This work was supported by a grant from the RGC-NSFC Joint Fund sponsored by the Research Grants Council of Hong Kong (Project No. N_HKUST615/14). We would like to acknowledge Nanosystem Fabrication Facility (NFF) of HKUST for the device / system fabrication.

1. Fabrication of a Cr/Au adhesion layer using liftoff
<ol>
	<li>Spincoat HPR504 positive photoresist of 1.2 &#181;m thickness onto a quartz wafer using a spread speed of 1,000 rpm for 5 s and a spin speed of 4,000 rpm for 30 s.</li>
	<li>Softbake the photoresist on the quartz wafer at 110 &#176;C for 5 min on a hot plate.</li>
	<li>Using a mask aligner, expose the wafer such that locations for Cr/Au deposition are exposed with ultraviolet (UV) light. The exposure power density and time is 16 mW/cm2 and 7.5 s respectively (exposure energy density = 120 mJ/cm2).</li>
	<li>Develop the wafer by submersing it in positive resist developer FHD-5 for 1 min. Rinse the wafer with deionized (DI) water after the development process.</li>
	<li>Dry the wafer using a nitrogen (N2) gun. Put the wafer in an oven for 5 min at 120 &#176;C.</li>
	<li>Using electron beam (e-beam) evaporation, deposit a 5 nm Cr layer, followed by a 50 nm Au layer on the wafer. The deposition rates are 1 &#197;/s and 2 &#197;/s, respectively.</li>
	<li>Place the e-beam evaporated wafer into a container. Pour copious amount of acetone inside.</li>
	<li>Close the container using a lid. Place the lidded container in an ultrasonic cleaner for 10 min or until the liftoff process has completed.</li>
	<li>Flush the wafer using isopropanol (IPA) followed by DI water. Dry it using N2 gun and oven afterwards. 
	NOTE: The protocol can be paused here.</li>
</ol>
2. Fabrication of thin film Ag electrodes on the adhesion layer using liftoff
<ol>
	<li>Spincoat AZ P4620 positive photoresist of 7 &#181;m thickness onto the wafer using a spread speed of 1,000 rpm for 5 s and a spin speed of 4,000 rpm for 30 s.</li>
	<li>Softbake the photoresist on the wafer at 90 &#176;C for 450 s on a hot plate.</li>
	<li>Using a mask aligner, expose the wafer such that locations for Ag deposition are exposed with UV. The exposure power density and time is 16 mW/cm2 and 45 s respectively (exposure energy density = 720 mJ/cm2).</li>
	<li>Develop the wafer by submersing it in FHD-5 for 2 min. Rinse the wafer with DI water after the development process.</li>
	<li>Dry the wafer using a N2 gun. Put the wafer in an oven for 5 min at 120 &#176;C.</li>
	<li>Sputter a 1 &#181;m Ag layer on the wafer. The sputtering rate is ~86 nm/min.</li>
	<li>Place the sputtered wafer into a container. Pour copious amount of acetone inside.</li>
	<li>Close the container using a lid. Place the lidded container in an ultrasonic cleaner for 10 min or until the liftoff process has completed.</li>
	<li>Flush the wafer using IPA followed by DI water. Dry it using N2 gun and oven afterwards.</li>
</ol>
3. Passivation of the wafer to expose only the electrodes and contact pads
<ol>
	<li>Passivate the whole wafer surface with a 2 &#181;m silicon dioxide (SiO2) layer using plasma enhanced chemical vapor deposition (PECVD).
	<ol>
		<li>Passivate a small silicon dummy sample (a silicon wafer fragment) together the wafer simultaneously.</li>
		<li>Measure the thickness of the oxide layer of the dummy sample. 
		NOTE: The protocol can be paused here.</li>
	</ol>
	</li>
	<li>Spincoat AZ 5214E dual tone photoresist of 1.4 &#181;m thickness onto the wafer using a spread speed of 1000 rpm for 5 s and a spin speed of 3000 rpm for 30 s.</li>
	<li>Softbake the photoresist on the wafer at 90 &#176;C for 150 s on a hot plate.</li>
	<li>Using a mask aligner, expose the wafer such that the locations for pad opening are exposed with UV. The exposure power density and time is 16 mW/cm2 and 2.25 s respectively (exposure energy density = 36 mJ/cm2).</li>
	<li>Develop the wafer by submersing it in FHD-5 for 75 s. Rinse the wafer with DI water after the development process.</li>
	<li>After briefly drying the wafer using N2 gun, further dry and hard bake the wafer in an oven for 15 min at 120 &#176;C.</li>
	<li>Perform descum of photoresist on the wafer for 1 min using a plasma asher to ensure complete removal of unwanted photoresist.</li>
	<li>Perform reactive ion etching on the wafer and the dummy sample to expose the thin film electrodes and contact pads.
	<ol>
		<li>After performing the etching process for a short period of time (e.g., 5-10 min), stop the operation and take out the dummy sample.</li>
		<li>Measure the thickness of the oxide layer on top of the dummy sample. Compare it with the result obtained in step 3.1.2.</li>
		<li>Calculate the rate of SiO2 etching of the machine to fine tune the etching duration in order to achieve a 10% overetch.</li>
		<li>Continue the etching process without the dummy sample.</li>
	</ol>
	</li>
	<li>Resist strip the etched wafer by plasma ashing for 30 min, followed by a positive photoresist stripper MS2001 bath at 70 &#176;C for 5 min.</li>
	<li>Flush the wafer using DI water. Dry the wafer using N2 gun and oven. 
	NOTE: The protocol can be paused here.</li>
</ol>
4. Preparation for the fabrication of thin film Ag/AgCl electrodes (chip)
<ol>
	<li>Dice cut the wafer to obtain different testing chips.</li>
	<li>Polish the electrode surfaces on the chips using fine sandpaper.</li>
	<li>Bond the contact pads on the chip to an external printed circuit board for interfacing purposes in further steps.</li>
	<li>3D-print an acrylic hollow rectangular container to hold the electrolyte on the thin film electrodes. The dimensions of the rectangular container should allow placement of a wire and a pipette inside the void comfortably.</li>
	<li>Mix a small amount of polydimethylsiloxane (PDMS) prepolymer and its curing agent thoroughly. The ratio should be 10:1. 
	NOTE: It is very common to degas the PDMS mixture to obtain high quality PDMS devices; however, it is not needed in this case as the mixture is only used as an adhesive.</li>
	<li>Place the acrylic container on the diced chip in a fashion such that all the silver electrodes are inside the cavity of the container.
