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Graphene or Molybdenite? Which Replaces Silicon in the Transistor of the Future?

Graphene is winning fans, awards and application possibilities seemingly daily. But the elephant in the room, if you will, when discussing graphene, is the problem of it lacking a band gap.

Huge strides have been made in overcoming that shortcoming, but let’s just say that not having a band gap in its nature is more than a small liability for graphene in electronic applications.

Into this mix, researchers at Ecole Polytechnique Federale de Lausanne’s (EPFL) Laboratory of Nanoscale Electronics and Structures (LANES) had their research published this week in the journal Nature Nanotechnology that offers the humble and abundant mineral molybdenite (MoS2) as an attractive alternative to silicon as a two-dimensional material (like graphene is) for replacing the three-dimensional silicon in transistors.

"It's a two-dimensional material, very thin and easy to use in nanotechnology. It has real potential in the fabrication of very small transistors, light-emitting diodes (LEDs) and solar cells," says EPFL Professor Andras Kis in an article that reports on the research.

The big advantage it has over graphene in the search for a replacement to silicon: it has a band gap. And when it comes to being better than silicon, the advantages are impressive.

"In a 0.65-nanometer-thick sheet of MoS2, the electrons can move around as easily as in a 2-nanometer-thick sheet of silicon," explains Kis. "But it's not currently possible to fabricate a sheet of silicon as thin as a monolayer sheet of MoS2."

The researchers also report that transistors made from molybdenite will use 100,000 times less energy in a standby state than traditional silicon transistors.

As explained in the Nature abstract, molybdenite does not have to stand in competition with graphene, but could complement graphene “in applications that require thin transparent semiconductors, such as optoelectronics and energy harvesting.”

Two Good Gate Materials That Are Great Together

Okay, I couldn’t help myself. When I saw this story in which researchers at Georgia Tech had developed a top-gate organic field-effect transistor for plastic electronics that used a bilayer gate dielectric, I thought of those old Reese’s Peanut Butter Cup commercials: “Two great tastes that taste great together.”

“Rather than using a single dielectric material, as many have done in the past, we developed a bilayer gate dielectric,” said Bernard Kippelen, director of the Center for Organic Photonics and Electronics and professor in Georgia Tech’s School of Electrical and Computer Engineering.

The bilayer gate consists of a fluorinated polymer known as CYTOP and a high-k metal-oxide layer created by atomic layer deposition.

As noted in the Georgia Tech press release: “CYTOP is known to form few defects at the interface of the organic semiconductor, but it also has a very low dielectric constant, which requires an increase in drive voltage. The high-k metal-oxide uses low voltage, but doesn’t have good stability because of a high number of defects on the interface.”

So, in a sort of ‘let’s give it a shot’ spirit, the researchers wondered if they combined the two materials whether they would cancel out each other’s drawbacks. Answer: Yes.

“When we started to do the test experiments, the results were stunning. We were expecting good stability, but not to the point of having no degradation in mobility for more than a year,” said Kippelen.

“By having the bilayer gate insulator we have two different degradation mechanisms that happen at the same time, but the effects are such that they compensate for one another,” explains Kippelen.  “So if you use one it leads to a decrease of the current, if you use the other it leads to a shift of the threshold voltage and over time to an increase of the current. But if you combine them, their effects cancel out.”

The results have even surpassed the researchers’ expectations. “I had always questioned the concept of having air-stable field-effect transistors, because I thought you would always have to combine the transistors with some barrier coating to protect them from oxygen and moisture. We’ve proven ourselves wrong through this work,” said Kippelen.

While the applications for the transistor run the gamut of plastic electronics, including smart bandages, RFID tags, plastic solar cells, light emitters for smart cards, the transistors have only been demonstrated to date on glass substrates.

After seeing if they can get the transistors to work on plastic substrates, they will look into whether they can manufacture them with ink jet printing.

Nanotechnology and the State of the Union Address

Earlier this week in the President’s State of the Union Address, a 16-year-old girl by the name Amy Chyao accompanied the First Lady at her seat.

