Research reported in this publication was supported by the King Abdullah University of Science and Technology (KAUST).

CAUTION: Please consult all relevant material safety data sheets (MSDS) before use. Several of the chemicals used in these fabrications are acutely toxic and carcinogenic. Nanomaterials may pose additional hazards compared to their bulk counterparts. Please use all appropriate safety practices when performing a nanocrystal reaction, including the use of engineering controls (fume hood) and personal protective equipment (safety glasses, gloves, lab coat, full length pants, closed-toe shoes).
1. Preparations of aluminum templates
<ol>
	<li>Cleaning the aluminum discs
	<ol>
		<li>Wash the Al discs in a beaker with deionized (DI) water. Repeat 3 times.</li>
		<li>Hold the Al disc with tweezers and wash with acetone followed by isopropyl alcohol (IPA) and DI water.</li>
		<li>Place the Al discs in a beaker with acetone and sonicate for 10 min.</li>
	</ol>
	</li>
	<li>Electropolishing of aluminum discs
	<ol>
		<li>Prepare the electropolishing solution, 3 M perchloric acid in ethanol. Cool the electropolishing solution in a fridge at 4 &#176;C before use.</li>
		<li>Wash the Al discs in a beaker with DI water. Repeat 3 times.</li>
		<li>Grip the cleaned Al template with the dressing forceps and immerse it inside the beaker filled with electropolishing solution along with the platinum (Pt) mesh electrode. Keep the forceps out of the solution as much as possible.</li>
		<li>Stir the solution at 400 rpm.</li>
		<li>Connect the Al disc to the positive terminal and Pt to the negative terminal of the power supply. Apply a voltage of 20 V while the current is limited to 2 A.</li>
		<li>Polish the discs for 3 min and wash the discs with DI water.</li>
	</ol>
	</li>
</ol>
2. Hard anodization
<ol>
	<li>Preparing the cells
	<ol>
		<li>Wash the cell parts (copper plate, PDMS/rubber O rings, cell, Pt mesh cap) with DI water.</li>
		<li>Take the electropolished Al discs out of the DI water and place it on the cell holes with O-rings. Check carefully that there are no leaks.</li>
	</ol>
	</li>
	<li>Anodization
	<ol>
		<li>Fill the assembled cell with 0.3 M oxalic acid and place it on the cold plate at 4 &#176;C.</li>
		<li>Once the oxalic acid is between 2-5 &#176;C, apply 40 V for 20 min (mild anodization). Then, increase the voltage in steps of 0.1 V/s up to 140 V.</li>
		<li>Keep this voltage constant for 45 min. The anodized template will be a bright golden color.</li>
		<li>Open the cell and wash the Al disc with DI water and dry with nitrogen (N2).</li>
	</ol>
	</li>
</ol>
3. Preparation for deposition
<ol>
	<li>Removal of Al back
	<ol>
		<li>Prepare a copper solution with 0.1 M of CuCl2&#183;2H2O and 6 M of HCl.</li>
		<li>Place the anodized template in a cell (with a 10 mm hole diameter) with the back side facing upwards.</li>
		<li>Pour the copper solution and a magnetic stirrer into the cell and agitate at 300 rpm.</li>
		<li>After around 15 min, the solution becomes transparent. Replace it with fresh solution and agitate for 5 min more.</li>
		<li>Wash the discs with DI water and dry with N2.</li>
	</ol>
	</li>
	<li>Opening the pores
	<ol>
		<li>Place the sample (back side facing upwards) in a Petri dish on a pH strip.</li>
		<li>Deposit 10 wt% phosphoric acid to completely cover the membrane. Add more phosphoric acid every hour to avoid dryness.</li>
		<li>After 6.5 h, wash with DI water, and dry with N2.</li>
	</ol>
	</li>
	<li>Gold sputtering
	<ol>
		<li>Prepare the sputtering machine. Open the inert gas valve and vent the chamber.</li>
		<li>Tape the Al disc onto the sputter stage with the back side facing up.</li>
		<li>Adjust parameters to deposit 200 nm and run the profile.</li>
	</ol>
	</li>
</ol>
4. Deposition of nanowires
<ol>
	<li>Prepare a solution of 0.2 M of iron (II) sulfate, 0.16 M of boric acid and 0.05 M of L-ascorbic acid.</li>
	<li>Mount the Al membrane into the cell (15 mm diameter hole)</li>
	<li>Pour the solution into the cell and connect the source meter with the negative contact attached to the copper plate and the positive contact to the platinum mesh.</li>
	<li>Apply a constant current of 2.5 mA to start electrodeposition. The length of the nanowire is directly proportional to the electrodeposition time.</li>
</ol>
5. Membrane removal and washing of nanowires
<ol>
	<li>Gold etching
	<ol>
		<li>Break the membrane using a tweezer. Select small pieces (around 1 or 2 mm2).</li>
		<li>Prepare one or more small pieces for dry etching using reactive ion etching (RIE) equipment. Glue the pieces to a dummy wafer using lubricant, keeping the gold face up.</li>
		<li>Etch the gold in the RIE equipment for 2 min using the following parameters: T = 25 &#176;C, P = 150 W and argon flow rate = 25 cm3/min. Repeat in shorter cycles if some gold is still present.</li>
	</ol>
	</li>
	<li>Nanowire release
	<ol>
		<li>Prepare the chrome solution using 0.2 M of CrO3 and 0.5 M of H3PO4.</li>
		<li>Fill a 1.5 mL microtube tube with 1 mL of the chrome solution and the small pieces of membrane containing nanowires.</li>
		<li>Leave the solution working for 24 h at 40 &#176;C.</li>
		<li>When the nanowires are completely released, no black particles should be observed with the naked eye.</li>
		<li>Wash the nanowires by placing the microtube in a magnetic rack and replacing the chrome solution with 1 mL of ethanol.</li>
		<li>Repeat the washing process at least 10 times.</li>
	</ol>
	</li>
</ol>

