The Authors would like to acknowledge the individuals in the Quantum Electronics and Photonics Division of the National Institute of Standards and Technology in Boulder, CO&#160;for their assistance.

The GaN NWs used in these experiments were grown by catalyst-free molecular beam epitaxy (MBE) on Si(111) substrates9. The general procedure for preparing the NW suspension from the substrate with the as-grown NWs is illustrated in Figure 1.
1. Nanowire Suspension Preparation
<ol>
	<li>Cleave a small (&#60;5 mm x&#160;5 mm) piece of the as-grown NWs on the substrate.</li>
	<li>Fill a small capped vial with about 1 ml of isopropanol (IPA).</li>
	<li>Place the cleaved piece into the vial, close the cap and sonicate for about 30 sec in order to remove the NWs from the substrate. Once prepared, the NW suspension remains viable for an extended period of time.</li>
</ol>
2. Substrate Preparation
The substrates used are heavily-doped (&#961; ~0.001-0.005 &#937;-cm) 3-inch Si wafers with 200 nm of thermally-grown SiO2 on both sides.
<ol>
	<li>If electrical contact to the silicon substrate is desired, etch off the oxide from the backside of the wafer using an RIE etcher with the following conditions, O2 flow = 20 sccm, CHF3 flow = 50 sccm, -240 V bias, 120 W for 20 min. This step also helps avoid charging of the specimen during electron microscopy.</li>
	<li>After oxide etch, clean wafer by submersing it face down using a tripod holder into a 1,000 ml beaker with ~100 ml of acetone for 5 min.</li>
	<li>Remove wafer from acetone and immediately rinse off both sides of the wafer using a squirt bottle of IPA over an empty 1,000 ml beaker, which will be used for solvent waste.</li>
	<li>Repeat step 2 using IPA.</li>
	<li>Repeat step 3 rinsing the wafer with deionized H2O over the solvent waste beaker.</li>
	<li>Repeat step 2 using deionized H2O.</li>
	<li>Blow wafer dry using compressed dry N2.</li>
</ol>
3. Nanowire Dispersal
<ol>
	<li>Cleave the clean wafer into 4 equal quarters. Each quarter will have 3 contact patterns deposited on it in the general areas shown in Figures 2A and B.</li>
	<li>Sonicate the vial of NW suspension prior to dispensing in order to get a uniform NW concentration in the suspension.</li>
	<li>Set the micropipette to the desired drop size (3-30 &#956;l) and draw the NW suspension from the vial.</li>
	<li>Take substrate piece (&#188; wafer) that will be used for dispersal and ensure that it is level so that the NW suspension does not migrate away to the edges.</li>
	<li>Dispense the NW suspension onto the (oxidized) front surface of the Si substrate in the general area where the contact pattern will be deposited. If desired, deposit additional drops of the NW suspension in the same area, after the solvent from the previous drop has completely evaporated.</li>
	<li>Repeat step 3.4 for the other 2 areas where a contact pattern will be. Do not dispense the NW suspension from more than 1 growth run on a single substrate piece (&#188; wafer) in order to avoid cross contamination. The dispersal of the NW suspension onto the substrate is illustrated in Figures 2C and D.</li>
	<li>After the final drop of NW suspension has evaporated, place the sample face down in a tripod holder and gently dip in successive baths of acetone and isopropanol to remove unwanted impurities. Rinse in deionized H2O, and blow dry in N2. Do not use squirt bottles or sonicate the solvent during this cleaning process to avoid excess removal of NWs from the surface.</li>
</ol>
4. Photolithography of Contact Pattern
Use standard photolithographic techniques to create the contact pattern in a clean room with ambient conditions of ~20 &#176;C and ~40% relative humidity. Mask aligner intensity (step&#160;4.6), exposure time (step 4.8) and develop time (step 4.9) will be equipment-dependent and should be adjusted to produce maximum pattern definition with a lift-off-resist (LOR) undercut of about 0.5 &#956;m.
<ol>
	<li>Spin lift-off resist onto sample using a two step recipe of (1) 300 rpm for 10 sec (2) 2,000 rpm for 45 sec.</li>
	<li>Place sample on a hot plate to bake for 5 min at 150-170 &#176;C.</li>
	<li>Remove sample and allow to cool for 30 sec and then spin on photoresist using a two step recipe of (1) 1,000 rpm for 3 sec (2) at 5,000 rpm for 45 sec.</li>
	<li>Place sample on a hot plate to bake for 1 min at 115 &#176;C.</li>
	<li>Allow sample to cool for 1 min.</li>
	<li>Calibrate mask aligner to a light intensity of ~1.90 mW/cm2.</li>
	<li>Load sample into mask aligner, with the proper mask for the contact pattern installed.</li>
	<li>Bring sample into contact with mask and expose the sample for 24 sec.</li>
	<li>Develop photoresist by swirling sample in a beaker of developer for 21 sec.</li>
	<li>Rinse with deionized H2O and blow dry with N2.</li>
</ol>
5. Sample Pretreatment Prior to Metal Deposition
Prior to loading the samples into the electron-beam evaporator for metal deposition, give the patterned wafer a UV ozone treatment and a HCl:H2O bath.
<ol>
	<li>Load samples into UV Ozone generator for 10 min with an ultra-high-purity O2 flow rate of 80 sccm.</li>
	<li>After UV ozone treatment, place samples in a HCl:H2O (1:10) solution for 1 min at room temperature.</li>
