Name: Fabrication of Periodic Gold Nanocup Arrays Using Colloidal Lithography (Video) | JoVE Journal
Uploaded: 2017-09-02T13:00:00.000+00:00
Duration: 8 min 21 s
Description: Learn to fabricate periodic gold nanocup arrays using colloidal lithography with self-assembled polystyrene nanospheres. This scalable technique enables tunable plasmon resonance for applications in chemical sensing and photovoltaics.

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5 minutes

This research was performed at the Pacific Northwest National Laboratory (PNNL), which is operated by Battelle Memorial Institute for the Department of Energy (DOE) under Contract No. DE-AC05-76RL01830. The authors gratefully acknowledge support from the U.S. Department of State through the Key Verification Assets Fund (V Fund) under Interagency Agreement SIAA15AVCVPO10.

1. Material Preparation
<ol>
	<li>Place several 2 in. silicon wafers into a quartz carrier for cleaning and load the silicon wafers into the plasma etching system. Pump the vacuum chamber down until it reaches at least 75 mTorr. This may take a few minutes.</li>
	<li>Begin the flow of O2 (30 sccm) gas and allow the pressure to stabilize. Set the etch time to 15 min. Once the chamber pressure has stabilized initiate the radiofrequency (RF) 13.56 MHz 250 W plasma. 
	NOTE: This step cleans the silicon wafers of any organic contaminates and functionalizes the surface with hydroxylated (-OH) moieties thereby ensuring a hydrophilic surface.</li>
	<li>While waiting for the plasma cleaning step to finish, remove the commercially purchased polystyrene nanospheres (170 nm diameter, 10% solids, 0.5% sodium dodecyl sulfate) from the refrigerator (4 &#176;C). Allow the container to warm to room temperature.</li>
	<li>Briefly vortex (1 min) and sonicate (35 kHz, 1 min) the polystyrene nanospheres to minimize nanosphere agglomeration.</li>
	<li>Into a clean glass vial, measure 1.0 mL of the 170 nm polystyrene nanospheres and add 1.0 mL of H2O to obtain a 5% solids colloidal suspension.</li>
	<li>After 15 min, stop the flow of O2, vent the vacuum chamber, and remove the freshly cleaned wafers.</li>
</ol>
2. Spin-coating of Polystyrene Nanospheres Template
<ol>
	<li>Unload the cleaned silicon wafers from the plasma etcher. Then mount a 2 in. wafer onto the spin-coater. Ensure it is properly centered and that the O-ring is clear of any debris. Initiate the vacuum and ensure that the wafer is securely attached to the stage.</li>
	<li>Set the spin parameters of the spin-coater. These parameters vary based on the nanosphere size. For a solution of 5% 170 nm nanospheres, set the spin-coater to a 1 step process with a spin time of 1 min, a speed of 3,000 rpm, and an acceleration of 2,000 rpm/s.</li>
	<li>Using a disposable syringe, withdraw ~ 1 mL of colloidal suspension from the vial. Set aside the vial. Take a 5 &#181;m syringe filter and place it at the end of the syringe. Depress the syringe until a droplet of suspension clears the tip. The filter removes undesirable aggregates and particulate that can significantly reduce film quality.</li>
	<li>Deposit enough suspension directly on the center of the wafer such that approximately 2/3 of the surface is covered. Try to minimize bubbles because those can impact film quality. Close the spin-coater lid and press Start. During this process, it may be possible to see thin film interference effects on the surface of the wafer as the nanospheres self-assemble. This will vary based on the nanosphere diameter.</li>
	<li>Remove the spin-coated wafer after deactivating the vacuum. Wipe the bowl and lid of the spin-coater to remove excess nanospheres.</li>
</ol>
3. Film Quality Assessment and Preparation for Etching
<ol>
	<li>Visually assess the quality of the self-assembled film by looking for noticeable defects such as streaks or holes that may have been caused by particulate during the spin-coating process.</li>
	<li>Assess the film quality by placing the wafer under an optical microscope. Grain boundaries and some defects are normal. If the wafer has large uncoated areas or obvious multilayers, it is necessary to adjust the spin parameters to obtain a more uniform film. Electron microscopy may also be used to assess film quality.</li>
	<li>Turn on the light source to the microscope and focus onto the surface of the silicon wafer using a 20X objective. Assess the quality at several spots throughout the wafer to ensure uniformity.</li>
	<li>The final film quality check is to use scanning electron microscopy (SEM) to visualize the nanosphere self-assembly at the nanoscale. It is possible to assess the degree of multilayers, holes, and grain boundaries/defects across small portions of the wafer relatively quickly using this technique.</li>
	<li>Once a sufficient film has been obtained, place the wafer into an oven (107 &#176;C) for 2 min to anneal the self-assembled nanospheres. This helps encourage adhesion to the substrate and yields a better nanopatterned surface after etching.</li>
</ol>
4. Etching, Metal Deposition, and Optical Characterization
<ol>
	<li>Load the annealed wafer into the plasma etcher and initiate the pump down process.</li>
	<li>Once the vacuum chamber reaches at least 75 mTorr, begin the flow of O2 (20 sccm) gas and wait for the pressure to stabilize. Initiate the RF plasma (75 W) for 165 s.</li>
	<li>Once the RF plasma cycle has completed, stop the flow of O2 and vent the chamber.</li>
	<li>The substrate is now etched and ready for metal deposition. Transport the sample to a sputter coater and deposit a thin (20 nm) layer of gold. Varying deposition angles may be used to alter the optical properties of the nanocups. In this case, metal deposition normally incident to the substrate was performed.</li>
	<li>After the metal deposition, the substrate may be characterized using optical spectroscopy. Focus the microspectrophotometer onto the surface of the metallized substrate and measure the reflectance spectra. For 170 nm etched nanosphere arrays, the LSPR was at 615 nm.</li>
	<li>Using pressure-sensitive adhesive tape, gently place the film in contact with the substrate. It may be necessary to remove any air bubbles that formed at the interface using a tweezers.</li>
	<li>Once the tape is in contact with the substrate, the tape may be immediately peeled off to remove the nanocups from the substrate surface. Gently peel back the tape and the result is a flexible and conformal film of gold nanocups.</li>
</ol>

