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.
