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Black Phosphorus Takes a Step Toward CMOS

Researchers working at the Institute for Basic Science Center for Integrated Nanostructure Physics at Sungkyunkwan University (SKKU) in South Korea have discovered that they can manipulate black phosphorus to behave as an n-type (excess electrons) semiconductor, a p-type (excess holes), or as if it were ambipolar (both n- or p-type) simply by changing its thickness and its bandgap or by using a different metal to contact it with. (Today’s digital logic, CMOS, requires both n-type and p-type transistors.)

With this knowledge, the Korean researchers were able to fabricate a transistor from the material that can operate at lower voltages than a silicon-based transistor. (Though it wasn’t the first black phosphorus transistor ever made.)

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Scalable Production for Graphene Nanoribbons Boosts Potential in Electronics

The form of graphene that holds possibly the most promise for use in electronics is the graphene nanoribbon. The narrowness of these ribbons is the key. At widths ranging from a few nanometers to no more than few hundred nanometers the edge of the graphene determines its properties: Very narrow nanoribbons act as a semiconductors and wider ones behave as conductors.

There are a number of methods for producing graphene nanoribbons that can be categorized as either “top down” approaches, like lithographic techniques, or “bottom up” methods where the graphene nanoribbons self assemble into a desired form.

Researchers at the University of Wisconsin-Madison recently developed a technique based on a bottom-up approach and managed to do it without the limitation of previous methods. The result looks to be a production technique that is compatible with semiconductor manufacturing methods and can be scaled up to bulk production.

In research published in the journal Nature Communications, the Wisconsin team were able to grow graphene on a conventional germanium semiconductor wafer.

"Graphene nanoribbons that can be grown directly on the surface of a semiconductor like germanium are more compatible with planar processing that's used in the semiconductor industry, and so there would be less of a barrier to integrating these really excellent materials into electronics in the future," said Michael Arnold, an associate professor UW-Madison, in a press release.

Previous bottom-up approaches only worked on metal substrates, and, to date, have not been able to produce nanoribbons with the necessary length to be useful for electronics.

The UW-Madison researchers overcame this limitation by putting a bit of a twist on chemical vapor deposition (CVD)—a technique that has served as a kind of bridge of late between scalability and purity in graphene production. In CVD techniques, the graphene is grown from a vaporous precursor on a metal substrate like copper or nickel in a furnace. What the UW-Madison team did was to start the process with methane, which attaches to the germanium surface and breaks down into various hydrocarbons. These different hydrocarbons then begin to react with each other to form graphene.

The key to the technique is its tenability. The researchers can slow the growth rate of the graphene and thereby both lengthen the nanoribbons and make them narrower by decreasing the amount of methane in the CVD furnace chamber.

"What we've discovered is that when graphene grows on germanium, it naturally forms nanoribbons with these very smooth, armchair edges," Arnold said in the release. "The widths can be very, very narrow and the lengths of the ribbons can be very long, so all the desirable features we want in graphene nanoribbons are happening automatically with this technique."

Novel Process Cuts Costs and Improves Performance of Quantum Dots

While the long road to commercial success for quantum dots seemed to reach its end when Samsung’s first quantum dot televisions shipped this spring, the material still is relatively expensive to make, limiting its commercial impact.

Now researchers at the University of Illinois Urbana Champaign (UIUC) have attempted to address the expensive production costs of quantum dots, and at the same time, improve their performance and efficiency.

In research published in the journal Applied Physics Letters, the UIUC team developed a method to pull out more efficient and polarized light from quantum dots over a large-scale area. The technique involves combining quantum dots with photonic crystal technology.

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Molybdenum Ditelluride: Like 2-D Silicon, But Better

A team of researchers from Korea and Japan have had a breakthrough with a semiconductor material that they claim could be a candidate to replace silicon in future electronics. In the 7 August issue of Science they report the creation of a transistor where the channel consists of layers of a two-dimensional material molybdenum ditelluride (MoTe2). 

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Graphene Offers Promise of Thermoelectric Material for Next-Generation Vehicles

Thermoelectric materials have been a tantalizingly promising technology for producing electricity from heat that would otherwise just be wasted. The basic premise of thermoelectric materials is that an electrical current is generated as a result of a difference in temperature between one side of the material and the other.

This would seem to be an obvious way to generate an electrical current from your computer or your car just based on the heat they produce. But heretofore, the available materials had poor thermoelectric conversion efficiency or were prohibitively expensive for commercial uses—they just didn’t produce that much current for the buck.

But when traditional materials fail, in come the nanomaterials. We’ve covered multi-walled carbon nanotubes for this use, along with nanowires and nanopillars.

Now, researchers at the University of Manchester in the U.K.,in collaboration with the company European Thermodynamics Ltd., have called upon graphene to make thermoelectric materials more useful.

In research published in the journal Applied Materials and Interfaces, the joint academic-industrial team added a small amount of graphene to strontium titanium dioxide (STO), a thermoelectric material that, by itself, generates a current only at extremely high temperatures. The graphene made a big difference: STO’s operating temperature was expanded to room temperature.

