Name: fabrication of refractive-index-matched devices for biomedical microfluidics
Uploaded: 2018-09-10T14:45:00
Description: Watch this Scientific Journal Video about Fabrication of Refractive-index-matched Devices for Biomedical Microfluidics at JoVE.com

This work was supported by the University of Utah office of the Vice President for Research, as well as by funds in conjunction with grant P30 CA042014 awarded to the Huntsman Cancer Institute and to the CRR Program at the Huntsman Cancer Institute.

1. Fabrication of the Polydimethylsiloxane Negative
<ol>
	<li>Preparation of polydimethylsiloxane
	<ol>
		<li>Measure 18 g of PDMS silicone elastomer and 1.8 g of the curing reagent. Pour the curing reagent into a measuring boat containing the elastomer.</li>
		<li>Mix the elastomer and the curing reagent vigorously for 1 min and put the mixture into a vacuum chamber for 30 min.</li>
		<li>Remove the PDMS from the vacuum, pour 15 g onto the negative using a cookie cutter (radius = 3.8 cm) to keep the P...

<ol>
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MY133-V2000 can be used as an alternative to traditional soft lithography fabrication materials such as PDMS. Previous work has shown that materials with a high index of refraction, such as PDMS, introduce significant artifacts near the channel walls due to the mismatching indices of refraction between the fabrication material and the aqueous solution inside the channel13. MY133-V2000 enables matching the refractive index of the microfluidic device to the aqueous solutions commonly used in biomedi...

The development of microfluidic technology has enabled a wide range of new biomedical techniques that leverage the unique physics of microscopic-scale flows1,2. This includes the diagnostic techniques built on microfluidic platforms that quantify clinically relevant biomarkers, including cell stiffness3, surface markers4, and growth5. By manipulating single cells, microfluidic devices can also be used to measure biomarker heterogeneity, for example as an indicator of malignancy6. The ability to combine microfluidic applications with microscopy has further increased the utility of these platforms by allowing for devices that measure multiple biomarkers simultaneously7.
QPM is a microscopy technique that measures the phase shift as light passes through and interacts with the matter inside transparent samples. The mass of individual cells can be calculated from QPM measurements, by using the known relationship between the refractive index and the biomass density8,9. Previous work has shown that QPM is capable of measuring clinically relevant parameters such as cell growth10,11 and cell mechanical properties via disorder strength12. When combined with microfluidics, QPM can potentially be used to measure cell behavior in a highly controlled environment in vitro. One of the primary challenges facing combining QPM with microfluidics is the high refractive index of most polymers used to construct microfluidic channels via soft lithography13.
An important challenge facing the combination of microfluidics with various microscopy techniques is the mismatch between the refractive index of the device material relative to the refractive index of water14,15. One method to address this is through the use of a low refractive index polymer such as CYTOP16 or MY133-V200013. The latter is a fluorinated ultraviolet (UV)-curable acrylate polymer that has a refractive index similar to water (n = 1.33) and that is compatible with soft lithography techniques, allowing for a smooth integration into many established microfluidic device fabrication workflows. This makes MY133-V2000 not only suitable for microfluidic device fabrication, but also allows it to be readily combined with QPM and other microscopy approaches, to measure cell behavior both at the colony and on a single-cell scale. MY133-V2000 eliminates artifacts due to phase unwrapping by producing little, if any, phase shift as light passes through the water-MY133 interface.
Although eliminating the mismatch in refractive index, one major challenge associated with the devices fabricated from fluorinated polymers, such as MY133-V2000, is the low adherence to other materials such as glass or PDMS. The present work demonstrates the fabrication of an MY133-V2000 microfluidic device using soft lithography. Using O2 plasma to treat the surface of both the channel and the PDMS substrate combined with a custom-fabricated acrylic holder ensures that the device adheres to the substrate, creating a sealed channel. This device is suitable for cell culture and QPM to measure the mass of cells in the channel, which has important applications for measuring the growth of live cells and the intracellular transport of cell biomass, both of which have clinical relevance in diagnostic medicine and drug discovery.

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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 MY133-V2000, a fluorinated polymer with a low refractive index matching that of water. A key feature of this protocol is how to overcome the lack of adhesion that is characteristic of fluorinated polymers by using oxygen plasma and by fabricating the device within an acrylic holder to provide the extra mechanical force required to seal the channel against the PDMS substrate (Figure 1). The low refractive index of the...