	<ol>
		<li>Using a toothpick or a fine rod, smear the uncured PDMS mixture on the outer edge where the container and the chip touch each other.</li>
		<li>Carefully place the chip on a flat hot plate and cure the PDMS for 2 h at 80 &#176;C or until the container is securely affixed onto the chip.</li>
	</ol>
	</li>
</ol>
5. Preparation for the fabrication of thin film Ag/AgCl electrodes (reagents)
<ol>
	<li>Using DI water and concentrated hydrochloric acid (HCl), obtain 0.01 M HCl solution.</li>
	<li>Using DI water and potassium chloride (KCl) powder, obtain 3.5 M KCl solution and 0.1 M KCl solution. 
	NOTE: The protocol can be paused here.</li>
</ol>
6. Preparation for the fabrication of thin film Ag/AgCl electrodes (macro electrodes)
<ol>
	<li>Cut some silver wires.</li>
	<li>Polish the surface of the silver wires with fine sandpaper.</li>
	<li>Submerse 80% of the silver wires into household bleach for 1 h. 
	NOTE: The color of the wire will change from silvery to dark purple. This shows the formation of AgCl on the surface of the silver wire.</li>
	<li>Flush the Ag/AgCl wire with DI water.</li>
	<li>Make an Ag/AgCl reference electrode using one of the Ag/AgCl wires referencing to Hassel et al. with modifications15. 
	NOTE: The modifications are using a pipette instead of a glass capillary, using 3.5 M KCl as the electrolyte, ditching the polymer block and the gold-plated connector and replace it with parafilm.</li>
	<li>Store the Ag/AgCl electrodes by submersing them in 3.5 M KCl solution. Make sure the silver part does not come in contact with the solution.
	<ol>
		<li>Cut several pieces of Ag/AgCl wires and put them into the KCl solutions mentioned in step 5.2. 
		NOTE: The protocol can be paused here.</li>
	</ol>
	</li>
</ol>
7. Cathodic cleaning of the micro Ag electrodes
NOTE: All of the following processes use the CHI660D electrochemical analyzer/workstation and its accompanying software.
<ol>
	<li>Flush the chip using IPA followed by DI water.</li>
	<li>Pour 0.01 M HCl solution into the acrylic container.</li>
	<li>Wipe dry the macro Ag/AgCl reference electrode&#39;s pipette exterior (fabricated in step 6.5) and a macro Ag/AgCl electrode (fabricated in step 6.3) using laboratory clean wipes.</li>
	<li>Connect the chip and the macro electrodes to the analyzer such that a thin film Ag electrode on the chip is defined as the working electrode, the macro Ag/AgCl reference electrode is defined as the reference electrode, and the bare macro Ag/AgCl electrode is defined as the counter electrode.</li>
	<li>Place the macro electrodes into the container. Use blu-tack as the lid of the container to anchor the macro electrodes.</li>
	<li>Place the setup into a Faraday cage.</li>
	<li>In the CHI660D software, click on the Setup tab at the top left corner of the window. Then click Technique | Amperometric i-t Curve | OK to perform cathodic cleaning of the electrodes.</li>
	<li>In the pop-up menu, modify the parameters for cathodic cleaning.
	<ol>
		<li>Set the Init E (V) as -1.5.</li>
		<li>Set the Sample Interval (sec) as 0.1 (Default).</li>
		<li>Set the Run Time (sec) as 900.</li>
		<li>Set the Quiet Time (sec) to be 0 (Default).</li>
		<li>Set the Scales during Run as 1 (Default).</li>
		<li>Set the Sensitivity (A/V) appropriately. For an 80 &#181;m x 80 &#181;m electrode, set it as 1e-006.</li>
	</ol>
	</li>
	<li>Press OK. Start the process by pressing the Start icon under the menu bar.</li>
	<li>Let the experiment run and finish.</li>
	<li>Open the Faraday cage.</li>
	<li>Remove the macro reference and counter electrode. Wipe dry their surfaces.</li>
	<li>Pour the used electrolyte in a waste container. Flush the acrylic container using DI water.</li>
</ol>
8. Fabrication of single layer AgCl on top of the thin film Ag electrodes
<ol>
	<li>Pour 0.1 M KCl solution into the acrylic container.</li>
	<li>Connect the chip and the macro electrodes to the analyzer such that the cleaned thin film Ag electrode on the chip is defined as the working electrode, the macro Ag/AgCl reference electrode is defined as the reference electrode, and the bare macro Ag/AgCl electrode is defined as the counter electrode.</li>
	<li>Place the macro electrodes into the container. Use blu-tack as the lid of the container to anchor the macro electrodes.</li>
	<li>Place the setup into a Faraday cage.</li>
	<li>In the CHI660D software, click on the Setup tab at the top left corner of the window, then click Technique | Chronopotentiometry | OK to perform galvanostatic fabrication of single layer AgCl on silver electrodes.</li>
	<li>In the pop-up menu, modify the parameters for such process.
	<ol>
		<li>Set the Cathodic Current (A) as 0 (Default).</li>
		<li>Set the Anodic Current (A) such that the current density applied to the thin film electrode is 0.5 mA/cm2.</li>
		<li>Keep the High and Low E limit and Hold Time as default.</li>
		<li>Set the Cathodic Time (sec) as 10 (Default).</li>
		<li>Set the Anodic Time (sec) correspondingly to achieve the degree of AgCl coverage needed. 
		NOTE: With reference to Faradays Law of Electrolysis, the time needed for 100% coverage is 262 s. The time needed varies linearly with the coverage percentage.</li>
		<li>Set the Initial Polarity as Anodic.</li>
		<li>Set the Data Storage Intvl (sec) as 0.1 (Default).</li>
		<li>Set the Number of Segments as 1 (Default).</li>
		<li>Set the Current Switching Priority as Time.</li>
		<li>Uncheck the Auxiliary Signal Recording When Sample Interval &#62; = 0.0005s (Default).</li>
	</ol>
	</li>
	<li>Press OK. Start the process by pressing the Start icon under the menu bar.</li>
	<li>Let the experiment run and finish.</li>
	<li>Open the Faraday cage.</li>
	<li>Remove the macro reference and counter electrode. Wipe dry their surfaces.</li>
	<li>Submerse the macro electrodes in 3.5 M KCl solution for storage.</li>
	<li>Pour the used electrolyte in a waste container. Flush the container using DI water.</li>
	<li>Cover the opening of the acrylic container using parafilm for further processing.</li>
</ol>

