>Fisher Chemical</td><td>11 278</td><td></td></tr><tr><td>Vacuum pump 115VAC 60Hz</td><td>Fisher Chemical</td><td>01 096</td><td></td></tr><tr><td>1-Octadecene (ODE), 90%</td><td>Sigma Aldrich</td><td>O806</td><td>Technical grade, distilled and dried over 4&amp;Aring; sieves</td></tr></tbody></table>

synthesis of in37p20(o2cr)51 clusters and their conversion to inp quantum dots

This text presents a method for the synthesis of In37P20(O2C14H27)51 clusters and their conversion to indium phosphide quantum dots. The In37P20(O2CR)51 clusters have been observed as intermediates in the synthesis of InP quantum dots from molecular precursors (In(O2CR)3, HO2CR, and P(SiMe3)3) and may be isolated as a pure reagent for subsequent study and use as a single-source precursor. These clusters readily convert to crystalline and relatively monodisperse samples of quasi-spherical InP quantum dots when subjected to thermolysis conditions in the absence of additional precursors above 200 &#176;C. The optical properties, morphology, and structure of both the clusters and quantum dots are confirmed using UV-Vis spectroscopy, photoluminescence spectroscopy, transmission electron microscopy, and powder X-ray diffraction. The molecular symmetry of the clusters is additionally confirmed by solution-phase 31P NMR spectroscopy. This protocol demonstrates the preparation and isolation of atomically-precise InP clusters, and their reliable and scalable conversion to InP QDs.

InP clusters and quantum dots are characterized by UV-Vis absorption and PL spectroscopy, XRD, TEM, and NMR spectroscopy. For the InP clusters, an asymmetric absorption feature is observed, with a peak maximum at 386 nm (Figure 1a). Despite the true monodispersity of the sample, this lowest energy peak exhibits a broad linewidth, which narrows upon decrease in temperature. This has been attributed to a set of discrete electronic transitions that are specific to the vibrational motions of the low-symmetry nanocluster lattice17. No appreciable PL QY is observed for clusters at 298 K despite the lack of obvious trap states that would arise from undercoordinated indium or phosphorus ions.
The non-stoichiometric, In-rich cluster (where In is present in a 1.85:1 ratio relative to phosphorus) results in a structure that corresponds with neither the zinc blende nor the wurzite XRD patterns of bulk InP (Figure 1b). Instead, the InP clusters attain a low-symmetry, pseudo-C2v structure that is best described by a set of intersecting polytwistane units25. The core diameter is in the 1-2 nm range depending on the axis from which it is viewed (Figure 1c). This low symmetry structure is reflected in the solution-phase 31P NMR spectrum of the cluster. The 31P NMR spectrum of myristate-capped InP clusters shows 11 distinct peaks (2 P atoms on the C2 axis that each give a unique peak and the remaining 18 P each have a symmetry equivalent, resulting in an additional 9 peaks) ranging from -187&#160;to -242&#160;ppm (Figure 1d)26. The broadness observed in the 31P NMR spectrum varies as a function of solvent and concentration, and purification method as has been recently described for related nanoscale systems27.
The optical spectra of InP QDs synthesized from clusters using the method described here display a lowest energy excitonic transition (LEET) at 564 nm and the corresponding PL emission peak at 598 nm with a full width at half maximum of 52 nm and trap emission evident at redder wavelengths (Figure 2a). It is worth noting that while the two synthetic methods (heat-up and hot-injection) yield InP QDs of comparable optical quality, the hot injection method typically leads to a sample with higher monodispersity due to the rapid nucleation at elevated temperature13. The typically low PL quantum yields obtained directly from the synthesis without further surface treatment (shelling, F- etching, or Lewis acid coordination) are hypothesized to result from a mixture of hole and electron traps present at the surface of these nanocrystals18,28.&#160;The XRD pattern of the resultant InP QDs confirms the zinc blende phase (Figure 2b). Peak broadening in the XRD data occurs due to the finite size of the highly crystalline structures, which in the case of InP QDs is 3.1 nm +/- 0.5 nm in diameter (Figure 2c, a size histogram can be found in ref. 13).
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/59425/59425fig1v2.jpg" /> 
Figure 1. Representative characterization data for InP clusters. (A) UV-Vis spectrum of InP clusters. (B) XRD pattern for purified InP clusters showing deviation from the expected bulk zinc blende (black trace) and wurtzite (grey trace) InP pattern. (C) TEM image of isolated InP clusters. (D) 31P NMR spectrum of InP clusters collected at 202 MHz in C6D6 at 298 K. <a href="https://www.jove.com/files/ftp_upload/59425/59425fig1v2large.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/59425/59425fig2.jpg" /> 
Figure 2. Representative characterization data for InP quantum dots prepared from InP clusters. (A) UV-Vis (solid) and PL (dotted) spectra of InP QDs prepared from myristate-capped InP clusters using the hot-injection protocol. (B) XRD pattern of purified InP QDs showing agreement with the bulk zinc blende InP pattern. (C) TEM images of InP QDs grown from clusters using the hot-injection protocol. <a href="https://www.jove.com/files/ftp_upload/59425/59425fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about Synthesis of In37P20O2CR51 Clusters and Their Conversion to InP Quantum Dots at JoVE.com

