We acknowledge the support from the Microscopy Core Facility at Utah State University for the SEM result. We also acknowledge the National Science Foundation (Award #162344) for the DC Magnetron Sputtering System, the National Science Foundation (Award #133792) for the (Field Electron and Ion) FEI Quanta 650, and the Department of Energy, Nuclear Energy University Program for the FEI Nova Nanolab 600.

NOTE: The large-area fabrication of controllable and customizable gold nanoparticle films is achieved by following the detailed protocol. The protocol follows three major areas that are (1) substrate preparation, (2) dewetting and etching, and (3) characterization.
1. Substrate Preparation
<ol>
	<li>Clean the substrate (100 nm SiO2 on Si) using an acetone rinse followed by an isopropyl alcohol rinse and then dry using a stream of N2 gas.</li>
	<li>Load the substrate into the thermal evaporator system and evacuate to reach the desired pressure for the deposition of the metal film. Ensure that the chamber is evacuated to a pressure on the order of 10-6 Torr for the removal of air and water vapor in the chamber.</li>
	<li>Using the thermal evaporator, deposit the gold film at the desired thickness (5 nm in this case). The gold source material was obtained in the form of 0.5 mm diameter wire of gold (99.99% pure). Note that the thickness control for all the deposition stages is performed by the calibration of the machine, considering all important parameters and post measurement of the thickness. At both deposition stages, the argon pressure is a couple of millitorrs (1-5 mTorr), and the range is given as different pressures are chosen to calibrate for the deposition rate.</li>
	<li>Vent and remove the substrate with deposited metal film from the thermal evaporator system. The protocol can be paused here.</li>
	<li>Load the substrate with deposited metal film into the direct current (DC) magnetron sputter deposition system and evacuate to reach the desired pressure for deposition of the capping film (Table of Materials).
	<ol>
		<li>To locate the sample in the machine, put the sample in the load lock and the device transfers the sample to the main deposition chamber to ensure a sufficient level of vacuum. Note that the deposition of the alumina capping layer tales place in the next step and this step is explaining the process of placing the sample in the apparatus and how the sample is transferred to the main deposition chamber.</li>
	</ol>
	</li>
	<li>Deposit the capping layer of the desired material and thickness. Note that the deposition of the alumina follows a similar procedure and condition of the gold layer deposition, variable thickness alumina in this case. The alumina source material was obtained in the form of a 50.8 mm diameter, 6.35 mm thick sputter target of aluminum oxide (99.5% pure).</li>
	<li>Vent the DC magnetron sputter deposition chamber and remove the prepared sample. (Table of Materials). The protocol can be paused here.</li>
</ol>
2. Dewetting and Etching
<ol>
	<li>Place the prepared sample onto a pre-heated hot plate. For the 5 nm gold film capped with alumina, heat the sample at 300 &#176;C and allow the sample to dewet for 1 h. The protocol can be paused here.</li>
	<li>Etch the alumina while leaving the gold and underlying SiO2/Si substrate with a 3:1:1=H2O:NH4OH:H2O2 (in wt%) solution at 80 &#176;C for 1 h. Note that the process is performed in a hood and all precautions for dealing with corrosive and environmental hazardous material should be taken. The protocol can be paused here.</li>
</ol>
3. Characterization
<ol>
	<li>Prepare the sample to be vacuum compatible by rinsing with acetone and isopropyl alcohol followed by drying with N2.</li>
	<li>Image the nanoparticle films using scanning electron microscopy (SEM) under high vacuum and at high magnification (50,000X magnification in this case to resolve the minimum sized nanoparticles). The protocol can be paused here.</li>
	<li>Perform image analysis to obtain information of nanoparticle size and spacing distributions. The image analysis is done using a MATLAB-based code that thresholds the grayscale image, performs noise reduction and particle filling routines15.</li>
</ol>