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M., Rivera-Fernandez, S., Jesus, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Shape+matters:+Synthesis+and+biomedical+applications+of+high+aspect+ratio+magnetic+nanomaterials.">Shape matters: Synthesis and biomedical applications of high aspect ratio magnetic nanomaterials.</a> Nanoscale. 7, 8233-8260 (2015).</li><li>Alnassar, M., Alfadhel, A., Ivanov, Y. P., Kosel, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Magnetoelectric+polymer+nanocomposite+for+flexible+electronics.">Magnetoelectric polymer nanocomposite for flexible electronics.</a> Journal of Applied Physics. 117, 17D711 (2015).</li><li>Contreras, M. 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I., 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=Functionalized+magnetic+nanowires+for+chemical+and+magneto-mechanical+induction+of+cancer+cell+death.">Functionalized magnetic nanowires for chemical and magneto-mechanical induction of cancer cell death.</a> Scientific Reports. 6, 35786 (2016).</li><li>Shore, D., 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=Electrodeposited+Fe+and+Fe-Au+nanowires+as+MRI+contrast+agents.">Electrodeposited Fe and Fe-Au nanowires as MRI contrast agents.</a> Chemical Communications. 52, 12634-12637 (2016).</li><li>Garc&#237;a-Mart&#237;n, J., 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=Imaging+magnetic+vortices+by+magnetic+force+microscopy:+Experiments+and+modelling.">Imaging magnetic vortices by magnetic force microscopy: Experiments and modelling.</a> Journal of Physics D: Applied Physics. 37, 965 (2004).</li><li>Piraux, L., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Giant+magnetoresistance+in+magnetic+multilayered+nanowires.">Giant magnetoresistance in magnetic multilayered nanowires.</a> Applied Physics Letters. 65, 2484-2486 (1994).</li><li>Piraux, L., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Template-grown+NiFe/Cu/NiFe+nanowires+for+spin+transfer+devices.">Template-grown NiFe/Cu/NiFe nanowires for spin transfer devices.</a> Nano Letters. 7, 2563-2567 (2007).</li><li>Wang, Z., 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=Spin-wave+quantization+in+ferromagnetic+nickel+nanowires.">Spin-wave quantization in ferromagnetic nickel nanowires.</a> Physical Review Letters. 89, 027201 (2002).</li><li>Wernsdorfer, W., 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=Measurements+of+magnetization+switching+in+individual+nickel+nanowires.">Measurements of magnetization switching in individual nickel nanowires.</a> Physical Review B. 55, 11552 (1997).</li><li>Kou, X., 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=Memory+effect+in+magnetic+nanowire+arrays.">Memory effect in magnetic nanowire arrays.</a> Advanced Materials. 23, 1393-1397 (2011).</li><li>Mohammed, H., Vidal, E. 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I., 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=Nanowire+for+energy:+A+review.">Nanowire for energy: A review.</a> Applied Physics Reviews. 5, 041305 (2018).</li><li>Zongjie, W., Suwon, L., Kyo-in, K., Keekyoung, K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Nanowire-Based+Sensors+for+Biological+and+Medical+Applications.">Nanowire-Based Sensors for Biological and Medical Applications.</a> IEEE Transactions on Nanobioscience. 15 (3), 186 (2016).</li></ol>

The authors have nothing to disclose.