Watch this Scientific Journal Video about Fabrication of Refractive-index-matched Devices for Biomedical Microfluidics at JoVE.com

Fabrication of Refractive-index-matched Devices for Biomedical Microfluidics

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 ...

This method can help address an important challenge in combining microscopy with microfluidics by enabling refractive index matching between the microchannel structure and aqueous media inside the channel. The main advantage of this technique is that it is compatible with fabrication methods commonly used to create microfluidic devices. Another key advantage of our approach is method used for sealing microchannels made of materials with low adhesion, which is a common problem among well refractive index polymers.Visual demonstration of this method is crucial, as the steps to seal the channel can be difficult to get right. For example, it is important that reservoirs are unobstructed during clamping. Begin this procedure with fabrication of polydimethylsiloxane, or PDMS, negative, as described in the text protocol.Then, fill the PDMS negative with 400 microliters of MY-133-V-2000. The negative must be slightly overfilled. Place the MY-133-V-2000 filled PDMS negative into the vacuum chamber for two hours to remove any bubbles.After removing the MY-133-V-2000 from the vacuum chamber, press a glass slide against the top of the slightly overfilled negative to create a flat surface of MY-133-V-2000 and to prevent oxygen from inhibiting polymerization. Next, insert the MY-133-V-2000 into a 400 watt UV oven. Set the UV radiation to 50%of the maximum intensity for 300 seconds to cure the microchannel.This is a power of approximately 4.5 joules per square centimeter, roughly double the minimum curing power recommended by the manufacturer. After cutting the acrylic as described in the text protocol, place two small drops on the edge of the acrylic and then use a disposable tool to evenly spread the glue. Place the glass substrate onto the acrylic and allow the glue to dry using the weight of the glass to hold it in place.Coat the exposed glass with 100 microliters of PDMS using a positive displacement pipette. Then, insert the base layer into a vacuum spin coater to evenly coat the glass with a PDMS film approximately 10 micrometers thick using a speed of 1500 RPM for two minutes. Remove the base layer from the spin coater and carefully wipe away any excess PDMS that coated the acrylic with acetone and paper towel.Be careful not to disturb the uncured PDMS. Now, bake the base layer with the PDMS in an oven at 65 degrees Celsius for two hours to cure the PDMS. Pipette one milliliter of PDMS onto a glass slide and then use another glass slide to evenly spread the PDMS until it starts to ooze out the sides.Cure this on a hot plate at 150 degrees Celsius for 10 minutes. Cut the cured PDMS into a rectangle with the same dimensions as the MY-133-V-2000 device. Then, using the midlayer of the acrylic as a mold, punch holes for the reservoir and cut a square viewing window in the PDMS to make the PDMS gasket.Now, remove the cured MY-133-V-2000 from the UV oven and remove the channel from the negative and place it onto a flat substrate with the channel side facing up. Remove the base layer containing the cured PDMS from the 65 degree oven and place it onto the substrate. Then, place the substrate with both the MY-133-V-2000 channel and the base layer into the oxygen plasma cleaner.Set the vacuum pressure to 200 millitorr and the radio frequency level to high. Proceed to surface treat the channel and glass substrate for 30 seconds. The oxygen plasma should glow blue when the plasma cleaner is switched on.After 30 seconds, remove both the microchannel and the base layer from the plasma cleaner. Using forceps, immediately place the MY-133-V-2000 channel side down in the rectangular cutout of the base layer acrylic, such that it contacts the PDMS. Now, place the PDMS gasket on top of the MY-133-V-2000 device, lining up the holes in the gasket with the reservoirs in the device.The use of this PDMS gasket is crucial