<ol><li>Pillai, S., Catchpole, K. R., Trupke, T., Green, M. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Surface+plasmon+enhanced+silicon+solar+cells%5BTitle%5D)+AND+%22Journal+of+Applied+Physics%22%5BJournal%5D)">Surface plasmon enhanced silicon solar cells.</a> Journal of Applied Physics. 101 (9), 093105(2007).</li>
<li>Ding, B., Lee, B. J., Yang, M., Jung, H. S., Lee, J. -K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Surface-Plasmon+Assisted+Energy+Conversion+in+Dye-Sensitized+Solar+Cells%5BTitle%5D)+AND+%22Advanced+Energy+Materials%22%5BJournal%5D)">Surface-Plasmon Assisted Energy Conversion in Dye-Sensitized Solar Cells.</a> Advanced Energy Materials. 1 (3), 415-421 (2011).</li>
<li>Tehrani, S., Chen, E., Durlam, M., DeHerrera, M., Slaughter, J. M., Shi, J., Kerszykowski, G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((High+density+submicron+magnetoresistive+random+access+memory+%28invited%29%5BTitle%5D)+AND+%22Journal+of+Applied+Physics%22%5BJournal%5D)">High density submicron magnetoresistive random access memory (invited).</a> Journal of Applied Physics. 85 (8), 5822-5827 (1999).</li>
<li>Ross, C. A., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Fabrication+of+patterned+media+for+high+density+magnetic+storage%5BTitle%5D)+AND+%22Journal+of+Vacuum+Science+%26+Technology+B%22%5BJournal%5D)">Fabrication of patterned media for high density magnetic storage.</a> Journal of Vacuum Science & Technology B. 17, 3168(1999).</li>
<li>Gu, M., Zhang, Q., Lamon, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Nanomaterials+for+optical+data+storage%5BTitle%5D)+AND+%22Nature+Reviews+Materials%22%5BJournal%5D)">Nanomaterials for optical data storage.</a> Nature Reviews Materials. 1, 16070(2016).</li>
<li>Mock, J. J., Barbic, M., Smith, D. R., Schultz, D. A., Schultz, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Shape+effects+in+plasmon+resonance+of+individual+colloidal+silver+nanoparticles%5BTitle%5D)+AND+%22The+Journal+of+Chemical+Physics%22%5BJournal%5D)">Shape effects in plasmon resonance of individual colloidal silver nanoparticles.</a> The Journal of Chemical Physics. 116 (15), 6755-6759 (2002).</li>
<li>Su, K. -H. A., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Interparticle+Coupling+Effects+on+Plasmon%5BTitle%5D)+AND+%22Resonances+of+Nanogold+Particles%2C+Nano+Letters%22%5BJournal%5D)">Interparticle Coupling Effects on Plasmon.</a> Resonances of Nanogold Particles, Nano Letters. 3 (8), 1087-1090 (2003).</li>
<li>Lee, K., El-Sayed, M. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Gold+and+Silver+Nanoparticles+in+Sensing+and+Imaging%3A+Sensitivity+of+Plasmon+Response+to+Size%2C+Shape%2C+and+Metal+Composition%5BTitle%5D)+AND+%22The+Journal+of+Physical+Chemistry+B%22%5BJournal%5D)">Gold and Silver Nanoparticles in Sensing and Imaging: Sensitivity of Plasmon Response to Size, Shape, and Metal Composition.</a> The Journal of Physical Chemistry B. 110 (39), 19220-19225 (2006).</li>
<li>Grzelczak, M., Prez-Juste, J., Mulvaney, P., Liz-Marzn, L. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Shape+control+in+gold+nanoparticle+synthesis%5BTitle%5D)+AND+%22Chemical+Society+Reviews%22%5BJournal%5D)">Shape control in gold nanoparticle synthesis.</a> Chemical Society Reviews. 37 (9), 1783-1791 (2008).</li>
<li>Ye, J., Thompson, C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Templated+Solid-State+Dewetting+to+Controllably+Produce+Complex+Patterns%5BTitle%5D)+AND+%22Advanced+Materials%22%5BJournal%5D)">Templated Solid-State Dewetting to Controllably Produce Complex Patterns.</a> Advanced Materials. 23 (13), 1567-1571 (2011).</li>
<li>Huang, J., Kim, F., Tao, A., Connor, S., Yang, P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Spontaneous+formation+of+nanoparticle+stripe+patterns+through+dewetting%5BTitle%5D)+AND+%22Nature+Materials%22%5BJournal%5D)">Spontaneous formation of nanoparticle stripe patterns through dewetting.</a> Nature Materials. 4, 896-900 (2005).</li>
<li>Hughes, R. A., Menumerov, E., Neretina, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((When+lithography+meets+self-assembly%3A+a+review+of+recent+advances+in+the+directed+assembly+of+complex+metal+nanostructures+on+planar+and+textured+surfaces%5BTitle%5D)+AND+%22Nanotechnology%22%5BJournal%5D)">When lithography meets self-assembly: a review of recent advances in the directed assembly of complex metal nanostructures on planar and textured surfaces.</a> Nanotechnology. 28 (28), 282002(2017).</li>
<li>Kim, D., Giermann, A. L., Thompson, C. V. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Solid-state+dewetting+of+patterned+thin+films%5BTitle%5D)+AND+%22Applied+Physics+Letters%22%5BJournal%5D)">Solid-state dewetting of patterned thin films.</a> Applied Physics Letters. 95 (25), 251903(2009).</li>
<li>Fowlkes, J. D., Doktycz, M. J., Rack, P. D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((An+optimized+nanoparticle+separator+enabled+by+electron+beam+induced+deposition%5BTitle%5D)+AND+%22Nanotechnology%22%5BJournal%5D)">An optimized nanoparticle separator enabled by electron beam induced deposition.</a> Nanotechnology. 21 (16), 165303(2010).</li>
<li>White, B. C. A., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((The+Effect+of+Different+Thickness+Alumina+Capping+Layers+on+the+Final+Morphology+of+Dewet+Thin+Ni+Films%5BTitle%5D)+AND+%22Applied+Physics+A%22%5BJournal%5D)">The Effect of Different Thickness Alumina Capping Layers on the Final Morphology of Dewet Thin Ni Films.</a> Applied Physics A. 124 (3), 233(2018).</li>
</ol>