As in any other nanomaterial production, high-quality solutions and materials are required in this protocol. Electropolishing and electrodepositing solutions can be reused several times. However, the anodization solution should only be used once and be freshly made. After removing the Al back, the membranes are extremely weak and can be broken if not handled carefully. The N2 should not be directly applied when drying the membranes. All processes prior to anodization are equally important for the self-ordering of pore structures. Surface impurities, pits, and scratches may lead to poorly ordered nanopores.
The thickness of the alumina membrane generated in step 2 is usually around 60&#160;&#181;m, much longer than the nanowire we require. If longer nanowires are needed, this protocol can be adapted to make thicker membranes by increasing the time of anodization. These nanopores can be used as templates for forming arrays of standing nanowires or released by a subsequent chemical removal of alumina structure. Furthermore, different metals can be electrodeposited using the same setup, including multisegmented nanowires15, by changing the solution and the applied current. Rate deposition would be different for each metal.
The main advantage of the anodization method presented is the high quality of the pores: constant diameter along tenths of micrometers, small diameter distribution, and high pore density. Further, this technique is efficient, economical, and highly reproducible. It can be done safely at ambient conditions in the general laboratory. Nanowires promise a lot in future energy conversion devices (including photovoltaics, thermoelectrics, and betavoltaics16) and as biological and medical sensors17. All of these applications will require extensive material and device development.

Cylindrical magnetic nanowires have attracted an enormous amount of interest in the last decade for a variety of promising applications. Nanowires are novel materials that possess unique properties, mainly due to a high aspect ratio and shape anisotropy1. Because of these properties, nanowires are considered unique systems and excellent model objects for a number of practical applications: flow sensors2, magnetic separation3, bio-inspired tactile sensors4, energy harvesting5, cancer treatments2,6, drug delivery7,8, and MRI contrast agents3,9. Nanowires are also considered ideal for other applications: magnetic force microscopy10, giant magnetoresistance11, spin transfer torque12,13, and data storage devices14,15.
In order to exploit these nanowires to their full advantage, a reproducible fabrication method that yields nanowires of high quality and specific properties is required. The anodization of aluminum produces self-organized, highly ordered cylindrical pores with controllable pore diameters. Because of this, AAO templates are preferred in nanotechnology applications over expensive lithographic techniques. Using these membranes as scaffolds, nanowires can be created by direct current (DC), alternating current (AC), or pulsed DC electrodeposition. Controlling the fabrication process of the membrane and the deposition of the nanowires, a wide range of magnetic nanowires can be created for particular applications1. Here, we report the fabrication of Fe nanowires, including the formation of the porous alumina membrane that is used as the template, electrodeposition into templates using electrolyte solution, and the release of the nanowires into the solution.