Synthesis of In37P20O2CR51 Clusters and Their Conversion to InP Quantum Dots

A protocol for the synthesis of In37P20(O2C14H27)51 clusters and their conversion to indium phosphide quantum dots is presented.

Synthesis of In37P20(O2CR)51 Clusters and Their Conversion to InP Quantum Dots

This text presents a method for the synthesis of In37P20(O2C14H27)51 clusters and their conversion to indium phosphide quantum dots. The In37P20(O2CR)51 ...

synthesis-in37p20-o2cr-51-clusters-their-conversion-to-inp-quantum

Research

JoVE Journal

Chemistry

9.6K Views.  University of Washington. A protocol for the synthesis of In37P20(O2C14H27)51 clusters and their conversion to indium phosphide quantum dots is presented.

Video: Synthesis of In37P20O2CR51 Clusters and Their Conversion to InP Quantum Dots

Compact Quantum Dots for Single-molecule Imaging

Single-molecule imaging is an important tool for understanding the mechanisms of biomolecular function and for visualizing the spatial and temporal heterogeneity of molecular behaviors that underlie cellular biology 1-4. To image an individual molecule of interest, it is typically conjugated to a fluorescent tag (dye, protein, bead, or quantum dot) and observed with epifluorescence or total internal reflection fluorescence (TIRF) microscopy. While dyes and fluorescent proteins have been the mainstay of fluorescence imaging for decades, their fluorescence is unstable under high photon fluxes necessary to observe individual molecules, yielding only a few seconds of observation before complete loss of signal. Latex beads and dye-labeled beads provide improved signal stability but at the expense of drastically larger hydrodynamic size, which can deleteriously alter the diffusion and behavior of the molecule under study.
Quantum dots (QDs) offer a balance between these two problematic regimes. These nanoparticles are composed of semiconductor materials and can be engineered with a hydrodynamically compact size with exceptional resistance to photodegradation 5. Thus in recent years QDs have been instrumental in enabling long-term observation of complex macromolecular behavior on the single molecule level. However these particles have still been found to exhibit impaired diffusion in crowded molecular environments such as the cellular cytoplasm and the neuronal synaptic cleft, where their sizes are still too large 4,6,7.
Recently we have engineered the cores and surface coatings of QDs for minimized hydrodynamic size, while balancing offsets to colloidal stability, photostability, brightness, and nonspecific binding that have hindered the utility of compact QDs in the past 8,9. The goal of this article is to demonstrate the synthesis, modification, and characterization of these optimized nanocrystals, composed of an alloyed HgxCd1-xSe core coated with an insulating CdyZn1-yS shell, further coated with a multidentate polymer ligand modified with short polyethylene glycol (PEG) chains (Figure 1). Compared with conventional CdSe nanocrystals, HgxCd1-xSe alloys offer greater quantum yields of fluorescence, fluorescence at red and near-infrared wavelengths for enhanced signal-to-noise in cells, and excitation at non-cytotoxic visible wavelengths. Multidentate polymer coatings bind to the nanocrystal surface in a closed and flat conformation to minimize hydrodynamic size, and PEG neutralizes the surface charge to minimize nonspecific binding to cells and biomolecules. The end result is a brightly fluorescent nanocrystal with emission between 550-800 nm and a total hydrodynamic size near 12 nm. This is in the same size range as many soluble globular proteins in cells, and substantially smaller than conventional PEGylated QDs (25-35 nm).

We describe the preparation of colloidal quantum dots with minimized hydrodynamic size for single-molecule fluorescence imaging. Compared to conventional quantum dots, these nanoparticles are similar in size to globular proteins and are optimized for single-molecule brightness, stability against photodegradation, and resistance to nonspecific binding to proteins and cells.

