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Graphene and Germanium: A Happy Marriage With Exceptional Conductivity

Graphene became the subject of much research because its electrical, mechanical, and optical properties make it an excellent material for electronics. The conductivity of freestanding graphene is comparable to that of copper. However, using graphene in electronic components requires a substrate to support it, and researchers were faced with a problem: graphene's electrical properties degrade when bonded to most substrates. For example, bonded to silicon dioxide, a material widely used in electronics because of its good insulating properties, graphene's conductivity decreases by two to three orders of magnitude.

Now a team of researchers has shown that graphene, when deposited on a germanium substrate covered with a thin germanium oxide layer, acquires excellent electrical properties, and its conductivity even improves compared to pure graphene. The team, from the University of Wisconsin-Madison and University of Notre Dame, reported their findings in ACS Nano earlier this month.

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Graphene Biosensor Is Faster and More Sensitive Than ELISA

With its attractive electrical conductivity properties and its large surface to volume ratio, graphene has always presented an attractive possibility for researchers looking to develop new generations of biosensors.

Researchers at Swansea University in the UK have been able to exploit those properties in graphene by developing a technique to produce it over a large area with consistent quality.

In research published in the Institute of Physics journal 2D Materials, the researchers achieved improved size and quality by abandoning the traditional exfoliation technique and instead producing it with an epitaxial growth method that deposits the graphene on a large, semi-insulating substrate of silicon carbide.

After creating device patterns on the graphene with semiconductor processing techniques, the researchers attached bioreceptor molecules. The molecules serve to bind target molecules that are found in blood, saliva or urine. In this case, the target molecule was 8-hydroxydeoxyguanosine (8-OHdG), which is produced when DNA is damaged. When it appears at elevated levels, it is a reliable indicator of an increased risk of developing several cancers.

When the 8-OHdG molecules are present in a sample, they cause a change in the channel resistance in the biosensor. Based on this method, the researchers were able to detect the molecule at concentrations as low as 0.1 nanograms per milliliter. This five times as sensitive as enzyme-linked immunosorbent assays (ELISAs), which are currently used for biomarker analysis. Not only was the graphene-based nanosensor more sensitive, it was a good deal faster than an ELISA test, completing its analysis of a sample in minutes.

“Now that we’ve created the first proof-of-concept biosensor using epitaxial graphene, we will look to investigate a range of different biomarkers associated with different diseases and conditions, as well as detecting a number of different biomarkers on the same chip," said Dr. Owen Guy, a co-author of the study, in a press release.

Are Multiferroics the Ultimate Replacement for Flash Memory?

Researchers at The City College of New York with collaborators from Drexel, Columbia, Brookhaven National Laboratory, and China’s South University of Science and Technology, have developed a new kind of material, called a complex oxide, that one researcher described as potentially leading to the “ultimate replacement for flash memory”.

The work, which was published in the Nature online journal Scientific Reports, involved the development of a single material that combines both magnetic and ferroelectric properties—a multiferroic. By joining these two properties it becomes possible to control charges using magnetic fields and spins simply by applying a voltage. This could lead to new designs in both logic circuits and spintronics, the materials' discoverers claim.

A few years back, research out of Tyndall National Institute in Ireland suggested that it could be possible to use atomic layer deposition to lay down rare earth oxides and create “a one terabyte USB stick in the near future.”

This latest research appears to further the prospects of that outcome by developing a process to build the new complex oxides using common elements: barium, titanium, and manganese. The novel material belongs to the Hollandite crystal group, which is a mineral composed of manganate of barium and manganese. 

For nearly two decades, scientists have predicted that inorganic substances like this had a ferroelectric nature, and this work has confirmed that prediction.

“The Holy Grail in this field is the combination of both magnetic and ferroelectric elements at room temperature with a sufficient magnitude of interaction,” said Stephen O’Brien, associate professor of chemistry at The City College, in a press release. He added that the material could be the “ultimate replacement for flash memory” or smaller devices with massive storage capacities.