<table><tbody><tr><td>Acetone</td><td>Sigma Aldrich</td><td>CAS 67-64-1</td><td></td></tr><tr><td>Aluminium Discs 99.999%</td><td>GoodFellow</td><td>AL000957</td><td>Thickness: 0.50mm +/- 10%, Diameter 25.0mm +/- 0.5mm</td></tr><tr><td>Big Beaker</td><td></td><td></td><td>1000 mL</td></tr><tr><td>Boric acid</td><td>Sigma Aldrich</td><td>101942058</td><td>99%</td></tr><tr><td>Cables</td><td></td><td></td><td></td></tr><tr><td>Chromium (VI) oxide</td><td>fisher chemical</td><td>A98-212</td><td></td></tr><tr><td>Cold plate</td><td>Thermo Scientific</td><td>Accel 500 LC</td><td></td></tr><tr><td>Computer</td><td></td><td></td><td>Used with LabView to control the Sourcemeter</td></tr><tr><td>Copper (II) chloride</td><td></td><td></td><td></td></tr><tr><td>Copper plate</td><td></td><td></td><td>Custom made</td></tr><tr><td>DC Power Source</td><td>Agilent</td><td>E3646A</td><td></td></tr><tr><td>DI Water</td><td></td><td></td><td></td></tr><tr><td>Dressing Forceps</td><td>fisher scientific</td><td>12-460-164</td><td>30.5 cm length, serrated tips</td></tr><tr><td>Ethanol</td><td>VWR International Ltd. (US)</td><td>20823.327</td><td></td></tr><tr><td>Fume hood</td><td>Flores valles</td><td></td><td></td></tr><tr><td>Hydrochloric acid</td><td>VWR International Ltd. (US)</td><td>20255.290</td><td></td></tr><tr><td>Iron (II) sulfate</td><td>Merck</td><td>1.03965.1000</td><td></td></tr><tr><td>L-Ascorbic acid</td><td>MP biomedicals</td><td>100769</td><td></td></tr><tr><td>Magnetic rack</td><td>life technologies</td><td>DynaMag 2</td><td></td></tr><tr><td>Magnetic stirrer and hot plate</td><td>IKA</td><td>RCT basic</td><td></td></tr><tr><td>Mechanical stirrer</td><td>Aslong</td><td>JGB37-520</td><td></td></tr><tr><td>Mixer and heater</td><td>Eppendorf</td><td>ThermoMixer F1.5</td><td></td></tr><tr><td>Nylon cell</td><td></td><td></td><td>Custom made</td></tr><tr><td>Oxalic Acid</td><td>VWR International Ltd. (US)</td><td>20063.365-5L</td><td></td></tr><tr><td>PDMS O-ring</td><td></td><td></td><td>Custom made</td></tr><tr><td>Perchloric acid</td><td>VWR International Ltd. (US)</td><td>20583.327</td><td>70-72 %</td></tr><tr><td>Petri dish</td><td></td><td></td><td>Or any other container</td></tr><tr><td>pH strip</td><td></td><td></td><td>Any pH strip</td></tr><tr><td>Phosphoric acid</td><td>acros organics</td><td>201140010</td><td>85%wt</td></tr><tr><td>Platinum</td><td>Goodfellow</td><td>PT005115</td><td>Diameter 0.05mm, 99.9% purity</td></tr><tr><td>Platinum wire</td><td>Goodfellow</td><td>PT05120</td><td>Diameter: 0.2 mm, Purity: 99.95%</td></tr><tr><td>Power Supply</td><td>Rhode &amp;amp; Scharz</td><td>NGPX 35/10</td><td></td></tr><tr><td>Retort stand (x2)</td><td></td><td></td><td></td></tr><tr><td>Screws</td><td></td><td></td><td></td></tr><tr><td>Small beaker</td><td></td><td></td><td>50 mL</td></tr><tr><td>Source meter</td><td>Keithley</td><td>2400-C</td><td></td></tr><tr><td>Sputter</td><td>Quorum</td><td>Q300T D</td><td></td></tr><tr><td>Tape</td><td></td><td></td><td>Any temperature resistant tape</td></tr><tr><td>Teflon propeller</td><td></td><td></td><td></td></tr><tr><td>Ultrasonic cleaner</td><td></td><td></td><td></td></tr></tbody></table>

iron nanowire fabrication by nano-porous anodized aluminum and its characterization

Magnetic nanowires possess unique properties that have attracted the interest of different fields of research, including basic physics, biomedicine, and data storage. We demonstrate a fabrication method for iron (Fe) nanowires&#160;via electrochemical deposition into anodic&#160;alumina oxide (AAO) templates. The templates are fabricated by anodization of aluminum (Al) discs, and the pore length and diameter are controlled by changing the anodizing conditions. Pores with an average diameter of around 120 nm are created using oxalic acid as the electrolyte. Using this method, cylindrical nanowires are synthesized, which are released by dissolving the alumina using a selective chemical etchant.

