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Researchers Seek Source of Frequency Fluctuations in Nanoresonators

The expectation has been that nanoelectromechanical systems (NEMS) will replace its technological predecessor microelectromechanical systems (MEMS) in the varied range of applications for which MEMS are currently used. However, the emergence of the usurper technology has not been without some struggles.

There have been durability issues with NEMS devices and there has also been the nagging issue that they never seem to perform quite up to their theoretical limits.

Now in joint research between the Commissariat a l'Energie Atomique (the French Atomic Energy and Alternative Energies Commission or CEA) and the University of Grenbole-Alpes, researchers have taken on the challenge of addressing this shortfall in performance for NEMS devices—in particular nanoresonators—by looking for better ways to measure their performance.

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New Measurement Technique Tests Nanomaterials During Production

The addition of nanomaterials such as graphene and carbon nanotubes to polymers and other materials in order to impart conductivity or mechanical strength is fairly commonplace at this point. It is now possible to buy a nanomaterial for a specific application and know the kind of functionalization that the nanomaterial needs, as well all the dispersion parameters required to extract the full effect of the nanomaterial’s properties in the polymer.

While the growing maturity of this value chain would seem to indicate that all is clear to start churning out materials that are either conductive or super strong or both (and in large part it is), it has still remained difficult to perform non-destructive testing of these materials during production operations such as roll-to-roll processing.

Now researchers at the U.S. National Institute of Standards and Technology (NIST) have developed a non-destructive measurement technique to ensure that nano-enabled materials are reaching their specified technical properties while being mass-produced in a roll-to-roll process.

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Graphene and Quantum Dots Turn a Mobile Device Into a Heart-Rate Monitor

The Mobile World Congress (MWC) held in Barcelona, Spain, last week hosted a Graphene Pavilion that included a number of research institutes operating under the umbrella of the Graphene Flagship, the European Commission’s €1 billion ($1.1 billion) investment aimed at centralizing graphene research throughout Europe. There were some other companies at the event that are not directly tied to the Graphene Flagship, such as Haydale and Zap&Go. (I visited both booths and posted videos that described their technologies.)

Another booth I visited while at the MWC was that of the Institute of Photonic Sciences (ICFO) in Barcelona, Spain. While this blog has covered a fair amount of the research coming out of ICFO over the years, our coverage has mostly followed the more long-range research projects that have attempted to leverage two-dimensional materials and plasmonics for the creation of a new generation of integrated circuits based around photons rather than electrons.

The prospect of integrated photonic circuits based on plasmonics remains firmly in the future. But ICFO was demonstrating what it has been able to fabricate based on its work with graphene and quantum dots. Essentially, what researchers there have built and demonstrated (see the video, below) is a transparent and flexible photodetector.

Last year, we reported on other research out of ICFO in which they developed a graphene-based photodetector that was capable of converting absorbed light into an electrical voltage in less than 50 femtoseconds, bringing switching to the brink of terahertz speeds. And, four years ago, we covered ICFO’s work in combining graphene and quantum dots, which, at the time, they believed would be ideal for automotive night-vision technologies.

We can only speculate on how this latest technology ties into that previous research. When asked if a description of the underlying technology had been published anywhere, the researchers explained that it hadn’t and that they could not provide any further details until it has. 

Nonetheless, the demonstration of the technology did capture the imagination of many folks visiting the booth. One of the applications that ICFO has devised for the transparent, flexible photodetector was as a heart rate monitor. You can see the demonstration of the technology in the video.

Basically what happens is that when a finger is placed on the photodetector, the digit acts as an optical modulator, changing the amount of light hitting the photodetector as your heart beats and sends blood through your fingertip. This change in signal is what generates a pulse rate on the screen of the mobile device.

It was a nifty application of the technology and certainly inspired a lot of gee-whizzes from those who saw it. However, the ICFO researchers didn’t really envision this as a shot at a viable commercial technology, but more of a demonstration of what is possible. I wasn’t quite so sure that they should have been so dismissive. It seemed to me like it would be cool little technology to have on one’s mobile device. We’ll have to see what they publish, maybe they envision a better application potential.

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Scanning Probe Thermometry: A New Tool That Can Take the Temperature of Nanoelectronics

If you were to point to one invention that triggered what has come to be known as the field of nanotechnology, then you would be on pretty safe ground to cite the work of IBM Zurich scientists in creating first the scanning tunneling microscope (STM)  and later the atomic force microscope (AFM).  Both of these were the first tools to give us the capabilities to investigate, characterize and manipulate matter on the nanoscale.

Now, once again, scientists at IBM Zurich have provided us with a new tool to see the nanoscale world: They’ve developed a technique that uses an AFM to measure the temperature of an object on the nanoscale. The resulting device could provide a greater understanding of the capabilities of nanoscale devices, such as a new generation of transistors based on nanomaterials.