Single-molecule imaging is an important tool for  understanding the mechanisms of biomolecular function and for visualizing the  spatial and temporal ...

Engineering

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 ...

Seeded Synthesis of CdSe/CdS Rod and Tetrapod Nanocrystals

We demonstrate a method for the synthesis of multicomponent nanostructures consisting of CdS and CdSe with rod and tetrapod morphologies. A seeded synthesis strategy is used in which spherical seeds of CdSe are prepared first using a hot-injection technique. By controlling the crystal structure of the seed to be either wurtzite or zinc-blende, the subsequent hot-injection growth of CdS off of the seed results in either a rod-shaped or tetrapod-shaped nanocrystal, respectively. The phase and morphology of the synthesized nanocrystals are confirmed using X-ray diffraction and transmission electron microscopy, demonstrating that the nanocrystals are phase-pure and have a consistent morphology. The extinction coefficient and quantum yield of the synthesized nanocrystals are calculated using UV-Vis absorption spectroscopy and photoluminescence spectroscopy. The rods and tetrapods exhibit extinction coefficients and quantum yields that are higher than that of the bare seeds. This synthesis demonstrates the precise arrangement of materials that can be achieved at the nanoscale by using a seeded synthetic approach.

A protocol for the seeded synthesis of rod-shaped and tetrapod-shaped multicomponent nanostructures consisting of CdS and CdSe is presented.

We demonstrate a method for the synthesis of multicomponent nanostructures consisting of CdS and CdSe with rod and tetrapod morphologies. A seeded ...

Photochemical Oxidative Growth of Iridium Oxide Nanoparticles on CdSe@CdS Nanorods

We demonstrate a procedure for the photochemical oxidative growth of iridium oxide catalysts on the surface of seeded cadmium selenide-cadmium sulfide (CdSe@CdS) nanorod photocatalysts. Seeded rods are grown using a colloidal hot-injection method and then moved to an aqueous medium by ligand exchange. CdSe@CdS nanorods, an iridium precursor and other salts are mixed and illuminated. The deposition process is initiated by absorption of photons by the semiconductor particle, which results with formation of charge carriers that are used to promote redox reactions. To insure photochemical oxidative growth we used an electron scavenger. The photogenerated holes oxidize the iridium precursor, apparently in a mediated oxidative pathway. This results in the growth of high quality crystalline iridium oxide particles, ranging from 0.5 nm to about 3 nm, along the surface of the rod. Iridium oxide grown on CdSe@CdS heterostructures was studied by a variety of characterization methods, in order to evaluate its characteristics and quality. We explored means for control over particle size, crystallinity, deposition location on the CdS rod, and composition. Illumination time and excitation wavelength were found to be key parameters for such control. The influence of different growth conditions and the characterization of these heterostructures are described alongside a detailed description of their synthesis. Of significance is the fact that the addition of iridium oxide afforded the rods astounding photochemical stability under prolonged illumination in pure water (alleviating the requirement for hole scavengers).

A protocol for the photochemical oxidative growth of small crystalline iridium oxide nanoparticles on the surface of CdSe@CdS seeded rod nanoparticles is presented.

We demonstrate a procedure for the photochemical oxidative growth of iridium oxide catalysts on the surface of seeded cadmium selenide-cadmium sulfide ...

Synthesis of Cd-free InP/ZnS Quantum Dots Suitable for Biomedical Applications

Fluorescent nanocrystals, specifically quantum dots, have been a useful tool for many biomedical applications. For successful use in biological systems, quantum dots should be highly fluorescent and small/monodisperse in size. While commonly used cadmium-based quantum dots possess these qualities, they are potentially toxic due to the possible release of Cd2+ ions through nanoparticle degradation. Indium-based quantum dots, specifically InP/ZnS, have recently been explored as a viable alternative to cadmium-based quantum dots due to their relatively similar fluorescence characteristics and size. The synthesis presented here uses standard hot-injection techniques for effective nanoparticle growth; however, nanoparticle properties such as size, emission wavelength, and emission intensity can drastically change due to small changes in the reaction conditions. Therefore, reaction conditions such temperature, reaction duration, and precursor concentration should be maintained precisely to yield reproducible products. Because quantum dots are not inherently soluble in aqueous solutions, they must also undergo surface modification to impart solubility in water. In this protocol, an amphiphilic polymer is used to interact with both hydrophobic ligands on the quantum dot surface and bulk solvent water molecules. Here, a detailed protocol is provided for the synthesis of highly fluorescent InP/ZnS quantum dots that are suitable for use in biomedical applications.

