Elastic conductors for new sensing applications

201306047919620Researchers from North Carolina State University have developed elastic conductors made from silver nanowires, as the basis of stretchable electronic devices.

The silver nanowires can be printed to fabricate patterned stretchable conductorsStretchable circuitry could be used, for example, to create tactile, strain and motion sensors in wearable or conformable applications.

Dr Yong Zhu, an assistant professor of mechanical and aerospace engineering at NC State, and Feng Xu, a PhD student in Zhu’s lab have developed elastic conductors using silver nanowires. Silver has very high electric conductivity. The technique developed at NC State embeds silver nanowires in a polymer that can withstand significant stretching without adversely affecting the material’s conductivity. This makes it attractive as a component for use in stretchable electronic devices.

Simple fabrication

Silver nanowires are placed on a silicon plate and a liquid polymer is poured over the silicon substrate, which flows around the silver nanowires. High heat turns the polymer from a liquid into an elastic solid, trapping the nanowires in the polymer. The polymer is peeled off the silicon plate.

Zhu says the elastic conductor technology could be commercially viable within five years. Fabrication is simple and is compatible with printing and patterning techniques, including screen and inkjet. Zhu’s team has made some prototypes, filed for patents and discussions about next steps towards commercialisation are taking place. When the polymer is stretched and relaxed, the surface containing nanowires buckles, creating a composite that is wavy on the side that contains silver nanowires and flat on the other.

After the nanowire-embedded surface has buckled, the material can be stretched up to 50% of its elongation, or tensile strain, without affecting the conductivity of the silver nanowires, because the buckled shape of the material allows the nanowires to stay in a fixed position in relation to each other, as the polymer is being stretched.

The research was supported by the National Science Foundation.

Tetrapod Nanocrystals as Fluorescent Stress Probes of Electrospun Nanocomposites

Abstract Image



A nanoscale, visible-light, self-sensing stress probe would be highly desirable in a variety of biological, imaging, and materials engineering applications, especially a device that does not alter the mechanical properties of the material it seeks to probe. Here we present the CdSe–CdS tetrapod quantum dot, incorporated into polymer matrices via electrospinning, as an in situ luminescent stress probe for the mechanical properties of polymer fibers. The mechanooptical sensing performance is enhanced with increasing nanocrystal concentration while causing minimal change in the mechanical properties even up to 20 wt % incorporation. The tetrapod nanoprobe is elastic and recoverable and undergoes no permanent change in sensing ability even upon many cycles of loading to failure. Direct comparisons to side-by-side traditional mechanical tests further validate the tetrapod as a luminescent stress probe. The tetrapod fluorescence stress–strain curve shape matches well with uniaxial stress–strain curves measured mechanically at all filler concentrations reported.

Shedding a Light on the Cause for Nanoparticle Size Distribution

QDOTS imagesCAKXSY1K 8(Nanowerk News) When buying shoes it does not matter  how good-looking the shoes might be if the size does not fit. This is similar  with nanoparticles, which are made by the so-called emulsionsolvent evaporation  process. This process allows for the production of nanoparticles with high  purity. Nevertheless they can still be improved: so far, their size distribution  cannot be fully controlled. However, a defined size is of prime importance for  future applications, whether it is for drug delivery or for intelligent  coatings.
An interdisciplinary and international research collaboration at  the Max Planck Institute for Polymer Research in Mainz was able to rule out  coalescence as reason for the borad nanoparticle size distribution. Coalescence  describes the tendency of colloidal droplets to melt together. For the first  time, Daniel Crespy, who is group leader in the department of Katharina  Landfester, was able to prove that the coalescence between droplets during the  process is not significantly responsible for the broad size distribution of the  particles.
“This study elucidates the mechanism of a common process used  for the preparation of nanoparticles,“ says Daniel Crespy about his research  results.
The chemist labeled the original materials prior to the  preparation of the nanoparticles. Some polymers were labeled with red and others  with blue dyes. During the synthesis, the polymers and a solventwere emulsified  in water. After the evaporation of the solvent, solid nanoparticles are  obtained. This is a common method to produce all types of nanoparticles.  Crespy’s trick: Upon adding both red- and blue-labeled polymers to the solvent,  nanoparticles with both colors were obtained. The so-called negative control  shows that if red and blue particles are mixed, no aggregation occurs because  species with both dyes were not detected.
What happens if a red emulsion from polymer and solvent is mixed  with a blue emulsion? Less than every twelfth particle –around 8 percent – were  labeled with both red and blue dyes, which means that coalescence does not play  a significant role in the process.
For the first time, the scientists were able to directly  quantify the occurrence of coalescence. Together with Kaloian Koynov, who is  physicist and expert for spectroscopic methods at the MPI-P, Crespy could  monitor the coalescence of nanometer sized droplets by fluorescence correlation  spectroscopy.
The experimental results were finally confirmed by simulations  based on Monte-Carlo algorithms performed by Davide Donadio, group leader of a  Max Planck Research Group. Thanks to this study (“Particle Formation in the Emulsion-Solvent Evaporation  Process”), the reason for the broad size distribution could be attributed to  the process itself.
Source: Max Planck Institute for Polymer Research