After electropolishing, the Al disks reflect light well, as seen in Figure 1. If any small scratches or dots are observed, discard the disk. The plot of the applied current during the anodization process should be smooth and follow the three steps of anodization. In case of contaminated solution, excessive defects on the disk surface, incorrect preparation of the cell (see Figure 2), or the solution being too warm, the applied current plot curves will show peaks and irregularities. Two actual anodization curves are shown in Figure 3, including pictures of the samples. Anodization takes place on one side of the Al disk (top side). After removing the Al back, the membrane should be clearly visible from both sides. The pore opening can be checked using scanning electron microscopy (SEM) on the bottom side. Figure 4 shows a sample in which the pores were not completely opened. The deposition rate of Fe nanowires for membranes of this size is around 300 nm/min. As an example, Fe nanowire of around 1 &#181;m is shown in Figure 5. Note that this image was taken after breaking the membrane.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/60111/60111fig1.jpg" /> 
Figure 1: Aluminum disks. Before polishing (left) and after polishing (right). Marks on top of the polished disk are caused by the forceps. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/60111/60111fig01largev2.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/60111/60111fig2.jpg" /> 
Figure 2: Anodization cell. (A) Components of the cell. (B) Detail of the Al disk positioned over the PDMS O-ring. (C) Cell assembled. (D) Cell located over the cold plate and with the mechanical stirrer. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/60111/60111fig02largev2.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" class="xfigimg" src="/files/ftp_upload/60111/60111fig3.jpg" /> 
Figure 3: Applied current versus time during anodization for a successful (left) and unsuccessful (right) anodization. The three steps of anodization can be easily recognized. The stable 40 V (0&#8211;20 min); the constant increase up to 140 V (20&#8211;36:40 min), shown first as an increase of applied current and later as a constant current; and third, the stable 145 V until the end of the process. When anodization occurs properly, curves are smooth like the one on the left. When the curves show peaks or chaotic behavior (right) the sample would be burnt. In this case, the Al disk diameter was 25 mm. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/60111/60111fig03largev2.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" class="xfigimg" src="/files/ftp_upload/60111/60111fig4.jpg" /> 
Figure 4: SEM image of a membrane from the bottom side. This image shows the morphology of a membrane next to its edge. At any other point of the membrane, the membrane shows open pores like the ones in the picture. If the pores are not open properly, the hexagonal structure that is shown at the edge of the picture would be visible anywhere in the membrane. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/60111/60111fig04largev2.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 5" class="xfigimg" src="/files/ftp_upload/60111/60111fig5.jpg" /> 
Figure 5: Cross section SEM image of iron nanowires inside the membrane. The Fe nanowire is clearly recognizable from the alumina membrane due to its higher electron density. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/60111/60111fig05largev2.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

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Iron Nanowire Fabrication by Nano-Porous Anodized Aluminum and its Characterization

In this work, we describe a&#160;protocol to fabricate iron nanowires, including the formation of the porous alumina membrane that is used as the template, electrodeposition into templates using electrolyte solution, and the release of the nanowires into the solution.

Magnetic nanowires possess unique properties that have attracted the interest of different fields of research, including basic physics, biomedicine, and ...

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8.2K Views.  King Abdullah University of Science and Technology. Ons fabricageprotocol is gebaseerd op de elektrodepositie met een nanoporeuze aluminiumoxide, de enige methode die de nanodraden eigenschappen kan geven die we nodig hebben. Het belangrijkste voordeel van deze techniek, is het vergemakkelijkt een eenvoudige en snelle fabricage van zeer geordende nano-poreuze aluminiumoxide sjablonen met een breed scala van porie maten. Magnetische nano materialen hebben aangetrokken veel belangstelling als gevolg van hun Het kan nuttig zijn in of in vitro dia...

Video: Iron Nanowire Fabrication by Nano-Porous Anodized Aluminum and its Characterization

Fabricating Nanogaps by Nanoskiving

There are several methods of fabricating nanogaps with controlled spacings, but the precise control over the sub-nanometer spacing between two electrodes-and generating them in practical quantities-is still challenging. The preparation of nanogap electrodes using nanoskiving, which is a form of edge lithography, is a fast, simple and powerful technique. This method is an entirely mechanical process which does not include any photo- or electron-beam lithographic steps and does not require any special equipment or infrastructure such as clean rooms. Nanoskiving is used to fabricate electrically addressable nanogaps with control over all three dimensions; the smallest dimension of these structures is defined by the thickness of the sacrificial layer (Al or Ag) or self-assembled monolayers. These wires can be manually positioned by transporting them on drops of water and are directly electrically-addressable; no further lithography is required to connect them to an electrometer.