In this protocol, the synthesis of Cd-free InP/ZnS quantum dots (QDs) is detailed. InP-based QDs are gaining popularity due to the toxicity of Cd2+ ions that may be released through nanoparticle degradation. After synthesis, QDs are solubilized in water using an amphiphilic polymer for use in biomedical applications.

Fluorescent nanocrystals, specifically quantum dots, have been a useful tool for many biomedical applications. For successful use in biological systems, ...

Synthesis of Ligand-free CdS Nanoparticles within a Sulfur Copolymer Matrix

Aliphatic ligands are typically used during the synthesis of nanoparticles to help mediate their growth in addition to operating as high-temperature solvents. These coordinating ligands help solubilize and stabilize the nanoparticles while in solution, and can influence the resulting size and reactivity of the nanoparticles during their formation. Despite the ubiquity of using ligands during synthesis, the presence of aliphatic ligands on the nanoparticle surface can result in a number of problems during the end use of the nanoparticles, necessitating further ligand stripping or ligand exchange procedures. We have developed a way to synthesize cadmium sulfide (CdS) nanoparticles using a unique sulfur copolymer. This sulfur copolymer is primarily composed of elemental sulfur, which is a cheap and abundant material. The sulfur copolymer has the advantages of operating both as a high temperature solvent and as a sulfur source, which can react with a cadmium precursor during nanoparticle synthesis, resulting in the generation of ligand free CdS. During the reaction, only some of the copolymer is consumed to produce CdS, while the rest remains in the polymeric state, thereby producing a nanocomposite material. Once the reaction is finished, the copolymer stabilizes the nanoparticles within a solid polymeric matrix. The copolymer can then be removed before the nanoparticles are used, which produces nanoparticles that do not have organic coordinating ligands. This nascent synthesis technique presents a method to produce metal-sulfide nanoparticles for a wide variety of applications where the presence of organic ligands is not desired.

Herein we present a method to synthesize ligand-free cadmium sulfide (CdS) nanoparticles based on a unique sulfur copolymer. The sulfur copolymer operates as a high temperature solvent and a sulfur source during the nanoparticle synthesis and stabilizes the nanoparticles after the reaction.

Aliphatic ligands are typically used during the synthesis of nanoparticles to help mediate their growth in addition to operating as high-temperature ...

High Resolution Phonon-assisted Quasi-resonance Fluorescence Spectroscopy

High resolution optical spectroscopy methods are demanding in terms of either technology, equipment, complexity, time or a combination of these. Here we demonstrate an optical spectroscopy method that is capable of resolving spectral features beyond that of the spin fine structure and homogeneous linewidth of single quantum dots (QDs) using a standard, easy-to-use spectrometer setup. This method incorporates both laser and photoluminescence spectroscopy, combining the advantage of laser line-width limited resolution with multi-channel photoluminescence detection. Such a scheme allows for considerable improvement of resolution over that of a common single-stage spectrometer. The method uses phonons to assist in the measurement of the photoluminescence of a single quantum dot after resonant excitation of its ground state transition. The phonon&#39;s energy difference allows one to separate and filter out the laser light exciting the quantum dot. An advantageous feature of this method is its straight forward integration into standard spectroscopy setups, which are accessible to most researchers.

The manuscript describes a method of phonon-assisted quasi-resonant fluorescence spectroscopy that incorporates both laser-limited resolution and photoluminescence (PL) spectroscopy. This method utilizes optical phonons to provide linewidth-limited resolution spectra of atom-like semiconductor structures in the energy domain. The method is also easily realized with a single spectrometer optical spectroscopy setup.

High resolution optical spectroscopy methods are demanding in terms of either technology, equipment, complexity, time or a combination of these. Here we ...

