Name: fabrication of periodic gold nanocup arrays using colloidal lithography
Uploaded: 2017-09-02T13:00:00
Description: Watch this Scientific Journal Video about Fabrication of Periodic Gold Nanocup Arrays Using Colloidal Lithography at JoVE.com

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 wafe...

<ol>
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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.

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	<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 boun...

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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