because PDMS is more elastic than MY-133-V-2000 and can therefore be compressed more. This allows the force to be transferred from the acrylic to the microchannel without breaking the glass. At this point, place the unfinished device on a raised platform above the bench to provide a clamping surface for device assembly.Then, extract three milliliters of acrylic cement using a syringe. Distribute enough acrylic cement on top of the base layer to provide a thin coating. Make the coat as even as possible and do not let the material seep into the channel.Now, place the mid layer acrylic piece on top of the base layer acrylic. Make sure that the holes line up with the reservoirs of the MY-133-V-2000 channel. Clamp the mid layer down onto the base layer as tightly as possible and hold it for two minutes while the cement hardens to bond the pieces of acrylic together.Ensure that the reservoirs are not obstructed during this step to prevent air from being trapped underneath the MY-133-V-2000 device. Air trapped between the MY-133-V-2000 and the substrate will remain trapped if the reservoirs are obstructed. This will cause leaks in the finished device.Therefore, it is critical that the holes are not obstructed during this step. After the cement hardens, remove the pressure from the device. Place the acrylic cement on the midlayer in the locations of anticipated contact with the top layer of the acrylic.Now, place the top layer of the acrylic on the midlayer piece of the acrylic, ensuring that the holes are aligned with the reservoirs. Allow it to rest on the bench for two minutes while the acrylic cement dries. Test the MY-133-V-2000 device by adding 10 microliters of food dye or deionized water into one of the reservoir holes to test for the adhesion and flow.Insert a tube into the opposite reservoir and connect the other end to a vacuum trap. Turn the vacuum on to pull the dye or water through the channel to ensure that a successful device was created. Check under a microscope to verify that there are no leaks in the channel or reservoir.Using the vacuum, remove the dye from both reservoirs. Then, rinse the reservoirs with ethanol. Place the ethanol in one reservoir.Pull the ethanol through using the vacuum. Allow the ethanol to sit at room temperature for 10 minutes. Spray the channel thoroughly with ethanol and put it into a sterile polystyrene dish.Wrap it tightly with Parafilm and store until needed. The heat map shown here represents the mass of FOD adherent MCF-7 cells in a microchannel. Few artifacts are visible near the walls of the microchannel.In this measurement, a single aderent MCF-7 cell can be seen on the edge of the microchannel next to the wall. The lack of artifacts near the wall allows precise quantitative measurements to be obtained, even when cells are in close proximity to the channel wall. This final figure shows a pair of MCF-7 cells seated near the microchannel wall.The mass of these cells at each pixel can be precisely measured using quantitative phase microscopy. While following this protocol, it's important not to obstruct the reservoirs when clamping the device while the acrylic cement dries. Obstructing the holes traps air between the MY-133-V-2000 and the substrate.This will cause leaks in the channel during flow. The use of MY-133-V-2000 as a fabrication material for microfluidic devices provides a distinct advantage over traditional materials by greatly reducing artifacts near microchannel walls. This allows quantitative measurements to be made in close proximity to microchannel structures.Devices made with this protocol are compatible with any commonly used microscopy technique. This protocol is based on cell pathography so it's compatible with other advanced microfluidic techniques such as cell sorters or gradient generators for studying cell behavior. Overall, this technique arms researchers with a new tool for biomedical microfluidic assays based on microscopy.The extra precision it grants to quantitative measurements in microchannels has important applications in many fields, including drug discovery and single cell analysis.