Combining Solid-state and Solution-based Techniques: Synthesis and Reactivity of Chalcogenidoplumbates(II or IV)

The phases of &#34;PbCh2&#34; (Ch = Se, Te) are obtained from solid-state syntheses (i.e., by the fusion of the elements under inert conditions in silica glass ampules). Reduction of such phases by elemental alkaline metals in amines affords crystalline chalcogenidoplumbate(II) salts comprised of [PbTe3]2- or [Pb2Ch3]2- anions, depending upon which sequestering agent for the cations is present: crown ethers, like 18-crown-6, or cryptands, like [2.2.2]crypt. Reactions of solutions of such anions with transition-metal compounds yield (poly-)chalcogenide anions or transition-metal chalcogenide clusters, including one with a &#181;-PbSe ligand (i.e., the heaviest-known CO homolog).
In contrast, the solid-state synthesis of a phase of the nominal composition &#34;K2PbSe2&#34; by successive reactions of the elements and by the subsequent solvothermal treatment in amines yields the first non-oxide/halide inorganic lead(IV) compound: a salt of the ortho-selenidoplumbate(IV) anion [PbSe4]4-. This was unexpected due to the redox potentials of Pb(IV) and Se(-II). Such methods can further be applied to other elemental combinations, leading to the formation of solutions with binary [HgTe2]2- or [BiSe3]3- anions, or to large-scale syntheses of K2Hg2Se3 or K3BiSe3 via the solid-state route.
All compounds are characterized by single-crystal X-ray diffraction and elemental analysis; solutions of plumbate salts can be investigated by 205Pb and 77Se or 127Te NMR techniques. Quantum chemical calculations using density functional theory methods enable energy comparisons. They further allow for insights into the electronic configuration and thus, the bonding situation. Molecular Rh-containing Chevrel-type compounds were found to exhibit delocalized mixed valence, whereas similar telluridopalladate anions are electron-precise; the cluster with the &#181;-PbSe ligand is energetically favored over a hypothetical CO analog, in line with the unsuccessful attempt at its synthesis. The stability of formal Pb(IV) within the [PbSe4]4- anion is mainly due to a suitable stabilization within the crystal lattice.

Combining Solid-state and Solution-based Techniques: Synthesis and Reactivity of ChalcogenidoplumbatesII or IV

The synthesis of chalcogenidoplumbates(II,IV) via the in situ reduction of nominal "PbCh2" (Ch = Chalcogen) and via a solid-state reaction and subsequent solvothermal reactions is presented. Additionally, reactivities of plumbate(II) solutions are portrayed, which yield the heaviest-known CO homolog known to date: the &#181;-PbSe ligand.

The phases of &#34;PbCh2&#34; (Ch = Se, Te) are obtained from solid-state syntheses (i.e., by the fusion of the elements under inert conditions in silica ...

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.

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 ...

Resonance Fluorescence of an InGaAs Quantum Dot in a Planar Cavity Using Orthogonal Excitation and Detection

The ability to perform simultaneous resonant excitation and fluorescence detection is important for quantum optical measurements of quantum dots (QDs). Resonant excitation without fluorescence detection &#8211; for example, a differential transmission measurement &#8211; can determine some properties of the emitting system, but does not allow applications or measurements based on the emitted photons. For example, the measurement of photon correlations, observation of the Mollow triplet, and realization of single photon sources all require collection of the fluorescence. Incoherent excitation with fluorescence detection &#8211; for example, above band-gap excitation &#8211; can be used to create single photon sources, but the disturbance of the environment due to the excitation reduces the indistinguishability of the photons. Single photon sources based on QDs will have to be resonantly excited to have high photon indistinguishability, and simultaneous collection of the photons will be necessary to make use of them. We demonstrate a method to resonantly excite a single QD embedded in a planar cavity by coupling the excitation beam into this cavity from the cleaved face of the sample while collecting the fluorescence along the sample&#39;s surface normal direction. By carefully matching the excitation beam to the waveguide mode of the cavity, the excitation light can couple into the cavity and interact with the QD. The scattered photons can couple to the Fabry-Perot mode of the cavity and escape in the surface normal direction. This method allows complete freedom in the detection polarization, but the excitation polarization is restricted by the propagation direction of the excitation beam. The fluorescence from the wetting layer provides a guide to align the collection path with respect to the excitation beam. The orthogonality of the excitation and detection modes enables resonant excitation of a single QD with negligible laser scattering background.

Resonant excitation of a single self-assembled quantum dot can be achieved using an excitation mode orthogonal to the fluorescence collection mode. We demonstrate a method using the waveguide and Fabry-Perot modes of a planar microcavity surrounding the quantum dots. The method allows complete freedom in the detection polarization.

The ability to perform simultaneous resonant excitation and fluorescence detection is important for quantum optical measurements of quantum dots (QDs). ...

Synthesis of In₃₇P₂₀(O₂CR)₅₁ Clusters and Their Conversion to InP Quantum Dots

Summary

Explore More Videos

Chapters in this video