“Current oxide thermoelectric materials are limited by their operating temperatures which can be around 700 degrees Celsius,” said Robert Freer, one of the lead University of Manchester researchers, in a press release. “This has been a problem which has hampered efforts to improve efficiency by utilizing heat energy waste for some time.

Another handicap limiting the usefulness of thermoelectric materials is that their energy conversion efficiencies hover around 1 percent. But the Manchester team reports that their new hybrid material will convert 3 to 5 percent of heat into electricity. They reason that because a vehicle loses 70 percent of the energy in fuel via waste heat and friction, applying this material for improved thermal energy recovery will lead to a substantial boost in energy efficiency.

Ampliflying Light 10,000 Times

A new type of device could amplify the light emitted by a nanometer-scale object as much as 10,000 times, improving low-light photography and bringing previously hard-to-see items into view.

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Graphene and Carbon Nanotubes Together Produce a Digital Switch

The two darlings of carbon nanomaterials, carbon nanotubes and graphene, increasingly are joining forces even as they are having  their obituaries read while still hardly out of the lab. We’ve seen them being used in hybrid energy storage applications and for supercapacitors.

Now researchers at the Michigan Technological University (MTU) have combined these two nanomaterials to tackle a far more difficult application field: electronics. Specifically, the researchers have created digital switches by making a sandwich of carbon nanotubes and graphene.

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A New Spin on Silicon

A group of scientists has stumbled upon a previously unknown characteristic of silicon, one that could make for faster, optical computers.

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Solution-based Process Could Produce Tuned Graphene in Bulk

Researchers in Taiwan have developed a solution-based process for producing graphene that is tuned to exhibit specific electrical and mechanical properties. While solution-based exfoliation of graphene has been possible for some time, this new approach uses pulses of an electrical voltage rather than a constant voltage to produce the desired effects.

The researchers believe that their work, which was reported in the journal Nanotechnology, could pave the way for new applications for graphene in drug delivery or electronics.

Graphene production methods have been seen as an obstacle to the use of the material in a range of applications for which it has been targeted.  The mechanical exfoliation of graphene sheets from graphite, while producing the best quality graphene for electronic applications, is decidedly un-scalable. And production methods that are more scalable lack the quality necessary for these same electronic applications. A solution-based process that can be ramped up to yield high volumes of graphene that possesses the electrical and mechanical properties one desires would cause a dramatic upshift in graphene’s commercial development.

“Whilst electrochemistry has been around for a long time it is a powerful tool for nanotechnology because it’s so finely tunable,” said Mario Hofmann, a researcher at National Cheng Kung University in Taiwan, in a press release. “In graphene production we can really take advantage of this control to produce defects.”

The trick to getting exactly the right defects in the graphene depended not only on using a pulsed voltage, but also being able to carefully monitor how the graphene was changing in the solvent process. To monitor this change, the researchers found that they could simply observe the transparency of the solution.

As part of their work, the researchers tested the quality of the graphene produced via their method as a transparent conductor (the application for which graphene is being considered as a potential replacement for indium tin oxide). The resistance of their graphene films (at 50 percent transparency) was 30 times that of other graphene-based transparent conductors.

In future research, the Taiwan-based team will look at how altering the duration of the pulses impacts the exfoliation process both in terms of producing greater quantities of final product as well as gaining greater control on the defects engineered into the graphene.

Is Graphene Really in Need of a Killer App?

The grace period for the eventual commercial success of a nanomaterial, it would seem, is limited. There’s been an abundance of hand wringing in the last few years over the fact that carbon nanotubes have offered us little more to date than advanced composites. Now we are beginning to see that same frustration play out with graphene, to the point where Nature News has published an article questioning whether graphene’s killer app is ever going to come.

It would seem that ten years is the length of time the public expects for a material to go from its first synthesis in the lab to having a huge commercial impact. That was the case for giant magnetoresistance (GMR). That effect was first produced in the lab in 1988 and made it into commercial hard disk drives by 1997.

In this somewhat unfair comparison, it must be understood that GMR-based hard drives did not need to completely uproot an entire industry to succeed. For graphene, a main target has been silicon CMOS, an entrenched technology if ever there was one. But silicon CMOS will not take us into the future indefinitely. You need no more confirmation of this than the money that is being invested by the tech giants to find a replacement.

We have passed the decade mark since graphene was first synthesized and the public is getting restless, especially about what its commercial potential actually is and how to make profitable investments in the technology.

During this past decade, we have seen the first flushes of excitement about graphene occur in electronics, where it seemed to offer a way around the problems with carbon nanotubes of getting them where you wanted and interconnecting them. But that excitement needed to be tempered with the vexing issue of graphene lacking a band gap. While a band gap has been successfully engineered into graphene, it doesn’t appear that digital logic applications are the path of least resistance for the material.

Articles like the one in Nature make all the valid points about graphene production outstripping demand, something we also saw befall carbon nanotubes. However, maybe we need not look any further for a killer app, because of the likelihood that silicon’s days are numbered. In the meantime, we seem to be finding critical applications for graphene, and other nanomaterials, that we didn’t initially consider, such as water purification membranesenergy storage, and energy generation applications.



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
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