	<li>Rinse samples with deionized H2O and blow dry gently with N2.</li>
</ol>
6. Electron-beam Evaporation of Contact Metals
<ol>
	<li>Immediately after pretreatment, mount the samples to the platen using screws and clips and secure platen in the e-beam evaporator. Make sure that sufficient Ni and Au are available for deposition.</li>
	<li>Pump down chamber until the pressure is below 1.3 x 10-3 Pa (1 &#956;Torr) (may require to pump overnight).</li>
	<li>Set high voltage to 10 kV and begin rotation of samples at 5 rpm. Make sure shutter is closed.</li>
	<li>Select the Ni crucible and enter parameters for Ni into the crystal rate monitor. Deposit 50 nm (500 &#8491;) of Ni at a deposition rate of ~0.1 nm/s.</li>
	<li>Once Ni source has sufficiently cooled down (~15 min), switch to Au crucible, change the parameters to those for Au, and deposit 100 nm (1,000 &#8491;) of Au at a deposition rate of ~0.1 nm/sec.</li>
	<li>Once Au crucible has cooled down (~10 min), vent chamber and unload sample platen. If unsure of metal thickness, use a profilometer to determine the Ni/Au thickness.</li>
</ol>
7. Contact Metal Lift-off
<ol>
	<li>Remove samples from platen and place in a photoresist stripper bath at room temperature for a few hours to lift off the metal deposited on the photoresist. If desired, elevate the bath temperature to around 50-60 &#176;C to accelerate lift-off.</li>
	<li>If the metal does not come off completely, use a squirt bottle of Remover PG to forcefully remove the remaining undesired metal from the surface. Do not sonicate the sample because this can cause the nanowires to break free from the metal.</li>
	<li>Rinse off the Remover PG from sample with IPA into a solvent waste beaker and place sample into a beaker of clean IPA.</li>
	<li>Repeat step 7.3 using deionized H2O and then blow the sample dry using N2.</li>
</ol>
8. Contact Anneal
Test devices before the contact anneal in order to compare these with annealed devices. Perform the contact anneal of the Ni/Au films using a rapid thermal annealer (RTA) with ultra-high-purity N2/O2 (3:1) as the process gas.
<ol>
	<li>Wait at least 24 hr after removal of contact metal before annealing samples, to ensure the metal contacts have reached equilibrium after deposition.</li>
	<li>Purge the RTA by running the N2/O2 at a temperature of 650 &#176;C for 5 min without any samples.</li>
	<li>Load samples and adjust flow rate of N2/O2 to 1.4 SLPM.</li>
	<li>Anneal samples at 550 &#176;C for 10 min. The temperature ramp rate was ~500 &#176;C/min.</li>
</ol>
9. Ni/Au Film Removal
Since removal of the Ni/Au film is a destructive process, devices are typically imaged and tested before this step. The procedure for the Ni/Au film removal is illustrated in Figure 3.
<ol>
	<li>Wait at least 24 hr after contact anneal before removing the Ni/Au film.</li>
	<li>Cleave a piece of the sample from the area of interest. Make sure size of sample is large enough in order to grab the edges with tweezers without touching the area of interest.</li>
	<li>Secure a SEM pin stub mount in a holder such that the mount can be easily moved or taken out.</li>
	<li>Place a piece of conductive carbon tape flat onto the mount surface. The piece of carbon tape should be larger than the area of the film that is to be removed. A conductive carbon tab that is the precise size of the mount surface works very well for this application. Apply the carbon tape or tab to the mount surface slowly and carefully to ensure that the surface is as smooth as possible.</li>
	<li>Before removing the backing of the tape, using a finger, press hard on the tape so that it is firmly adhering to the mount surface. This is very important so that the tape does not come off with the sample.</li>
	<li>Take a cleaved sample piece and gently lay the area of interest directly onto the carbon tape along the edge of the mount such that the area of the sample that was used for handling with tweezers is hanging off the edge (Figure 3A). Once sample has been placed onto tape, do not remove it for repositioning in order to avoid destroying the Ni/Au film.</li>
	<li>Press firmly down onto the back of the sample using forceps (Figure 3B). Take care not to press too hard on the sample around the mount edges because this can cause the sample to break.</li>
	<li>To remove the substrate, take a clean razor blade or scalpel and gently nudge it in between the substrate and the tape along the edges of the sample (Figure 3C). Carefully repeat this process, nudging the razor blade a little further each time until the substrate can easily be peeled off by grabbing it with tweezers. Using too much force can break the sample. To keep the tape from coming off with substrate, place a pair of forceps or a probe onto the area of the tape that is adjacent to the substrate when prying the substrate off (Figure 3C). The annealed Ni/Au film should remain adhered to the tape (Figure 3D). On average, this step will take about 1 min to complete.</li>
	<li>Image the newly removed Ni/Au film using SEM as soon as possible so as to observe the newly exposed interface of the film before it becomes contaminated.</li>
</ol>

