The authors are grateful for the financial support from Ministry of Science and Technology.

1. Preparation of Silver Precursor Ink
<ol>
	<li>Add 1.85 g of diethanolamine (DEA) in 3.15 ml deionized water.</li>
	<li>Dissolve 0.15 g of silver nitrate in 5 ml deionized water.</li>
	<li>Mix the aqueous silver nitrate solution with DEA at a 1:1 volume ratio to have 10 ml silver precursor ink right before use.</li>
</ol>
2. Fabrication of Stretchable Conductive Thin Films
<ol>
	<li>Preparation of AgNW ink
	<ol>
		<li>Dilute 2 ml of 0.5 wt% AgNWs in isopropanol with 18 ml deionized water.</li>
		<li>Place it in ultrasonic bath for 30 sec at 25 &#176;C.</li>
	</ol>
	</li>
	<li>Fabrication of AgNW thin films by auto-spray coating
	<ol>
		<li>Cut standard microscope slides into pieces of size equal to 1&#215;2.5 cm2. Prepare 16 glass pieces of this size and clean them with ethanol-wetted lens cleaning tissue.</li>
		<li>Transfer 16 ml AgNW ink (from section 1) into the paint cup of the airbrush with a pipette. Mount the airbrush on a computer-controlled robot for spray coating.</li>
		<li>Place 8 of glass pieces in a 4&#215;2 arrangement on the stage and fix them with heat-resistant tapes. The total area of all glass substrates on the stage is 4&#215;5 cm2.</li>
		<li>Set working pressure and heating stage temperature at 3 bar and 100 &#176;C, separately.</li>
		<li>Open the robot control software. Click to select brush motion command sequence under the &#34;Command&#34; column. Type in necessary input parameters to complete the auto-spraying program as shown in Figure 1. Run the program. 
		Note: The &#34;Line Speed&#34; command makes the airbrush travel at 200 mm/sec. By the &#34;Brush Area&#34; command, the airbrush moves back and forth in the direction of the short side of the glass substrate array while the stage moves along the direction of the long side and the space between two strokes is 5 mm. The &#34;Line Start&#34; and &#34;Line End&#34; commands determine the positions of starting and end points of the auto-spraying operation. The positions of them depend on the position of the glass substrate array on the stage. The &#34;Wait Point&#34; command sets a waiting time of 20 sec in the end of each auto-spraying cycle. &#34;Loop Address&#34; command enables multiple spray cycles and the number of auto-spraying cycles is 15 times. More detailed instruction of the commands can be found in the manufacturer&#39;s protocol.</li>
		<li>Change the number of auto-spraying cycles into 30 times. Repeat steps 2.2.3 - 2.2.5 to fabricate AgNW thin films of 30 spraying cycles.</li>
		<li>After spray coating, bake silver nanowire thin films on a hot plate at 120 &#176;C for 10 min.</li>
	</ol>
	</li>
	<li>&#160;Chemical soldering process
	<ol>
		<li>Cast 400 &#956;l of silver precursor ink over each spray-coated silver nanowire thin film on the glass substrate.</li>
		<li>Bake the films on a hot plate at 100 &#176;C for 40 min.</li>
		<li>Rinse the reactive coatings carefully with deionized water to remove unreactive chemical residues and air-dry the coated films.</li>
	</ol>
	</li>
	<li>Cast-peeling process
	<ol>
		<li>&#160;Cast 200 &#956;l commercially available water-based PU emulsion over each silver nano-composite thin film on the glass substrate.</li>
		<li>Air-dry the films for 10 hr to ensure full solidification.</li>
		<li>Peel off the samples from the glass substrates as free standing composite films.</li>
	</ol>
	</li>
</ol>
3. Characterization
<ol>
	<li>&#160;Stretching test
	<ol>
		<li>Turn on the linear motorized stage and wait 10 min for the machine to warm up.</li>
		<li>Open the stage control software. Set the number of the moving steps of the motor as 8,000. Click &#34;X+&#34; in the stage control software to move the mobile stage until it touches the fixed stage and click &#34;SET 0&#34; to set the position of the mobile stage as zero in the stage control software. 
		Note: The mobile stage moves 0.00125 mm in one step of the motor. For example, the mobile stage moves 1 cm if the motor moves 8,000 steps. The plus sign of &#34;X+&#34; means that the mobile stage moves in the direction of approaching fixed stage while the negative sign of &#34;X-&#34; means moving away from the fixed stage.</li>
		<li>Click &#34;X-&#34; to move the mobile stage to leave 1 cm space between the mobile and fixed stage. Secure both ends of the sample with wired holders onto the stages. Thereby, the stretching area of the sample is 1&#215;1 cm2. The setup of stretching machine is shown in Figure 2.</li>
		<li>Use the alligator clips, which are the other ends of the wiring cables to the stage holders (Figure 2), to connect to the digital multimeter for resistance measurements.</li>
		<li>Set the number of the moving steps of the motor as 800. Click &#34;X-&#34; to move the mobile stage 1 mm (10% strain) away from the fixed stage to stretch the sample and record the resistance. Repeat this step until the resistance increases significantly (~150% strain).</li>
	</ol>
	</li>
	<li>Stability test
	<ol>
		<li>Prepare the test as in steps 3.1.2 - 3.1.4.</li>
		<li>Open the digital multimeter software. Connect the digital multimeter to the computer. Long press the &#34;REL &#916;&#34; button on the digital multimeter until a computer icon appears in the upper left corner of the digital multimeter monitor. Click &#34;USB connection&#34; in the digital multimeter software and the software starts to record measured resistances.</li>
		<li>Enter commands into the input fields in the program panel of the stage control software as shown in Figure 3. The command &#34;:U-4000&#34; means to move the mobile stage 4,000 steps away from the fixed stage while the command &#34;:U4000&#34; means to move the mobile stage 4,000 steps back to the fixed stage (4,000 for 50% strain, 8,000 for 100% strain). The number of stretching cycles is 15 times. The default speed of the mobile stage is 1 mm/sec.</li>
		<li>Click &#34;Run 123&#34; in the program panel of the stage control software to execute the automatic program. The mobile stage moves in a reciprocating motion to stretch the sample with elongation cycles of triangular waveform.</li>
		<li>Click the save icon in the multimeter software and export the data of the resistance response profiles as an .xls file.</li>
	</ol>
	</li>
	<li>LED lighting test
	<ol>
		<li>Prepare the test as in steps 3.1.2 - 3.1.3. Connect the wired holders in series with a LED and a power supply.</li>
		<li>Turn on the power supply. Increase the voltage to 9 V to light the LED.</li>
		<li>Click &#34;X-&#34; to move the mobile stage 1 mm (10% strain) away from the fixed stage to stretch the sample and take a picture to record the brightness of the LED. Repeat this step until the light of LED becomes dim. Be careful that the auto-exposure of the camera should be turned off while taking pictures.</li>
	</ol>
	</li>
</ol>