The authors have nothing to disclose.

The protocol is a feasible and easy process for a nano-manufacturing process for producing nanoparticles on a substrate over large areas with controllable characteristics. The dewetting phenomenon, which leads to the production of particles, is based on the tendency of the dewetted layer to achieve minimum surface energy. The control over the size and shape of the particles is targeted with the deposition of a second surface on the main layer to tune the surface energies, and the final equilibrium between the adhesion and the energy required to bend the capping layer on the particles determines different dewetting regimes, which lead to different surface morphologies. This protocol has been designed and demonstrated based on equipment and processes that are typically accessible to anyone with basic microfabrication equipment and process capabilities. In the demonstrated approach, additional control over the final nanoparticle distribution can be achieved by changing the metal film thickness, the cap layer thickness, the substrate material, and the cap layer material. Between these process variables, a wide range of nanoparticle size and spacing can be achieved. 
Adding additional steps or substituting techniques used in the current protocol can provide additional modification of the process resulting in more control over the nanoparticle distributions, including wider range of nanoparticle size and spacing, narrowing of the nanoparticle distributions, or the ability to produce multimodal nanoparticle films. This protocol was designed and demonstrated with an emphasis on accessibility and low cost. If more range is desired, the use of a rapid thermal annealing system or laser irradiation will change the heating rate and provide more nanoparticle control. If a multimodal nanoparticle distribution is desired, intermediate steps of lithography (electron beam lithography or photolithography) can be added before metal deposition or before cap layer deposition. The lithography step(s) will result in a variable thickness metal or cap layer across the surface and thus a different nanoparticle distribution. 
Another modification that can be made easily is in the desired metal, depending on the specific application of the nanoparticle film. Here, the demonstration used gold because of the plasmonic properties, but similarly, a metallic nanoparticle or other plasmonic nanoparticle, or even a core-shell nanoparticle could be desired. This is achieved by changing the metal film material. This change will affect the resulting nanoparticle distribution because of the differences in surface energies, but the same trends would be expected. Note that the thickness of the capping layer provides control over the resulting nanoparticle size and spacing. For new material systems, an understanding of the extent of control will be needed. 
This protocol was designed to eliminate the issue of large-area substrate-based nanoparticle fabrication for applications ranging from solar energy conversion to high-density data storage. These applications require a large-area of nanoparticles with well-defined and controlled nanoparticles. The techniques that are used in research labs to study the impact that nanoparticles have in these applications involved expensive equipment and time intensive processes, making them unfeasible for industrial applications. This protocol has demonstrated the level of control needed based on affordable and fast processing steps.
This protocol has the potential to be a revolutionary technique for the production of any nanoparticle films that require substrate-based processing. This demonstration was only done with a single material system, but more research will be done in the short term to explore the full capabilities of control and customization that is provided by this protocol.