The fabrication of electrically addressable, high-aspect-ratio (> 1000:1) metal nanowires separated by gaps of single nanometers using either sacrificial layers of aluminum and silver or self-assembled monolayers as templates is described. These nanogap structures are fabricated without a clean room or any photo- or electron-beam lithographic processes by a form of edge lithography known as nanoskiving.

There are several methods of fabricating nanogaps with controlled spacings, but the precise control over the sub-nanometer spacing between two ...

Ultrahigh Density Array of Vertically Aligned Small-molecular Organic Nanowires on Arbitrary Substrates

In recent years &pi;-conjugated organic semiconductors have emerged as the active material in a number of diverse applications including large-area, low-cost displays, photovoltaics, printable and flexible electronics and organic spin valves. Organics allow (a) low-cost, low-temperature processing and (b) molecular-level design of electronic, optical and spin transport characteristics. Such features are not readily available for mainstream inorganic semiconductors, which have enabled organics to carve a niche in the silicon-dominated electronics market. The first generation of organic-based devices has focused on thin film geometries, grown by physical vapor deposition or solution processing. However, it has been realized that organic nanostructures can be used to enhance performance of above-mentioned applications and significant effort has been invested in exploring methods for organic nanostructure fabrication.	
A particularly interesting class of organic nanostructures is the one in which vertically oriented organic nanowires, nanorods or nanotubes are organized in a well-regimented, high-density array. Such structures are highly versatile and are ideal morphological architectures for various applications such as chemical sensors, split-dipole nanoantennas, photovoltaic devices with radially heterostructured "core-shell" nanowires, and memory devices with a cross-point geometry. Such architecture is generally realized by a template-directed approach. In the past this method has been used to grow metal and inorganic semiconductor nanowire arrays. More recently &pi;-conjugated polymer nanowires have been grown within nanoporous templates. However, these approaches have had limited success in growing nanowires of technologically important &pi;-conjugated small molecular weight organics, such as tris-8-hydroxyquinoline aluminum (Alq3), rubrene and methanofullerenes, which are commonly used in diverse areas including organic displays, photovoltaics, thin film transistors and spintronics.	
Recently we have been able to address the above-mentioned issue by employing a novel "centrifugation-assisted" approach. This method therefore broadens the spectrum of organic materials that can be patterned in a vertically ordered nanowire array. Due to the technological importance of Alq3, rubrene and methanofullerenes, our method can be used to explore how the nanostructuring of these materials affects the performance of aforementioned organic devices. The purpose of this article is to describe the technical details of the above-mentioned protocol, demonstrate how this process can be extended to grow small-molecular organic nanowires on arbitrary substrates and finally, to discuss the critical steps, limitations, possible modifications, trouble-shooting and future applications.

We report a simple method for fabricating an ultrahigh density array of vertically ordered small-molecular organic nanowires. This method allows for synthesis of complex heterostructured hybrid nanowire geometries, which can be inexpensively grown on arbitrary substrates. These structures have potential applications in organic electronics, optoelectronics, chemical sensing, photovoltaics and spintronics.

In  recent years &pi;-conjugated  organic semiconductors have emerged as the active material in a number of  diverse applications including large-area, ...

Engineering

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 ...

Ohmic Contact Fabrication Using a Focused-ion Beam Technique and Electrical Characterization for Layer Semiconductor Nanostructures

Layer semiconductors with easily processed two-dimensional (2D) structures exhibit indirect-to-direct bandgap transitions and superior transistor performance, which suggest a new direction for the development of next-generation ultrathin and flexible photonic and electronic devices. Enhanced luminescence quantum efficiency has been widely observed in these atomically thin 2D crystals. However, dimension effects beyond quantum confinement thicknesses or even at the micrometer scale are not expected and have rarely been observed. In this study, molybdenum diselenide (MoSe2) layer crystals with a thickness range of 6-2,700 nm were fabricated as two- or four-terminal devices. Ohmic contact formation was successfully achieved by the focused-ion beam (FIB) deposition method using platinum (Pt) as a contact metal. Layer crystals with various thicknesses were prepared through simple mechanical exfoliation by using dicing tape. Current-voltage curve measurements were performed to determine the conductivity value of the layer nanocrystals. In addition, high-resolution transmission electron microscopy, selected-area electron diffractometry, and energy-dispersive X-ray spectroscopy were used to characterize the interface of the metal&#8211;semiconductor contact of the FIB-fabricated MoSe2 devices. After applying the approaches, the substantial thickness-dependent electrical conductivity in a wide thickness range for the MoSe2-layer semiconductor was observed. The conductivity increased by over two orders of magnitude from 4.6 to 1,500 &#937;&#8722;1 cm&#8722;1, with a decrease in the thickness from 2,700 to 6 nm. In addition, the temperature-dependent conductivity indicated that the thin MoSe2 multilayers exhibited considerably weak semiconducting behavior with activation energies of 3.5-8.5 meV, which are considerably smaller than those (36-38 meV) of the bulk. Probable surface-dominant transport properties and the presence of a high surface electron concentration in MoSe2 are proposed. Similar results can be obtained for other layer semiconductor materials such as MoS2 and WS2.