<ol>
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Mater. 23 (27), 3052-3056 (2011).</li><li>Sekitani, T., Someya, T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Stretchable,+Large-area+Organic+Electronics.">Stretchable, Large-area Organic Electronics.</a> Adv. Mater. 22 (20), 2228-2246 (2010).</li><li>Lipomi, D. J., Tee, B. C., Vosgueritchian, M., Bao, Z. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Stretchable+organic+solar+cells.">Stretchable organic solar cells.</a> Adv. Mater. 23 (15), 1771-1775 (2011).</li><li>Liang, J., Li, L., Niu, X., Yu, Z., Pei, Q. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Elastomeric+polymer+light-emitting+devices+and+displays.">Elastomeric polymer light-emitting devices and displays.</a> Nat. 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The authors have nothing to disclose.

The chemical soldering process can help reinforce the contact between silver nanowires. As shown in Figure 4b, the wire/wire junctions are covered with silver after applying the reactive silver ink over the spray coated AgNW thin film. The silver recovery relies strongly on the formaldehyde generated from DEA degradation, and thus the soldering process or silver reduction can be accelerated with increasing temperature.22 Because the metal surfaces of AgNWs provide effective electron exchange sites, silver nanoparticles are preferentially reduced along the AgNW surfaces, and wrap over the wire-wire junctions to form amalgamated contacts. This chemical soldering process, however, needs a rinsing process to remove the excessive reactive solutions afterwards. One needs to rinse the solder AgNW thin films slowly and carefully due to the low adhesion between AgNWs and glass before drying. After rinsing and drying, the AgNW thin films can be easily transferred to PU by the cast-and-peel method. With a mold or spacer on top of the glass, one can also easily adjust the thickness of the PU layers in the composite stretchable conductors.
The composite stretchable conductors show great electrical conductivity at high strain conditions regardless of the amounts of spray coated AgNWs. As a result of the firmly joined nanowire contacts, the AgNW network remains intact under stretching conditions and provide great electron transfer paths. As shown in Figure 5, the sheet resistances of unsoldered stretchable conductors increase rapidly as the applied strains increase because the elongation of the conductors leads to the dislocation of the unsoldered wire-wire junctions and reduce the conductive paths of AgNW network. On the other hand, the sheet resistances of soldered stretchable conductors remain as low as ~100 &#937;/sq at a large strain of 120%, indicating that reduced silver nanoparticles do anneal the contacts between AgNWs and enhance the connectivity of AgNWs to prevent the dislocation of the AgNW mesh. Once an effective percolating AgNW network is formed, the composite stretchable conductors can exhibit high conductivity regardless of deposited AgNW amounts. As evidenced in the resistance-strain curves in Figure 5, the stretchable conductor with half AgNW amount (15 vs. 30 spraying cycles) shows virtually the same electrical conductivity under stretching process, indicating that the amount of AgNWs has negligible effects on conductivity after chemical soldering. As a result, an even lower AgNW amount can be used as long as the initial spray coated AgNW thin films are conductive before the chemical soldering process.
The stretchable conductors show fairly good mechanical stability even under large dynamic strains with fast stretching rates. As shown in Figure 6, the soldered AgNW mesh network remain intact when one applies a triangular elongation cycles with a large strain of 50% deformation. Thus, the electrical resistance of the stretchable conductor remains nearly unchanged with a very small variation of ~5 &#937;/sq, which is consistent with the resistance change (~4 &#937;/sq) under a static strain of 50%. However, when an even large strain amplitude (100%) is applied, large resistance variation is observed, indicating structural changes in the AgNW network integrity in the pulsating process. The dynamic resistance variation is much larger than that in static stretching. Compared with the data in Figure 5, the static resistance is about 25 &#937;/sq at 100% strain while the dynamic resistance is 90 &#937;/sq at first peak and increases up to 400 &#937;/sq in the dynamic pulsation. At the large strain of 100%, the sample cannot sustain the dynamic stress and some structure of the soldered Ag nanowire network may be damaged, leading to the inconsistency between the sheet resistance under static and dynamic strains. Moreover, while stretching direction reverses, where peak resistance occurs, an instantaneous large velocity acceleration is applied by the mobile stage and can lead to further structural damages, which reflect in the increases in the dynamic peak resistances. Besides, parts of the difference in the static and dynamic resistances may come from possible temporary dislocations between AgNWs and the PU substrates under dynamic strains. Those dislocation can be restored, as evidenced by the fact that the resistance returns to the original value after the pulsation stops. Thus, to avoid resistance variation, one needs to be carefully evaluate the adhesion and mechanical compatibility between AgNW mesh and the elastic substrates.
