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="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Nanoplasmonic+waveguides:+towards+applications+in+integrated+nanophotonic+circuits.">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/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Dielectric+shell+isolated+and+graphene+shell+isolated+nanoparticle+enhanced+Raman+spectroscopies+and+their+applications.">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://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Large+Area+Plasmonic+Color+Palettes+with+Expanded+Gamut+Using+Colloidal+Self-Assembly.">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/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Electron-beam+lithography+of+plasmonic+nanorod+arrays+for+multilayered+optical+storage.">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/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Fabrication+and+Functionalization+of+Periodically+Aligned+Metallic+Nanocup+Arrays+Using+Colloidal+Lithography+with+a+Sinusoidally+Wrinkled+Substrate.">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/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Symmetry+breaking+in+individual+plasmonic+nanoparticles.">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="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Plasmon+hybridization+in+stacked+metallic+nanocups.">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/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Color+Rendering+Plasmonic+Aluminum+Substrates+with+Angular+Symmetry+Breaking.">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/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Angle-+and+Spectral-Dependent+Light+Scattering+from+Plasmonic+Nanocups.">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/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Light-Bending+Nanoparticles.">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/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Optical+antenna+enhanced+spontaneous+emission.">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://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Plasmonic+black+metals+in+resonant+nanocavities.">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://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Efficient+Nanosecond+Photoluminescence+from+Infrared+PbS+Quantum+Dots+Coupled+to+Plasmonic+Nanoantennas.">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 Troubleshooting:
If the film quality is poor, it may be necessary to pre-filter the nanosphere solution. Here, we used a 5 &#181;m syringe filter but it may be advantageous to use syringe filters down to 0.22 &#181;m, depending on the nanosphere diameter. The etching process can be adjusted to get the desired optical response. The quality of the etch should be evaluated using SEM to ensure non-touching and evenly spaced polymeric nanospheres. Once the etch parameters have been established for a particular system, it is possible to reproducibly manufacture several wafers in a batch with similar plasmon resonances. Metal deposition at varying angles will tune the nanocup&#39;s anisotropic optical properties.
Critical Steps:
The nanospheres must be properly stored and handled to achieve high quality films. Allow the nanospheres to warm to room temperature and briefly vortex followed by sonication to help ensure monodisperse nanospheres. The silicon substrate must be plasma cleaned and used immediately in order to ensure a highly hydrophilic surface. Finally, the self-assembled film should both be inspected by eye as well as through optical microscopy. Minimal defects should be observed, otherwise it will be necessary to adjust spin conditions.
Limitations:
This is a highly scalable technique but it does have several limitations that must be kept in mind. The self-assembly process is excellent at producing large arrays of nanospheres but it is challenging to fabricate nanostructures with three-dimensional anisotropy. Complex nanostructures are best fabricated by electron beam lithography or focused ion beam milling. These nanostructures, however, do not scale well and are exceedingly expensive to manufacture.
Overall, this protocol demonstrates how to fabricate nanoplasmonic films. Nanoplasmonic films have a variety of applications in areas such as nonlinear optical materials7, photovoltaics12, and light emitting diodes13.

