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Graphene Nanoribbons Get Electrons to Behave Like Photons

An international team of researchers has developed a novel way to produce graphene nanoribbons that enables electrons to travel though it without resistance at room temperature, a property known as ballistic transport. The ballistic transport properties measured by the researchers exceeds previous theoretical limits for graphene by a factor of ten, opening up the potential in the opinion of the researchers for graphene to usher in a new era in electronics despite the material's lack of a band gap. 

The team of researchers, including those at Georgia Institute of Technology along with others from Leibniz Universität Hannover in Germany, the Centre National de la Recherche Scientifique (CNRS) in France and Oak Ridge National Laboratory in the United States, has developed a process for producing graphene nanoribbons on a silicon wafer that results in such high electron mobility that the electrons behave more like photons do when in an optical fiber.

The lack of inherent band gap in graphene—its inability to effectively stop and start the flow of electrons—has remained a major stumbling block in developing it for use in electronics. Of course, there have been various approaches to engineering a band gap into graphene, such as applying a strong electrical field to bilayer graphene. However, Walt de Heer, a Regent's professor in the School of Physics at the Georgia Institute of Technology, believes that we should stop trying to get graphene to behave like silicon, and instead design a new type of electronics that exploits graphene’s unique properties. This is along the lines of what Samsung proposed 18 months ago by developing new switches to exploit graphene’s electron mobility.

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Graphene Circuit Competes Head-to-Head With Silicon Technology

IBM has built on their previous graphene research and developed what is being reported as the best graphene-based integrated circuit (IC) built to date, with 10 000 times better performance than previously reported efforts.

This graphene-based IC serves as a radio frequency receiver that performs signal amplification, filtering and mixing. In tests, the IBM team was able to use the circuit to send text messages (in this case, “IBM”) without any distortion.

“This is the first time that someone has shown graphene devices and circuits to perform modern wireless communication functions comparable to silicon technology,” IBM Research director of physical sciences Supratik Guha said in a release.

The IC, which is fully described in the journal Nature Communications (“Graphene radio frequency receiver integrated circuit”), overcomes major problems previously encountered with graphene-based ICs that cause the transistor performance to degrade.

The key to overcoming this issue was a new manufacturing method. Simply put, the graphene is added late in the process to prevent it from being damaged during other manufacturing steps.

Despite the improved manufacturing method for the IC, the IBM researchers still depended on a costly method for producing the graphene that was used. They believe that if a high-quality graphene could be produced in a roll-to-roll process, the IC would become easier and cheaper to produce.

This latest circuit builds on the first integrated circuit built from graphene—developed by IBM in 2011—that was a broadband radio-frequency mixer, a fundamental component of radios that processes signals by finding the difference between two high-frequency wavelengths.

While others have judged the odds that graphene will yield benefits in electronic applications as slim to none because it lacks an inherent band gap, IBM has stayed on a steady course to test those assumptions. In early 2010, Big Blue researchers engineered a band gap into graphene large enough to pursue the use of graphene in infrared (IR) and terahertz (THz) detectors and emitters. Then a year later, IBM followed up with a graphene transistor capable of operating at 100 gigahertz that has the same gate length as silicon chips with speeds of 40 GHz.  Of course, a transistor on its own can’t do much of anything, so about six months later, IBM reported building the first integrated graphene circuit that was the precursor to this latest version.

In describing the impact of the research, Shu-Jen Han of IBM Research said in an IBM blog:

“Our demonstration has the potential to improve today’s wireless devices’ communication speed, and lead the way toward carbon-based electronics device and circuit applications beyond what is possible with today’s silicon chips. Integrating graphene radio frequency (RF) devices into today’s low-cost silicon technology could also be a way to enable pervasive wireless communications allowing such things as smart sensors and RFID tags to send data signals at significant distances.”

With IBM's apparent relentless pursuit of an IC for a radio frequency receiver, it would seem that seeing these devices in our telephones at some point in the future could be a realistic prospect.