<ol>
	<li>Lu, W., Lieber, 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=Semiconductor+nanowires.">Semiconductor nanowires.</a> J. Phys. D: Appl. Phys. 39, R387-R406 (2006).</li><li>Mater Res, A. n. n. u. R. e. v. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> 34, 83-122 (2004).</li><li>Pettersen, S. V., Grande, A. P., 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=Formation+and+electronic+properties+of+oxygen+annealed+Au/Ni+and+Pt/Ni+contacts+to+p-type.">Formation and electronic properties of oxygen annealed Au/Ni and Pt/Ni contacts to p-type.</a> 22, 186-193 (2007).</li><li>Chen, L. C., Ho, J. K., 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=The+Effect+of+Heat+Treatment+on+Ni/Au+Ohmic+Contacts+to+p-Type.">The Effect of Heat Treatment on Ni/Au Ohmic Contacts to p-Type.</a> 176, 773-777 (1999).</li><li>Liday, 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=Investigation+of+NiOx-based+contacts+on+p-GaN.">Investigation of NiOx-based contacts on p-GaN.</a> J. Mater. Sci. Mater. Electron. 19, 855-862 (2008).</li><li>Narayan, J., Wang, H., Oh, T. H., Choi, H. K., Fan, J. C. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Formation+of+epitaxial+Au/Ni/Au+ohmic+contacts+to+p-GaN.">Formation of epitaxial Au/Ni/Au ohmic contacts to p-GaN.</a> Appl. Phys. Lett. 81 (21), 3978-3980 (2002).</li><li>Ho, J. K., Jong, C. S., 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=Low-resistance+ohmic+contacts+to+p-type+GaN+achieved+by+the+oxidation+of+Ni/Au+films.">Low-resistance ohmic contacts to p-type GaN achieved by the oxidation of Ni/Au films.</a> J. Appl. Phys. Lett. 86 (8), 4491-4497 (1999).</li><li>Herrero, A. M., Blanchard, P., 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=Microstructure+evolution+and+development+of+annealed+Ni/Au+contacts+to+GaN+nanowires.">Microstructure evolution and development of annealed Ni/Au contacts to GaN nanowires.</a> Nanotechnology. 23 (36), 5203.1-5203.10 (2012).</li><li>Bertness, K. A., Roshko, A., Sanford, N. A., Barker, J. M., Davydov, A. V. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Spontaneously+grown+GaN+and+AlGaN+nanowires.">Spontaneously grown GaN and AlGaN nanowires.</a> J. Crystal Growth. 287, 522-527 (2006).</li><li>Herrero, A. M., Bertness, K. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Optimization+of+Dispersion+and+Surface+Pretreatment+for+Single+GaN+Nanowire+Devices.">Optimization of Dispersion and Surface Pretreatment for Single GaN Nanowire Devices.</a> J. Vac. Sci. Tech. B. 30 (6), 2201.1-2201.5 (2012).</li></ol>