Large-area nanoparticle film fabrication is critically important for the adoption of recent technological advances in solar energy conversion and high-density data storage with the use of plasmonic nanoparticles1,2,3,4,5. Interestingly, it is the magnetic properties of some of these plasmonic nanoparticles, which provide these nanoparticles with the ability to manipulate and control light at the nanoscale. This controllability of light provides the possibility to enhance light entrapment of the incident light at the nanoscale and increase the absorptivity of the surface. Based on these same properties and having the ability to have nanoparticles in either a magnetized and a non-magnetized state, scientists are also defining a new platform for high-density digital data storage. In each of these applications, it is critical that a large area and affordable nanofabrication technique is developed that allows for the control of nanoparticle size, spacing, and shape.
The available techniques to produce nanoparticles are mostly based on nanoscale lithography, which have significant scalability and cost issues. There have been multiple different studies that have attempted to address the scalability problem of these techniques, but to date, no process exists that provides the level of control needed for nanoparticle fabrication and is cost and time effective enough for adoption in industrial applications6,7,8,9,10,11. Some recent research efforts improved the controllability of pulsed laser induced dewetting (PLiD) and templated solid-state dewetting12,13,14, but they still have significant required lithography steps and thus the scalability problem.
In this manuscript, we present the protocol of a nanofabrication method that will address this scalability and cost issue that has plagued the adoption and use of nanoparticle films in widespread industrial applications. This processing method allows the control over the produced nanoparticle size and spacing by manipulating the surface energies which dictate the self-assembly of the nanoparticles formed. Here, we demonstrate the use of this technique using a thin gold film to produce gold nanoparticles, but we have recently published a slightly different version of this method using a nickel film and thus this technique can be used with any desired metal. The goal of this method is to produce nanoparticle films while minimizing the cost and complexity of the process and thus we have modified our previous approach, which used atomic layer deposition and nanosecond laser irradiation on a Ni-alumina system and replaced them with physical vapor deposition and a hot plate. The result of our work on a Ni-alumina system also showed an acceptable level of control on the morphology of the surface after the dewetting15.

<table><tbody><tr><td>100 nm SiO&lt;sub&gt;2&lt;/sub&gt;/Si Substrate</td><td>University Wafer</td><td>Thermal Oxide Wafer</td><td></td></tr><tr><td>Alumina Sputter Target (99.5%)</td><td>Kurt J. Lesker</td><td>Alumina Target</td><td></td></tr><tr><td>Gold Wire (99.99%)</td><td>Kurt J. Lesker</td><td>Gold Wire</td><td></td></tr><tr><td>H&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;2&lt;/sub&gt;</td><td>Sigma-Aldrich</td><td></td><td></td></tr><tr><td>Hot Plate</td><td>Thermo Scientific</td><td>Cimarec</td><td></td></tr><tr><td>NH&lt;sub&gt;4&lt;/sub&gt;OH</td><td>Sigma-Aldrich</td><td></td><td></td></tr><tr><td>Scanning Electron Microscope</td><td>FEI</td><td>Quanta 650</td><td></td></tr><tr><td>Scanning Electron Microscope</td><td>FEI</td><td>Nova Nanolab 600</td><td></td></tr><tr><td>Sputter Deposition System</td><td>AJA International</td><td>Orion-5</td><td></td></tr><tr><td>Thermal Evaporator</td><td>Edwards</td><td>360</td><td></td></tr></tbody></table>

large area substrate-based nanofabrication of controllable and customizable gold nanoparticles via capped dewetting

Recent scientific advances in the utilization of metallic nanoparticle for enhanced energy conversion efficiency, improved optical device performance, and high-density data storage have demonstrated the potential benefit of their use in industrial applications. These applications require precise control over nanoparticle size, spacing, and sometimes shape. These requirements have resulted in the use of time and cost intensive processing steps to produce nanoparticles, thus making the transition to industrial application unrealistic. This protocol will resolve this issue by providing a scalable and affordable method for the large-area production of nanoparticle films with improved nanoparticle control compared to the current techniques. In this article, the process will be demonstrated with gold, but other metals can also be used.