The stretchable conductors can serve directly as an ideal elastic interconnects in many electronic applications. Figure 7 shows the dynamic observation when a stretchable conductor is connected with a LED in series. When supplied with a constant voltage, the brightness of the LED light remains almost unchanged even with a strain up to 110%. This synthesized composite stretchable conductor can be easily applied in any electrical devices as elastic conductive tracks. However, to further implementation in real electrical devices, one needs miniaturized electrode patterns for complete circuitry. Thus, more research on creating stretchable circuits with printing techniques is still underway.
In summary, this work presents a simple method to fabricate highly stretchable conductors at low temperatures. The chemically soldered AgNW networks on PU can accommodate substantial elastic strains and exhibit excellent electrical conductivity as well as mechanical stability in stretching process. Moreover, the chemically soldered conductors show nearly identical electric and mechanical performances regardless of the amount of AgNWs, indicating a possible reduction in material waste. We believe that this stretchable conductor material can directly serve as effective interconnects in wearable and stretchable optoelectronic devices, such as LED and solar cells, for the next generation electronics.

Deformable electronic devices with large stretchability have been identified as critical parts to the realization of wearable and portable electronics in the next generation.1 Those stretchable electronic devices not only show great flexibility as those electronic devices on plastic sheets,2, 3 but also exhibit excellent performance under severe stretching or twisting conditions.4 To realize the stretchable electronics, materials with great electric performance under large deformation is needed. Recent advancements in material sciences have shown the possibility to synthesize such functional materials and have used them to design stretchable optoelectronic devices 5-9 with great tolerance to complex shape deformations. Among all the electronic functional materials, stretchable conductors are necessary to supply electrical power to those optoelectronic devices and thus are of critical importance for device performance. Because regular conducting materials, like metal or indium tin oxide, lack of mechanical robustness under large deformation, interconnects made of these materials are unable to exhibit good electrical conductivity under stretching process. Thus, elastic substrates covered with a thin layer of flexible conductive materials, such as carbon nanotube,1 graphene,10 or AgNWs,11-14 are designed for conductors with excellent stretchability. Because of the high bulk conductivity, AgNW thin films have been shown to be the most promising material for composite stretchable conductors.13 The percolating networks of AgNW thin films can effectively accommodate large elastic deformations in stretching process with great electrical conductance, and are considered as a promising stretchable electrode candidate. To implement AgNW thin films as stretchable conductors, it is necessary to have effective electrical contacts between AgNWs. After liquid deposition and drying on substrate surfaces, AgNWs regularly stack together to form a percolating mesh with loose contact points, which yield in large electrical resistances. Thus, one needs to anneal the contacts between nanowires by high-temperature or high-pressure annealing methods15-20 to reduce the contact resistances.
In contrast to these annealing processes in the literature, here, we will demonstrate a simple chemical method to anneal AgNW network connections under regular laboratory conditions.21 The fabrication process is shown in Figure 4A. A reactive ink is used to sinter the spray coated AgNW thin films on a glass plate. After reaction, the contacts between nanowires are covered with silver and hence the AgNW network is soldered chemically together. A cast-and-peel method is then used to transfer the soldered AgNW network to a stretchable PU substrate to form a composite conductor, which can exhibit no obvious change in electrical conductivity even at large tensile strain of 120%.