No conflicts of interest declared.

The technique presented allows for analysis of the contact/substrate and contact/NW microstructure of single NW devices. The main advantages of this technique are its low cost and simplicity. It allows for qualitative and quantitative analysis of the contact interface on a large scale with the substrate as well as on a submicrometer scale with individual NWs. The use of carbon tape for the film removal and SEM pin stubs for sample mounting make it possible for analysis using characterization techniques that require clean...

Single-NW devices are made by dispersing a NW suspension onto an insulating substrate and forming contact pads on the substrate via conventional photolithography and metal deposition, which results in randomly formed two-terminal devices. A thick SiO2 film on a Si wafer is typically used as an insulating substrate1,2. For metals deposited on a SiO2 surface, a common problem resulting from heat treatment is the occurrence of void formation at the metal/SiO2 interface. In addition to cracking and delamination of the metal film, this void formation can negatively affect device performance from an increase in resistance caused by a reduction of the contact area. Ni/Au contacts oxidized in N2/O2 atmospheres are the predominant contact scheme applied to p-GaN3-7. During heat treatment in a N2/O2, the Ni diffuses to the surface to form NiO and the Au diffuses down to the substrate surface.
In this work, excessive void formation at the contact/NW and contact/SiO2 interfaces was shown to occur during annealing of Ni/Au contacts to NWs on SiO28. The surface morphology of the annealed Ni/Au film, however, does not indicate the existence of voids or the degree to which void formation has occurred. To address this problem, we developed a technique for the removal of Ni/Au contacts and GaN NWs from SiO2/Si substrates in order to analyze the interface of the contact with the substrate and NWs. This technique can be used for the removal of any contact structure that has poor adhesion to the substrate. The Ni/Au films with GaN NWs embedded in them are removed from the SiO2 substrate with carbon tape. The carbon tape is adhered to a standard pin mount for characterization by use of scanning electron microscopy (SEM) along with several other tools. The detailed procedure for the fabrication of single GaN NW devices and analysis of their contact interface morphology are described.