The protocol described here has been used for multiple metals and has shown the ability to produce nanoparticles on a substrate over large-area, with controllable size and spacing. Figure 1 shows the protocol with representative results showing the ability to control the fabricated nanoparticle size and spacing. When following this protocol, the result, which is the fabricated nanoparticle film with size and spacing distributions, will be dependent upon the choice of metal, the choice of substrate, the choice of capping layer material, the metal thickness, and the capping layer thickness. By adjusting any one of these parameters, a shift and change in these distributions would be expected. As an example, the 5 nm gold film on SiO2 with a Al2O3 capping layer of thicknesses of 0 nm, 5 nm, 10 nm, and 20 nm result in average nanoparticle radii of 14.2 nm, 18.4 nm, 17.3 nm, and 15.6 nm, respectively, an average nanoparticle spacing of 36.9 nm, 56.9 nm, 51.3 nm, and 47.2 nm, respectively.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/58827/58827fig1.jpg" /> 
Figure 1: Graphical image of the protocol and representative results. The histograms presented are the pith (Top left) and Radii (Bottom left) distribution of the particle. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/58827/58827fig1large.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/58827/58827fig2.jpg" /> 
Figure 2: The SEM image of No-capping layer (a) and samples with 5 (b), 10 (c) and 20 nm (d) capping layer. The change in particle sizes and distributions are apparent comparing the images. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/58827/58827fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about Large Area Substrate-Based Nanofabrication of Controllable and Customizable Gold Nanoparticles Via Capped Dewetting at JoVE.com

Large Area Substrate-Based Nanofabrication of Controllable and Customizable Gold Nanoparticles Via Capped Dewetting

This protocol details a novel nano-manufacturing technique that can be used to make controllable and customizable nanoparticle films over large areas based on the self-assembly of dewetting of capped metal films.

Recent scientific advances in the utilization of metallic nanoparticle for enhanced energy conversion efficiency, improved optical device performance, and ...

large-area-substrate-based-nanofabrication-controllable-customizable

Nanyang Technological University

Research

JoVE Journal

Engineering

5.6K Views.  Utah State University. Ons protocol biedt een techniek om de nanodeeltjesverdeling, vervaardigd op een substraat, aan te passen met het veranderen van de ontstabriceerde dynamiek, zonder de materiaaldikte te veranderen. Onze techniek is eenvoudig, maar biedt nog steeds een scala aan deeltjesgroottes. Andere technieken die er zijn vereisen uitgebreide lithografische stappen om de deeltjescontrole te bieden.Onze controle van deeltjesdistributie biedt een techniek voor nanodeeltjesfabricage die onderzoek ten go...

Video: Large Area Substrate-Based Nanofabrication of Controllable and Customizable Gold Nanoparticles Via Capped Dewetting

Template Directed Synthesis of Plasmonic Gold Nanotubes with Tunable IR Absorbance

A nearly parallel array of pores can be produced by anodizing aluminum foils in acidic environments1, 2. Applications of anodic aluminum oxide (AAO) membranes have been under development since the 1990's and have become a common method to template the synthesis of high aspect ratio nanostructures, mostly by electrochemical growth or pore-wetting. Recently, these membranes have become commercially available in a wide range of pore sizes and densities, leading to an extensive library of functional nanostructures being synthesized from AAO membranes. These include composite nanorods, nanowires and nanotubes made of metals, inorganic materials or polymers 3-10. Nanoporous membranes have been used to synthesize nanoparticle and nanotube arrays that perform well as refractive index sensors, plasmonic biosensors, or surface enhanced Raman spectroscopy (SERS) substrates 11-16, as well as a wide range of other fields such as photo-thermal heating 17, permselective transport 18, 19, catalysis 20, microfluidics 21, and electrochemical sensing 22, 23. Here, we report a novel procedure to prepare gold nanotubes in AAO membranes. Hollow nanostructures have potential application in plasmonic and SERS sensing, and we anticipate these gold nanotubes will allow for high sensitivity and strong plasmon signals, arising from decreased material dampening 15.