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 border="0" cellpadding="0" cellspacing="0" width="762">
	<tbody>
		<tr height="43">
			<td height="43" style="height:43px;width:204px;">Polystyrene microspheres</td>
			<td style="width:152px;">Bangs Laboratories, Inc.</td>
			<td style="width:209px;">PS02N</td>
			<td style="width:197px;">170 nm &#8211; 580 nm diameter</td>
		</tr>
		<tr height="43">
			<td height="43" style="height:43px;width:204px;">Silicon wafers</td>
			<td style="width:152px;">El-CAT, Inc.</td>
			<td style="width:209px;">3489</td>
			<td style="width:197px;">300 mm thick, one side polished [100]</td>
		</tr>
		<tr height="22">
			<td height="22" style="height:22px;width:204px;">Adhesive tape</td>
			<td style="width:152px;">3M</td>
			<td style="width:209px;">Scotch 600</td>
			<td style="width:197px;"></td>
		</tr>
		<tr height="22">
			<td height="22" style="height:22px;width:204px;">Spin coater</td>
			<td style="width:152px;">Laurell</td>
			<td style="width:209px;">WS-650-23B</td>
			<td style="width:197px;"></td>
		</tr>
		<tr height="22">
			<td height="22" style="height:22px;width:204px;">Plasma etcher</td>
			<td style="width:152px;">Nordson March&#160;</td>
			<td style="width:209px;">AP-600</td>
			<td style="width:197px;"></td>
		</tr>
		<tr height="22">
			<td height="22" style="height:22px;width:204px;">Microspectrophotometer</td>
			<td style="width:152px;">CRAIC</td>
			<td style="width:209px;">380-PV</td>
			<td style="width:197px;"></td>
		</tr>
		<tr height="22">
			<td height="22" style="height:22px;width:204px;">Sonicator</td>
			<td style="width:152px;">VWR</td>
			<td style="width:209px;">97043-932</td>
			<td style="width:197px;"></td>
		</tr>
		<tr height="22">
			<td height="22" style="height:22px;width:204px;">Scintillation vials</td>
			<td style="width:152px;">Wheaton</td>
			<td style="width:209px;">986734</td>
			<td style="width:197px;"></td>
		</tr>
		<tr height="22">
			<td height="22" style="height:22px;width:204px;">5 um syringe filter</td>
			<td>Millex</td>
			<td>SLSV025LS</td>
			<td></td>
		</tr>
		<tr height="22">
			<td height="22" style="height:22px;width:204px;">Oxygen gas</td>
			<td>Oxarc</td>
			<td>PO249&#160;</td>
			<td>Industrial Grade 99.5% purity</td>
		</tr>
		<tr height="22">
			<td height="22" style="height:22px;width:204px;">Vaccum pump</td>
			<td>Kurt J. Lesker</td>
			<td>Edwards 28</td>
			<td></td>
		</tr>
		<tr height="22">
			<td height="22" style="height:22px;">Disposable syringes</td>
			<td>Air Tite Products Co.</td>
			<td>14-817-25</td>
			<td>1 mL capacity</td>
		</tr>
		<tr height="22">
			<td height="22" style="height:22px;width:204px;">Water</td>
			<td>Sigma-Aldrich</td>
			<td>W4502</td>
			<td></td>
		</tr>
	</tbody>
</table>

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 &#176;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 boundaries are typically observed even in high-quality films, but with careful attention to detail, it is possible to nearly eliminate point defects. Deposition of 20 nm of gold using sputter coating resulted in a plasmonically-active film and was characterized using optical reflectance spectroscopy (Figure 3). The plasmonic film was transferred from the rigid silicon substrate to a flexible film using commonly available adhesive tape. The tape was placed in contact with the plasmonically-active film and allowed to adhere to the film for 1 min. The tape was then gently removed from the substrate, resulting in a transfer of the gold nanocups to the film (Figure 4).
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/56204/56204fig1.jpg" /> 
Figure 1: Representative scanning electron micrographs of self-assembled nanostructures fabricated using colloidal lithography. (a) Self-assembled monolayer of a typical array of polystyrene nanospheres before etching, (b) periodically spaced polystyrene nanospheres after annealing and etching (75 W, 20 sccm O2 for 165 s), and (c) periodically spaced gold nanocups with 20 nm of gold (Au) deposited at a normal incidence with respect to the substrate. Scale bar: 100 nm. Magnification: 100 kX. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/56204/56204fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/56204/56204fig2.jpg" /> 
Figure 2: Optical microscopy of self-assembled films to evaluate quality. (a) Film with good monolayer coverage and minimal defects. Grain boundaries are observed with minimal defects and holes. (b) Film consisting of monolayer and multilayer regions. (c) Film with major defects and incomplete monolayer coverage. Scale bar: 20 &#181;m. Magnification: 20X. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/56204/56204fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" class="xfigimg" src="/files/ftp_upload/56204/56204fig3.jpg" /> 
Figure 3: Optical reflectance characterization of the fabricated gold nanocup array. Optical reflectance spectra showing a strong plasmonic resonance at ~ 615 nm. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/56204/56204fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" class="xfigimg" src="/files/ftp_upload/56204/56204fig4.jpg" /> 
Figure 4: Resulting flexible, transparent film after peeling gold nanocups from sacrificial silicon (Si) wafer. (a) Schematic of lift-off procedure. (b) Optical image of peeled film. (c) Photograph focused past the film to demonstrate transparency. (d) Representative optical transmission spectra of a film after lift-off. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/56204/56204fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about Fabrication of Periodic Gold Nanocup Arrays Using Colloidal Lithography at JoVE.com

Fabrication of Periodic Gold Nanocup Arrays Using Colloidal Lithography

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 ...