<ol>
	<li>Bakker, E., Telting-Diaz, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Electrochemical+sensors.">Electrochemical sensors.</a> Analytical Chemistry. 74 (12), 2781-2800 (2002).</li><li>Jobst, G., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Thin-Film+Microbiosensors+for+Glucose-Lactate+Monitoring.">Thin-Film Microbiosensors for Glucose-Lactate Monitoring.</a> Analytical Chemistry. 68 (18), 3173-3179 (1996).</li><li>Matsumoto, T., Ohashi, A., Ito, N. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Development+of+a+micro-planar+Ag/AgCl+quasi-reference+electrode+with+long-term+stability+for+an+amperometric+glucose+sensor.">Development of a micro-planar Ag/AgCl quasi-reference electrode with long-term stability for an amperometric glucose sensor.</a> Analytica Chimica Acta. 462 (2), 253-259 (2002).</li><li>Suzuki, H., Hirakawa, T., Sasaki, S., Karube, I. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=An+integrated+three-electrode+system+with+a+micromachined+liquid-junction+Ag/AgCl+liquid-junction+Ag/AgCl+reference+electrode.">An integrated three-electrode system with a micromachined liquid-junction Ag/AgCl liquid-junction Ag/AgCl reference electrode.</a> Analytica Chimica Acta. 387 (1), 103-112 (1999).</li><li>Ives, D. J. G., Janz, G. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Reference Electrodes - theory and practice. , (1961).</li><li>Huynh, T. M., Nguyen, T. S., Doan, T. C., Dang, C. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Fabrication+of+thin+film+Ag/AgCl+reference+electrode+by+electron+beam+evaporation+method+for+potential+measurements.">Fabrication of thin film Ag/AgCl reference electrode by electron beam evaporation method for potential measurements.</a> Advances in Natural Sciences: Nanoscience and Nanotechnology. 10 (1), 015006 (2019).</li><li>Katan, T., Szpak, S., Bennion, D. 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C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=An+improved+approach+for+fabricating+Ag/AgCl+reference+electrodes.">An improved approach for fabricating Ag/AgCl reference electrodes.</a> Electrochimica Acta. 71, 252-257 (2012).</li><li>Safari, S., Selvaganapathy, P. R., Derardja, A., Deen, M. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Electrochemical+growth+of+high-aspect+ratio+nanostructured+silver+chloride+on+silver+and+its+application+to+miniaturized+reference+electrodes.">Electrochemical growth of high-aspect ratio nanostructured silver chloride on silver and its application to miniaturized reference electrodes.</a> Nanotechnology. 22 (31), 315601 (2001).</li><li>Tjon, K. C. 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The authors have nothing to disclose.