<table border="1">
	<tbody>
		<tr>
			<td></td>
			<td></td>
			<td></td>
			<td>REAGENTS and MATERIALS</td>
		</tr>
		<tr>
			<td>Lift-off resist</td>
			<td>MicroChem</td>
			<td>LOR 5A</td>
			<td>Varies according to application</td>
		</tr>
		<tr>
			<td>Photoresist</td>
			<td>Shipley</td>
			<td>1813</td>
			<td>Varies according to application</td>
		</tr>
		<tr>
			<td>Developer</td>
			<td>Rohm and Haas Electronic Materials</td>
			<td>MF CD-26</td>
			<td>Varies according to application</td>
		</tr>
		<tr>
			<td>Photoresist stripper</td>
			<td>MicroChem</td>
			<td>Nano Remover PG</td>
			<td>Varies according to application</td>
		</tr>
		<tr>
			<td>Ni source</td>
			<td>International Advanced Materials</td>
			<td>99.999% purity</td>
			<td></td>
		</tr>
		<tr>
			<td>Au source</td>
			<td>International Advanced Materials</td>
			<td>99.999% purity</td>
			<td></td>
		</tr>
		<tr>
			<td>SiO2/Si wafers</td>
			<td>Silicon Valley Microelectronics</td>
			<td>3-inch &#60;100&#62; N/As 0.001-0.005 Ohm-cm, 200 nm thermal oxide</td>
			<td></td>
		</tr>
		<tr>
			<td>Carbon tape</td>
			<td>SPI Supplies</td>
			<td>5072, 8 mm wide</td>
			<td></td>
		</tr>
		<tr>
			<td></td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td></td>
			<td></td>
			<td></td>
			<td>Solvents are standard semiconductor or research grade. Vendor is not important for the experimental outcome.</td>
		</tr>
		<tr>
			<td></td>
			<td></td>
			<td></td>
			<td>Reactive ion etch gases and thermal annealing gases are high purity grade. Vendor is not important for the experimental outcome.</td>
		</tr>
		<tr>
			<td></td>
			<td></td>
			<td></td>
			<td>EQUIPMENT</td>
		</tr>
		<tr>
			<td>Ultrasonic cleaner</td>
			<td>Cole-Palmer</td>
			<td>EW-08849-00</td>
			<td>Low power</td>
		</tr>
		<tr>
			<td>Micropipette</td>
			<td>Rainin</td>
			<td>PR-200</td>
			<td>Metered, disposal tips</td>
		</tr>
		<tr>
			<td>Reactive ion etcher</td>
			<td>SemiGroup</td>
			<td>RIE 1000 TP</td>
			<td>Other vendors also used with different process parameters</td>
		</tr>
		<tr>
			<td>Mask aligner</td>
			<td>Karl Suss</td>
			<td>MJB3</td>
			<td>Other vendors also used with different process parameters</td>
		</tr>
		<tr>
			<td>UV ozone cleaner</td>
			<td>Jelight</td>
			<td>Model 42</td>
			<td>Other vendors also used with different process parameters</td>
		</tr>
		<tr>
			<td>E-beam evaporator</td>
			<td>CVC</td>
			<td>SC-6000</td>
			<td>Other vendors also used with different process parameters</td>
		</tr>
		<tr>
			<td></td>
			<td></td>
			<td></td>
			<td>* Manufacturers and product names are given solely for completeness. These specific citations neither constitute an endorsement of the product by NIST nor imply that similar products from other companies would be less suitable.</td>
		</tr>
	</tbody>
</table>

analysis of contact interfaces for single gan nanowire devices

Single GaN nanowire (NW) devices fabricated on SiO2 can exhibit a strong degradation after annealing due to the occurrence of void formation at the contact/SiO2 interface. This void formation can cause cracking and delamination of the metal film, which can increase the resistance or lead to a complete failure of the NW device. In order to address issues associated with void formation, a technique was developed that removes Ni/Au contact metal films from the substrates to allow for the examination and characterization of the contact/substrate and contact/NW interfaces of single GaN NW devices. This procedure determines the degree of adhesion of the contact films to the substrate and NWs and allows for the characterization of the morphology and composition of the contact interface with the substrate and nanowires. This technique is also useful for assessing the amount of residual contamination that remains from the NW suspension and from photolithographic processes on the NW-SiO2 surface prior to metal deposition. The detailed steps of this procedure are presented for the removal of annealed Ni/Au contacts to Mg-doped GaN NWs on a SiO2 substrate.

An example of SEM analysis on annealed Ni/Au films removed from the SiO2 substrate using carbon tape is shown in Figure 4. The surface of a Ni/Au contact prior to removal is shown in Figure 4A. The underside of the same area of that particular Ni/Au film after removal is shown in Figure 4B. Comparison of the surface and underside morphology can help determine if there is a relationship between the two. For example, when the two images are compared, it can be s...

Watch this Scientific Journal Video about Analysis of Contact Interfaces for Single GaN Nanowire Devices at JoVE.com

Analysis of Contact Interfaces for Single GaN Nanowire Devices

A technique was developed that removes Ni/Au contact metal films from their substrate to allow for the examination and characterization of the contact/substrate and contact/NW interfaces of single GaN nanowire devices.

Single GaN nanowire (NW) devices fabricated on SiO2 can exhibit  a strong degradation after annealing due to the  occurrence of void formation at the ...

analysis-of-contact-interfaces-for-single-gan-nanowire-devices

Universidad de Cartagena UDEC

Research

JoVE Journal

Engineering

9.3K Views.  National Institute of Standards and Technology. A technique was developed that removes Ni/Au contact metal films from their substrate to allow for the examination and characterization of the contact/substrate and contact/NW interfaces of single GaN nanowire devices.