<ol><li>Fang, Y., Sun, M. <a target="_blank" href="https://www.nature.com/lsa/journal/v4/n6/full/lsa201567a.html">Nanoplasmonic waveguides: towards applications in integrated nanophotonic circuits.</a> Light Sci Appl. 4, e294(2015).</li>
<li>Li, J. F., Anema, J. R., Wandlowski, T., Tian, Z. Q. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Dielectric+shell+isolated+and+graphene+shell+isolated+nanoparticle+enhanced+Raman+spectroscopies+and+their+applications%5BTitle%5D)+AND+%22Chemical+Society+Reviews%22%5BJournal%5D)">Dielectric shell isolated and graphene shell isolated nanoparticle enhanced Raman spectroscopies and their applications.</a> Chemical Society Reviews. 44 (23), 8399-8409 (2015).</li>
<li>Wang, L., et al. <a target="_blank" href="http://pubs.acs.org/doi/abs/10.1021/acsphotonics.5b00725">Large Area Plasmonic Color Palettes with Expanded Gamut Using Colloidal Self-Assembly.</a> ACS Photonics. , (2016).</li>
<li>Taylor, A. B., Michaux, P., Mohsin, A. S. M., Chon, J. W. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Electron-beam+lithography+of+plasmonic+nanorod+arrays+for+multilayered+optical+storage%5BTitle%5D)+AND+%22Optics+Express%22%5BJournal%5D)">Electron-beam lithography of plasmonic nanorod arrays for multilayered optical storage.</a> Optics Express. 22 (11), 13234-13243 (2014).</li>
<li>Endo, H., Mochizuki, Y., Tamura, M., Kawai, T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Fabrication+and+Functionalization+of+Periodically+Aligned+Metallic+Nanocup+Arrays+Using+Colloidal+Lithography+with+a+Sinusoidally+Wrinkled+Substrate%5BTitle%5D)+AND+%22Langmuir%22%5BJournal%5D)">Fabrication and Functionalization of Periodically Aligned Metallic Nanocup Arrays Using Colloidal Lithography with a Sinusoidally Wrinkled Substrate.</a> Langmuir. 29 (48), 15058-15064 (2013).</li>
<li>Wang, H., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Symmetry+breaking+in+individual+plasmonic+nanoparticles%5BTitle%5D)+AND+%22Proceedings+of+the+National+Academy+of+Sciences%22%5BJournal%5D)">Symmetry breaking in individual plasmonic nanoparticles.</a> Proceedings of the National Academy of Sciences. 103 (29), 10856-10860 (2006).</li>
<li>Wollet, L., et al. <a target="_blank" href="https://www.osapublishing.org/ome/abstract.cfm?uri=ome-2-10-1384">Plasmon hybridization in stacked metallic nanocups.</a> Optical Materials Express. 2 (10), 1384-1390 (2012).</li>
<li>Duempelmann, L., Casari, D., Luu-Dinh, A., Gallinet, B., Novotny, L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Color+Rendering+Plasmonic+Aluminum+Substrates+with+Angular+Symmetry+Breaking%5BTitle%5D)+AND+%22ACS+Nano%22%5BJournal%5D)">Color Rendering Plasmonic Aluminum Substrates with Angular Symmetry Breaking.</a> ACS Nano. 9 (12), 12383-12391 (2015).</li>
<li>King, N. S., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Angle-+and+Spectral-Dependent+Light+Scattering+from+Plasmonic+Nanocups%5BTitle%5D)+AND+%22ACS+Nano%22%5BJournal%5D)">Angle- and Spectral-Dependent Light Scattering from Plasmonic Nanocups.</a> ACS Nano. 5 (9), 7254-7262 (2011).</li>
<li>Mirin, N. A., Halas, N. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Light-Bending+Nanoparticles%5BTitle%5D)+AND+%22Nano+Letters%22%5BJournal%5D)">Light-Bending Nanoparticles.</a> Nano Letters. 9 (3), 1255-1259 (2009).</li>
<li>Eggleston, M. S., Messer, K., Zhang, L., Yablonovitch, E., Wu, M. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Optical+antenna+enhanced+spontaneous+emission%5BTitle%5D)+AND+%22Proceedings+of+the+National+Academy+of+Sciences%22%5BJournal%5D)">Optical antenna enhanced spontaneous emission.</a> Proceedings of the National Academy of Sciences. 112 (6), 1704-1709 (2015).</li>
<li>Bora, M., et al. <a target="_blank" href="http://aip.scitation.org/doi/abs/10.1063/1.4802910?journalCode=apl">Plasmonic black metals in resonant nanocavities.</a> Applied Physics Letters. 102 (25), 251105(2013).</li>
<li>Akselrod, G. M., et al. <a target="_blank" href="http://pubs.acs.org/doi/abs/10.1021/acsphotonics.6b00357">Efficient Nanosecond Photoluminescence from Infrared PbS Quantum Dots Coupled to Plasmonic Nanoantennas.</a> ACS Photonics. , (2016).</li>
</ol>