The physical properties of an Ag/AgCl electrode is controlled by the morphology and the structure of the AgCl deposited on the electrode. In this paper, we presented a protocol to precisely control the coverage of a single layer of AgCl on the surface of the silver electrode. An integral part of the protocol is a modified form of the Faraday&#39;s Law of Electrolysis, which is used to control the degree of AgCl on the thin film silver electrodes. It can be written as:
<img alt="Equation" src="...

The Ag/AgCl electrode is one of the most used electrodes in the field of electrochemistry. It is most commonly used as the reference electrode in electrochemical systems due to its ease of fabrication, non-toxic property and stable electrode potential1,2,3,4,5,6.
Researchers have attempted to understand the mechanism of Ag/AgCl electrodes. The layer of chloride salt on the electrode has been found to be a fundamental material in the characteristic redox reaction of the Ag/AgCl electrode in a chloride containing electrolyte. For the oxidation path, the silver at the imperfection sites on the surface of the electrode combines with the chloride ions in the solution to form soluble AgCl complexes, in which they diffuse to the edges of the AgCl deposited on the surface of the electrode for precipitation in the form of AgCl. The reduction path involves the formation of soluble AgCl complexes using the AgCl on the electrode. The complexes diffuse to the silver surface and reduces back to elemental silver7,8.
The morphology of the AgCl layer is a pivotal influence in the physical property of Ag/AgCl electrodes. Various works showed that the large surface area is key to form reference Ag/AgCl electrodes with highly reproducible and stable electrode potentials9,10,11,12. Therefore, researchers have investigated methods to create Ag/AgCl electrodes with a large surface area. Brewer et al. discovered that using constant voltage instead of constant current to fabricate Ag/AgCl electrodes would result in a highly porous AgCl structure, increasing the surface area of the AgCl layer11. Safari et al. took advantage of the mass transport limitation effect during AgCl formation on the surface of silver electrodes to form AgCl nanosheets on top of them, increasing the surface area of the AgCl layer significantly12.
There is a rising trend to design AgCl electrode for sensing applications. A low contact impedance is crucial for sensing electrodes. Thus, it is important to understand how the surface coating of AgCl would affect its impedance property. Our previous research showed that the degree of AgCl coverage on the silver electrode has a pivotal influence on the impedance characteristic of the electrode/electrolyte interface13. However, to correctly estimate the contact impedance of thin film Ag/AgCl electrodes, the AgCl layer formed must be smooth and have well-controlled coverage. Therefore, a method to form smooth AgCl layers with designated degrees of AgCl coverage is needed. Works have been done to address this need partially. Brewer et al. and Pargar et al. discussed that a smooth AgCl can be achieved using a gentle constant current, fabricating the AgCl layer on top of the silver electrode11,14. Katan et al. formed a single layer of AgCl on their silver samples and observed the size of individual AgCl particles8. Their research found that the thickness of a single layer of AgCl is around 350 nm. The aim of this work is to develop a protocol to form fine and well-controlled films of AgCl with predicted impedance properties on top of silver electrodes.

<table><tbody><tr><td>AST Peva-600EI E-Beam Evaporation System</td><td>Advanced System Technology</td><td></td><td>For Cr/Au Deposition</td></tr><tr><td>AZ 5214 E Photoresist</td><td>MicroChemicals</td><td></td><td>Photoresist for pad opening</td></tr><tr><td>AZ P4620 Photoresist</td><td>AZ Electronic Materials</td><td></td><td>Photoresist for Ag liftoff</td></tr><tr><td>Branson/IPC 3000 Plasma Asher</td><td>Branson/IPC</td><td></td><td>Ashing</td></tr><tr><td>Branson 5510R-MT Ultrasonic Cleaner</td><td>Branson Ultrasonics</td><td></td><td>Liftoff</td></tr><tr><td>CHI660D</td><td>CH Instruments, Inc</td><td></td><td>Electrochemical Analyser</td></tr><tr><td>Denton Explorer 14 RF/DC Sputter</td><td>Denton Vacuum</td><td></td><td>For Ag Sputtering</td></tr><tr><td>FHD-5</td><td>Fujifilm</td><td>800768</td><td>Photoresist Development</td></tr><tr><td>HPR 504 Photoresist</td><td>OCG Microelectronic Materials NV</td><td></td><td>Photoresist for Cr/Au liftoff</td></tr><tr><td>Hydrochloric acid fuming 37%</td><td>VMR</td><td>20252.420</td><td>Making diluted HCl for cathodic cleaning</td></tr><tr><td>J.A. Woollam M-2000VI Spectroscopic Elipsometer</td><td>J.A. Woollam</td><td></td><td>Measurement of silicon dioxide passivation layer thickness on dummy</td></tr><tr><td>Multiplex CVD</td><td>Surface Technology Systems</td><td></td><td>Silicon dioxide passivation</td></tr><tr><td>Oxford RIE Etcher</td><td>Oxford Instruments</td><td></td><td>For Pad opening</td></tr><tr><td>Potassium Chloride</td><td>Sigma-Aldrich</td><td>7447-40-7</td><td>Making KCl solutions</td></tr><tr><td>SOLITEC 5110-C/PD Manual Single-Head Coater</td><td>Solitec Wafer Processing, Inc.</td><td></td><td>For spincoating of photoresist</td></tr><tr><td>SUSS MA6</td><td>SUSS MicroTec</td><td></td><td>Mask Aligner</td></tr><tr><td>Sylgard 184 Silicone Elastomer Kit</td><td>Dow Corning</td><td></td><td>Adhesive for container on chip</td></tr></tbody></table>

fabrication of thin film silver/silver chloride electrodes with finely controlled single layer silver chloride