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Research

JoVE Journal

Engineering

Fabrication, Operation and Flow Visualization in Surface-acoustic-wave-driven Acoustic-counterflow Microfluidics

Surface acoustic waves (SAWs) can be used to drive liquids in portable microfluidic chips via the acoustic counterflow phenomenon. In this video we present the fabrication protocol for a multilayered SAW acoustic counterflow device. The device is fabricated starting from a lithium niobate (LN) substrate onto which two interdigital transducers (IDTs) and appropriate markers are patterned. A polydimethylsiloxane (PDMS) channel cast on an SU8 master mold is finally bonded on the patterned substrate. Following the fabrication procedure, we show the techniques that allow the characterization and operation of the acoustic counterflow device in order to pump fluids through the PDMS channel grid. We finally present the procedure to visualize liquid flow in the channels. The protocol is used to show on-chip fluid pumping under different flow regimes such as laminar flow and more complicated dynamics characterized by vortices and particle accumulation domains.

In this video we first describe fabrication and operation procedures of a surface acoustic wave (SAW) acoustic counterflow device. We then demonstrate an experimental setup that allows for both qualitative flow visualization and quantitative analysis of complex flows within the SAW pumping device.

Surface acoustic waves (SAWs) can be used to drive liquids in portable microfluidic chips via the acoustic counterflow phenomenon. In this video we ...

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.

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 ...

Fabrication and Testing of Microfluidic Optomechanical Oscillators

Cavity optomechanics experiments that parametrically couple the phonon modes and photon modes have been investigated in various optical systems including microresonators. However, because of the increased acoustic radiative losses during direct liquid immersion of optomechanical devices, almost all published optomechanical experiments have been performed in solid phase. This paper discusses a recently introduced hollow microfluidic optomechanical resonator. Detailed methodology is provided to fabricate these ultra-high-Q microfluidic resonators, perform optomechanical testing, and measure radiation pressure-driven breathing mode and SBS-driven whispering gallery mode parametric vibrations. By confining liquids inside the capillary resonator, high mechanical- and optical- quality factors are simultaneously maintained.

Parametric optomechanical excitations have recently been experimentally demonstrated in microfluidic optomechanical resonators by means of optical radiation pressure and stimulated Brillouin scattering. This paper describes the fabrication of these microfluidic resonators along with methodologies for generating and verifying optomechanical oscillations.

Cavity optomechanics experiments that parametrically couple the phonon modes and photon modes have been investigated in various optical systems including ...

Fabrication of Gate-tunable Graphene Devices for Scanning Tunneling Microscopy Studies with Coulomb Impurities

Owing to its relativistic low-energy charge carriers, the interaction between graphene and various impurities leads to a wealth of new physics and degrees of freedom to control electronic devices. In particular, the behavior of graphene&#8217;s charge carriers in response to potentials from charged Coulomb impurities is predicted to differ significantly from that of most materials. Scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) can provide detailed information on both the spatial and energy dependence of graphene&#39;s electronic structure in the presence of a charged impurity.&#160;The design of a hybrid impurity-graphene device, fabricated using controlled deposition of impurities onto a back-gated graphene surface, has enabled several novel methods for controllably tuning graphene&#8217;s electronic properties.1-8 Electrostatic gating enables control of the charge carrier density in graphene and the ability to reversibly tune the charge2 and/or molecular5 states of an impurity. This paper outlines the process of fabricating a gate-tunable graphene device decorated with individual Coulomb impurities for combined STM/STS studies.2-5 These studies provide valuable insights into the underlying physics, as well as signposts for designing hybrid graphene devices.

This paper details the fabrication process of a gate-tunable graphene device, decorated with Coulomb impurities for scanning tunneling microscopy studies. Mapping the spatially dependent electronic structure of graphene in the presence of charged impurities unveils the unique behavior of its relativistic charge carriers in response to a local Coulomb potential.

Owing to its relativistic low-energy charge carriers, the interaction between graphene and various impurities leads to a wealth of new physics and degrees ...

Fabrication and Operation of Acoustofluidic Devices Supporting Bulk Acoustic Standing Waves for Sheathless Focusing of Particles

Acoustophoresis refers to the displacement of suspended objects in response to directional forces from sound energy. Given that the suspended objects must be smaller than the incident wavelength of sound and the width of the fluidic channels are typically tens to hundreds of micrometers across, acoustofluidic devices typically use ultrasonic waves generated from a piezoelectric transducer pulsating at high frequencies (in the megahertz range). At characteristic frequencies that depend on the geometry of the device, it is possible to induce the formation of standing waves that can focus particles along desired fluidic streamlines within a bulk flow. Here, we describe a method for the fabrication of acoustophoretic devices from common materials and clean room equipment. We show representative results for the focusing of particles with positive or negative acoustic contrast factors, which move towards the pressure nodes or antinodes of the standing waves, respectively. These devices offer enormous practical utility for precisely positioning large numbers of microscopic entities (e.g., cells) in stationary or flowing fluids for applications ranging from cytometry to assembly.

Acoustofluidic devices use ultrasonic waves within microfluidic channels to manipulate, concentrate and isolate suspended micro and nanoscopic entities. This protocol describes the fabrication and operation of such a device supporting bulk acoustic standing waves to focus particles in a central streamline without the aid of sheath fluids.

Acoustophoresis refers to the displacement of suspended objects in response to directional forces from sound energy. Given that the suspended objects must ...

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 ...

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 ...