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Nanyang Technological University

Research

JoVE Journal

Engineering

7.1K Views.  Energy and Environment Directorate. We demonstrate the fabrication of periodic gold nanocup arrays using colloidal lithographic techniques and discuss the importance of nanoplasmonic films. 

Video: Fabrication of Periodic Gold Nanocup Arrays Using Colloidal Lithography

Micropunching Lithography for Generating Micro- and Submicron-patterns on Polymer Substrates

Conducting polymers have attracted great attention since the discovery of high conductivity in doped polyacetylene in 19771. They offer the advantages of low weight, easy tailoring of properties and a wide spectrum of applications2,3. Due to sensitivity of conducting polymers to environmental conditions (e.g., air, oxygen, moisture, high temperature and chemical solutions), lithographic techniques present significant technical challenges when working with these materials4. For example, current photolithographic methods, such as ultra-violet (UV), are unsuitable for patterning the conducting polymers due to the involvement of wet and/or dry etching processes in these methods. In addition, current micro/nanosystems mainly have a planar form5,6. One layer of structures is built on the top surfaces of another layer of fabricated features. Multiple layers of these structures are stacked together to form numerous devices on a common substrate. The sidewall surfaces of the microstructures have not been used in constructing devices. On the other hand, sidewall patterns could be used, for example, to build 3-D circuits, modify fluidic channels and direct horizontal growth of nanowires and nanotubes.
A macropunching method has been applied in the manufacturing industry to create macropatterns in a sheet metal for over a hundred years. Motivated by this approach, we have developed a micropunching lithography method (MPL) to overcome the obstacles of patterning conducting polymers and generating sidewall patterns. Like the macropunching method, the MPL also includes two operations (Fig. 1): (i) cutting; and (ii) drawing. The "cutting" operation was applied to pattern three conducting polymers4, polypyrrole (PPy), Poly(3,4-ethylenedioxythiophen)-poly(4-styrenesulphonate) (PEDOT) and polyaniline (PANI). It was also employed to create Al microstructures7. The fabricated microstructures of conducting polymers have been used as humidity8, chemical8, and glucose sensors9. Combined microstructures of Al and conducting polymers have been employed to fabricate capacitors and various heterojunctions9,10,11. The "cutting" operation was also applied to generate submicron-patterns, such as 100- and 500-nm-wide PPy lines as well as 100-nm-wide Au wires. The "drawing" operation was employed for two applications: (i) produce Au sidewall patterns on high density polyethylene (HDPE) channels which could be used for building 3D microsystems12,13,14, and (ii) fabricate polydimethylsiloxane (PDMS) micropillars on HDPE substrates to increase the contact angle of the channel15.

A micropunching lithography approach is developed to generate micro- and submicron-patterns on top, sidewall and bottom surfaces of polymer substrates. It overcomes the obstacles of patterning conducting polymers and generating sidewall patterns. This method allows rapid fabrication of multiple features and is free of aggressive chemistry.

Conducting polymers have attracted great attention since the discovery of high conductivity in doped polyacetylene in 19771. They offer the advantages of ...

Fabrication of Spatially Confined Complex Oxides

Complex materials such as high Tc superconductors, multiferroics, and colossal magnetoresistors have electronic and magnetic properties that arise from the inherent strong electron correlations that reside within them. These materials can also possess electronic phase separation in which regions of vastly different resistive and magnetic behavior can coexist within a single crystal alloy material. By reducing the scale of these materials to length scales at and below the inherent size of the electronic domains, novel behaviors can be exposed. Because of this and the fact that spin-charge-lattice-orbital order parameters each involve correlation lengths, spatially reducing these materials for transport measurements is a critical step in understanding the fundamental physics that drives complex behaviors. These materials also offer great potential to become the next generation of electronic devices 1-3. Thus, the fabrication of low dimensional nano- or micro-structures is extremely important to achieve new functionality. This involves multiple controllable processes from high quality thin film growth to accurate electronic property characterization. Here, we present fabrication protocols of high quality microstructures for complex oxide manganite devices. Detailed descriptions and required equipment of thin film growth, photo-lithography, and wire-bonding are presented.