Photo: IBM Research - Zurich

Graphene Composite Offers Critical Fix for Sodium-ion Batteries

Sodium-ion batteries offer an attractive alternative to Li-ion batteries not because they outperform Li-ion batteries, but mainly because of lower costs due to the the nearly unlimited supply of sodium. They are also an attractive alternative in part because unlike their sodium-sulfur battery cousins they can be made in similar sizes to Li-ion batteries.

However, the commercial development of sodium-ion batteries has been hampered by the materials used in the negative electrodes. These swell to as much as 400 to 500 percent their original size, leading to mechanical damage and loss of electrical contact.

Now researchers at Kansas State University have developed a composite, paper-like material made from two 2-dimensional materials—molybdenum disulfide and graphene nanosheets—that has been shown to overcome this shortcoming.

In research published in the journal ACS Nano (“MoS2/Graphene Composite Paper for Sodium-Ion Battery Electrodes”),  the 2-D composite material developed proved resistant to the “alloying” reaction that electrode materials typically suffer when in contact with sodium.

The research also marks the first time that the flexible paper electrode was used in an anode for sodium-ion battery that operates at room temperature.

The Kansas State researchers began their work by looking for better ways to synthesize 2-D materials for rechargeable battery applications. This led them to create the large-area composite paper that they used for negative electrodes.

The paper is a composite of acid-treated layered molybdenum disulfide and chemically modified graphene in an interleaved structured. “The interleaved and porous structure of the paper electrode offers smooth channels for sodium to diffuse in and out as the cell is charged and discharged quickly,” said Gurpreet Singh, an assistant professor at Kansas State and one of the authors of the paper, in a press release. “This design also eliminates the polymeric binders and copper current collector foil used in a traditional battery electrode.”

While the researchers are looking for opportunities to commercialize their technology for rechargeable battery applications, they also feel that they have contributed to the fundamental understanding of how to synthesize 2-D materials.

"This method should allow synthesis of gram quantities of few-layer-thick molybdenum disulfide sheets, which is very crucial for applications such as flexible batteries, supercapacitors, and polymer composites,” Singh said.

Image: Gurpreet Singh

Can Graphene Enable Thermal Transistors?

One of the critical functions for electronic devices is a process known as rectification in which an electrical current can be forced to move in one direction and then the opposite one. Rectification makes possible electronics devices such as transistors, diodes and memory circuits.

Now researchers at Purdue University's School of Mechanical Engineering and Birck Nanotechnology Center have determined that a material based on graphene is capable of producing this rectification effect not on electric current but on heat flow.

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‘Borophene’ Might Be Joining Graphene in the 2-D Material Club

The world of two-dimensional (2-D) materials has just gotten a little more crowded. If graphene, boron nitride, molybendum disulfide and silicene weren’t quite enough, we now may have something to join the mix in the 2-D universe that will go by the name “borophene.”

In experiments and simulations based at Brown University, in Providence, R.I., researchers took a big step toward a long theorized material made up of single-atom sheets of boron. The researchers haven’t actually produced borophene, but they did make a needed precursor structure that proves that the material is possible.

In research that was published in the journal Nature Communications (“Planar hexagonal B36 as a potential basis for extended single-atom layer boron sheets”)  a cluster of 36 boron atoms were formed into a structure that resembles theoretical predictions for borphene: a symmetrical, one-atom thick disc with a perfect hexagonal hole in the middle.

The prediction was that boron, which has one fewer electron than carbon, wouldn’t be able to form into the honeycomb-lattice pattern that graphene takes. Instead it would likely form into a triangular lattice and a hexagonal hole would form in the middle of the sheet. That prediction pretty closely resembles what the researchers were able to produce in the lab.