<table border="0" cellpadding="0" cellspacing="0" width="833">
	<tbody>
		<tr height="21">
			<td height="21" style="height:21px;width:173px;">Silver nanowire</td>
			<td style="width:195px;">Sigma-Aldrich</td>
			<td style="width:117px;">778095-25ML</td>
			<td style="width:349px;">AgNW, 120 nm in diameter and 20-50 mm in length, 0.5 wt% in IPA</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Silver nitrate crystal</td>
			<td>Macron Fine Chemicals</td>
			<td>MK216903</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Diethanolamine</td>
			<td>Sigma-Aldrich</td>
			<td>D8885-500G</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Polyurethane emulsion</td>
			<td>First Chemical</td>
			<td>20130326036</td>
			<td>35 wt% water-based anionic polyester-polyurethane emulsion</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Airbrush</td>
			<td>Taiwan Airbrush &#38; Equipment</td>
			<td>AFC-sensor&#160;</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Desktop robot</td>
			<td>Dispenser Tech</td>
			<td>DT-200&#160;</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Digital dispenser controller</td>
			<td>Dispenser Tech</td>
			<td>9000E&#160;</td>
			<td></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:173px;">Auto-spraying program</td>
			<td style="width:195px;">Dispenser Tech</td>
			<td style="width:117px;">Smart robot edit version 3.0.0.5</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Air compressor&#160;</td>
			<td>PUMA Industrial</td>
			<td>NCS-10&#160;</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Linear motorized stage</td>
			<td>TANLIAN E-O</td>
			<td>Customized</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:173px;">Stage control software</td>
			<td style="width:195px;">TANLIAN E-O</td>
			<td style="width:117px;">Customized</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Digital multimeter</td>
			<td>HILA INTERNATIONAL</td>
			<td>DM-2690TU</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:173px;">Digital multimeter software</td>
			<td style="width:195px;">HILA INTERNATIONAL</td>
			<td style="width:117px;">NA</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Power supply</td>
			<td>CHERN TAIH</td>
			<td>CT-605</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">LED</td>
			<td>PChome</td>
			<td>M08330766</td>
			<td>http://www.pcstore.com.tw/sun-flower/M08330766.htm</td>
		</tr>
	</tbody>
</table>

a fabrication method for highly stretchable conductors with silver nanowires

Stretchable electronics are identified as a key technology for electronic applications in the next generation. One of the challenges in fabrication of stretchable electronic devices is the preparation of stretchable conductors with great mechanical stability. In this study, we developed a simple fabrication method to chemically solder the contact points between silver nanowire (AgNW) networks. AgNW nanomesh was first deposited on a glass slide via spray coating method. A reactive ink composed of silver nanoparticle (AgNPs) precursors was applied over the spray coated AgNW thin films. After heating for 40 min, AgNPs were preferentially generated over the nanowire junctions to solder the AgNW nanomesh, and reinforced the conducting network. The chemically modified AgNW thin film was then transferred to polyurethane (PU) substrates by casting method. The soldered AgNW thin films on PU exhibited no obvious change in electrical conductivity under stretching or rolling process with elongation strains up to 120%.