O’Brien is apparently not alone in his optimism for this material, with the noted “father of integrated ferroelectrics,” J.F. Scott of the University of Cambridge, making it known that he believes that multiferroics might hold the future for the ultimate memory device.

Weird New Graphene Effect Makes Electrons Scoot Sideways

Electrons are like people – they follow the path of least resistance. In a conductive material, this means running in the same direction as the electric field.

But like people, electrons sometimes ignore the rules. Physicists from MIT and the University of Manchester have developed a new graphene-based material in which electrons move at controllable angles. The research could spawn new types of energy efficient transistors and have huge implications for how electronics are developed, they claim.

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Wine Critics Watch Out: Artificial Tongues Are Getting Better

As it turns out we humans are not as good as we think at discerning differences in wine. While some argue considerable expertise does exist around wine tasting, others have branded that expertise junk (or should that be drunk?) science.

To overcome the junk science aspect of wine tasting, artificial tongue technologies, sometimes referred to as electronic tongues, have been advanced over the years as an objective way to discern wines based on their taste, free from the human wine critic's personal prejudices.

To further the state-of-the-art in artificial tongue technologies, researchers at the Interdisciplinary Nanoscience Centre (iNANO), at Aarhus University, have developed a nanosensor that is capable of measuring the effect of astringency in your mouth when you drink wine.

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Electricity Makes Mortar for Nanotube Bricks

Each allotrope of carbon—diamond, graphite, graphene, and fullerenes—has its unique set of interesting properties. So finding a way to get carbon to form a hybrid of these allotropes has been an enticing concept. The problem with making such hybrids is that it usually entails extreme chemical, temperature, or pressure conditions, leading to a lack of control over the final product.

Now researchers from Northeastern University, MIT, and the Korea Advanced Institute of Science and Technology (KAIST) have developed a simple, highly-scalable method for creating inter-allotropic transformations and hybridizations of carbon that appear across large-area ​carbon networks. Using alternating pulses of electricity across single-walled carbon nanotubes (SWNTs) they transform them into larger-diameter SWNTs, multi-walled CNTs of varying morphologies, or multi-layered graphene nanoribbons. They reported the details in  the journal Nature Communications.

The key feature of the method is that it produces molecular junctions for the carbon nanotubes that have superior electrical and thermal conductivity compared to carbon nanotubes arrays that are junction-free.

To visualize the difference between a CNT array with molecular junctions and one without, the researchers say that the one without is like a wall of bricks without mortar, while the one with molecular junctions is like a brick wall made using mortar.

“We have filled in the gaps with cement,” said co-​​author Swastik Kar, an assistant pro­fessor of physics at Northeastern, in the press release. “We started with single-​​walled carbon nanotubes,” he added, “and then used this pioneering method to bring them together.”

The researchers believe that CNT arrays using these junctions could be useful for reinforcing composite materials. In the last few years, we have begun to see the use of CNTs in composites that actually improve the strength of the composite as opposed to just replacing a regular resin material. (In research back in 2012, scientists in Switzerland demonstrated how using magnetic forces could orient the carbon nanotubes in the composite to impart even greater strength.)

While stronger composites are indeed an attractive characteristic for these new CNT arrays, their improved electrical and thermal conductivity properties should be attractive for electronic applications as well.

Electronic Skin Made From Nanoparticles Offers Early Breast Cancer Detection

Researchers at the Nebraska Center for Materials and Nanoscience at the University of Nebraska have developed a prototype electronic skin made from nanoparticles that they claim can offer an early detection method for breast cancer.

The researchers, who published their findings in the journal ACS Applied Materials & Interfaces, developed a thin-film tactile device, also known as “electronic skin”, in which the contact pressure that corresponds to the shape of the object can be mapped by measuring the local deformation of the tactile-device film.

The research team built the tactile device layer-by-layer using spin coating of polymers in combination with the deposition of 10-nanometer (nm) gold nanoparticles, which are often used in cancer detection and treatment techniques—along with 3-nm cadmium sulfide nanoparticles. The overall multilayer structure consisted of three layers of gold nanoparticles and two layers of cadmium sulfide nanoparticles separated by nine layers of the polymers. All of this was then deposited onto a indium-tin oxide (ITO) glass substrate. The ITO served as the bottom electrode while aluminum foil was used as the top electrode.