Solution-suspendable gold nanotubes with controlled dimensions can be synthesized by electrochemical deposition in porous anodic aluminum oxide (AAO) membranes using a hydrophobic polymer core. Gold nanotubes and nanotube arrays hold promise for applications in plasmonic biosensing, surface-enhanced Raman spectroscopy, photo-thermal heating, ionic and molecular transport, microfluidics, catalysis and electrochemical sensing.

A nearly parallel array of pores can be produced by anodizing aluminum foils in acidic environments1, 2. Applications of anodic aluminum oxide (AAO) ...

Chemistry

A Technique to Functionalize and Self-assemble Macroscopic Nanoparticle-ligand Monolayer Films onto Template-free Substrates

This protocol describes a self-assembly technique to create macroscopic monolayer films composed of ligand-coated nanoparticles1,2. The simple, robust and scalable technique efficiently functionalizes metallic nanoparticles with thiol-ligands in a miscible water/organic solvent mixture allowing for rapid grafting of thiol groups onto the gold nanoparticle surface. The hydrophobic ligands on the nanoparticles then quickly phase separate the nanoparticles from the aqueous based suspension and confine them to the air-fluid interface. This drives the ligand-capped nanoparticles to form monolayer domains at the air-fluid interface.&#160; The use of water-miscible organic solvents is important as it enables the transport of the nanoparticles from the interface onto template-free substrates.&#160; The flow is mediated by a surface tension gradient3,4 and creates macroscopic, high-density, monolayer nanoparticle-ligand films.&#160; This self-assembly technique may be generalized to include the use of particles of different compositions, size, and shape and may lead to an efficient assembly method to produce low-cost, macroscopic, high-density, monolayer nanoparticle films for wide-spread applications.

A simple, robust and scalable technique to functionalize and self-assemble macroscopic nanoparticle-ligand monolayer films onto template-free substrates is described in this protocol.

This protocol describes a self-assembly technique to create macroscopic monolayer films composed of ligand-coated nanoparticles1,2. The simple, robust and ...

Synthesis, Characterization, and Functionalization of Hybrid Au/CdS and Au/ZnS Core/Shell Nanoparticles

Plasmonic nanoparticles are an attractive material for light harvesting applications due to their easily modified surface, high surface area and large extinction coefficients which can be tuned across the visible spectrum. Research into the plasmonic enhancement of optical transitions has become popular, due to the possibility of altering and in some cases improving photo-absorption or emission properties of nearby chromophores such as molecular dyes or quantum dots. The electric field of the plasmon can couple with the excitation dipole of a chromophore, perturbing the electronic states involved in the transition and leading to increased absorption and emission rates. These enhancements can also be negated at close distances by energy transfer mechanism, making the spatial arrangement of the two species critical. Ultimately, enhancement of light harvesting efficiency in plasmonic solar cells could lead to thinner and, therefore, lower cost devices. The development of hybrid core/shell particles could offer a solution to this issue. The addition of a dielectric spacer between a gold nanoparticles and a chromophore is the proposed method to control the exciton plasmon coupling strength and thereby balance losses with the plasmonic gains. A detailed procedure for the coating of gold nanoparticles with CdS and ZnS semiconductor shells is presented. The nanoparticles show high uniformity with size control in both the core gold particles and shell species allowing for a more accurate investigation into the plasmonic enhancement of external chromophores.

The synthesis of uniform gold nanoparticles coated with semiconductor shells of CdS or ZnS is performed. The semiconductor coating is conducted by first depositing a silver sulfide shell and exchanging the silver cations for zinc or cadmium cations.

Plasmonic nanoparticles are an attractive material for light harvesting applications due to their easily modified surface, high surface area and large ...