No doubt Ms. Chyao’s presence was a bit of stage craft to underscore the future of America’s ingenuity and innovation because Ms. Chyao, who is still a high school junior, managed to synthesize a nanoparticle that when exposed to infrared light even when it is inside the body can be triggered like a bomb to kill cancer cells. Ms. Chyao performed her research and synthesis in the lab of Kenneth J. Balkus, Jr., a chemistry professor at the University of Texas at Dallas.

This is a remarkable achievement and even more so from someone still so young, so we would have to agree with Prof. Balkus’ assessment that “At some point in her future, she’ll be a star.”

However, Chyao was given to us as a shining example of the US potential for innovation, and, as a result, its competitiveness. So beyond stage craft, what is the assessment of innovation for the US in a time of emerging technologies such as nanotechnology?

As President Obama attempts to rally the nation with “This is our Sputnik moment”, Andrew Maynard over on his 20/20 blog tries to work out what innovation means in our current context as compared to what it meant 50 years ago at the dawn of the space race.

The problems we now face as Maynard points “are increasingly complex technologies and a vastly more interconnected  world” that have forever changed “the dynamic between having a good idea and coming up with a sustainable solution.”

According to Maynard, technology innovation has to be re-examined so at least we have the tools, institutions and understanding of how to do it effectively and efficiently in this new context.

Maynard and Tim Harper of Cientifica published a paper last week on this topic through World Economic Forum entitled Building a Sustainable Future: Rethinking the Role of Technology Innovation in an Increasingly Interdependent, Complex and Resource-constrained World

I have touched on the themes of the paper previously here on this blog when the two authors were first arguing their idea at The Summit of the Global Agenda at the annual World Economic Forum Meeting in Dubai back in 2009.

The ideas struck me then, and have increased since, as being the kind of ideas that you stop and ask yourself: “Why didn’t I think of that?”

In other words, they’re the kind of ideas that will meet more than their fair share of resistance for the simple reason that many will believe that if they didn’t actually think of it themselves, they should have.

For me, the key issue Harper and Maynard address is that we all acknowledge that we have some major issues that we need to address (energy, food, water) and often technology is held out as some kind of savior in solving them. But we allow the technology that we need to be held up by the vagaries of the markets and the profits and interests of a handful of people. Maybe we should examine establishing a framework by which it is more likely that the technologies we need are developed rather than just those that might best turn a profit. 

While maintaining the status quo may temporarily lead us to greater competitiveness, it will hardly matter if we can’t find some way to ensure that the technologies we need get developed just as vigorously and swiftly as those that promise the biggest profit margins.

Electron Multiplication for Thin Film Solar Gets Some Skeptics

I have been very reluctant to get on the bandwagon that nanotechnology offered us any clear, never mind easy, solutions to getting solar power to be more efficient in generating electricity.

But I am always willing to consider the possibility that nanotechnology holds the key to making cheap and highly efficient solar power. One of the nano-related alternatives I discussed was the use of quantum dots for either electron multiplication or creating so-called “hot-carrier” cells.

As I had explained previously, “Electron multiplication involves making multiple electron-hole pairs for each incoming photon while with hot carrier cells the extra energy supplied by a photon that is usually lost as heat is exploited to make in higher-energy electrons which in turn leads to a higher voltage.”

The concept of electron multiplication has been a line of research vigorously pursued since 2004 when it was first proposed. In my blog on the subject, I highlighted research coming from the University of Minnesota and Texas that had investigated further the possibility of creating multiple charge carriers from one photon.

But Eran Rabani, a researcher at Tel Aviv University, was not so convinced by the research on electron multiplication.

"Our theory shows that current predictions to increase efficiencies won't work,” Rabani is quoted as saying in the linked article above. “The increase in efficiencies cannot be achieved yet through Multiexciton Generation, a process by which several charge carriers (electrons and holes) are generated from one photon."