Video: Analysis of Contact Interfaces for Single GaN Nanowire Devices

Monolayer Contact Doping of Silicon Surfaces and Nanowires Using Organophosphorus Compounds

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

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

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

Thermal Measurement Techniques in Analytical Microfluidic Devices

Thermal measurement techniques have been used for many applications such as thermal characterization of materials and chemical reaction detection. Micromachining techniques allow reduction of the thermal mass of fabricated structures and introduce the possibility to perform high sensitivity thermal measurements in the micro-scale and nano-scale devices. Combining thermal measurement techniques with microfluidic devices allows performing different analytical measurements with low sample consumption and reduced measurement time by integrating the miniaturized system on a single chip. The procedures of thermal measurement techniques for particle detection, material characterization, and chemical detection are introduced in this paper.

Here, we present three protocols for thermal measurements in microfluidic devices.

Thermal measurement techniques have been used for many applications such as thermal characterization of materials and chemical reaction detection. ...

Seedless Growth of Bismuth Nanowire Array via Vacuum Thermal Evaporation

Here a seedless and template-free technique is demonstrated to scalably grow bismuth nanowires, through thermal evaporation in high vacuum at RT. Conventionally reserved for the fabrication of metal thin films, thermal evaporation deposits bismuth into an array of vertical single crystalline nanowires over a flat thin film of vanadium held at RT, which is freshly deposited by magnetron sputtering or thermal evaporation. By controlling the temperature of the growth substrate the length and width of the nanowires can be tuned over a wide range. Responsible for this novel technique is a previously unknown nanowire growth mechanism that roots in the mild porosity of the vanadium thin film. Infiltrated into the vanadium pores, the bismuth domains (~ 1 nm) carry excessive surface energy that suppresses their melting point and continuously expels them out of the vanadium matrix to form nanowires. This discovery demonstrates the feasibility of scalable vapor phase synthesis of high purity nanomaterials without using any catalysts.

A protocol for seedless and high yield growth of bismuth nanowire arrays through vacuum thermal evaporation technique is presented.

Here a seedless and template-free technique is demonstrated to scalably grow bismuth nanowires, through thermal evaporation in high vacuum at RT. ...

Solvent Bonding for Fabrication of PMMA and COP Microfluidic Devices

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

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

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

Scanning Light Scattering Profiler (SLPS) Based Methodology to Quantitatively Evaluate Forward and Backward Light Scattering from Intraocular Lenses

The scanning light scattering profiler (SLSP) methodology has been developed for the full-angle quantitative evaluation of forward and backward light scattering from intraocular lenses (IOLs) using goniophotometer principles. This protocol describes the SLSP platform and how it employs a 360&#176; rotational photodetector sensor that is scanned around an IOL sample while recording the intensity and location of scattered light as it passes through the IOL medium. The SLSP platform can be used to predict, non-clinically, the propensity for current and novel IOL designs and materials to induce light scatter. Non-clinical evaluation of light scattering properties of IOLs can significantly reduce the number of patient complaints related to unwanted glare, glistening, optical defects, poor image quality, and other phenomena associated with the unintended light scattering. Future studies should be conducted to correlate SLSP data with clinical results to help identify which measured light scatter is most problematic for patients that have undergone cataract surgery subsequent to IOL implantation.

Scanning Light Scattering Profiler SLPS Based Methodology to Quantitatively Evaluate Forward and Backward Light Scattering from Intraocular Lenses

This protocol describes the scanning light scattering profiler (SLSP) that enables the full-angle quantitative evaluation of forward and backward scattering of light from intraocular lenses (IOLs) using goniophotometer principles.

The scanning light scattering profiler (SLSP) methodology has been developed for the full-angle quantitative evaluation of forward and backward light ...

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 ...