"It’s beautiful,” said Lai-Sheng Wang, professor of chemistry at Brown, in a press release. “It has exact hexagonal symmetry with the hexagonal hole we were looking for. The hole is of real significance here. It suggests that this theoretical calculation about a boron planar structure might be right.”

Producing the boron structure was not anything like using the “Scotch Tape” method for producing graphene. The researchers were able to make the cluster of 36 boron atoms by first shooting a laser at bulk boron to vaporize it into boron atoms. They then shot the vapor of boron atoms with a jet of helium that froze the atoms into clusters. After being frozen, they were zapped again with a laser. This second laser kicks an electron out, which is funneled down a tube of sorts. By measuring the speed of the electron, they can determine how strongly the cluster holds onto its electrons, which is known as the electron binding energy spectrum. This spectrum is distinct, like a fingerprint, to the cluster’s structure.

Based on this binding energy spectrum, which indicated that it was very low energy compared to other boron clusters, certain structures were possible—3000 possible structures in fact. By using a supercomputer the researchers could go through each possibility. When Wang saw that one of the possibilities was a planar disc with the hexagonal hole, he knew that was the structure to investigate.

The researchers plowed on investigating all 3000 structures with a supercomputer and finally concluded that the B36 structure was the one that most closely matched the spectrum measured in the physical experiments.

While theoretically borophene should have even more interesting electronic characteristics than graphene, it’s not clear from this work whether anyone will ever be able to make it.

Image: Wang Lab/Brown University

Two-Dimensional Materials Get Into Hydrogen Gas Production

One of the inconvenient truths about fuel cells for powering automobiles—a key to the establishment of the so-called hydrogen economy—is that it is extremely costly and energy intensive to isolate hydrogen gas.

The last couple of years have produced research using different nanomaterials that can do the job. Last year, University of Buffalo researchers created silicon nanoparticles that generated hydrogen gas nearly instantaneously when water was added to them. In that process, the nanomaterial didn’t need light or electricity to produce the hydrogen. Of course, the downside was that producing the silicon nanoparticles required a fair amount of energy itself, so it wasn’t clear whether this was a viable solution to overcoming the energy costs of hydrogen production.

The main push in nanomaterials for hydrogen gas separation has been artificial photosynthesis approaches in which sunlight rather than electricity is used to split the hydrogen from a water molecule. These efforts stood in contrast to other nanomaterial solutions that entailed simply replacing the platinum catalyst in the standard electrocatalytic process with a nanomaterial.

Now researchers at North Carolina State University (NCSU) have demonstrated that molybdenum sulfide (MoS2) can be used as an effective catalyst for producing hydrogen gas in a solar water-splitting process.

In research published in the journal Nano Letters (“Layer-Dependent Electrocatalysis of MoS2 for Hydrogen Evolution”), the NCSU team demonstrated that while MoS2 it is not as effective a catalyst as platinum, its relative low cost could make it an attractive alternative.

“We found that the thickness of the thin film is very important,” says Dr. Linyou Cao, an assistant professor of materials science and engineering at NCSU, in a press release. “A thin film consisting of a single layer of atoms was the most efficient, with every additional layer of atoms making the catalytic performance approximately five times worse.”

The researchers also have indicated that MoS2 thin films have an ideal band gap for solar water splitting. In a Q&A with an NCSU blog, Cao said:

"The band gap of monolayer MoS2 spans over the redox potentials of water. Its valence band is lower than the potential of water oxidation, and the conduction band is higher than that of water reduction. Additionally, its band gap, about 1.8eV, nicely matches the spectrum of solar radiation."

It should be interesting to see if this discovery that MoS2 makes for an ideal material in solar water splitting compares favorably to other nanomaterials used in artificial photosynthesis approaches.

Photo: North Carolina State University

Cause of 2-D Molybdenum Disulfide's Electronic Shortcomings Revealed

When molybdenum disulfide (MoS2) entered the conversation related to two-dimensional (2-D) alternatives to graphene in electronic applications, some thought that MoS2 had an edge as a transistor material. That thought was inspired by the material's intrinsic rather than engineered band gap, unlike graphene.