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New 2-D Material Hits the Goldilocks "Just Right" Button

Most of the so-called flatlands, the universe of two-dimensional (2-D) materials, is reminiscent of the beds and bowls of porridge sampled by Goldilocks during her short stay at the home of the three bears. Some 2-D materials, like silicene, are unable to remain two-dimensional for very long. Others, like graphene or boron nitride, remain stable but can’t be pressed into service as anything other than metallic conductors or insulators. (Another group of materials tick the stability box and behave as semiconductors, but are not one-atom-thick.)

Now researchers at the University of Kentucky, in collaboration with scientists from Daimler in Germany and the Institute for Electronic Structure and Laser (IESL) in Greece, may have found a formulation that is just right: a 2-D material that is both stable at high temperatures and under mechanical stress and easily fashioned into a semiconductor.

In research described in the journal Physical Review B, Rapid Communications, the researchers discovered that a combination of silicon, boron and nitrogen—all of which are cheap and abundant elements—led to the formation of an extremely stable one-atom-thick material.

In a phone interview, Madhu Menon, the physicist at the University of Kentucky who led the research, said that the stability was created by the fact that silicon and nitrogen form strong double bonds in 2-D form. This stands in contrast to silicene, the two-dimensional form of silicon.

“Silicene is not stable because the silicon atoms do not like to stay as a two-dimensional system,” Menon told IEEE Spectrum. “Silicon likes to have more than three neighbors, so that is why the surface gets puckered. And if you wait long enough, it will go into its 3-D silicon form.”

He added: “This proposed material is quite unique. The double bond between the silicon and nitrogen that leads to its stability in 2-D form was quite a revelation for me. This is unusual for this to happen.”

In the video below, Menon describes the new material and its potential electronic applications.

This new material is like graphene in that it too is a metallic conductor and must be functionalized in order to behave as a semiconductor. However, because its surface is made up of silicon atoms, it’s possible to functionalize the surface to gain a band gap. But with graphene, you cannot easily dope the surface but only the edges—a more complicated and expensive process.

One of the more interesting properties of the proposed silicon-boron-nitrogen material, according to Menon, is that when it is made into nanotubes it always acts as a metallic conductor. Nanotubes made from graphene can be semiconductors or metallic conductors. The prospect of having a material that will consistently form microscopic one-dimensional conductors is very attractive for electronic applications that depend on the availability of conducting channels.

Menon and his colleagues are eager to make the material in the lab, but they will need additional funding to proceed with that work. In the meantime, Menon will continue to characterize the material in simulations, examining its thermal properties and figuring out whether it can form n-type or p-type semiconductors.

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Piezoelectric Graphene Ink Enables Thin-Film Pressure Sensors of Any Size

UK-based Haydale Graphene Industries Plc has established itself over the years as one of the go-to companies if you wanted graphene to have just the right properties for the device you were aiming to develop. If you wanted the graphene to have high conductivity, or maybe conductivity was not as critical as its thermal properties, Haydale was the place you would go to get graphene that did exactly what you wanted.

The task of functionalizing and dispersing graphene so that it bonds with the resin or polymer matrix in which it is being used has proven trickier than many companies had initially thought. Scores of novices have tried mixing batches of graphene into their products, only to have it make the products worse rather than better.

The backbone of Haydale’s business has been providing expertise on how to extract graphene’s attractive properties. But now the company is moving up the value chain, offering its own device based on its functionalized graphene.

Through a development agreement with the Welsh Centre for Printing and Coating (WCPC) at Swansea University, Haydale has taken what it has learned about using graphene to impart electrical conductivity and, together with WCPC, developed a material that, when used in a pressure sensitive sheet, conducts electricity only when pressure is applied. In the video below, Rob Haslett, the Sector Manager of Functional Inks at Haydale, explains how the sensor was developed and how Haydale expects the technology to be applied.

The thin plastic sheet has conductive silver tracks oriented vertically on one side, and silver tracks oriented horizontally on the other side. In between these tracks is a graphene-based conductive ink that is responsive to pressure, making it possible to measure the pressure at any point across the film’s surface.

When the sheet is attached to a computer or intelligent device, it can deliver a readout indicating not only where the pressure is occurring on the film, but also how strong that pressure is. This difference in pressure is shown through different shades of color: Lighter shades mean light pressure and darker shades indicate heavier pressure. This makes it possible to map pressure the way a cartographer would.

The beauty of the sensor is that it is relatively inexpensive and easy to manufacture, according to Haslett. Further, the piezoelectric ink based sensor can be made to any size and shape. So it’s conceivable that you could cover an entire floor with it. Haslett also suggests that the sheet could be shaped into a glove or an insert for a shoe, offering potential applications as a “smart” shoe for sports or medical rehabilitation. Haslett and his colleagues have also considered a number of other applications, including security systems that could detect when something has been moved. 

Haydale says it is looking for partners to work with in developing the sensor for some applications that they might have overlooked.