Atmospheric Pressure Fabrication of Large-Sized Single-Layer Rectangular SnSe Flakes

Tin selenide (SnSe) belongs to the family of layered metal chalcogenide materials with a buckled structure like phosphorene, and has shown potential for applications in two-dimensional nanoelectronics devices. Although many methods to synthesize SnSe nanocrystals have been developed, a simple way to fabricate large-sized single-layer SnSe flakes remains a great challenge. Herein, we show the experimental method to directly grow large-sized single-layer rectangular SnSe flakes on commonly used SiO2/Si insulating substrates using a straightforward two-step fabrication method in an atmospheric pressure quartz tube furnace system. The single-layer rectangular SnSe flakes with an average thickness of ~6.8 &#197; and lateral dimensions of about 30 &#181;m &#215; 50 &#181;m were fabricated by a combination of vapor transport deposition technique and nitrogen etching route. We characterized the morphology, microstructure, and electrical properties of the rectangular SnSe flakes and obtained excellent crystallinity and good electronic properties. This article about the two-step fabrication method can help researchers grow other similar two-dimensional, large-sized, single-layer materials using an atmospheric pressure system.

A protocol is presented demonstrating a two-step fabrication technique to grow large-sized single-layer rectangular shaped SnSe flakes on low-cost SiO2/Si dielectrics wafers in an atmospheric pressure quartz tube furnace system.

Tin selenide (SnSe) belongs to the family of layered metal chalcogenide materials with a buckled structure like phosphorene, and has shown potential for ...

Fabrication of Refractive-index-matched Devices for Biomedical Microfluidics

The use of microfluidic devices has emerged as a defining tool for biomedical applications. When combined with modern microscopy techniques, these devices can be implemented as part of a robust platform capable of making simultaneous complementary measurements. The primary challenge created by the combination of these two techniques is the mismatch in refractive index between the materials traditionally used to make microfluidic devices and the aqueous solutions typically used in biomedicine. This mismatch can create optical artifacts near the channel or device edges. One solution is to reduce the refractive index of the material used to fabricate the device by using a fluorinated polymer such as MY133-V2000 whose refractive index is similar to that of water (n = 1.33). Here, the construction of a microfluidic device made out of MY133-V2000 using soft lithography techniques is demonstrated, using O2 plasma in conjunction with an acrylic holder to increase the adhesion between the MY133-V2000 fabricated device and the polydimethylsiloxane (PDMS) substrate. The device is then tested by incubating it filled with cell culture media for 24 h to demonstrate the ability of the device to maintain cell culture conditions during the course of a typical imaging experiment. Finally, quantitative phase microscopy (QPM) is used to measure the distribution of mass within the live adherent cells in the microchannel. This way, the increased precision, enabled by fabricating the device from a low index of refraction polymer such as MY133-V2000 in lieu of traditional soft lithography materials such as PDMS, is demonstrated. Overall, this approach for fabricating microfluidic devices can be readily integrated into existing soft lithography workflows in order to reduce optical artifacts and increase measurement precision.

This protocol describes the fabrication of microfluidic devices from MY133-V2000 to eliminate artifacts that often arise in microchannels due to the mismatching refractive indices between microchannel structures and an aqueous solution. This protocol uses an acrylic holder to compress the encapsulated device, improving adhesion both chemically and mechanically.

The use of microfluidic devices has emerged as a defining tool for biomedical applications. When combined with modern microscopy techniques, these devices ...

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.

Silver nanowire transparent electrodes have been employed as window layers for Cu(In,Ga)Se2 thin-film solar cells. Bare silver nanowire electrodes ...

Crack Monitoring in Resonance Fatigue Testing of Welded Specimens Using Digital Image Correlation

A procedure using digital image correlation (DIC) to detect cracks on welded specimens during fatigue tests on resonance testing machines is presented. It is intended as a practical and reproducible procedure to identify macroscopic cracks at an early stage and monitor crack propagation during fatigue tests. It consists of strain field measurements at the weld using DIC. Images are taken at fixed load cycle intervals. Cracks become visible in the computed strain field as elevated strains. This way, the whole width of a small-scale specimen can be monitored to detect where and when a crack initiates. Subsequently, it is possible to monitor the development of the crack length. Because the resulting images are saved, the results are verifiable and comparable. The procedure is limited to cracks initiating at the surface and is intended for fatigue tests under laboratory conditions. By visualizing the crack, the presented procedure allows direct observation of macrocracks from their formation until rupture of the specimen.

Digital image correlation is used in fatigue tests on a resonance testing machine to detect macroscopic cracks and monitor crack propagation in welded specimens. Cracks on the specimen surface become visible as increased strains.

A procedure using digital image correlation (DIC) to detect cracks on welded specimens during fatigue tests on resonance testing machines is presented. It ...