However, as researchers learned more about the material, clouds began to appear in the bright and sunny picture of MoS2. Among the drawbacks are less-than-ideal electron mobility and sub-threshold slope.

In collaborative research between IBM's Thomas J. Watson Research Center and Yale University, the culprit behind MoS2’s underwhelming electronic properties has been revealed. The issue turns out to be traps, an issue with which people who study semiconductors are painfully familiar. Traps are impurities or dislocations that can trap an electron or hole and hold it until a pair is completed. It took decades of research to reduce the traps' density at the dielectric-semiconductor interface to a level that allows for high performance silicon transistors.

The research, which was published in the journal Nature Communications (“Electronic transport and device prospects of monolayer molybdenum disulphide grown by chemical vapour deposition”), systematically quantifies the density-of-states and response time of band tail trapping states.

“Our work presents the first systematic understanding of the traps states in 2-D semiconductors based on transition metal dichalcogenides,” explains Tony Low, a researcher at IBM’s Nanometer Scale Science & Technology group and one of the authors of the report. “This work lays the groundwork for future engineering effort in eliminating these traps. Such concerted engineering effort is needed in order for us to harness the real potential of this new material for electronics and photonics.”

The researchers revealed that one limitation on MoS2's electron mobility was the fact that there were actually two types of charge carriers.

“Initially, we found a very low mobility,” said Fengnian Xia, a member of the research group who is an assistant professor in Yale’s electrical engineering department, in a press release. “But after careful analysis, we noticed...some carriers are trapped within the band gap, so these carriers are not really mobile. But the other carriers are in the band, where they exhibit much higher mobility.”

One of the critical features of this research is that they did not use MoS2 produced through mechanical cleavage—the so-called “Scotch-tape” method—but through chemical vapor deposition (CVD), which better lends itself to bulk production.

The researchers say that the trapped carriers are not intrinsic to MoS2 and can be addressed by improving the quality of the material.

Image: Ben Mills/Wikipedia; iStockphoto

Graphene and Perovskite Are a Winning Combination for Photovoltaics

When it comes to graphene and photovoltaics, for the most part it’s only been a story about replacing the indium tin oxide (ITO) used as the transparent electrodes of organic solar cells.

But last year Spanish researchers in collaboration with teams from the Massachusetts Institute of Technology and Max Planck Institute for Polymer Research in Germany started to change the game and took graphene into the conversion and conduction layers of a photovoltaic cell.

Now, Spanish scientists at the Universitat Jaume I in collaboration this time with researchers from Oxford University have developed a photovoltaic system in which graphene and titanium dioxide combine to serve as the charge collector while perovskite acts as the sunlight absorber.

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Theoretical 3-D Semimetal Comes to Life and Mimics Graphene's Properties

While many believe that the key to producing the next generation of chips lies in developing better manufacturing techniques for nanomaterials rather than just creating new nanomaterials, there are others who simply can’t resist the temptation of producing theoretical materials in the real world.

Until 10 years ago, graphene was one of those theoretical materials that had never actually been produced. Now that graphene has been here in the real world with us for a decade it seems there are those who want to move on to spending another decade measuring and characterizing a completely new material that mimics graphene.

One material that has intrigued theorists for the last few years is something called a three-dimensional (3-D) topological Dirac semimetal. Graphene itself is a Dirac semimetal in that at low energies its "electrons are effectively relativistic but with a velocity about 300 times smaller than the velocity of light." But graphene is two-dimensional. A 3D version of this class of materials was believed to have many of the intriguing electronic characteristics of graphene, like high electron mobility, but with it being in three dimensions it could potentially eliminate at least some of the tricky aspects of working with a material that only has two dimensions.

Now a research team from Stanford Linear Accelerator Center (SLAC) and Lawrence Berkeley National Laboratory has produced one of these 3-D topological Dirac semimetals.