Rabani has published two articles on his research, one is in the journal Chemical Physical Letters and the other in Nano Letters

While Rabani seems to be dismissing this line of research and the possibility that more than one electron pair can be generated from one photon, he believes that by eliminating this line of research it will open up other research directions that are more promising for solar technology.

However, it’s not clear that this has permanently closed the door on Multiexciton Generation as Rabani quote seems to indicate: “The increase in efficiencies cannot be achieved YET through Multiexciton Generation.”

Graphene Demonstrates Capabilities in Spintronics

Perhaps two of the most important recent developments in advanced materials for electronics have been giant-magneto resistance (GMR) and graphene.

While the GMR phenomenon was identified over twenty years ago, it wasn’t until 2008 that it garnered a Nobel Prize for its discoverers. However, in the years between its discovery and its Nobel Prize recognition it transformed hard-disk drive (HDD) memories into the terabyte stratosphere of today from the few gigabyte level they were at just a decade ago.

Graphene didn’t have to wait twenty years for its discoverers to earn a Nobel Prize. It managed to do that in just six years.

Whether there is something to the shorter elapsed time, I am not sure. But where GMR use of “spintronics” as opposed to “electronics” has remained in the HDD component of the computer, research is now showing that graphene may be able to extend spintronics into other areas of computers.

Researchers from the Nano-Science Center at the Niels Bohr Institute, University of Copenhagen, in collaboration with Japanese researchers, have discovered that bending graphene into a curved shape influences the spin of the electrons.

While the term “graphene” is oddly never used in the article on this research, graphene has not been considered a prime candidate for advancing spintronics because when it is laid out flat the electrons do not affect the spin and its direction remains random rather than patterned.

"However, our results show that if the graphite layer is curved into a tube with a diameter of just a few nanometers, the spin of the individual electrons are suddenly strongly influenced by the motion of the electrons. When the electrons on the nanotube are further forced to move in simple circles around the tube the result is that all the spins turn in along the direction of the tube", explain the researchers Thomas Sand Jespersen and Kasper Grove-Rasmussen at the Nano-Science Center at the Niels Bohr Institute.

The research, which was originally published in the journal Nature Physics, breaks out of the long-standing belief that this phenomenon could only be achieved with a single electron on a “perfect carbon nanotube.”

The researchers have not only demonstrated that this kind of alignment can be achieved with any number of electrons and with carbon nanotubes that have defects and impurities, but also have shown that you can control the strength of the effect and even turn it off completely.

These developments make a path to real-world applications seem far clearer.

Building 3D Nano Structures with RNA Resistant to Enzymes

The field of so-called RNA nanotechnology has been around for awhile, but it has had difficulty advancing as much as DNA nanotechnology despite its more flexible capabilities because of an enzyme known as RNase that eats RNA within minutes, making it extremely difficult to build anything with RNA without it being cut up into pieces in short order.

But Peixuan Guo and his colleagues at the University of Cincinnati have developed a method by which they replace a chemical group in the RNA making it become resistant to the RNase enzyme. Guo has been spearheading research with RNA and with this latest paper published in the American Chemical Society’s journal Nano he and his team may have established a way to take advantage of this macromolecule for building nanostructures from the bottom up.

Guo and his team focused their research on the ribose rings that along with alternating phosphate groups make up the backbone of RNA. By replacing one of the rings the researchers made it so the RNase could not attach itself to the RNA.

The advantages of RNA over DNA in bottom-up fabrication are that it can be manipulated like DNA “while possessing noncanonical base-pairing, versatile function, and catalytic activity similar to proteins.”

The Foresight Institute’s blog Nanodot has excellent coverage of this development that was written by Jim Lewis, who, according to his article, has spent most of his research career working with RNA.

Guo will be using the RNA in “gearing a powerful nanomotor that packages viral DNA into the protein shells of a bacterial virus named phi29.” 

Lewis’ closing sentence in his Foresight report sums it up eloquently: “It will be interesting to watch over the next several years if this variety of 3D structures leads to useful structures and devices for the development of molecular machine systems and ultimately productive nanosystems.”