This paper aims to present a protocol to form smooth and well-controlled films of silver/silver chloride (Ag/AgCl) with designated coverage on top of thin film silver electrodes. Thin film silver electrodes sized 80 &#181;m x 80 &#181;m and 160 &#181;m x 160 &#181;m were sputtered on quartz wafers with a chromium/gold (Cr/Au) layer for adhesion. After passivation, polishing and cathodic cleaning processes, the electrodes underwent galvanostatic oxidation with consideration of Faraday&#39;s Law of Electrolysis to form smooth layers of AgCl with a designated degree of coverage on top of the silver electrode. This protocol is validated by inspection of scanning electron microscope (SEM) images of the surface of the fabricated Ag/AgCl thin film electrodes, which highlights the functionality and performance of the protocol. Sub-optimally fabricated electrodes are fabricated as well for comparison. This protocol can be widely used to fabricate Ag/AgCl electrodes with specific impedance requirements (e.g., probing electrodes for impedance sensing applications like impedance flow cytometry and interdigitated electrode arrays).

Figure 1 shows an 80 &#181;m x 80 &#181;m Ag/AgCl electrode with a designed AgCl coverage of 50% fabricated following this protocol. By observation, the area of the AgCl patch is around 68 &#181;m x 52 &#181;m, which corresponds to around 55% of AgCl coverage. This shows that the protocol can finely control the amount of AgCl coverage on the thin film Ag electrodes. The AgCl layer fabricated is also very smooth, as evident by the clumping of adjacent AgCl par...

Watch this Scientific Journal Video about Fabrication of Thin Film Silver/Silver Chloride Electrodes with Finely Controlled Single Layer Silver Chloride at JoVE.com

Fabrication of Thin Film Silver/Silver Chloride Electrodes with Finely Controlled Single Layer Silver Chloride

This paper aims to present a method to form smooth and well-controlled films of silver chloride (AgCl) with designated coverage on top of thin film silver electrodes.

This paper aims to present a protocol to form smooth and well-controlled films of silver/silver chloride (Ag/AgCl) with designated coverage on top of thin ...

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14.0K Views.  The Hong Kong University of Science and Technology. Dit protocol maakt de afzetting van een gladde enkele laag zilverchloride met aangewezen dekking op dunne film zilveren elektroden. Dit is de eerste keer dat een techniek die precies kan controleren de dekking van enkele laag zilverchloride op dunne films van elektrode wordt geïntroduceerd. Om te beginnen spoelt u de chip door met isopropanol, gevolgd door DI-water.Giet 0,01 molaire zoutzuuroplossing in de acrylcontainer. Veeg met behulp van laboratorium schone doekjes droog de macro ...

Video: Fabrication of Thin Film Silver/Silver Chloride Electrodes with Finely Controlled Single Layer Silver Chloride

Polycrystalline Silicon Thin-film Solar cells with Plasmonic-enhanced Light-trapping