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New process to make nanospheres could enable advances across multiple industries

QDOTS imagesCAKXSY1K 8(Nanowerk News) A patent-pending technology to produce  nanospheres developed by a research team at North Dakota State University,  Fargo, could enable advances across multiple industries, including electronics,  manufacturing, and biomedical sectors.


The environmentally-friendly process produces polymer-based  nanospheres (tiny microscopic particles) that are uniform in size and shape,  while being low-cost and easily reproducible. The process developed at NDSU  allows scale-up of operation to high production levels, without requiring  specialized manufacturing equipment.

The environmentally-friendly process oxidizes ozone in water to produce  polymer-based nanospheres, ranging from 70 to 400 nanometers in diameter, that  are uniform in size and shape, stay suspended in solution, and are easily  removed using a centrifuge. The scanning electron microscopy image depicts the  uniform spherical morphology of these nanospheres.

A 3 a.m. Eureka! moment

Dr. Victoria Gelling, associate professor in the Department of  Coatings and Polymeric Materials at NDSU, had a “Eureka!” moment when she woke  early one morning – 3 a.m., to be precise, an hour when most of us are still  sleeping. Dr. Gelling used early morning creativity to imagine a new way to  oxidize monomers, which are relatively small and simple molecules, into  polymers, which are larger, more complex molecules that can be used to create  synthetic materials. Dr. Gelling hypothesized that oxidizing ozone in water  might accomplish this task.

Later that day in the lab, Dr. Gelling and her team tested the  hypothesis. On the first try, they created a suspension of nearly perfectly  rounded, uniformly-sized nanospheres, ranging from 70 to 400 nanometers in  diameter. In addition to their uniform size, the nanospheres stay suspended in  the solution, and are easily removed using a centrifuge.

“The synthesis of the nanospheres is rather simple, with no  other chemicals required other than water, ozone, and the small molecules which  will become the polymers,” said Dr. Gelling. “We also have tight control of the  size, as they are beautiful, perfect marbles.”

Given their uniform size and shape, the nanospheres could have  uses across multiple industries. According to Dr. Gelling, such nanospheres  could be used to:

  • Produce  high-performance electronic devices and energy-efficient digital displays
  • Create  materials with high conductivity and smaller parts for consumer electronics
  • Deliver  medicine directly to diseased cells in the body
  • Provide  antibacterial coating on dressing for wounds
  • Develop  nanosensors to aid in early disease detection
  • Create  coatings that provide increased protection against corrosion and  abrasion

Watch the Video Here: http://youtu.be/ndK-NzULfAk

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Self-assembling Solar-harvesting Films Reveals New Low-Cost Tool for 3D Circuit Printing

4 March 2013 (created 4 March 2013)

QDOTS imagesCAKXSY1K 8Scientists from Imperial College London, working at the Institut Laue-Langevin, have presented a new way of positioning nanoparticles in plastics, with important applications in the production of coatings and photovoltaic material that harvest energy from the sun.  The study used neutrons to understand the role that light – even ambient light – plays in the stabilisation of these notoriously unstable thin films. As a proof of concept the team have shown how the combination of heat and low intensity visible and UV light could in future be used as a precise, low-cost tool for 3D printing of self-assembling, thin-film circuits on these films.
Thin films made up of long organic molecule chains called polymers and fullerenes (large football-shaped molecules composed entirely of carbon) are used mainly in polymer solar cells where they emit electrons when exposed to visible or ultraviolet sun rays. These so-called photovoltaic materials can generate electrical power by converting solar radiation into direct electrical current.
Polymer solar cells are of significant interest for low-power electronics, such as autonomous wireless sensor networks used to monitor everything from ocean temperature to stress inside a car engine. These fullerene-polymer mixtures are particularly appealing because they are lightweight, inexpensive to make, flexible, customisable on the molecular level, and relatively environmentally-friendly.
However current polymer solar cells only offer about one third of the efficiency of other energy harvesting materials, and are very unstable.
In order to improve science’s understanding of the dynamics of these systems and therefore their operational performance, the team carried out neutron reflectometry experiments at the ILL, the world’s flagship centre for neutron science, on a simple model film made up of pure fullerenes with a flexible polymer. Neutron reflectometry is a non-destructive technique that allows you to ‘shave’ layers off these thin films to look at what happens to the fullerenes and the polymers separately, at atomic scale resolution, throughout their depth.
Whilst previous theories suggested that thin film stabilisation was linked to the formation of an expelled fullerene nanoparticle layer at the substrate interface, neutron reflectometry experiments showed that the carbon “footballs” remain evenly distributed throughout the layer. Instead, the team revealed that the stabilisation of the films was caused by a form of photo-crosslinking of the fullerenes. The process imparts greater structural integrity to films, which means that ultrathin films, (down to 10000 times smaller than a human hair) readily become stable with trace amounts of fullerene.
The implications of this finding are significant, particularly in the potential to create much thinner plastic devices which remain stable, with increased efficiency and lifetime (whilst the smaller amount of material required minimises their environmental impact).