We describe the approaches for the device fabrication and electrical characterization of molybdenum diselenide (MoSe2) layer semiconductor nanostructures with different thicknesses. In addition, the fabrication of ohmic contacts for MoSe2-layer nanocrystals by the focused-ion beam deposition method using platinum (Pt) as a contact metal is described.

Layer semiconductors with easily processed two-dimensional (2D) structures exhibit indirect-to-direct bandgap transitions and superior transistor ...

Fabrication of Low Temperature Carbon Nanotube Vertical Interconnects Compatible with Semiconductor Technology

We demonstrate a method for the low temperature growth (350 &#176;C) of vertically-aligned carbon nanotubes (CNT) bundles on electrically conductive thin-films. Due to the low growth temperature, the process allows integration with modern low-&#954; dielectrics and some flexible substrates. The process is compatible with standard semiconductor fabrication, and a method for the fabrication of electrical 4-point probe test structures for vertical interconnect test structures is presented. Using scanning electron microscopy the morphology of the CNT bundles is investigated, which demonstrates vertical alignment of the CNT and can be used to tune the CNT growth time. With Raman spectroscopy the crystallinity of the CNT is investigated. It was found that the CNT have many defects, due to the low growth temperature. The electrical current-voltage measurements of the test vertical interconnects displays a linear response, indicating good ohmic contact was achieved between the CNT bundle and the top and bottom metal electrodes. The obtained resistivities of the CNT bundle are among the average values in the literature, while a record-low CNT growth temperature was used.

A method for the growth of low temperature vertically-aligned carbon nanotubes, and the subsequent fabrication of vertical interconnect electrical test structures using semiconductor fabrication is presented.

We demonstrate a method for the low temperature growth (350 &#176;C) of vertically-aligned carbon nanotubes (CNT) bundles on electrically conductive ...

Synthesis and Characterization of Fe-doped Aluminosilicate Nanotubes with Enhanced Electron Conductive Properties

The goal of the protocol is to synthesize Fe-doped aluminosilicate nanotubes of the imogolite type with the formula (OH)3Al2-xFexO3SiOH. Doping with Fe aims at lowering the band gap of imogolite, an insulator with the chemical formula (OH)3Al2O3SiOH, and at modifying its adsorption properties towards azo-dyes, an important class of organic pollutants of both wastewater and groundwater.
Fe-doped nanotubes are obtained in two ways: by direct synthesis, where FeCl3 is added to an aqueous mixture of the Si and Al precursors, and by post-synthesis loading, where preformed nanotubes are put in contact with a FeCl3&#8226;6H2O aqueous solution. In both synthesis methods, isomorphic substitution of Al3+ by Fe3+ occurs, preserving the nanotube structure. Isomorphic substitution is indeed limited to a mass fraction of ~1.0% Fe, since at a higher Fe content (i.e., a mass fraction of 1.4% Fe), Fe2O3 clusters form, especially when the loading procedure is adopted. The physicochemical properties of the materials are studied by means of X-ray powder diffraction (XRD), N2 sorption isotherms at -196 &#176;C, high resolution transmission electron microscopy (HRTEM), diffuse reflectance (DR) UV-Vis spectroscopy, and &#950;-potential measurements. The most relevant result is the possibility to replace Al3+ ions (located on the outer surface of the nanotubes) by post-synthesis loading on preformed imogolite without perturbing the delicate hydrolysis equilibria occurring during nanotube formation. During the loading procedure, an anionic exchange occurs, where Al3+ ions on the outer surface of the nanotubes are replaced by Fe3+ ions. In Fe-doped aluminosilicate nanotubes, isomorphic substitution of Al3+ by Fe3+ is found to affect the band gap of doped imogolite. Nonetheless, Fe3+ sites on the outer surface of nanotubes are able to coordinate organic moieties, like the azo-dye Acid Orange 7, through a ligand-displacement mechanism occurring in an aqueous solution.