One of major approaches to cheaper solar cells is reducing the amount of semiconductor material used for their fabrication and making cells thinner. To compensate for lower light absorption such physically thin devices have to incorporate light-trapping which increases their optical thickness. Light scattering by textured surfaces is a common technique but it cannot be universally applied to all solar cell technologies. Some cells, for example those made of evaporated silicon, are planar as produced and they require an alternative light-trapping means suitable for planar devices. Metal nanoparticles formed on planar silicon cell surface and capable of light scattering due to surface plasmon resonance is an effective approach.
The paper presents a fabrication procedure of evaporated polycrystalline silicon solar cells with plasmonic light-trapping and demonstrates how the cell quantum efficiency improves due to presence of metal nanoparticles.
To fabricate the cells a film consisting of alternative boron and phosphorous doped silicon layers is deposited on glass substrate by electron beam evaporation. An Initially amorphous film is crystallised and electronic defects are mitigated by annealing and hydrogen passivation. Metal grid contacts are applied to the layers of opposite polarity to extract electricity generated by the cell. Typically, such a ~2 &mu;m thick cell has a short-circuit current density (Jsc) of 14-16 mA/cm2, which can be increased up to 17-18 mA/cm2 (~25% higher) after application of a simple diffuse back reflector made of a white paint.
To implement plasmonic light-trapping a silver nanoparticle array is formed on the metallised cell silicon surface. A precursor silver film is deposited on the cell by thermal evaporation and annealed at 23&deg;C to form silver nanoparticles. Nanoparticle size and coverage, which affect plasmonic light-scattering, can be tuned for enhanced cell performance by varying the precursor film thickness and its annealing conditions. An optimised nanoparticle array alone results in cell Jsc enhancement of about 28%, similar to the effect of the diffuse reflector. The photocurrent can be further increased by coating the nanoparticles by a low refractive index dielectric, like MgF2, and applying the diffused reflector. The complete plasmonic cell structure comprises the polycrystalline silicon film, a silver nanoparticle array, a layer of MgF2, and a diffuse reflector. The Jsc for such cell is 21-23 mA/cm2, up to 45% higher than Jsc of the original cell without light-trapping or ~25% higher than Jsc for the cell with the diffuse reflector only.
Introduction
Light-trapping in silicon solar cells is commonly achieved via light scattering at textured interfaces. Scattered light travels through a cell at oblique angles for a longer distance and when such angles exceed the critical angle at the cell interfaces the light is permanently trapped in the cell by total internal reflection (Animation 1: Light-trapping). Although this scheme works well for most solar cells, there are developing technologies where ultra-thin Si layers are produced planar (e.g. layer-transfer technologies and epitaxial c-Si layers) 1 and or when such layers are not compatible with textures substrates (e.g. evaporated silicon) 2. For such originally planar Si layer alternative light trapping approaches, such as diffuse white paint reflector 3, silicon plasma texturing 4 or high refractive index nanoparticle reflector 5 have been suggested.
Metal nanoparticles can effectively scatter incident light into a higher refractive index material, like silicon, due to the surface plasmon resonance effect 6. They also can be easily formed on the planar silicon cell surface thus offering a light-trapping approach alternative to texturing. For a nanoparticle located at the air-silicon interface the scattered light fraction coupled into silicon exceeds 95% and a large faction of that light is scattered at angles above critical providing nearly ideal light-trapping condition (Animation 2: Plasmons on NP). The resonance can be tuned to the wavelength region, which is most important for a particular cell material and design, by varying the nanoparticle average size, surface coverage and local dielectric environment 6,7. Theoretical design principles of plasmonic nanoparticle solar cells have been suggested 8. In practice, Ag nanoparticle array is an ideal light-trapping partner for poly-Si thin-film solar cells because most of these design principle are naturally met. The simplest way of forming nanoparticles by thermal annealing of a thin precursor Ag film results in a random array with a relatively wide size and shape distribution, which is particularly suitable for light-trapping because such an array has a wide resonance peak, covering the wavelength range of 700-900 nm, important for poly-Si solar cell performance. The nanoparticle array can only be located on the rear poly-Si cell surface thus avoiding destructive interference between incident and scattered light which occurs for front-located nanoparticles 9. Moreover, poly-Si thin-film cells do not requires a passivating layer and the flat base-shaped nanoparticles (that naturally result from thermal annealing of a metal film) can be directly placed on silicon further increases plasmonic scattering efficiency due to surface plasmon-polariton resonance 10.
The cell with the plasmonic nanoparticle array as described above can have a photocurrent about 28% higher than the original cell. However, the array still transmits a significant amount of light which escapes through the rear of the cell and does not contribute into the current. This loss can be mitigated by adding a rear reflector to allow catching transmitted light and re-directing it back to the cell. Providing sufficient distance between the reflector and the nanoparticles (a few hundred nanometers) the reflected light will then experience one more plasmonic scattering event while passing through the nanoparticle array on re-entering the cell and the reflector itself can be made diffuse - both effects further facilitating light scattering and hence light-trapping. Importantly, the Ag nanoparticles have to be encapsulated with an inert and low refractive index dielectric, like MgF2 or SiO2, from the rear reflector to avoid mechanical and chemical damage 7. Low refractive index for this cladding layer is required to maintain a high coupling fraction into silicon and larger scattering angles, which are ensured by the high optical contrast between the media on both sides of the nanoparticle, silicon and dielectric 6. The photocurrent of the plasmonic cell with the diffuse rear reflector can be up to 45% higher than the current of the original cell or up to 25% higher than the current of an equivalent cell with the diffuse reflector only.

Polycrystalline silicon thin-film solar cells on glass are fabricated by deposition of boron and phosphorous doped silicon layers followed by crystallisation, defect passivation and metallisation. Plasmonic light-trapping is introduced by forming Ag nanoparticles on the silicon cell surface capped with a diffused reflector resulting in ~45% photocurrent enhancement.

One of major approaches to cheaper solar cells is reducing the amount of semiconductor material used for their fabrication and making cells thinner. To ...

Engineering

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) ...

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 ...

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 ...

Dynamic Electrochemical Measurement of Chloride Ions

This protocol describes the dynamic measurement of chloride ions using the transition time of a silver silver chloride (Ag/AgCl) electrode. Silver silver chloride electrode is used extensively for potentiometric measurement of chloride ions concentration in electrolyte. In this measurement, long-term and continuous monitoring is limited due to the inherent drift and the requirement of a stable reference electrode. We utilized the chronopotentiometric approach to minimize drift and avoid the use of a conventional reference electrode. A galvanostatic pulse is applied to an Ag/AgCl electrode which initiates a faradic reaction depleting the Cl&#713; ions near the electrode surface. The transition time, which is the time to completely deplete the ions near the electrode surface, is a function of the ion concentration, given by the Nernst equation. The square root of the transition time is in linear relation to the chloride ion concentration. Drift of the response over two weeks is negligible (59 &#181;M/day) when measuring 1 mM [Cl&#713;]using a current pulse of 10 Am-2. This is a dynamic measurement where the moment of transition time determines the response and thus is independent of the absolute potential. Any metal wire can be used as a pseudo-reference electrode, making this approach feasible for long-term measurement inside concrete structures.

Dynamic measurement of chloride ions is presented. Transition time of an Ag/AgCl electrode, during a chronopotentiometric technique, can give the concentration of chloride ions in electrolyte. This method does not require a stable conventional reference electrode.