"Nanoscoop" Material Promises 40x Faster Charging of Batteries

Researchers at Rensselaer Polytechnic Institute (RPI) in Troy, NY have developed a new type of nanomaterial they have dubbed “nanoscoops” because of its resemblance to an ice cream cone. The novel material promises to enable li-ion batteries to charge 40 to 60 times faster than conventional batteries.

The research, which was led by Professor Nikhil Koratkar and initially published in the journal Nano Letters, demonstrated how a “nanoscoop” electrode was able to achieve its faster charge in 100 continuous charge/discharge cycles.

“Charging my laptop or cell phone in a few minutes, rather than an hour, sounds pretty good to me,” said Koratkar in a RPI press release. “By using our nanoscoops as the anode architecture for Li-ion rechargeable batteries, this is a very real prospect. Moreover, this technology could potentially be ramped up to suit the demanding needs of batteries for electric automobiles.”

While this technology could increase the charge/discharge rates of li-ion batteries, I didn’t see any discussion within the coverage of the research whether it will improve the watt-hours of energy per kilogram (Wh/kg), which I noted last month to much criticism Secretary Steven Chu had suggested should reach a level of 1000Wh/kg in electric vehicles to compete with fossil fuels.

At any rate, Koratkar also intriguingly suggests in the piece that the nanoscoop material could enable the bringing together of supercapacitors—used for power-intensive functions such as starting the car—and traditional batteries—that provide high energy density for normal driving—into one single battery unit.

The trick to the nanoscoops capabilities lies in its composition, structure and size. In composition and structure it sounds like a good ice cream cone. They are “made from a carbon (C) nanorod base topped with a thin layer of nanoscale aluminum (Al) and a “scoop” of nanoscale silicon (Si)”.

It is this structure that accounts for the material’s ability to accept and discharge Li ion batteries at extremely fast rates and without causing damage. The layered structure of the nanoscoop transfers strain from the carbon layer to the aluminum layer and then finally to the silicon.

The next steps for the researchers will be to overcome the lack of overall mass of the electrode. They are investigating growing longer scoops or perhaps stacking numerous scoops on top of each other.

Just Because It's Smaller Doesn't Make It Nanotechnology

Andrew Maynard is often the first stop for mainstream journalists when they need to cover the story of nanotech. This is no doubt due to Maynard’s unique blend of scientific knowledge and his ability to communicate the science effectively to both the layman and his fellow scientist.

So when PBS decided to launch a new program as part of the Nova series entitled Making Stuff, hosted by New York Times technology columnist David Pogue, they must have been a little concerned that Maynard was less than enthusiastic about a clip from the program’s coverage of bioengineered materials both in terms of its ethical and safety point of view, the latter of which is Maynard’s bailiwick.

So David Levin, Nova's resident podcaster, contacted Maynard and they produced a podcast entitled The Dangers of Nanotech. Well, if anyone thought the program was being too soft on the safety aspect of bioengineered materials, it appears they more than made up for it by creating an alarmist title for a podcast on nanotechnology.

But Maynard tries mightily to fight back the alarmism by stating out front, “At the moment, the health issues [around nanotech] are very speculative.” And he continues on this vein balancing concerns with what we really know, not an easy task to perform.

But there is a notable omission in the whole discussion. We get initially the typical scare screed: “Nanotechnology is in everything from our pants to sunscreen, but how safe is it really?” And then not once in this nine-minute podcast do we get a discussion on the safety of the products that contain nanotech. It is like deciding to do a safety segment on electronics and you decide to spend the entire presentation on the toxicity of mercury. That’s all fine and good but shouldn’t we talk about the lifecycle of the products, just for 30 seconds or so.

The actual of topic of "Making Stuff" with nanotechnology is scheduled to air initially on January 26th, and is ingeniously entitled Making Stuff: Smaller. It is a pity that the description of the episode feels it necessary to trot out “micro-robots that probe the human body” as what the future holds as technology continues to go smaller. But I suppose that’s what really grabs the audience’s attention when it comes to nanotech.