The morphology of the AgNW thin film after chemical soldering process is shown in Figure 4B. Recovered AgNPs preferentially grow on the surface of AgNWs and wrap over the wire/wire junctions. Figure 5 shows the variation in sheet resistance with applied elongation strains for the unsoldered and the soldered thin films containing different amount of AgNWs. After the chemical soldering process, AgNW thin film conductors can maintain high conductivity under high strain conditions, regardless of the sprayed amount of AgNWs. Both soldered AgNW thin films show sheet resistance below 100 &#937;/sq when strains below 120% are applied. The composite stretchable conducting thin films show great mechanical stability in dynamic deformation process. Figure 6 shows the resistance variations of the stretchable conductor under elongation cycles of a triangular waveform at a fast strain rate of 0.05 sec-1. No obvious resistance change is observed with a strain amplitude of 50%. When the strain amplitude increases to 100% strain, the peak resistance increases with the number of the pulsation cycles, and the resistance of the film returns to the original value after the pulsation stops. Figure 7 demonstrates an electronic application of the chemically soldered thin films.
<img alt="Figure 1" src="/files/ftp_upload/53623/53623fig1.jpg" /> 
Figure 1. Screenshot of the robot control software. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/53623/53623fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" src="/files/ftp_upload/53623/53623fig2.jpg" /> 
Figure 2. Stretching machine configuration. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/53623/53623fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" src="/files/ftp_upload/53623/53623fig3.jpg" /> 
Figure 3. Screenshot of the stage control software. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/53623/53623fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" src="/files/ftp_upload/53623/53623fig4.jpg" /> 
Figure 4. Schematic diagram of fabrication process for the highly stretchable and conductive metal conductors. (A) The samples are prepared as indicated in the figure. (B) SEM images of chemical-soldered AgNW thin film before being transferred to PU substrate. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/53623/53623fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 5" src="/files/ftp_upload/53623/53623fig5.jpg" /> 
Figure 5. Stretching test. Comparison of electric resistances of unsoldered and soldered AgNW thin films with various sprayed AgNW amounts under stretching conditions. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/53623/53623fig5large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 6" src="/files/ftp_upload/53623/53623fig6.jpg" /> 
Figure 6. Stability test. Resistance response profiles of stretching films under elongation cycles of triangular waveform. The pulsating strain rate is 10 sec-1. The tested sample is made from AgNW thin films with 15 spraying cycles. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/53623/53623fig6large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 7" src="/files/ftp_upload/53623/53623fig7.jpg" /> 
Figure 7. Video of LED light connected with a stretchable conductor under various strain conditions. The brightness change of LED caused by the stretched conductor is shown in the figure. The testing sample is made from AgNW thin films with 15 spraying cycles. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/53623/Figure7.mp4">(Right click to download)</a>

Watch this Scientific Journal Video about A Fabrication Method for Highly Stretchable Conductors with Silver Nanowires at JoVE.com

A Fabrication Method for Highly Stretchable Conductors with Silver Nanowires

A simple synthesis method is used to chemically solder silver nanowire thin film to fabricate highly stretchable and conductive metal conductors.

Stretchable electronics are identified as a key technology for electronic applications in the next generation. One of the challenges in fabrication of ...

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9.8K Views.  National Taiwan University. A simple synthesis method is used to chemically solder silver nanowire thin film to fabricate highly stretchable and conductive metal conductors.

Video: A Fabrication Method for Highly Stretchable Conductors with Silver Nanowires

A Method to Fabricate Disconnected Silver Nanostructures in 3D

The standard nanofabrication toolkit includes techniques primarily aimed at creating 2D patterns in dielectric media. Creating metal patterns on a submicron scale requires a combination of nanofabrication tools and several material processing steps. For example, steps to create planar metal structures using ultraviolet photolithography and electron-beam lithography can include sample exposure, sample development, metal deposition, and metal liftoff. To create 3D metal structures, the sequence is repeated multiple times. The complexity and difficulty of stacking and aligning multiple layers limits practical implementations of 3D metal structuring using standard nanofabrication tools. Femtosecond-laser direct-writing has emerged as a pre-eminent technique for 3D nanofabrication.1,2 Femtosecond lasers are frequently used to create 3D patterns in polymers and glasses.3-7 However, 3D metal direct-writing remains a challenge. Here, we describe a method to fabricate silver nanostructures embedded inside a polymer matrix using a femtosecond laser centered at 800 nm. The method enables the fabrication of patterns not feasible using other techniques, such as 3D arrays of disconnected silver voxels.8 Disconnected 3D metal patterns are useful for metamaterials where unit cells are not in contact with each other,9 such as coupled metal dot10,11or coupled metal rod12,13 resonators. Potential applications include negative index metamaterials, invisibility cloaks, and perfect lenses.
In femtosecond-laser direct-writing, the laser wavelength is chosen such that photons are not linearly absorbed in the target medium. When the laser pulse duration is compressed to the femtosecond time scale and the radiation is tightly focused inside the target, the extremely high intensity induces nonlinear absorption. Multiple photons are absorbed simultaneously to cause electronic transitions that lead to material modification within the focused region. Using this approach, one can form structures in the bulk of a material rather than on its surface.
Most work on 3D direct metal writing has focused on creating self-supported metal structures.14-16 The method described here yields sub-micrometer silver structures that do not need to be self-supported because they are embedded inside a matrix. A doped polymer matrix is prepared using a mixture of silver nitrate (AgNO3), polyvinylpyrrolidone (PVP) and water (H2O). Samples are then patterned by irradiation with an 11-MHz femtosecond laser producing 50-fs pulses. During irradiation, photoreduction of silver ions is induced through nonlinear absorption, creating an aggregate of silver nanoparticles in the focal region. Using this approach we create silver patterns embedded in a doped PVP matrix. Adding 3D translation of the sample extends the patterning to three dimensions.

