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

Indium phosphide quantum dots are an exciting material for emerging and future optoelectronic technologies. Both academic and industry research labs are in need of high quality indium phosphide quantum dots. The indium phosphide cluster prepared with this material is an atomically precise and robust precursor for many nanoscale applications and it can be efficaciously synthesized on the gram scale.

Demonstrating the procedure will be Andrew Ritchhart and Nayon Park, two graduate students from my laboratory. To begin, place 2.65 grams of myristic acid in a flame or oven dried 50 milliliter three-neck round bottom flask equipped with a stir bar and connected to a Schlenk line. Put the flask under vacuum and refill it with nitrogen gas three times.

Then evacuate the flask and heat the myristic acid to 110 degrees Celsius while stirring at 200 rpm. Continue stirring under vacuum for two hours to remove water. Next, place the flask under a positive flow of nitrogen gas, turn off the heat, and add 0.93 grams of indium(III)acetate.

Evacuate the flask, turn the heat back on, and start stirring the mixture at 500 rpm. Once the mixture is completely liquid, continue stirring at 110 degrees Celsius under vacuum for six to 12 hours to obtain indium(III)myristate. Then in a nitrogen filled glove box, fill a syringe with 20 milliliters of anhydrous toluene.

Refill the reaction flask with nitrogen gas and add the toluene from the prepared syringe. Confirm that the solution begins refluxing. In the nitrogen filled glove box, combine 465 microliters of tris(trimethylsilyl)phosphine with 10 milliliters of anhydrous toluene.

Draw the solution into a syringe and block the end of the needle with a rubber stopper. Because it is pyrophoric, the tris(trimethylsilyl)phosphine syringe is carefully stoppered prior to removal from the glove box. Once ready to inject, it is swiftly unstoppered, inserted, and rapidly injected into the 110 degree solution.

Bring the syringe to the reaction flask and rapidly inject the tris(trimethylsilyl)phosphine solution into the stirring reaction mixture. Maintain reflux throughout the reaction. A minute after starting the reaction, take a 50 microliter aliquot, dilute it with three milliliters of toluene, and perform UV-Vis spectroscopy.

Take a spectrum of a fresh aliquot every five to 10 minutes to monitor the reaction progress. Once no further change is observed in the spectrum, remove the flask from heat to end the reaction. Let the indium phosphide cluster solution cool to 50 degrees Celsius and then remove the solvent under vacuum.

After that, seal the flask under a positive flow of nitrogen gas using a glass stopper, a T adapter, and electrical tape and bring the sealed flask into the nitrogen filled glove box. Resuspend the clusters in about one milliliter of toluene and centrifuge the mixture for 10 minutes to remove solid impurities. Decant the supernatant and discard the solids.

Add three milliliters of acetonitrile to the supernatant to precipitate the indium phosphide clusters and centrifuge the mixture again under the same conditions. Discard the supernatant and resuspend the pellet of indium phosphide clusters in about a milliliter of toluene. Repeat this process four times to finish washing the clusters.

Then resuspend the clusters in about half a milliliter of toluene and purify them by size exclusion chromatography. Remove the solvent from the collected fraction under vacuum to obtain the clusters as a waxy solid. Store the dry clusters under nitrogen for further use.

To begin the quantum dot synthesis, set up a 100 milliliter three-neck round bottom flask with a stir bar on a Schlenk line and prep the flask atmosphere as previously described. In a nitrogen filled glove box, draw 35 milliliters of 1-Octadecene into a syringe. Inject this solvent into the flask and heat it to 300 degrees Celsius under nitrogen while stirring.

Then in the glove box, dissolve 200 milligrams of indium phosphide clusters in five milliliters of anhydrous 1-Octadecene. Draw this solution into a syringe and inject it into the reaction flask. Stir the mixture at 500 rpm under nitrogen gas for 15 to 20 minutes.

When the reaction is complete, remove the flask from heat and let the mixture cool to room temperature. Remove the solvent by vacuum distillation at 160 degrees Celsius. Then in the glove box, dissolve the crude indium phosphide quantum dots in less than five milliliters of anhydrous toluene and transfer the solution to a 50 milliliter centrifuge tube.

Add about 40 milliliters of anhydrous acetonitrile and centrifuge the mixture for 10 minutes. Remove the supernatant and re-dissolve the precipitate in about five milliliters of anhydrous toluene. Perform this washing procedure twice more.

For storage, dissolve the purified indium phosphide quantum dots in anhydrous toluene. This prevents the formation of aggregates over time. The indium rich non-stoichiometric indium phosphide clusters showed an asymmetric absorption feature by UV-Vis spectroscopy with a peak maximum at 386 nanometers.

X-ray diffraction showed that the structure of the clusters corresponded with neither zinc blend nor work site structures of bulk indium phosphide. The clusters instead had a pseudo C to B structure resembling intersecting Polytwistane units. The low symmetry was reflected in the number of distinct peaks observed in the solution state phosphorus NMR spectrum.

The cluster core diameter was between one and two nanometers depending on the axis from which it was viewed. The indium phosphide quantum dots synthesized from these clusters showed a lowest energy excitonic transition at 564 nanometers and a photo luminescent emission peak at 598 nanometers. The quantum dots were about three nanometers in diameter.

X-ray diffraction showed that zinc blend was a good match for the quantum dot structure. The methods described here incorporate air and water-free technologies that are transferrable to many inorganic chemistry syntheses including the synthesis of quantum dots composed of other elements. The discovery of kinetically persistent indium phosphide cluster intermediates has mechanistically differentiated indium phosphide clusters for more well-developed quantum dot materials like cadmium selenide and lead sulfide.

This method requires the proper use of anhydrous reagents and air-free and water-free techniques to ensure good product quality and the safety of lab personnel. Tris(trimethylsilyl)phosphine is volatile and pyrophoric so use caution when handling and disposing of it. Researchers should wear appropriate PPE and be trained to handle fires in case of an emergency.