But just a tip to the program’s producers in case they’re interested, just because the robots are small doesn’t make them nanobots, or even nanotech-related. in fact, the practice of combining the concepts of nanotechnology and micro robots really just confuses the matter.

Graphene for Electrodes in Organic Solar Cells Could Reduce Costs

While organic solar cells have been promising an inexpensive way to exploit solar power in comparison to their silicon-based cousins, things have not panned out in the marketplace quite as expected with flexible solar cells being rolled out onto roofs like asphalt roofing material.

But researchers at MIT believe they have overcome at least one obstacle with organic solar cells by finding a material for the electrodes that will match organic cells’ flexibility and replace the expensive indium-tin-oxide (ITO).

The magic material is none other than graphene, the wonder material of the latter half of the first decade of the 21st century.

Of course, this is not the first time that graphene has been discussed in relation to organic solar cells, but actually getting the graphene to go where you want it to go remained an obstacle.

In a paper published in the Dec. 17 edition of the Institute of Physics journal Nanotechnology, MIT professors Jing Kong and Vladimir Bulovic demonstrated how they were able to overcome the material’s resistance to adhering to the panel. The solution turned out to be a doping process that introduced impurities into the graphene that made it bond with the panel.

After having overcome this manufacturing obstacle, the graphene performed much like ITO except that it was more flexible and also transparent to allow all available sunlight to pass through. But perhaps most importantly, carbon is far more abundant than the increasingly rare ITO, which would likely reduce the cost of the product.

Big Story in Nanotech for Last Decade: The Funding Gap

In a gesture to recognize its tenth anniversary, TNTLog asked its readers what has been the best and worst stories surrounding nanotechnology in the last 10 years.

In my estimation, the big story in nanotech would be the funding gap, or, as it might alternately be termed, the innovation gap.

It seems governments around the world are anxious to invest billions of dollars into nanotechnology and then leave the small start-ups that this investment germinates to die on the vine while they are left to search in vain for the funding that could bring their technologies to market.

While IBM and other large companies, especially in the electronics and chemical industries, have invested heavily in nanotech, and this likely accounts for a large portion of non-government investment in the field, funding for small to medium enterprises attempting to bring a product to market just has not been strong or effective enough.

I have argued that financial institutions that have typically funded start-ups like these, such as venture capitalists or other private capital institutions, just have not been effective at bridging the seven to ten year of funding that these start-ups need. 

I bring this all up because we have another cautionary tale of how a company with a promising technology and some initial funding, ends up in bankruptcy due to little more than under capitalization.

The story of Carbon Nanoprobes Inc.’s failure is presented to us in contrast to the success of Saladax Biomedical Inc. The story gives us no information on why Saladax was successful in raising more financing than Carbon Nanoprobes. It could be any number of reasons, but a very likely reason would be that the ROI horizon was so far off for Carbon Nanoprobes that it constituted a much riskier investment.

This has been the overarching issue for nanotechnology’s development in the last 10 years. A generation of investors has been so conditioned by the dot.com era and hedge funds fueled by derivatives and glorified Ponzi schemes that the prospect of having to wait 7 to 10 years for their investment to pay off is just beyond their attention span.

I suppose that some Darwinian-influenced economic theory would be plugged in right about now, i.e. these companies failed because they were not strong enough.

Fair enough. But we are facing challenges so grand now in terms of feeding our growing population, supplying energy and even having clean drinking water that maybe we should find some way of supporting innovation in areas where we desperately need it, instead of allowing those potential solutions to disappear because it presented too much risk for financiers’ portfolios.

Where are my solutions? I don’t have a solution of my own but I like the direction that both Andrew Maynard and Tim Harper are going with the innovation framework they are developing as part of their roles with the World Economic Forum. 

Perhaps there are better ways to separate the wheat from the chaff in emerging technologies than just letting the financiers decide.

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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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