Femtosecond-laser direct-writing is frequently used to create three-dimensional (3D) patterns in polymers and glasses. However, patterning metals in 3D remains a challenge. We describe a method for fabricating silver nanostructures embedded inside a polymer matrix using a femtosecond laser centered at 800 nm.

The standard nanofabrication toolkit includes  techniques primarily aimed at creating 2D patterns in dielectric media. Creating  metal patterns on a ...

Monolayer Contact Doping of Silicon Surfaces and Nanowires Using Organophosphorus Compounds

Monolayer Contact Doping (MLCD) is a simple method for doping of surfaces and nanostructures1. MLCD results in the formation of highly controlled, ultra shallow and sharp doping profiles at the nanometer scale. In MLCD process the dopant source is a monolayer containing dopant atoms.
In this article a detailed procedure for surface doping of silicon substrate as well as silicon nanowires is demonstrated. Phosphorus dopant source was formed using tetraethyl methylenediphosphonate monolayer on a silicon substrate. This monolayer containing substrate was brought to contact with a pristine intrinsic silicon target substrate and annealed while in contact. Sheet resistance of the target substrate was measured using 4 point probe. Intrinsic silicon nanowires were synthesized by chemical vapor deposition (CVD) process using a vapor-liquid-solid (VLS) mechanism; gold nanoparticles were used as catalyst for nanowire growth. The nanowires were suspended in ethanol by mild sonication. This suspension was used to dropcast the nanowires on silicon substrate with a silicon nitride dielectric top layer. These nanowires were doped with phosphorus in similar manner as used for the intrinsic silicon wafer. Standard photolithography process was used to fabricate metal electrodes for the formation of nanowire based field effect transistor (NW-FET). The electrical properties of a representative nanowire device were measured by a semiconductor device analyzer and a probe station.

A detailed procedure for surface doping of Silicon interfaces is provided. The ultra-shallow surface doping is demonstrated by using phosphorus containing monolayers and rapid annealing process. The method can be used for doping of macroscopic area surfaces as well as nanostructures.

Monolayer Contact Doping (MLCD) is a simple method for doping of surfaces and nanostructures1. MLCD results in the formation of highly controlled, ultra ...

Evaluating Plasmonic Transport in Current-carrying Silver Nanowires

Plasmonics is an emerging technology capable of simultaneously transporting a plasmonic signal and an electronic signal on the same information support1,2,3. In this context, metal nanowires are especially desirable for realizing dense routing networks4. A prerequisite to operate such shared nanowire-based platform relies on our ability to electrically contact individual metal nanowires and efficiently excite surface plasmon polaritons5 in this information support. In this article, we describe a protocol to bring electrical terminals to chemically-synthesized silver nanowires6 randomly distributed on a glass substrate7. The positions of the nanowire ends with respect to predefined landmarks are precisely located using standard optical transmission microscopy before encapsulation in an electron-sensitive resist. Trenches representing the electrode layout are subsequently designed by electron-beam lithography. Metal electrodes are then fabricated by thermally evaporating a Cr/Au layer followed by a chemical lift-off. The contacted silver nanowires are finally transferred to a leakage radiation microscope for surface plasmon excitation and characterization8,9. Surface plasmons are launched in the nanowires by focusing a near infrared laser beam on a diffraction-limited spot overlapping one nanowire extremity5,9. For sufficiently large nanowires, the surface plasmon mode leaks into the glass substrate9,10. This leakage radiation is readily detected, imaged, and analyzed in the different conjugate planes in leakage radiation microscopy9,11. The electrical terminals do not affect the plasmon propagation. However, a current-induced morphological deterioration of the nanowire drastically degrades the flow of surface plasmons. The combination of surface plasmon leakage radiation microscopy with a simultaneous analysis of the nanowire electrical transport characteristics reveals the intrinsic limitations of such plasmonic circuitry.

Silver nanowires can simultaneously transport electrons and optical information in the form of surface plasmons. A procedure is described here to realize such a shared circuitry and the limitations at propagating both information carriers are evaluated.