We describe the use of pulsed laser deposition (PLD), photolithography and wire-bonding techniques to create micrometer scale complex oxides devices. The PLD is utilized to grow epitaxial thin films. Photolithography and wire-bonding techniques are introduced to create practical devices for measurement purposes.

Complex materials such as high Tc superconductors, multiferroics, and colossal magnetoresistors have electronic and magnetic properties that arise from ...

Fabrication and Testing of Microfluidic Optomechanical Oscillators

Cavity optomechanics experiments that parametrically couple the phonon modes and photon modes have been investigated in various optical systems including microresonators. However, because of the increased acoustic radiative losses during direct liquid immersion of optomechanical devices, almost all published optomechanical experiments have been performed in solid phase. This paper discusses a recently introduced hollow microfluidic optomechanical resonator. Detailed methodology is provided to fabricate these ultra-high-Q microfluidic resonators, perform optomechanical testing, and measure radiation pressure-driven breathing mode and SBS-driven whispering gallery mode parametric vibrations. By confining liquids inside the capillary resonator, high mechanical- and optical- quality factors are simultaneously maintained.

Parametric optomechanical excitations have recently been experimentally demonstrated in microfluidic optomechanical resonators by means of optical radiation pressure and stimulated Brillouin scattering. This paper describes the fabrication of these microfluidic resonators along with methodologies for generating and verifying optomechanical oscillations.

Cavity optomechanics experiments that parametrically couple the phonon modes and photon modes have been investigated in various optical systems including ...

Atomically Traceable Nanostructure Fabrication

Reducing the scale of etched nanostructures below the 10 nm range eventually will require an atomic scale understanding of the entire fabrication process being used in order to maintain exquisite control over both feature size and feature density. Here, we demonstrate a method for tracking atomically resolved and controlled structures from initial template definition through final nanostructure metrology, opening up a pathway for top-down atomic control over nanofabrication. Hydrogen depassivation lithography is the first step of the nanoscale fabrication process followed by selective atomic layer deposition of up to 2.8 nm of titania to make a nanoscale etch mask. Contrast with the background is shown, indicating different mechanisms for growth on the desired patterns and on the H passivated background. The patterns are then transferred into the bulk using reactive ion etching to form 20 nm tall nanostructures with linewidths down to ~6 nm. To illustrate the limitations of this process, arrays of holes and lines are fabricated. The various nanofabrication process steps are performed at disparate locations, so process integration is discussed. Related issues are discussed including using fiducial marks for finding nanostructures on a macroscopic sample and protecting the chemically reactive patterned Si(100)-H surface against degradation due to atmospheric exposure.

We report a protocol for combining the atomic metrology of the Scanning Tunneling Microscope for surface patterning with selective Atomic Layer Deposition and Reactive Ion Etching. Using a robust process involving numerous atmospheric exposures and transport, 3D nanostructures with atomic metrology are fabricated.

Reducing the scale of etched nanostructures below the 10 nm range eventually will require an atomic scale understanding of the entire fabrication process ...

Indirect Fabrication of Lattice Metals with Thin Sections Using Centrifugal Casting

One of the typical methods to manufacture 3D lattice metals is the direct-metal additive manufacturing (AM) process such as Selective Laser Melting (SLM) and Electron Beam Melting (EBM). In spite of its potential processing capability, the direct AM method has several disadvantages such as high cost, poor surface finish of final products, limitation in material selection, high thermal stress, and anisotropic properties of parts. We propose a cost-effective method to manufacture 3D lattice metals. The objective of this study is to provide a detailed protocol on fabrication of 3D lattice metals having a complex shape and a thin wall thickness; e.g., octet truss made of Al and Cu alloys having a unit cell length of 5 mm and a cell wall thickness of 0.5 mm. An overall experimental procedure is divided into eight sections: (a) 3D printing of sacrificial patterns (b) melt-out of support materials (c) removal of residue of support materials (d) pattern assembly (e) investment (f) burn-out of sacrificial patterns (g) centrifugal casting (h) post-processing for final products. The suggested indirect AM technique provides the potential to manufacture ultra-lightweight lattice metals; e.g., lattice structures with Al alloys. It appears that the process parameters should be properly controlled depending on materials and lattice geometry, observing the final products of octet truss metals by the indirect AM technique.