This protocol describes the dynamic measurement of chloride ions using the transition time of a silver silver chloride (Ag/AgCl) electrode. Silver silver ...

Bridging the Bio-Electronic Interface with Biofabrication

Advancements in lab-on-a-chip technology promise to revolutionize both research and medicine through lower costs, better sensitivity, portability, and higher throughput. The incorporation of biological components onto biological microelectromechanical systems (bioMEMS) has shown great potential for achieving these goals. Microfabricated electronic chips allow for micrometer-scale features as well as an electrical connection for sensing and actuation. Functional biological components give the system the capacity for specific detection of analytes, enzymatic functions, and whole-cell capabilities. Standard microfabrication processes and bio-analytical techniques have been successfully utilized for decades in the computer and biological industries, respectively. Their combination and interfacing in a lab-on-a-chip environment, however, brings forth new challenges. There is a call for techniques that can build an interface between the electrode and biological component that is mild and is easy to fabricate and pattern.
Biofabrication, described here, is one such approach that has shown great promise for its easy-to-assemble incorporation of biological components with versatility in the on-chip functions that are enabled. Biofabrication uses biological materials and biological mechanisms (self-assembly, enzymatic assembly) for bottom-up hierarchical assembly. While our labs have demonstrated these concepts in many formats 1,2,3, here we demonstrate the assembly process based on electrodeposition followed by multiple applications of signal-based interactions. The assembly process consists of the electrodeposition of biocompatible stimuli-responsive polymer films on electrodes and their subsequent functionalization with biological components such as DNA, enzymes, or live cells 4,5. Electrodeposition takes advantage of the pH gradient created at the surface of a biased electrode from the electrolysis of water 6,7,. Chitosan and alginate are stimuli-responsive biological polymers that can be triggered to self-assemble into hydrogel films in response to imposed electrical signals 8. The thickness of these hydrogels is determined by the extent to which the pH gradient extends from the electrode. This can be modified using varying current densities and deposition times 6,7. This protocol will describe how chitosan films are deposited and functionalized by covalently attaching biological components to the abundant primary amine groups present on the film through either enzymatic or electrochemical methods 9,10. Alginate films and their entrapment of live cells will also be addressed 11. Finally, the utility of biofabrication is demonstrated through examples of signal-based interaction, including chemical-to-electrical, cell-to-cell, and also enzyme-to-cell signal transmission. 
Both the electrodeposition and functionalization can be performed under near-physiological conditions without the need for reagents and thus spare labile biological components from harsh conditions. Additionally, both chitosan and alginate have long been used for biologically-relevant purposes 12,13. Overall, biofabrication, a rapid technique that can be simply performed on a benchtop, can be used for creating micron scale patterns of functional biological components on electrodes and can be used for a variety of lab-on-a-chip applications.

This article describes a biofabrication approach: deposition of stimuli-responsive polysaccharides in the presence of biased electrodes to create biocompatible films which can be functionalized with cells or proteins. We demonstrate a bench-top strategy for the generation of the films as well as their basic uses for creating interactive biofunctionalized surfaces for lab-on-a-chip applications.

Advancements in lab-on-a-chip technology promise to revolutionize both research and medicine through lower costs, better sensitivity, portability, and ...

Bioengineering

Non-aqueous Electrode Processing and Construction of Lithium-ion Coin Cells

Research into new and improved materials to be utilized in lithium-ion batteries (LIB) necessitates an experimental counterpart to any computational analysis. Testing of lithium-ion batteries in an academic setting has taken on several forms, but at the most basic level lies the coin cell construction. In traditional LIB electrode preparation, a multi-phase slurry composed of active material, binder, and conductive additive is cast out onto a substrate. An electrode disc can then be punched from the dried sheet and used in the construction of a coin cell for electrochemical evaluation. Utilization of the potential of the active material in a battery is critically dependent on the microstructure of the electrode, as an appropriate distribution of the primary components are crucial to ensuring optimal electrical conductivity, porosity, and tortuosity, such that electrochemical and transport interaction is optimized. Processing steps ranging from the combination of dry powder, wet mixing, and drying can all critically affect multi-phase interactions that influence the microstructure formation. Electrochemical probing necessitates the construction of electrodes and coin cells with the utmost care and precision. This paper aims at providing a step-by-step guide of non-aqueous electrode processing and coin cell construction for lithium-ion batteries within an academic setting and with emphasis on deciphering the influence of drying and calendaring.

Non-aqueous electrode processing is central to the construction of coin cells and the evaluation of new electrode chemistries for lithium-ion batteries. A step-by-step guide to the basic practices needed as an electrochemical engineer working with batteries in an academic experimental setting is furnished.

Research into new and improved materials to be utilized in lithium-ion batteries (LIB) necessitates an experimental counterpart to any computational ...