A Simple Method for the Size Controlled Synthesis of Stable Oligomeric Clusters of Gold Nanoparticles under Ambient Conditions

Reducing dilute aqueous HAuCl4 with sodium thiocyanate (NaSCN) under alkaline conditions produces 2 to 3 nm diameter nanoparticles. Stable grape-like oligomeric clusters of these yellow nanoparticles of narrow size distribution are synthesized under ambient conditions via two methods. The delay-time method controls the number of subunits in the oligoclusters by varying the time between the addition of HAuCl4 to alkaline solution and the subsequent addition of reducing agent, NaSCN. The yellow oligoclusters produced range in size from ~3 to ~25 nm. This size range can be further extended by an add-on method utilizing hydroxylated gold chloride (Na+[Au(OH4-x)Clx]-) to auto-catalytically increase the number of subunits in the as-synthesized oligocluster nanoparticles, providing a total range of 3 nm to 70 nm. The crude oligocluster preparations display narrow size distributions and do not require further fractionation for most purposes. The oligoclusters formed can be concentrated &#62;300 fold without aggregation and the crude reaction mixtures remain stable for weeks without further processing. Because these oligomeric clusters can be concentrated before derivatization they allow expensive derivatizing agents to be used economically. In addition, we present two models by which predictions of particle size can be made with great accuracy.

We describe a simple method for producing highly stable oligomeric clusters of gold nanoparticles via the reduction of chloroauric acid (HAuCl4) with sodium thiocyanate (NaSCN). The oligoclusters have a narrow size distribution and can be produced with a wide range of sizes and surface coats.

Reducing dilute aqueous HAuCl4 with sodium thiocyanate (NaSCN) under alkaline conditions produces 2 to 3 nm diameter nanoparticles. Stable grape-like ...

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 ...

Microfabrication of Nanoporous Gold Patterns for Cell-material Interaction Studies

Nanostructured materials with feature sizes in tens of nanometers have enhanced the performance of several technologies, including fuel cells, biosensors, biomedical device coatings, and drug delivery tools. Nanoporous gold (np-Au), produced by a nano-scale self-assembly process, is a relatively new material that exhibits large effective surface area, high electrical conductivity, and catalytic activity. These properties have made np-Au an attractive material to scientific community. Most studies on np-Au employ macro-scale specimens and focus on fundamental science of the material and its catalytic and sensor applications. The macro-scale specimens limit np-Au's potential in miniaturized systems, including biomedical devices. In order to address these issues, we initially describe two different methods to micropattern np-Au thin films on rigid substrates. The first method employs manually-produced stencil masks for creating millimeter-scale np-Au patterns, while the second method uses lift-off photolithography to pattern sub-millimeter-scale patterns. As the np-Au thin films are obtained by sputter-deposition process, they are compatible with conventional microfabrication techniques, thereby amenable to facile integration into microsystems. These systems include electrically-addressable biosensor platforms that benefit from high effective surface area, electrical conductivity, and gold-thiol-based surface bioconjugation. We describe cell culture, immunostaining, and image processing techniques to quantify np-Au's interaction with mammalian cells, which is an important performance parameter for some biosensors. We expect that the techniques illustrated here will assist the integration of np-Au in platforms at various length-scales and in numerous applications, including biosensors, energy storage systems, and catalysts.

We report on techniques to micropattern nanoporous gold thin films via stencil printing and photolithography, as well as methods to culture cells on the microfabricated patterns. In addition, we describe image analysis methods to characterize morphology of the material and the cultured cells using scanning electron and fluorescence microscopy techniques.

Nanostructured materials with feature sizes in tens of nanometers have enhanced the performance of several technologies, including fuel cells, biosensors, ...

Bioengineering

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.

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 ...

Growth of Gold Dendritic Nanoforests on Titanium Nitride-coated Silicon Substrates

In this study, a high-power impulse magnetron sputtering system is used to coat a flat and firm titanium nitride (TiN) film on silicon (Si) wafers, and a fluoride-assisted galvanic replacement reaction (FAGRR) is employed for the rapid and easy deposition of gold dendritic nanoforests (Au DNFs) on the TiN/Si substrates. Scanning electron microscopy (SEM) images and energy-dispersive X-ray spectroscopy patterns of TiN/Si and Au DNFs/TiN/Si samples validate that the synthesis process is accurately controlled. Under the reaction conditions in this study, the thickness of the Au DNFs increases linearly to 5.10 &#177; 0.20 &#181;m within 15 min of the reaction. Therefore, the employed synthesis procedure is a simple and rapid approach for preparing Au DNFs/TiN/Si composites.