Here, we present a protocol to synthesize and characterize Fe-doped aluminosilicate nanotubes. The materials are obtained by either sol-gel synthesis upon addition of FeCl3&#8226;6H2O to the mixture containing the Si and Al precursors or by post-synthesis ionic exchange of preformed aluminosilicate nanotubes.

The goal of the protocol is to synthesize Fe-doped aluminosilicate nanotubes of the imogolite type with the formula (OH)3Al2-xFexO3SiOH. Doping with Fe ...

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 ...

Simultaneous Multi-surface Anodizations and Stair-like Reverse Biases Detachment of Anodic Aluminum Oxides in Sulfuric and Oxalic Acid Electrolyte

After reporting on the two-step anodization, nanoporous anodic aluminum oxides (AAOs) have been widely utilized in the versatile fields of fundamental sciences and industrial applications owing to their periodic arrangement of nanopores with relatively high aspect ratio. However, the techniques reported so far, which could be only valid for mono-surface anodization, show critical disadvantages, i.e., time-consuming as well as complicated procedures, requiring toxic chemicals, and wasting valuable natural resources. In this paper, we demonstrate a facile, efficient, and environmentally clean method to fabricate nanoporous AAOs in sulfuric and oxalic acid electrolytes, which can overcome the limitations that result from conventional AAO fabricating methods. First, plural AAOs are produced at one time through simultaneous multi-surfaces anodization (SMSA), indicating mass-producibility of the AAOs with comparable qualities. Second, those AAOs can be separated from the aluminum (Al) substrate by applying stair-like reverse biases (SRBs) in the same electrolyte used for the SMSAs, implying simplicity and green technological characteristics. Finally, a unit sequence consisting of the SMSAs sequentially combined with SRBs-based detachment can be applied repeatedly to the same Al substrate, which reinforces the advantages of this strategy and also guarantees the efficient usage of natural resources.

A protocol for fabricating nanoporous anodic aluminum oxides via simultaneous multi-surfaces anodization followed by stair-like reverse biases detachments is presented. It can be applied repeatedly to the same aluminum substrate, exhibiting a facile, high-yield, and environmentally clean strategy.

After reporting on the two-step anodization, nanoporous anodic aluminum oxides (AAOs) have been widely utilized in the versatile fields of fundamental ...

Fabrication of Gradient Nanopattern by Thermal Nanoimprinting Technique and Screening of the Response of Human Endothelial Colony-forming Cells

Nanotopography can be found in various extracellular matrices (ECMs) around the body and is known to have important regulatory actions upon cellular reactions. However, it is difficult to determine the relation between the size of a nanostructure and the responses of cells owing to the lack of proper screening tools. Here, we show the development of reproducible and cost-effective gradient nanopattern plates for the manipulation of cellular responses. Using anodic aluminum oxide (AAO) as a master mold, gradient nanopattern plates with nanopillars of increasing diameter ranges [120-200&#8239;nm (GP 120/200), 200-280&#8239;nm (GP 200/280), and 280-360&#8239;nm (GP 280/360)] were fabricated by a thermal imprinting technique. These gradient nanopattern plates were designed to mimic the various sizes of nanotopography in the ECM and were used to screen the responses of human endothelial colony-forming cells (hECFCs). In this protocol, we describe the step-by-step procedure of fabricating gradient nanopattern plates for cell engineering, techniques of cultivating hECFCs from human peripheral blood, and culturing hECFCs on nanopattern plates.

Here, we present a protocol for the fabrication of gradient nanopattern plates via thermal nanoimprinting and the method of screening responses of human endothelial progenitor cells to the nanostructures. By using the described technology, it is possible to produce a scaffold that can manipulate cell behavior by physical stimuli.

Nanotopography can be found in various extracellular matrices (ECMs) around the body and is known to have important regulatory actions upon cellular ...

Iron Nanowire Fabrication by Nano-Porous Anodized Aluminum and its Characterization

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