Fabrication of Periodic Gold Nanocup Arrays Using Colloidal Lithography

The overall goal of this procedure is to fabricate and characterize periodic arrays of gold nanocups using a low cost and straightforward technique called colloidal lithography. Gold nanocups or nanoshells have unique optical properties that have found application in areas such as nonlinear optics, metamaterials, and chemical sensing. In this video, you will learn how to fabricate flexible, transparent nano plasmonic array films using a low cost technique that is based on the self-assembly of highly monodisperse polystyrene nanospheres.

This approach has the benefit of scalability as compared with serial processes such as EUV lithography. These films consisting of periodic arrays of gold nanocups or nanoshells have applications ranging from chemical sensing, tamper indication, and photovoltaics. Furthermore, it is straightforward to tune the plasmon resonance of these films from the visible to the near-infrared dependent on the desired application.

This video will describe the fabrication of the gold nanocup arrays on a sacrificial silicone substrate, the optical characterization of the resulting structures, and transfer the films to a transparent adhesive film. First, place several silicone wafers into a quartz carrier for plasma cleaning. This ensures a clean hydrophilic surface that is essential for self-assembly of the polymeric nanospheres.

Next, place the quartz carrier into the plasma system and initiate the vacuum. The system was configured for a 250 watt 15-minute oxygen plasma clean with a flow rate of 30 SCCM of oxygen. While the silicone wafers are being plasma cleaned, remove the polystyrene nanosphere solution from the refrigerator and allow them to warm to room temperature.

To help ensure a uniform solution, briefly vortex mix the solution for one minute. Then sonicate the solution for one minute. In order to obtain a monolayer during spin coating, it is generally necessary to dilute the stock 10 weight percent nanosphere solution.

To do this, we use clean, high purity water to dilute the nanosphere solution to a five weight percent solid. Remove the clean silicone wafers from the plasma system and inspect them for any contaminants such as dust or organic residue. If a wafer is acceptable, mount it on the center of the spin coater and initiate the vacuum.

Next, set the spin coater to the appropriate speed, acceleration, and spin time. This will vary based on the size and the concentration of the polystyrene nanospheres used. In this case, set the spin coater to 3, 000 rpm 2, 000 rpm per second for one minute to produce monolayer coverage.

Taking a dilute nanosphere solution and using a plastic syringe, withdraw approximately one milliliter of nanosphere solution. Then fix a five micron filter onto the end of the syringe. This will help remove any agglomerans that will negatively impact the quality of the self-assembled film.

Hold the syringe over the boldest spin coater and depress the plunger until a drop of nanosphere solution has been released. Then in a fluid motion, dispense enough solution onto the center of the wafer until approximately 2/3 of the wafer is covered. Close the lid and initiate the spin cycle.

Wait until the spin cycle has been completed then release the vacuum, open the lid, and remove the wafer. Wipe down the inside of the spin coater, making sure to remove any excess nanosphere solution. Before continuing, it is essential to evaluate the quality of the spun cast film.

First, we can inspect by eye, looking for any defects caused by air bubbles, dust, or agglomerans. Then we can use optical microscopy to further evaluate the quality of the film. After turning on the light source and placing the sample under the objective, we can clearly evaluate the quality of the film including the formation of multilayers, defects, or holes.

In these images, we see a high quality film, a film containing multilayers, and a film containing holes. The final technique to evaluate film quality is to use scanning electron microscopy. This allows us to visualize the film at the nano scale.

After evaluating the quality of the film, we now need to anneal the nanospheres to the substrate. Place the substrate into an oven at 107 degrees Celsius for approximately two minutes. Remove the substrate from the oven and allow it to briefly cool to room temperature.

Next, transfer the annealed substrate to the plasma system for etching. Set the plasma system to 75 watts, 20 SCCM of oxygen, and 175 seconds of etch time. Begin the flow of oxygen.

Wait until the pressure has stabilized and initiate the RF plasma. As shown, oxygen plasma isotropically etches the polystyrene nanospheres, resulting in a periodic array of uniformly spaced spheres. After etching, deposit a 29 middle layer of gold using sputter coating.

Anisotropic structures may be made by varying the deposition angle. For these samples, gold was deposited normal to the substrate. Shown here on the left is a two-inch wafer of spun cast polystyrene nanospheres.

On the right is a wafer after gold deposition. A microspectrophotometer is used to measure the optical properties of the gold nanocup array on the silicone substrate. The plasmon resonance peak occurs at approximately 650 nanometers.

The final step of this protocol is to transfer the gold nanocup array to a flexible and conformal film. We use transparent, pressure-sensitive adhesive tape to perform this quick and straightforward lift off technique. Taking a strip of tape, carefully place it in contact with the gold-coated substrate.

You may find it's necessary to gently remove any air bubbles that may have formed between the wafer surface and the tape. Once the tape has made good contact with the substrate, gently peel the tape off. Once the tape has been completely removed, the result is a flexible and conformal film of periodically arranged nanocups.

Using this method, nano plasmonic films were fabricated using colloidal lithography, plasmic etching, and metallization where the polystyrene nanospheres functioned as a self-assemble template mask for metal nanocup deposition. The results of the fabrication process were verified using scanning electron microscopy. Here, we see the micrographs of the self-assembled monolayer of polystyrene nanospheres, the etched nanospheres, and the nanocups after metal deposition.

The optical properties of these films include good transmission of light up to approximately 70%in the visible spectrum while maintaining a clearly identifiable plasmon resonance. Colloidal lithography is a rapid potentially low cost technique to fabricate nano plasmonic films. This technique only requires commonly available equipment and nano plasmonic films can be prepared in just a few hours.

Here, we have showed how to use this technique to make films consisting of periodic arrays of gold nanocups with a plasmonic response in the visible spectrum. Once you have mastered this technique, you'll be able to fabricate a wide variety of nano plasmonic films with optical responses in both the visible and the near-infrared.