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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.

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.

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.

Tin Oxide: The First Stable p-type 2-D Semiconductor Material

Researchers at the University of Utah have developed the first stable intrinsic p-type (carrying positive charges) 2-D semiconductor material, tin oxide. If this semiconductor is mated with a n-type (carrying electrons) 2-D semiconductor in a transistor, it opens up the possibility of building power-saving two-dimensional complementary logic circuits like the ones in microprocessors today.

"Now we have everything—we have p-type 2-D semiconductors and n-type 2-D semiconductors," said Ashutosh Tiwari, an associate professor at the University of Utah and the leader of the research, in a press release. "Now things will move forward much more quickly."

The potential for 2-D materials—such as graphene and molybdenum disulfide—as an alternative to the three-dimensional silicon, raises hopes for smaller, faster, lower-power transistors.

However, the path to success for these 2-D materials in transistors has not always been clear, whether it be issues such as graphene not being a natural semiconductor or the charge carrier traps that compromise molybdenum disulfide. But possibly the biggest issue has been that all previous 2-D materials have only been stable n-type semiconductors. It has been possible to dope other 2-D materials, such as molybdenum disulfide and tungsten diselenide, to behave as p-type. However, this new tin oxide represents the first intrinsic p-type semiconductor in a 2D material.

In research described in the journal Advanced Electronic Materials, the Utah University researchers overcame this limitation by layering 2-D tin oxide (SnO) onto a sapphire-and-silicon-dioxide (SiO2) substrate. The researchers were then able to fabricate a few field-effect transistors (FETs) from it.

With this development, the attractive properties of 2-D materials in transistors are more fully exploitable. For instance, in 2-D materials charge transport is basically confined to a single plane, meaning electrical and thermal transport properties that can be much better than those of bulk silicon. That could mean chips that consume less power and throw-off less heat.

In an e-mail interview, Tiwari said that the next step in their research will be to build a CMOS (complementary metal oxide semiconductor) using their new p-type 2-D semiconductor.

This post was corrected on 18 February to indicate that some 2-D materials could act as p-type semiconductors when doped.

Quantum Computing With Ordinary CMOS Transistors

Future quantum computers might not be all that different from the one you’re using now. An international team of researchers have created a the most fundamental part of a quantum computer—the quantum bit, or qubit—using only a CMOS transistor that is not much different from those in today’s microprocessors.

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Electronic Qubit Integrated Into Solid-State Switch

Essential for so-called quantum networks and quantum computers is something called a quantum bit, or qubit, which would replace the traditional bits that are stored or transmitted in today’s computers and optical networks. It consists of an ion with an unpaired electron that has two spin states—up or down, or a 0 and 1 in a binary system. But under some conditions, these qubits can be made to have both the 0 and 1 state simultaneously—a quantum state.

One of the problems researchers have faced in incorporating qubits in optical networks is getting the photons to strongly interact with the qubits in a solid-state device. The aim has been to develop something akin to an electro-optic modulator that uses electronic signals to modulate properties of light in today’s optical networks.

Researchers at the University of Maryland have developed a novel design that may have achieved that aim by “combining the light-trapping of photonic crystals with the electron-trapping of quantum dots.

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Introducing One of the Best Thin-film Transistors Ever

Wide-band gap metal-oxide thin-film transistors (TFTs) have never been quite as popular as the ubiquitous metal-oxide semiconductor field-effect transistors (MOSFET). One of the main issues with TFTs has been that they are limited to n-type semiconductor materials that can only move negative charges through them, limiting their electrical output.

While different architectures have been investigated to overcome this, the problem has remained that there just haven’t been p-type wide-band inorganic semiconductor materials that have done the trick. The result has been that TFTs have been limited to low-power applications, such as display screens.

Now researchers at the University of Alberta in Canada say they have come up with a design that will take nearly any wide-band metal-oxide n-type semiconductor used in thin-film transistors and create a p-type channel, or inversion layer, through which positive charges can travel without the need of some new semiconductor material. As proof of their design, the Canadian researchers have created a TFT capable of conducting both electrons (negative charges) and holes (positive charges) resulting in far greater electrical output.

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

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