Plasmonics is an emerging technology capable of simultaneously transporting a plasmonic signal and an electronic signal on the same information ...

Preparation and Use of Photocatalytically Active Segmented Ag|ZnO and Coaxial TiO2-Ag Nanowires Made by Templated Electrodeposition

Photocatalytically active nanostructures require a large specific surface area with the presence of many catalytically active sites for the oxidation and reduction half reactions, and fast electron (hole) diffusion and charge separation. Nanowires present suitable architectures to meet these requirements. Axially segmented Ag|ZnO and radially segmented (coaxial) TiO2-Ag nanowires with a diameter of 200 nm and a length of 6-20 &#181;m were made by templated electrodeposition within the pores of polycarbonate track-etched (PCTE) or anodized aluminum oxide (AAO) membranes, respectively. In the photocatalytic experiments, the ZnO and TiO2 phases acted as photoanodes, and Ag as cathode. No external circuit is needed to connect both electrodes, which is a key advantage over conventional photo-electrochemical cells. For making segmented Ag|ZnO nanowires, the Ag salt electrolyte was replaced after formation of the Ag segment to form a ZnO segment attached to the Ag segment. For making coaxial TiO2-Ag nanowires, a TiO2 gel was first formed by the electrochemically induced sol-gel method. Drying and thermal annealing of the as-formed TiO2 gel resulted in the formation of crystalline TiO2 nanotubes. A subsequent Ag electrodeposition step inside the TiO2 nanotubes resulted in formation of coaxial TiO2-Ag nanowires. Due to the combination of an n-type semiconductor (ZnO or TiO2) and a metal (Ag) within the same nanowire, a Schottky barrier was created at the interface between the phases. To demonstrate the photocatalytic activity of these nanowires, the Ag|ZnO nanowires were used in a photocatalytic experiment in which H2 gas was detected upon UV illumination of the nanowires dispersed in a methanol/water mixture. After 17 min&#160;of illumination, approximately 0.2 vol% H2 gas was detected from a suspension of ~0.1 g of Ag|ZnO nanowires in a 50 ml 80 vol% aqueous methanol solution.

Preparation and Use of Photocatalytically Active Segmented Ag|ZnO and Coaxial TiO2-Ag Nanowires Made by Templated Electrodeposition

Procedures are outlined to prepare segmented and coaxial nanowires via templated electrodeposition in nanopores. As examples, segmented nanowires consisting of Ag and ZnO segments, and coaxial nanowires consisting of a TiO2 shell and a Ag core were made. The nanowires were used in photocatalytic hydrogen formation experiments.

Photocatalytically active nanostructures require a large specific surface area with the presence of many catalytically active sites for the oxidation and ...

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 ...

Fabrication Process of Silicone-based Dielectric Elastomer Actuators

This contribution demonstrates the fabrication process of dielectric elastomer transducers (DETs). DETs are stretchable capacitors consisting of an elastomeric dielectric membrane sandwiched between two compliant electrodes. The large actuation strains of these transducers when used as actuators (over 300% area strain) and their soft and compliant nature has been exploited for a wide range of applications, including electrically tunable optics, haptic feedback devices, wave-energy harvesting, deformable cell-culture devices, compliant grippers, and propulsion of a bio-inspired fish-like airship. In most cases, DETs are made with a commercial proprietary acrylic elastomer and with hand-applied electrodes of carbon powder or carbon grease. This combination leads to non-reproducible and slow actuators exhibiting viscoelastic creep and a short lifetime. We present here a complete process flow for the reproducible fabrication of DETs based on thin elastomeric silicone films, including casting of thin silicone membranes, membrane release and prestretching, patterning of robust compliant electrodes, assembly and testing. The membranes are cast on flexible polyethylene terephthalate (PET) substrates coated with a water-soluble sacrificial layer for ease of release. The electrodes consist of carbon black particles dispersed into a silicone matrix and patterned using a stamping technique, which leads to precisely-defined compliant electrodes that present a high adhesion to the dielectric membrane on which they are applied.

This manuscript shows the fabrication process for the manufacture of dielectric elastomer soft actuators based on silicone membranes. The three key stages of production are presented in detail: blade casting of thin silicone membranes; pad printing of compliant electrodes; and the assembly of all the components.

This contribution demonstrates the fabrication process of dielectric elastomer transducers (DETs). DETs are stretchable capacitors consisting of an ...