This study presents a feasible procedure for synthesizing gold dendritic nanoforests on titanium nitride/silicon substrates. The thickness of gold dendritic nanoforests increases linearly within 15 min of a synthesis reaction.

In this study, a high-power impulse magnetron sputtering system is used to coat a flat and firm titanium nitride (TiN) film on silicon (Si) wafers, and a ...

Gold Nanoparticle Synthesis

Gold nanoparticles (Au nanoparticles) that are ~12 nm in diameter were synthesized by rapidly injecting a solution of 150 mg (0.15 mmol) of tetrachloroauric acid in 3.0 g (3.7 mmol, 3.6 mL) of oleylamine (technical grade) and 3.0 mL of toluene into a boiling solution of 5.1 g (6.4 mmol, 8.7 mL) of oleylamine in 147 mL of toluene. While boiling and mixing the reaction solution for 2 hours, the color of the reaction mixture changed from clear, to light yellow, to light pink, and then slowly to dark red. The heat was then turned off, and the solution was allowed to gradually cool down to room temperature for 1 hour. The gold nanoparticles were then collected and separated from the solution using a centrifuge and washed three times; by vortexing and dispersing the gold nanoparticles in 10 mL portions of toluene, and then precipitating the gold nanoparticles by adding 40 mL portions of methanol and spinning them in a centrifuge. The solution was then decanted to remove any remaining byproducts and unreacted starting materials. Drying the gold nanoparticles in a vacuum environment produced a solid black pellet; which could be stored for long periods of time (up to one year) for later use, and then redissolved in organic solvents such as toluene.

A protocol for synthesizing ~12 nm diameter gold nanoparticles (Au nanoparticles) in an organic solvent is presented. The gold nanoparticles are capped with oleylamine ligands to prevent agglomeration. The gold nanoparticles are soluble in organic solvents such as toluene.

Gold nanoparticles (Au nanoparticles) that are ~12 nm in diameter were synthesized by rapidly injecting a solution of 150 mg (0.15 mmol) of ...

Versatile Technique to Produce a Hierarchical Design in Nanoporous Gold

The potential to generate variable pore sizes, simplistic surface modification, and a breadth of commercial uses in the fields of biosensors, actuators, drug loading and release, and the development of catalysts have unquestionably accelerated the usage of nanoporous gold (NPG)-based nanomaterials in research and development. This article describes the process of the generation of hierarchical bimodal nanoporous gold (hb-NPG) by employing a step-wise procedure involving electrochemical alloying, chemical dealloying techniques, and annealing to create both macro- and mesopores. This is done to improve the utility of NPG by creating a bicontinuous solid/void morphology. The area available for surface modification is enhanced by smaller pores, while molecular transport benefits from the network of larger pores. The bimodal architecture, which is the result of a series of fabrication steps, is visualized using scanning electron microscopy (SEM) as a network of pores that are less than 100 nm in size and connected by ligaments to larger pores that are several hundred nanometers in size. The electrochemically active surface area of the hb-NPG is assessed using cyclic voltammetry (CV), with a focus on the critical roles that both dealloying and annealing play in creating the necessary structure. The adsorption of different proteins is measured by solution depletion technique, revealing the better performance of hb-NPG in terms of protein loading. By changing the surface area to volume ratio, the created hb-NPG electrode offers tremendous potential for biosensor development. The manuscript discusses a scalable method to create hb-NPG surface structures, as they offer a large surface area for the immobilization of small molecules and improved transport pathways for faster reactions.

Nanoporous gold with a hierarchical and bimodal pore size distribution can be produced by combining electrochemical and chemical dealloying. The composition of the alloy can be monitored via EDS-SEM examination as the dealloying process advances. The material's loading capacity can be determined by studying protein adsorption onto the material.

The potential to generate variable pore sizes, simplistic surface modification, and a breadth of commercial uses in the fields of biosensors, actuators, ...

Large Area Substrate-Based Nanofabrication of Controllable and Customizable Gold Nanoparticles Via Capped Dewetting

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