In their tests, the researchers embedded objects that simulated lumps into a piece of silicone and pressed the device against it with the same pressure a clinician would use during a breast exam.

The results were significantly better than what a doctor might be able to detect. With the device, the researchers were able to detect an artificial lump as small as 5 millimeters wide that was embedded 20 mm into the silicone.

This compares favorably to clinical breast exams performed by medical professionals, in which they typically don’t find lumps as large as 21 mm wide. It's estimated that if doctors were able to detect irregularities when they’re half the size of those missed 21-mm lumps, a patient’s chances of survival would improve by more than 94 percent.

This test also offers some benefits over other detection techniques, such as magnetic resonance imaging (MRI), which can be very expensive, and mammography, which is often inadequate for young women or women with dense breast tissue.

The researchers also note that it could be used to screen patients for early signs of melanoma and other cancers.

Is the "Buckydiamondoid" the Future of Molecular Electronics?

What happens when you combine a buckyball with a diamondoid? As it turns out something wonderful for the prospects of molecular electronics. In fact, you get a new kind of material that conducts electricity in just one direction.

This conducting of electricity in one direction is the role of rectifiers, which take the form of diodes in computer chips. By shrinking these diodes down to the size of a nanoparticle it could shrink chip size while making devices faster and more powerful.

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First Graphene-enabled Flexible Display Demonstrated

In the UK’s concerted efforts to become a hub for graphene commercialization, one of the key partnerships between academic research and industry has been the one between the Cambridge Graphene Centre located at the University of Cambridge and a number of companies, including Nokia, Dyson, BaE systems, Philips and Plastic Logic. The last on this list, Plastic Logic, was spun out originally from the University of Cambridge in 2000. However, since its beginnings it has required a $200 million investment from RusNano to keep itself afloat back in 2011 for a time called Mountain View, California, home.

Nonetheless, it seems the connections to the old alma mater are still strong. Plastic Logic has developed in partnership with the Cambridge Graphene Centre for what it claims is the first graphene-based flexible display ever produced.

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Nanowire Circuit Guides Both Electricity and Light on the Same Wire

The field of plasmonics—the use surface plasmons generated when photons hit a metal structure—might enable photonic circuits that could do what electronic ICs do, but do it much faster—at the speed of light.  Without plasmonics, photonic circuits would be too large, because they need to accommodate wavelength of light.

In a step toward that goal, a joint research team from the University of Rochester and the Swiss Federal Institute of Technology in Zurich have developed a primitive circuit consisting of a silver nanowire and single-layer flake of molybdenum disulfide (MoS2). This simple circuit can efficiently guide both electricity and light along the same wire.

In the experiment, which was published in the journal Optica, a laser was used to trigger the plasmons on the surface of the wire. The plasmons coming off the nanowire triggered a photoluminescence in the MoS2, which is a two-dimensional material like graphene but has an inherent band gap. Excitons—basically energized electrons bound to positively charged holes that form when light hits a semiconductor—form in the MoS2, and decay into the nanowire plasmons. So, the international team demonstrated that the nanowire serves the dual purpose of exciting the MoS2 via plasmons and recollecting the decaying exciton as nanowire plasmons.

“We have found that there is pronounced nanoscale light-matter interaction between plasmons and atomically thin material that can be exploited for nanophotonic integrated circuits,” said Nick Vamivakas, assistant professor at the University of Rochester, in the press release.

The combination of subwavelength light guidiance and strong nanoscale light-matter interaction they demonstrated could help lead to compact and efficient on-chip optical processing, the researchers believe.

The next step in their research will be to demonstrate the primitive circuit with light emitting diodes.

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Nanoclast

IEEE Spectrum’s nanotechnology blog, featuring news and analysis about the development, applications, and future of science and technology at the nanoscale.

 
Editor
Dexter Johnson
Madrid, Spain
 
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Rachel Courtland
Associate Editor, IEEE Spectrum
New York, NY
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