The authors have nothing to disclose.

This protocol demonstrates a low-cost and efficient technique for fabricating periodic arrays of plasmonic gold nanocups. This technique is particularly advantageous because it avoids serial top-down processes such as electron beam lithography or focused ion beam milling. The presented technique shows that commercially available polymeric nanospheres can be self-assembled in a straightforward manner to serve as a nano-sized template for further processing.
Modifications and Troubleshoo...

The emergence of plasmonics in conjunction with improved nanofabrication and synthesis techniques have brought about a wide variety of exciting technologies such as sub-diffraction limited circuity, enhanced chemical detection, and optical sensing1,2,3. In this protocol, we demonstrate a scalable and relatively low-cost technique capable of fabricating nanopatterned plasmonic substrates using commercially available polymeric nanospheres and an etching step followed by metal deposition. Unlike other techniques for fabricating nanopatterned substrates, such as electron beam lithography4, this technique can quickly and efficiently be scaled to 300 mm wafers and beyond with minimal effort and uses a transfer step to produce flexible and conformal films5.
Since the Roman era, we have known that certain metals such as gold and silver can have brilliant optical properties when they are finely divided. Today, we understand that these metal particles exhibit an effect called the &#34;localized surface plasmon resonance&#34; (LSPR) when their dimensions approach the nanoscale. LSPR is analogous to a standing wave in which weakly bound electrons found in the metal oscillate coherently when light of certain frequencies illuminates the metal particles. Anisotropic nanostructures are of particular interest because unique optical resonances can emerge as a result of symmetry breaking6,7,8.
The illumination of half-shell (nanocup) structures with light can excite electric dipole or magnetic dipole plasmon modes, depending on factors such as the deposition angle of the metal, the orientation of the substrate with respect to the incident light, and the polarization of the incident light9. Nanocups have often been considered analogous to three-dimensional split-ring resonators, in which the resonance frequency can approximated as an LC-oscillator10,11. The resonance frequency for the size of polymeric nanospheres used here (170 nm), the amount of deposited gold (20 nm), and the etch rates yield resonance frequencies spanning the visible and near-IR.
The optical properties of the gold nanocups can be measured either in transmission or reflection, depending on the substrate used for spin-coating. In the presented protocol, we chose to use 2 in. silicon wafers as the substrate and perform reflectance measurements after metal deposition. The measurements were performed using a microscope coupled to a dispersive spectrometer with a halogen light source. We have also had success with using glass substrates, allowing for both transmission and reflection measurements immediately following the metal deposition. Furthermore, this technique can easily be scaled and is not limited to 2 in. wafers. Due to the wide commercial availability of high-quality monodisperse polymeric nanospheres, it is straightforward to tune the optical properties of these structures by simply starting with differently sized nanospheres.
In this protocol, a technique to fabricate anisotropic half-shell (or nanocup) gold nanostructures using a method called colloidal lithography is demonstrated. Colloidal lithography uses self-assembly of highly monodisperse polymeric nanosphere to quickly pattern a substrate that can be further processed into a plasmonic substrate after sputter coating a thin layer of gold. Likewise, it is possible to tune the anisotropy of the substrate by tilting the sample substrate during metal deposition. The resulting structures are polarization-sensitive because of the anisotropy of the formed nanostructure. Here, we demonstrate one particular case and perform optical characterization and lift-off to transfer the structures to a transparent, flexible film.