Fabrication of Fully Solution Processed Inorganic Nanocrystal Photovoltaic Devices

We demonstrate a method for the preparation of fully solution processed inorganic solar cells from a spin and spray coating deposition of nanocrystal inks. For the photoactive absorber layer, colloidal CdTe and CdSe nanocrystals (3-5 nm) are synthesized using an inert hot injection technique and cleaned with precipitations to remove excess starting reagents. Similarly, gold nanocrystals (3-5 nm) are synthesized under ambient conditions and dissolved in organic solvents. In addition, precursor solutions for transparent conductive indium tin oxide (ITO) films are prepared from solutions of indium and tin salts paired with a reactive oxidizer. Layer-by-layer, these solutions are deposited onto a glass substrate following annealing (200-400 &#176;C) to build the nanocrystal solar cell (glass/ITO/CdSe/CdTe/Au). Pre-annealing ligand exchange is required for CdSe and CdTe nanocrystals where films are dipped in NH4Cl:methanol to replace long-chain native ligands with small inorganic Cl- anions. NH4Cl(s) was found to act as a catalyst for the sintering reaction (as a non-toxic alternative to the conventional CdCl2(s) treatment) leading to grain growth (136&#177;39 nm) during heating. The thickness and roughness of the prepared films are characterized with SEM and optical profilometry. FTIR is used to determine the degree of ligand exchange prior to sintering, and XRD is used to verify the crystallinity and phase of each material. UV/Vis spectra show high visible light transmission through the ITO layer and a red shift in the absorbance of the cadmium chalcogenide nanocrystals after thermal annealing. Current-voltage curves of completed devices are measured under simulated one sun illumination. Small differences in deposition techniques and reagents employed during ligand exchange have been shown to have a profound influence on the device properties. Here, we examine the effects of chemical (sintering and ligand exchange agents) and physical treatments (solution concentration, spray-pressure, annealing time and annealing temperature) on photovoltaic device performance.

This protocol describes the synthesis and solution deposition of inorganic nanocrystals layer by layer to produce thin film electronics on non-conductive surfaces. Solvent-stabilized inks can produce complete photovoltaic devices on glass substrates via spin and spray coating following post-deposition ligand exchange and sintering.

We demonstrate a method for the preparation of fully solution processed inorganic solar cells from a spin and spray coating deposition of nanocrystal ...

Scalable Solution-processed Fabrication Strategy for High-performance, Flexible, Transparent Electrodes with Embedded Metal Mesh

Here, the authors report the embedded metal-mesh transparent electrode (EMTE), a new transparent electrode (TE) with a metal mesh completely embedded in a polymer film. This paper also presents a low-cost, vacuum-free fabrication method for this novel TE; the approach combines lithography, electroplating, and imprint transfer (LEIT) processing. The embedded nature of the EMTEs offers many advantages, such as high surface smoothness, which is essential for organic electronic device production; superior mechanical stability during bending; favorable resistance to chemicals and moisture; and strong adhesion with plastic film. LEIT fabrication features an electroplating process for vacuum-free metal deposition and is favorable for industrial mass production. Furthermore, LEIT allows for the fabrication of metal mesh with a high aspect ratio (i.e., thickness to linewidth), significantly enhancing its electrical conductance without adversely losing optical transmittance. We demonstrate several prototypes of flexible EMTEs, with sheet resistances lower than 1 &#937;/sq and transmittances greater than 90%, resulting in very high figures of merit (FoM) &#8211; up to 1.5 x 104 &#8211; which are amongst the best values in the published literature.

This protocol describes a solution-based fabrication strategy for high-performance, flexible, transparent electrodes with fully-embedded, thick metal mesh. Flexible transparent electrodes fabricated by this process demonstrate among the highest reported performances, including ultra-low sheet resistance, high optical transmittance, mechanical stability under bending, strong substrate adhesion, surface smoothness, and environmental stability.

Here, the authors report the embedded metal-mesh transparent electrode (EMTE), a new transparent electrode (TE) with a metal mesh completely embedded in a ...

Fabrication of Robust Nanoscale Contact between a Silver Nanowire Electrode and CdS Buffer Layer in Cu(In,Ga)Se2 Thin-film Solar Cells

Silver nanowire transparent electrodes have been employed as window layers for Cu(In,Ga)Se2 thin-film solar cells. Bare silver nanowire electrodes normally result in very poor cell performance. Embedding or sandwiching silver nanowires using moderately conductive transparent materials, such as indium tin oxide or zinc oxide, can improve cell performance. However, the solution-processed matrix layers can cause a significant number of interfacial defects between transparent electrodes and the CdS buffer, which can eventually result in low cell performance. This manuscript describes how to fabricate robust electrical contact between a silver nanowire electrode and the underlying CdS buffer layer in a Cu(In,Ga)Se2 solar cell, enabling high cell performance using matrix-free silver nanowire transparent electrodes. The matrix-free silver nanowire electrode fabricated by our method proves that the charge-carrier collection capability of silver nanowire electrode-based cells is as good as that of standard cells with sputtered ZnO:Al/i-ZnO as long as the silver nanowires and CdS have high-quality electrical contact. The high-quality electrical contact was achieved by depositing an additional CdS layer as thin as 10 nm onto the silver nanowire surface.

Fabrication of Robust Nanoscale Contact between a Silver Nanowire Electrode and CdS Buffer Layer in CuIn,GaSe2 Thin-film Solar Cells

In this protocol, we describe the detailed experimental procedure for the fabrication of a robust nanoscale contact between a silver nanowire network and CdS buffer layer in a CIGS thin-film solar cell.

Fabrication of Thin Film Silver/Silver Chloride Electrodes with Finely Controlled Single Layer Silver Chloride

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