An indirect additive manufacturing method combining a 3D printing of polymers with a centrifugal casting is outlined for manufacturing 3D octet truss metals (Al and Cu alloys) having a unit cell length of 5 mm with a wall thickness of 0.5 mm.

One of the typical methods to manufacture 3D lattice metals is the direct-metal additive manufacturing (AM) process such as Selective Laser Melting (SLM) ...

Fabrication and Characterization of Superconducting Resonators

Superconducting microwave resonators are of interest for a wide range of applications, including for their use as microwave kinetic inductance detectors (MKIDs) for the detection of faint astrophysical signatures, as well as for quantum computing applications and materials characterization. In this paper, procedures are presented for the fabrication and characterization of thin-film superconducting microwave resonators. The fabrication methodology allows for the realization of superconducting transmission-line resonators with features on both sides of an atomically smooth single-crystal silicon dielectric. This work describes the procedure for the installation of resonator devices into a cryogenic microwave testbed and for cool-down below the superconducting transition temperature. The set-up of the cryogenic microwave testbed allows one to do careful measurements of the complex microwave transmission of these resonator devices, enabling the extraction of the properties of the superconducting lines and dielectric substrate (e.g., internal quality factors, loss and kinetic inductance fractions), which are important for device design and performance.

Superconducting microwave resonators are of interest for detection of light, quantum computing applications and materials characterization. This work presents a detailed procedure for fabrication and characterization of superconducting microwave resonator scattering parameters.

Superconducting microwave resonators are of interest for a wide range of applications, including for their use as microwave kinetic inductance detectors ...

Colloidal Synthesis of Nanopatch Antennas for Applications in Plasmonics and Nanophotonics

We present a method for colloidal synthesis of silver nanocubes and the use of these in combination with a smooth gold film, to fabricate plasmonic nanoscale patch antennas. This includes a detailed procedure for the fabrication of thin films with a well-controlled thickness over macroscopic areas using layer-by-layer deposition of polyelectrolyte polymers, namely poly(allylamine) hydrochloride (PAH) and polystyrene sulfonate (PSS). These polyelectrolyte spacer layers serve as a dielectric gap in between silver nanocubes and a gold film. By controlling the size of the nanocubes or the gap thickness, the plasmon resonance can be tuned from about 500 nm to 700 nm. Next, we demonstrate how to incorporate organic sulfo-cyanine5 carboxylic acid (Cy5) dye molecules into the dielectric polymer gap region of the nanopatch antennas. Finally, we show greatly enhanced fluorescence of the Cy5 dyes by spectrally matching the plasmon resonance with the excitation energy and the Cy5 absorption peak. The method presented here enables the fabrication of plasmonic nanopatch antennas with well-controlled dimensions utilizing colloidal synthesis and a layer-by-layer dip-coating process with the potential for low cost and large-scale production. These nanopatch antennas hold great promise for practical applications, for example in sensing, ultrafast optoelectronic devices and for high-efficiency photodetectors.

A protocol for the colloidal synthesis of silver nanocubes and fabrication of plasmonic nanoscale patch antennas with sub-10 nm gaps is presented.

We present a method for colloidal synthesis of silver nanocubes and the use of these in combination with a smooth gold film, to fabricate plasmonic ...

Use of Sacrificial Nanoparticles to Remove the Effects of Shot-noise in Contact Holes Fabricated by E-beam Lithography

Nano-patterns fabricated with extreme ultraviolet (EUV) or electron-beam (E-beam) lithography exhibit unexpected variations in size. This variation has been attributed to statistical fluctuations in the number of photons/electrons arriving at a given nano-region arising from shot-noise (SN). The SN varies inversely to the square root of a number of photons/electrons. For a fixed dosage, the SN is larger in EUV and E-beam lithographies than for traditional (193 nm) optical lithography. Bottom-up and top-down patterning approaches are combined to minimize the effects of shot noise in nano-hole patterning. Specifically, an amino-silane surfactant self-assembles on a silicon wafer that is subsequently spin-coated with a 100 nm film of a PMMA-based E-beam photoresist. Exposure to the E-beam and the subsequent development uncover the underlying surfactant film at the bottoms of the holes. Dipping the wafer in a suspension of negatively charged, citrate-capped, 20 nm gold nanoparticles (GNP) deposits one particle per hole. The exposed positively charged surfactant film in the hole electrostatically funnels the negatively charged nanoparticle to the center of an exposed hole, which permanently fixes the positional registry. Next, by heating near the glass transition temperature of the photoresist polymer, the photoresist film reflows and engulfs the nanoparticles. This process erases the holes affected by SN but leaves the deposited GNPs locked in place by strong electrostatic binding. Treatment with oxygen plasma exposes the GNPs by etching a thin layer of the photoresist. Wet-etching the exposed GNPs with a solution of I2/KI yields uniform holes located at the center of indentations patterned by E-beam lithography. The experiments presented show that the approach reduces the variation in the size of the holes caused by SN from 35% to below 10%. The method extends the patterning limits of transistor contact holes to below 20 nm.