<table><tbody><tr><td>Polystyrene microspheres</td><td>Bangs Laboratories, Inc.</td><td>PS02N</td><td>170 nm &amp;ndash; 580 nm diameter</td></tr><tr><td>Silicon wafers</td><td>El-CAT, Inc.</td><td>3489</td><td>300 mm thick, one side polished [100]</td></tr><tr><td>Adhesive tape</td><td>3M</td><td>Scotch 600</td><td></td></tr><tr><td>Spin coater</td><td>Laurell</td><td>WS-650-23B</td><td></td></tr><tr><td>Plasma etcher</td><td>Nordson March&amp;nbsp;</td><td>AP-600</td><td></td></tr><tr><td>Microspectrophotometer</td><td>CRAIC</td><td>380-PV</td><td></td></tr><tr><td>Sonicator</td><td>VWR</td><td>97043-932</td><td></td></tr><tr><td>Scintillation vials</td><td>Wheaton</td><td>986734</td><td></td></tr><tr><td>5 um syringe filter</td><td>Millex</td><td>SLSV025LS</td><td></td></tr><tr><td>Oxygen gas</td><td>Oxarc</td><td>PO249&amp;nbsp;</td><td>Industrial Grade 99.5% purity</td></tr><tr><td>Vaccum pump</td><td>Kurt J. Lesker</td><td>Edwards 28</td><td></td></tr><tr><td>Disposable syringes</td><td>Air Tite Products Co.</td><td>14-817-25</td><td>1 mL capacity</td></tr><tr><td>Water</td><td>Sigma-Aldrich</td><td>W4502</td><td></td></tr></tbody></table>

fabrication of periodic gold nanocup arrays using colloidal lithography

Fabrication of Periodic Gold Nanocup Arrays Using Colloidal Lithography

Within recent years, the field of plasmonics has exploded as researchers have demonstrated exciting applications related to chemical and optical sensing in combination with new nanofabrication techniques. A plasmon is a quantum of charge density oscillation that lends nanoscale metals such as gold and silver unique optical properties. In particular, gold and silver nanoparticles exhibit localized surface plasmon resonances-collective charge density oscillations on the surface of the nanoparticle-in the visible spectrum. Here, we focus on the fabrication of periodic arrays of anisotropic plasmonic nanostructures. These half-shell (or nanocup) structures can exhibit additional unique light-bending and polarization-dependent optical properties that simple isotropic nanostructures cannot. Researchers are interested in the fabrication of periodic arrays of nanocups for a wide variety of applications such as low-cost optical devices, surface-enhanced Raman scattering, and tamper indication. We present a scalable technique based on colloidal lithography in which it is possible to easily fabricate large periodic arrays of nanocups using spin-coating and self-assembled commercially available polymeric nanospheres. Electron microscopy and optical spectroscopy from the visible to near-infrared (near-IR) was performed to confirm successful nanocup fabrication. We conclude with a demonstration of the transfer of nanocups to a flexible, conformal adhesive film.

Gold nanocups were prepared using 170 nm diameter polystyrene nanospheres. After annealing for 2 min at 107 °C and etching with a 75 W, 20 sccm O2 plasma for 165 s, the resulting film was characterized using SEM (Figure 1). To evaluate the quality of the spin-casted film, optical microscopy-in addition to visual inspection-may be used (Figure 2). High-quality films should be essentially free of defects. Grain boundarie...

We demonstrate the fabrication of periodic gold nanocup arrays using colloidal lithographic techniques and discuss the importance of nanoplasmonic films.

Within recent years, the field of plasmonics has exploded as researchers have demonstrated exciting applications related to chemical and optical sensing ...

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.

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Learn to fabricate periodic gold nanocup arrays using colloidal lithography with self-assembled polystyrene nanospheres. This scalable technique enables tunable plasmon resonance for applications in chemical sensing and photovoltaics.

Fabrication of Periodic Gold Nanocup Arrays Using Colloidal Lithography

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