The light sensitivity also suggests a unique and simple tool for imparting patterns and designs onto these notoriously unstable films. To prove the concept the team used a photomask to spatially control the distribution of light and added heat. The combination causes the fullerenes to self-assemble into well-defined connected and disconnected patterns, on demand, simply by heating the film until it starts to soften. This results in spontaneous topography and may form the basis of a low-cost tool for 3D printing of thin film circuits.

Other potential applications could include patterning of sensors or biomedical scaffolds.
In the future, the team is looking to apply its findings to conjugated polymers and fullerene derivatives, more common in commercial films, and industrial thin film coatings.

Source: From A neutron investigation into self-assembling solar-harvesting films reveals new low-cost tool for 3D circuit printing. This work is detailed in the paper “Patterning Polymer–Fullerene Nanocomposite Thin Films with Light” by Him Cheng Wong, Anthony M. Higgins, Andrew R. Wildes, Jack F. Douglas, João T. Cabral.

Effort to mass-produce flexible nanoscale electronics

Case Western Reserve University researchers have won a $1.2 million grant to develop technology for mass-producing flexible electronic devices at a whole new level of small.

As they’re devising new tools and techniques to make wires narrower than a particle of smoke, they’re also creating ways to build them in flexible materials and package the electronics in waterproofing layers of durable plastics.

The team of engineers, who specialize in different fields, ultimately aims to build flexible electronics that bend with the realities of life: Health-monitoring sensors that can be worn on or under the skin and foldable electronic devices as thin as a sheet of plastic wrap. And, further down the road, implantable nerve-stimulating electrodes that enable patients to regain control from paralysis or master a prosthetic limb.

Thinking bigger, the team believes the technology could be used to crank out rolls of thin-film solar panels that stand up to decades in the elements. Current thin-film panels are plagued with short life spans due to seepage between layers.

“The commercial development of nanoelectromechanical systems is limited by access to low-cost, high output—we call it ‘throughput’—processing tools,” says Christian Zorman, an associate professor of electrical engineering and computer science and lead researcher on the grant. “We’re trying to address that bottleneck.”

With this four-year National Science Foundation Scalable Nanomanufacturing Program grant, Zorman and his colleagues will push alternative technologies they’ve created to make wires and other metal structures less than 100 nm.

Currently, devices that combine electronic and mechanical functions are being made this small using electron beam lithography. But electron beams are too energetic to use on flexible plastics and require very high vacuum, which significantly limits throughput, is costly, and very time-consuming—all impediments to mass production.

Using inkjet printers to build small devices has proven cheap and effective, but getting down into the nanometers has been difficult.

Philip Feng, an assistant professor of electrical engineering and computer science, specializes in nanofabrication and devices. Joao Maia, an associate professor of macromolecular science and engineering, is an expert at making nanolayered polymers.

R. Mohan Sankaran, an associate chemical engineering professor, developed the technology to use microplasmas as a manufacturing tool. Zorman spent the last two decades developing techniques used to build microelectromechanical devices for harsh environments and biomedical applications.

When Feng and Zorman saw Sankaran’s work “we realized this could revolutionize nanoscale manufacturing,” Zorman says.

A plasma is a state of matter similar to a gas but a portion is ionized, that is particles are gaining or losing electrons and becoming charged. A spark is an example of a plasma, but it’s hot and uncontrollable.

Sankaran makes a controllable microplasma by ionizing argon gas as it is pumped out of a tube a hair-width across. “The plasma is like a pencil,” Sankaran says, “You can use it to draw a line or any pattern you want.”

