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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.

New Twist on Epitaxial Growth Opens New Possibilities for Two-Dimensional Materials

Some people believe that developing new manufacturing techniques for nanoscale devices, like new types of epitaxy in which crystals are grown on a substrate, may in fact be more critical to producing the next generations of chips than just creating new materials.

Along these lines, researchers at Oak Ridge National Laboratory (ORNL) and the University of Tennessee (UT) have developed a new technique for creating a two-dimensional hybrid material of graphene and boron nitride that has a seamless boundary.

“People call graphene a wonder material that could revolutionize the landscape of nanotechnology and electronics,” ORNL’s An-Ping Li said in a press release. “Indeed, graphene has a lot of potential, but it has limits. To make use of graphene in applications or devices, we need to integrate graphene with other materials.”

Last year, researchers at Rice University developed a process to combine graphene and boron nitride that used lithography techniques to weave the two 2-D materials together. This latest ORNL/UT work, which was published in the journal Science (“Heteroepitaxial Growth of Two-Dimensional Hexagonal Boron Nitride Templated by Graphene Edges”), is based on epitaxy, but with a bit of twist to make the two materials grow together.

The first twist is that the epitaxial growth is oriented horizontally rather than vertically. The graphene is grown on a copper substrate. The edges of the graphene are then etched away and the boron nitride is added through chemical vapor deposition. In this way, the boron nitride does not take on the crystalline structure of the copper substrate but that of the graphene.

“The graphene piece acted as a seed for the epitaxial growth in two-dimensional space, so that the crystallography of the boron nitride is solely determined by the graphene,” UT’s Gong Gu said in the release.

Perhaps even more important than joining the two materials together was that the boundaries between the two materials were atomically precise. It is this atomic precision of the one-dimensional interface between these two materials that could prove the key to seeing the production of practical devices.

In a reference to Nobel laureate Herbert Kroemer’s famous phrase “the interface is the device,” ORNL’s Li said, ““If we want to harness graphene in an application, we have to make use of the interface properties. By creating this clean, coherent, 1-D interface, our technique provides us with the opportunity to fabricate graphene-based devices for real applications.”

While this work could yield applications for devices built around 2-D materials, its effect on current research may have the most immediate impact.

“There is a vast body of theoretical literature predicting wonderful physical properties of this peculiar boundary, in absence of any experimental validation so far,” said Li, who leads an ORNL effort to study atomic-level structure-transport relationships using the lab’s unique four-probe scanning tunneling microscopy facility. “Now we have a platform to explore these properties.”

Image: ORNL

Organic Thin Film Transistors Approach Speed of Polysilicon Cousins

For years now, Zhenan Bao, a chemical engineering and materials science professor at Stanford University, has been coming up with new techniques to speed up the charge carrier mobility of organic transistors, which have labored under painfully slow speeds compared to their crystalline- or polycrystalline silicon cousins.

A little over two years ago, Bao developed a strain technique much like that used in silicon chips to increase the speed of organic semiconductors. At the time, it was believed that the strain technique could increase the frequencies at which organic circuits operate by as much as four times the rate of existing organic devices.

While even that much of an increase still left the organic circuits operating at one-hundredth the speed of crystalline silicon circuits, the hope was that the advance had opened up a path towards cheap, plastic, high-resolution TVs.

Now Bao and colleagues from the University of Nebraska at Lincoln have developed a new technique that they claim can raise organic semiconductors' operating speeds to levels approaching those of the polysilicon-based devices that control the pixels in advanced TVs.

Advanced research-stage organic transistors have achieved carrier mobility speeds between 5 and 15 centimeters squared per volt second (cm2/Vs), according to Bao, with typical organic transistors staying at about 1-2 cm2/Vs range. The organic transistors in these experiments were not uniform in performance, but their carrier mobilities clustered around 43 cm2/Vs (with one high-end outlier at 108cm2/Vs). Polysilicon transistors typically reach 100 cm2/Vs , with the latest research claiming speeds of 135-500cm2/Vs. So, a carrier mobility speed of 108cm2/Vs is certainly in the territory of polysilicon transistors

The technique, which is described in the journal Nature Communications (“Ultra-high mobility transparent organic thin film transistors grown by an off-centre spin-coating method”)  follows most of the traditional method for creating organic thin film transistors—placing a solution of carbon-rich molecules and a polymer on a spinning disk made of glass. The novelty of the new technique is that they spin the disk at a speed that is faster than usual and coat only a small portion of the disk's surface.

The result is a denser concentration and a more regular alignment of the organic molecules. This, in turn, yields much faster carrier mobility in the resulting thin film transistors.