In research published in the journal Science Express (“Discovery of a Three-Dimensional Topological Dirac Semimetal, Na3Bi”),  the research team investigated the potential of a sodium-bismuth compound, Na3Bi. This compound had been predicted to be one of these Dirac semimetals, and their research confirmed that it in fact did live up to its predictions.

Of course, calling this “a discovery” is a bit misleading since this material was first produced a long time ago. However, the technology did not exist at that time to measure the material's electronic structure. Still, even now that its electronic structure has been measured Na3Bi is no more usable than it had been when it was first stumbled upon. The researchers concede that the compound is too unstable for it to be used in electronic devices.

Nonetheless, the measurements that were taken seem to indicate that it has some of the attractive electronic properties of graphene. The real trick at this point will be to see if a more stable compound of semimetal can be produced. If this material is to have any impact, it looks like overcoming manufacturability issues will be the trick—just as it was with graphene. The researchers embark on further work to determine ways to produce the compound so that it’s more stable,

Joel E. Moore, a condensed matter physicist at the University of California-Berkeley and Berkeley Lab , was quoted in the press release about the research asking “whether these 3-D semimetals will support as many interesting phenomena as graphene does.” (Moore is described a different class of super-strange materials  topological insulators in the June 2011 issue of IEEE Spectrum.)

So, at present the new materials can’t be produced in a stable compound and there’s still some question as to whether it really does fully mimic graphene. But Moore believes that it could serve as a starting point for other states of matter, thereby leading to an onslaught of new examples that as Moore says, “should lead to a broader consideration by theorists of what interesting physics this class of materials might enable.”

Based on that assessment, it’s fair to say that if graphene actually does have commercial potential in a range of applications, it won’t be facing stiff competition in any of them from 3-D topological Dirac semimetals any time in the near future.

 

Study Shows Silicene Has 'Suicidal Tendencies'

When silicene, the two-dimensional version of silicon, was first introduced back in 2010, some called it a "wonder material." Silicene offered something akin to what graphene had been promising for half-a-decade but this time with an intrinsic (rather than engineered) band gap and without all the headaches of retooling an industry that had shaped itself around silicon for the last 50 years.

But research into silicene has been relatively quiet compared to graphene and its other 2D cousins. Now research out of the MESA+ Research Institute of the University of Twente in the Netherlands has discovered something about silicene that may help explain why silicene success remains elusive: As the researchers put it, the material has "suicidal tendencies."

The research, which was published in the journal Applied Physics Letters (“The Instability of Silicene on Ag”), has thrown into question the practical uses of silicene. “We find that silicene layers are intrinsically unstable against the formation of an 'sp3-like' hybridized, bulk-like silicon structure,” says the abstract of the research paper.

The Dutch researchers used electron microscopy to image in real time the formation of silicene on a film. They evaporated silicon atoms on a surface of silver so that a nearly-closed surface of silicene was formed.

The researchers didn't notice anything out of the ordinary up to this point, but then they observed that as soon as silicon atoms started to be deposited on the silicene layer “a silicon crystal" (silicon in a diamond crystal structure instead of in a honeycomb structure) appeared. Soon all of the material became crystalized, with only silicon remaining in the structure.

In the video below you can watch this all happen step by step. At the beginning, you see the silicene forming on the silver surface (this is the gray you see at the start). Then you see it turn gradually black—this is the formation of silicene islands on the surface of the silver. When this black takes over the surface, the silicene collapses into silicon crystals.

The reason that silicene always reverts back to silicon as soon as more layers are added onto it is that the regular crystal structure of silicon is more favorable than the honeycomb structure of the silicene. Silicene seems to just "kill" itself and simple silicon takes its place.

The researchers believe that attempts to create multiple layers of silicene will always intrinsically fail, but whether this means the end of silicene applications in electronics is hard to say at this point.

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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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