Solvent Bonding for Fabrication of PMMA and COP Microfluidic Devices

Thermoplastic microfluidic devices offer many advantages over those made from silicone elastomers, but bonding procedures must be developed for each thermoplastic of interest. Solvent bonding is a simple and versatile method that can be used to fabricate devices from a variety of plastics. An appropriate solvent is added between two device layers to be bonded, and heat and pressure are applied to the device to facilitate the bonding. By using an appropriate combination of solvent, plastic, heat, and pressure, the device can be sealed with a high quality bond, characterized as having high bond coverage, bond strength, optical clarity, durability over time, and low deformation or damage to microfeature geometry. We describe the procedure for bonding devices made from two popular thermoplastics, poly(methyl-methacrylate) (PMMA), and cyclo-olefin polymer (COP), as well as a variety of methods to characterize the quality of the resulting bonds, and strategies to troubleshoot low quality bonds. These methods can be used to develop new solvent bonding protocols for other plastic-solvent systems.

Solvent bonding is a simple and versatile method for fabricating thermoplastic microfluidic devices with high quality bonds. We describe a protocol to achieve strong, optically clear bonds in PMMA and COP microfluidic devices that preserve microfeature details, by a judicious combination of pressure, temperature, an appropriate solvent, and device geometry.

Thermoplastic microfluidic devices offer many advantages over those made from silicone elastomers, but bonding procedures must be developed for each ...

Challenges in Rheological Characterization of Highly Concentrated Suspensions &#8212; A Case Study for Screen-printing Silver Pastes

A comprehensive rheological characterization of highly concentrated suspensions or pastes is mandatory for a targeted product development meeting the manifold requirements during processing and application of such complex fluids. In this investigation, measuring protocols for a conclusive assessment of different process relevant rheological parameters have been evaluated. This includes the determination of yield stress, viscosity, wall slip velocity, structural recovery after large deformation and elongation at break as well as tensile force during filament stretching.
The importance of concomitant video recordings during parallel-plate rotational rheometry for a significant determination of rheological quantities is demonstrated. The deformation profile and flow field at the sample edge can be determined using appropriate markers. Thus, measurement parameter settings and plate roughness values can be identified for which yield stress and viscosity measurements are possible. Slip velocity can be measured directly and measuring conditions at which plug flow, shear banding or sample spillover occur can be identified clearly. 
Video recordings further confirm that the change in shear moduli observed during three stage oscillatory shear tests with small deformation amplitude in stage I and III but large oscillation amplitude in stage II can be directly attributed to structural break down and recovery. For the pastes investigated here, the degree of irreversible, shear-induced structural change increases with increasing deformation amplitude in stage II until a saturation is reached at deformations corresponding to the crossover of G' and G'', but the irreversible damage is independent of the duration of large amplitude shear. 
A capillary breakup elongational rheometer and a tensile tester have been used to characterize deformation and breakup behavior of highly filled pastes in uniaxial elongation. Significant differences were observed in all experiments described above for two commercial screen-printing silver pastes used for front side metallization of Si-solar cells.

A protocol for a robust and application relevant rheological characterization of highly concentrated suspensions is presented. Silver pastes used for screen-printing application in solar cell production are employed as model systems.

A comprehensive rheological characterization of highly concentrated suspensions or pastes is mandatory for a targeted product development meeting the ...

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 ...

Flow-assisted Dielectrophoresis: A Low Cost Method for the Fabrication of High Performance Solution-processable Nanowire Devices

Flow-assisted dielectrophoresis (DEP) is an efficient self-assembly method for the controllable and reproducible positioning, alignment, and selection of nanowires. DEP is used for nanowire analysis, characterization, and for solution-based fabrication of semiconducting devices. The method works by applying an alternating electric field between metallic electrodes. The nanowire formulation is then dropped onto the electrodes which are on an inclined surface to create a flow of the formulation using gravity. The nanowires then align along the gradient of the electric field and in the direction of the liquid flow. The frequency of the field can be adjusted to select nanowires with superior conductivity and lower trap density. 
In this work, flow-assisted DEP is used to create nanowire field effect transistors. Flow-assisted DEP has several advantages: it allows selection of nanowire electrical properties; control of nanowire length; placement of nanowires in specific areas; control of orientation of nanowires; and control of nanowire density in the device. 
The technique can be expanded to many other applications such as gas sensors and microwave switches. The technique is efficient, quick, reproducible, and it uses a minimal amount of dilute solution making it ideal for the testing of novel nanomaterials. Wafer scale assembly of nanowire devices can also be achieved using this technique, allowing large numbers of samples for testing and large-area electronic applications.

In this paper, flow assisted dielectrophoresis is demonstrated for the self-assembly of nanowire devices. The fabrication of a silicon nanowire field effect transistor is shown as an example.

Flow-assisted dielectrophoresis (DEP) is an efficient self-assembly method for the controllable and reproducible positioning, alignment, and selection of ...