The Stanford-Nebraska method is still highly experimental at this point. The researchers, who have dubbed their technique "off-center spin coating," have yet to gain a high level of control over the alignment of the organic materials or achieve uniform carrier mobility.

Despite these limitations, the researchers claim that the transparent thin film transistors they've created perform at levels comparable to that of polysilicon materials currently used in advanced displays.

Photo: Jinsong Huang and Yongbo Yuan

Better Condoms through Nanotechnology

The Bill and Melinda Gates Foundation has proven of late to be a spur to developing nanotechnology-based solutions to some of the world’s problems, like a system for sterilizing medical equipment even in places where there is no electricity.

The foundation's latest Grand Challenge Exploration grants are aimed at improving the humble condom. The Gates Foundation granted $100 000 to the University of Manchester to develop a condom in November of last year, reportedly using graphene, that would lead to thinner yet stronger condoms.

With the University of Manchester becoming a “hub” for graphene research, it makes sense that any efforts to use graphene for the improvement of condoms would take place there. But the Gates Foundation apparently didn’t want to limit the prospects of improving prophylactics to just graphene. Last week, it was announced that the Boston University School of Medicine (BUSM) and Boston Medical Center (BMC) have been awarded a $100 000 Grand Challenge grant to develop a better condom using nanotechnology.

"We are honored to be a recipient of a GCE grant project in order to examine this important public health issue," says Karen Buch MD, a third year radiology resident at BMC and Ducksoo Kim MD, professor of radiology at BUSM in a Boston Magazine article.  "We look forward to using nanotechnology to create a condom that is both effective and does not diminish sensation, which could help convince more people to use condoms and potentially reduce the incidence of sexually transmitted infections."

The nanotechnology that the Boston doctors intend to use for their improved condoms will be superdhydrophillic nanoparticles that coat the condom and trap water to make them more resilient and easier to use.

"We believe that by altering the mechanical forces experienced by the condom, we may ultimately be able to make a thinner condom which reduces friction, thereby reducing discomfort associated with friction [and] increases pleasure, thereby increasing condom use and decreases rates of unwanted pregnancy and infection transmission," Kim says in a press release.

So it appears the race is now on. Will hydrophilic nanoparticles or graphene be the nanomaterial of the future for condoms? Maybe both.

Photo: iStockphoto

Two-Dimensional Materials Could Make the Ink for Printable Electronics

Researchers at the National University of Singapore (NUS) have developed an exfoliation method for the two-dimensional (2D) material molybdenum disulfide that leads to crystals of the substance becoming high quality monolayer flakes. These flakes can made into a solution that could be used for printable photonics and electronics.

NUS researchers have been on a bit of a run lately in developing novel manufacturing techniques for 2D materials. Last month, researchers there developed a one-step method for producing graphene for wafer scale films. This latest work also presents improved manufacturing methods for 2D materials, but this time the material of choice is molybdenum disulfide (MoS2), which is itself gaining some favor over graphene in electronics applications. However, the exfoliation technique developed by the NUS team can be applied to other 2D materials such as such as tungsten diselenide and titanium disulfide.

These materials represent a class of chalcogenide compounds. When chalcogens, like sulfur or selenium, are combined with transition metals, like molybdenum or tungsten, they form transition metal dichalcogenides. So far only a few of these transition metal dichalcogenides have been investigated for their electronic properties,   but early indications have shown them to be promising for optoelectronic devices such as thin film solar, photodetectors and flexible logic circuits.

However, the process for turning them into a single, printable layer takes a long time and the yield is quite poor. To address this issue, the NUS researchers explored the use of metal adducts (a compound made from two or more substances) of naphthalene. The researchers created naphthalenide adducts of lithium, sodium and potassium and compared the exfoliation efficiency and quality of molybdenum disulfide produced from each. The research appears today's edition of Nature Communications.

The researchers were able to produce high quality single-layer molybdenum disulfide sheets with large flake sizes, and also demonstrated that exfoliated molybdenum disulfide flakes can be made into a printable solution. With this solution, the researchers were able to show that the ink could produce wafer-size films.

“At present, there is a bottleneck in the development of solution-processed two dimensional chalcogenides,” said Professor Loh Kian Ping, who heads the Department of Chemistry at the NUS, in a press release. “Our team has developed an alternative exfoliating agent using the organic salts of naphthalene and this new method is more efficient than previous solution-based methods. It can also be applied to other classes of two-dimensional chalcogenides. Considering the versatility of this method, it may be adopted as the new benchmark in exfoliation chemistry of two-dimensional chalcogenides.”

In future research, the NUS team will be looking at creating inks from different 2D chalcogenides using its novel method.

Photo: National University of Singapore

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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.

 
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Dexter Johnson
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Rachel Courtland
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