The Zap&Go graphene supercapacitor charger

Zap&Go's Graphene Supercapacitor Powers Portable Charger

At long last, there is a company that is about to launch a commercially available product based on a graphene-enabled supercapacitor. A UK-based startup called Zap&Go has found a way to exploit the attractive properties of graphene for supercapactiors to fabricate a portable charger and expects to make it available to consumers this year.

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Flexible Optical Metasurfaces Promise "Smart" Contact Lenses

The name of the game in optical metasurfaces is shortening the wavelengths of light. This yields devices that can manipulate light for information processing and also reduce the bulk of the devices based on traditional optics.

Metasurfaces have been pretty good at offering small, flat features, but the integrated metallic resonators they use to filter light according to specified frequencies have lacked efficiency. It was thought that dielectric resonators were an attractive alternative, but they present another problem: though fairly efficient, their frequency-filtering is hard to fine-tune.

Now researchers at RMIT University and the University of Adelaide have delivered the best of both worlds with a dielectric resonator that can be mechanically tuned. The property that makes these mechanically tunable dielectric resonators especially attractive is that they are embedded in a biocompatible polymer that renders them flexible. 

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Graphene's Role as a Superconductor Just Got Better

Graphene is an amazing conductor. The transport of electrons through graphene nanoribbons has even surpassed what scientists thought were the theoretical limits for the material—so much so that electrons moving through it seem to behave almost like photons.

Graphene’s amazing properties as a conductor has inspired some researchers to explore whether the single-atom-thick sheets of carbon could also be made into superconductors. Last year, an international research team from Canada and Germany was able to demonstrate that graphene can be made to behave that way when it’s doped with lithium atoms.

Now researchers in Japan (from Tohoku University and the University of Tokyo) have developed a new method for coaxing graphene to behave as a superconductor that has some important and distinctive differences from the previous research by the Canadian and German researchers.

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Cheap Plasmonic Interferometer Could Enable Prickless Glucose Monitor

Since 2012, IEEE Spectrum has been covering Domenico Pacifici at Brown University as he works to improve the capabilities of plasmonic interferometers. One major application would be glucose monitors that enable diabetics to check glucose levels through saliva instead of blood—no finger pricking necessary.

In his latest research, Pacifici and his team have developed a way to get a plasmonic interferometer to take measurements without the need for a coherent light source.

To have a coherent light, the light waves have to run in parallel, possess the same wavelength, and travel in-phase, which means the peaks and valleys of light waves are in alignment. Producing this kind of light demands expensive and bulky equipment. By eliminating that need, Pacifici’s team has created a far smaller and less expensive way to operate these devices.

“It has always been assumed that coherent light was necessary for plasmonic interferometry,” said Pacifici, in the press release. “But we were able to disprove that assumption.”

In research published in the Nature journal Scientific Reports, Pacifici and his team embedded light emitters in the form of fluorescent atoms directly into the sub-wavelength cavities of the plasmonic interferometers. The result was that even when the source light had very low coherence, the internal emitters could get the interferometer to operate as though the light was coming from a coherent light source.

Plasmonic interferometers operate on the same principles of plasmonics as every other plasmonic device does. When photons of light hit a metal surface, they rattle the electrons in the metal so much that they generate waves of electrons known as surface plasmons.

A plasmonic interferometer exploits this phenomenon by its very architecture, essentially a piece of metal that has a hole—or cavity—at its center and around that hole is carved co-centric grooves. The cavity is around 300 nanometers in diameter and the co-centric grooves are measured in microns.

When the light hits the surface of this device, some of the photons go into the cavity at the center of it while others hit the outer grooves and scatter. The scattered photons excite the electrons on the metal surface to the point where they become waves of surface plasmons. Just like waves on water, the waves move along the surface of the metal until they go into cavity at the center. Here they interfere with the photons that were originally drawn into the cavity generating an interference pattern: you can measure how the light weakens and strengthens coming out of the cavity.

By embedding fluorescent atoms in the cavity of the device, Pacifici’s team made the cavity produce its own surface plasmons. In this way, surface plasmons move out of the cavity onto the surface and then bounce off the co-centric grooves and back into the cavity. When these surface plasmons come in contact with the fluorescent atoms that were its source, it creates an interference with the directly transmitted photon. In this arrangement, the photons in the cavity and the plasmons are coming from the same emitter, so they are naturally coherent and interference occurs even though the emitters (the fluorescent atoms) are excited by incoherent light.

“The important thing here is that this is a self-interference process,” Pacifici said in the release. “It doesn’t matter that you’re using incoherent light to excite the emitters, you still get a coherent process.”

In addition to being able to use incoherent light sources, the architecture provides additional benefits, such as greater accuracy and the internal emitters mean that more delicate samples can be tested.

While this work is really just a test of concept, Pacifici believes that this is such a fundamentally different way for these devices to operate that it represents a significant breakthrough in the field.

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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
 
Contributor
Rachel Courtland
Associate Editor, IEEE Spectrum
New York, NY
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