Uniformly sized nanoparticles can remove fluctuations in contact hole dimensions patterned in poly(methyl methacrylate) (PMMA) photoresist films by electron beam (E-beam) lithography. The process involves electrostatic funneling to center and deposit nanoparticles in contact holes, followed by photoresist reflow and plasma- and wet-etching steps.

Nano-patterns fabricated with extreme ultraviolet (EUV) or electron-beam (E-beam) lithography exhibit unexpected variations in size. This variation has ...

Fabrication of 1-D Photonic Crystal Cavity on a Nanofiber Using Femtosecond Laser-induced Ablation

We present a protocol for fabricating 1-D Photonic Crystal (PhC) cavities on subwavelength-diameter tapered optical fibers, optical nanofibers, using femtosecond laser-induced ablation. We show that thousands of periodic nano-craters are fabricated on an optical nanofiber by irradiating with just a single femtosecond laser pulse. For a typical sample, periodic nano-craters with a period of 350 nm and with diameter gradually varying from 50 - 250 nm over a length of 1 mm are fabricated on a nanofiber with diameter around 450 - 550 nm. A key aspect of such a nanofabrication is that the nanofiber itself acts as a cylindrical lens and focuses the femtosecond laser beam on its shadow surface. Moreover, the single-shot fabrication makes it immune to mechanical instabilities and other fabrication imperfections. Such periodic nano-craters on nanofiber, act as a 1-D PhC and enable strong and broadband reflection while maintaining the high transmission out of the stopband. We also present a method to control the profile of the nano-crater array to fabricate apodized and defect-induced PhC cavities on the nanofiber. The strong confinement of the field, both transverse and longitudinal, in the nanofiber-based PhC cavities and the efficient integration to the fiber networks, may open new possibilities for nanophotonic applications and quantum information science.

We present a protocol for fabricating 1-D photonic crystal cavities on subwavelength diameter silica fibers (optical nanofibers) using femtosecond laser-induced ablation.

We present a protocol for fabricating 1-D Photonic Crystal (PhC) cavities on subwavelength-diameter tapered optical fibers, optical nanofibers, using ...

Fabrication of Ultra-thin Color Films with Highly Absorbing Media Using Oblique Angle Deposition

Ultra-thin film structures have been studied extensively for use as optical coatings, but performance and fabrication challenges remain.&#160; We present an advanced method for fabricating ultra-thin color films with improved characteristics. The proposed process addresses several fabrication issues, including large area processing. Specifically, the protocol describes a process for fabricating ultra-thin color films using an electron beam evaporator for oblique angle deposition of germanium (Ge) and gold (Au) on silicon (Si) substrates.&#160; Film porosity produced by the oblique angle deposition induces color changes in the ultra-thin film. The degree of color change depends on factors such as deposition angle and film thickness. Fabricated samples of the ultra-thin color films showed improved color tunability and color purity. In addition, the measured reflectance of the fabricated samples was converted into chromatic values and analyzed in terms of color. Our ultra-thin film fabricating method is expected to be used for various ultra-thin film applications such as flexible color electrodes, thin film solar cells, and optical filters. Also, the process developed here for analyzing the color of the fabricated samples is broadly useful for studying various color structures.

We present a detailed method for fabricating ultra-thin color films with improved characteristics for optical coatings. The oblique angle deposition technique using an electron beam evaporator allows improved color tunability and purity. Fabricated films of Ge and Au on Si substrates were analyzed by reflectance measurements and color information conversion.

Ultra-thin film structures have been studied extensively for use as optical coatings, but performance and fabrication challenges remain.&#160; We present ...