Colloidal Synthesis of Nanopatch Antennas for Applications in Plasmonics and Nanophotonics

The overall goal of this experimental procedure is to demonstrate a technique to fabricate nanopatch antennas which can enable tailored light-matter interactions such as strongly enhanced fluorescence. This method can help answer key questions in the nanophotonic and plasmonic communities such as how to achieve high fluorescence enhancement and control of other related subprocesses. The main advantage of this technique is that it enables large-scale fabrication of nanoantennas, where the critical feature size can be controlled on a single nanometer scale.

Demonstrating this procedure will be Thang Hoang, a postdoctoral associate, and Jiani Huang, a graduate student from my laboratory. Begin the synthesis procedure by dipping a cleaned round-bottomed flask into the heating bath, approximately 10mm deep into the liquid. Then, use a micropipette to place 10mL of ethylene glycol, or EG, into the round-bottomed flask.

Put the cap on the flask and wait for 20 minutes. The purpose of this step is to clean the flask with EG.After 20 minutes, remove the cap and then lift the round-bottomed flask out of the heating bath. Take the entire clamp out, as the EG solution is hot.

Pour the 10mL of EG into a disposal container, making sure that the magnetic stirring bar does not fall out. Place the flask back into the heating bath. Use a micropipette to add 5mL of EG into the flask and put the cap on.

After waiting for 5 minutes, take the cap off and use a micropipette to place 60 microliters of sodium hydrosulfide hydrate into the flask. Put the cap back on and wait for two minutes. After two minutes, take the cap off and use a micropipette to place 500 microliters of the hydrochloric acid solution into the flask.

Immediately, use a micropipette to add 1.25mL of the PVP solution into the flask before putting the cap back on and waiting for two minutes. After removing the cap, use a micropipette to place 400 microliters of the silver trifluoroacetate solution into the flask, and put the cap back on. Wait for 2.5 hours.

The silver nanocubes are forming during this step. Throughout this time, reduce the room light to a minimum. After 2.5 hours, turn the heater off, but leave the stirring on to avoid burning the fluid on the bottom.

Use the clamp to raise the flask above the heating bath, and remove the cap. Then, remove the flask from the heating bath to allow it to cool off faster. After about 20 minutes, add 5mL of acetone into the flask.

Vortex it to mix the solutions well. In the end, the total volume of the solution is 12mL. Using a micropipette, transfer the final solution to eight smaller 1.5mL plastic tubes.

Centrifuge these eight tubes at a speed of 5, 150 Gs for ten minutes. As a result, all the silver nanocubes are at the bottom of the tubes. Use a micropipette to remove the top supernatant, leaving approximately 100 microliters at the bottom of each tube.

Then, add 1mL of deionized water into each of these tubes. Vortex and sonicate the tubes. The nanocubes are now suspended in mainly deionized water.

Repeat the centrifugation-resuspension step once more. First, deposit a polyalleleamine hydrochloride, or PAH layer, by immersing the gold film into a PAH solution for five minutes. This results in a PAH layer on top of the gold film with a thickness of approximately 1 nanometer.

After five minutes, rinse the gold film using clean deionized water. There is now a single PAH layer on top of the gold film. Next, immerse the gold film with the single PAH layer into a sodium chloride solution for one minute.

Next, immerse the gold film with the single PAH layer into a polystyrene sulfonate, or PSS solution for five minutes. This results in a PSS layer with a thickness of approximately 1 nanometer on top of the PAH layer. Continue this process to deposit a total of five polyelectrolyte layers on the gold film.

Drop cast 100 microliters of a 25 micromolar cyanine-5 solution onto the sample surface. Then place a clean cover slip on top of the solution drop. Cyanine-5 molecules will incorporate into the top polyelectrolite layers uniformly.

After ten minutes, rinse the sample with deionized water, and dry it using clean nitrogen gas. To form nanopatch antennas, dilute the prepared nanocube solution by 100 times using deionized water, to enable the optical study of individual nanopatch antennas. Use a micropipette to place a drop of 20 microliters of the diluted nanocube solution onto a clean cover slip.

Place the sample in contact with the cover slip for two minutes. As a result, the silver nanocubes are immobilized on the top terminal PAH layer, because the nanocubes synthesized here are negatively charged, and the top PAH layer is positively charged. After two minutes, rinse the sample with deionized water and dry it using clean nitrogen gas.

Shown here are representative scanning electron microscopy images of the silver nanocubes obtained from this procedure. Here, the sample was fabricated using an undiluted solution of nanocubes. Whereas the sample was diluted ten, and one hundred times in these images.

In all cases, nanocubes of a relatively uniform size, characterized by sharp corners, with a radius of curvature of about 10 nanometers are observed. Shown here are representative optical characterizations of the final nanopatch antennas with embedded cyanine-5 dye molecules. Reflection measurements of an ensemble of nanopatch antennas show a characteristic plasmon resonance at 650 nanometers.

Scattering measurements of individual nanoantennas display a resonance at the same wavelength, but with a narrower width. Dark field images of the sample show diffraction-limited spots with a uniform red color, indicating that most nanopatch antennas have very similar resonances due to the good size homogeneity of the fabricated nanocubes. Finally, large fluorescence enhancement of the embedded cyanine-5 dye molecules is observed.

Once mastered, this fabrication technique can be completed in five hours if it is performed properly. After its development, this technique paved the way for researchers in the fields of nanophotonics and plasmonics to explore the fundamental applied metal interaction and potential applications in une-surface optoelectronic devices, including light-emitting diodes, high efficiency photoreflectors, and quantum information science. After watching this video, you should have a good understanding of how to fabricate nanopatch antennas utilizing colloidally synthesized silver nanocubes to enable enhanced light-matter interactions.