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.


Super Effective Camera Chips And Solar Cells Possible With Graphene

QDOTS imagesCAKXSY1K 8Theoretically expected, it was now experimentally proven by scientists for the first time that the fascinating new material graphene is also highly efficient at converting light into electricity–which makes it an ideal candidate to boost the sensitivity of imaging sensors and also to increase the maximum conversion efficiency of photovoltaic cells.











(Photo : Mitchell Ong, Stanford School )
This illustration shows lithium atoms (in red) adsorbed to a layer of graphene to create electricity when the graphene is bent, squeezed or twisted.

Current materials used for these applications include silicon and gallium arsenide, but they just generate a single electron for each photon absorbed. Since a photon contains more energy than one electron can carry, much of the energy contained in the incoming light is lost as heat. Graphene on the other hand can generate multiple electrons from absorbing one photon, according to theoretical research that was now confirmed in the lab as described this week in Nature Physics.

Previous work had inspired hope that graphene had this property, says Frank Koppens, a group leader at the Institute of Photonic Sciences in Spain, who led the research. To conduct the experiment, the researchers used two ultrafast light pulses. The first sent a known amount of energy into a single layer of graphene. The second served as a probe that counted the electrons the first one generated.

Koppens said he is “reasonably confident” that the group can enhance the performance of light sensors like those used in cameras, night vision goggles, and certain medical sensors quite soon–after all, his lab is already working on a prototype device to demonstrate the new found capability of graphene.

A second but more difficult application would be solar cells. The material could help to increase the theoretical efficiency limit to about 60%, about twice as much as the 30% limit possible with today’s silicon cells, which currently reach about 20% in the field and 25% in the lab. But Koppens cautions that key engineering challenges stand in the way of that, which includes figuring out how to extract power from a system at all.

The new paper illustrates a “very important concept,” since future devices will depend on an understanding of the physical processes that occur when graphene absorbs light, says says he and colleagues have a still-unpublished paper that describes a similar result. Demonstrating this property in graphene opens a promising new field of research, he says.

Graphene was already exciting as a photovoltaic material because of its unique optical properties, says Andrea Ferrari, a professor of nanotechnology at the University of  Cambridge in the U.K. who was not involved in this research. The material “can work with every possible wavelength you can think of,” he says. “There is no other material in the world with this behavior.” It is also flexible, robust, relatively cheap, and easily integrated with other materials. The new research “adds a third layer of interest to graphene for optics,” he says.

Among Koppens’s collaborators were MIT physics professor Leonid Levitov  and Justin Chien Wen Song, a graduate student in Levitov’s lab, who  helped Koppens interpret the data through theoretical modeling.

One of the major limitations of the use of nanomaterials at industrial scale is their high price. Although the price in recent years is declining and is expected to price of nanomaterials follow this trend in the medium term, there are still many applications where the low profit margin of the product does not allow the inclusion of nanomaterials. This is why it is considered necessary to develop new methods of synthesis of nanoparticles that allow the production of nanomaterials cheaply.

Some studies show that pyrolysis of biomass present an economical alternative to traditional synthesis methods for obtaining carbonaceous nanostructures. For example, researchers at the University of Nuevo Leon in Mexico, have shown that it is possible to obtain coal graphitizable from pyrolysis of walnutshell. These particles are obtained as a solid part in the synthesis process, but obtained solid and liquid fractions may also be utilized.

In addition to these amorphous carbon particles, it is possible to synthesize carbon nanoparticles by pyrolysis of biomass attached to the process of chemical vapor deposition, whose characteristics allow its use in energy.

This development could lead to a substantial drop in prices of nanomaterials as the process could be applied to other products from biomass or waste biomass.