To get down to nanometers, Feng must make stencils of nano-sized wires, circuits, and other desired forms. He’ll use a durable silicon carbide material Zorman has developed.

“To get to 100 nm or less,” Feng says, “we must study the laws of scaling, the materials used, and reactions that a microplasma can induce, such as the reactions on the surface of a polymer and inside the polymer, and to compare this process side-by-side with the electron beam lithography.”

As they scale down, Maia will focus on sealing the electronics from moisture.

“A lot of people are working on flexible electronics, but the problem is the product’s lifetime is short because moisture enters and decreases resistivity, shorts out or corrodes the electronics,” Maia says. “If you have to change out your flexible device every two weeks or two months, that’s not such a good thing.”

Maia will make sheets of polymers that include a nanolayer embedded with metal salts, such as silver nitride or gold chloride. These are the precursors of the wires and metallic structures needed to make the electronics.

The sheet will roll through a production line and pause under stencils. A set of microplasmas above the stencils will fire.

In preliminary tests on a stationary piece of film, electrons from the microplasma travel through the stencil and into the polymer where they turn the metal salts into conductive chains of metal particles that form wires and structures, like spray paint and a stencil form letters and numbers.

The sheet can then be dipped in a solution to dissolve the unexposed metal salts, to be recycled.

More layers or combinations of layers will be added to make the sheet watertight.

If multiple devices or packaging layers are needed, the sheets can be looped back through the process.

Nanoparticles: Making gold economical for sensing

Published online 10 October 2012

Gold nanocluster arrays developed at A*A*STAR are well suited for commercial applications of a high-performance sensing technique

Schematic of the nanocluster SERS substrate in planar chip and fiber-optic configurations. The dome shape of the gold nanoclusters reflects the shape of the hemispherical polymer nanostructures on the underlying surface. The red/green clusters represent the molecules being analyzed. The arrays are densely packed and regularly spaced (inset: electron micrograph of the arrays).

Reproduced, with permission, from Ref. 1 © 2012 American Chemical Society (inset); © 2012 A*STAR Institute of Materials Research and Engineering (main image)

Cancer, food pathogens and biosecurity threats can all be detected using a sensing technique called surface enhanced Raman spectroscopy (SERS). To meet ever-increasing demands in sensitivity, however, signals from molecules of these agents require massive enhancement, and current SERS sensors require optimization. An A*STAR-led research team recently fabricated a remarkably regular array of closely packed gold nanoparticle clusters that will improve SERS sensors1.

So-called ‘Raman scattering’ occurs when molecules scatter at wavelengths not present in the incident light. These molecules can be detected with SERS sensors by bringing them into contact with a nanostructured metal surface, illuminated by a laser at a particular wavelength. An ideal sensor surface should have: dense packing of metal nanostructures, commonly gold or silver, to intensify Raman scattering; a regular arrangement to produce repeatable signal levels; economical construction; and robustness to sustain sensing performance over time.

Few of the many existing approaches succeed in all categories. However, Fung Ling Yap and Sivashankar Krishnamoorthy at the A*STAR Institute of Materials Research and Engineering, Singapore, and co-workers produced closely packed nanocluster arrays of gold that incorporate the most desirable aspects for fabrication and sensing. In addition to flat surfaces, they also succeeded in coating fiber-optic tips with similarly dense nanocluster arrays (see image), which is a particularly promising development for remote-sensing applications, such as hazardous waste monitoring.

The researchers self-assembled their arrays by using surfaces coated with self-formed polymer nanoparticles, to which smaller gold nanoparticles spontaneously attached to form clusters. “It was surprising to reliably attain feature separations of less than 10 nanometers, at high yield, across macroscopic areas using simple processes such as coating and adsorption,” notes Krishnamoorthy.

By varying the size and density of the polymer features, Krishnamoorthy, Yap and co-workers tuned the cluster size and density to maximize SERS enhancements. Their technique is also efficient: less than 10 milligrams of the polymer and 100 milligrams of gold nanoparticles are needed to coat an entire 100 millimeter diameter wafer, or approximately 200 fiber tips. Both the polymer and the nanoparticles can be mass-produced at low cost. By virtue of being entirely ‘self-assembled’, the technique does not require specialized equipment or a custom-built clean room, so it is well suited to low-cost commercial implementation.

“We have filed patent applications for the work in Singapore, the USA and China,” says Krishnamoorthy. “The arrays are close to commercial exploitation as disposable sensor chips for use in portable SERS sensors, in collaboration with industry.”

The A*STAR-affiliated researchers contributing to this research are from the Institute of Materials Research and Engineering