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Graphene Propped Up Vertically on a Substrate Could Sustain Moore's Law

Let’s be clear from the beginning, recent research at Rice University with graphene is based on calculations, not physical manipulation of the material.

According to the physics, it should be possible to get graphene to stand up vertically on a substrate, like a wall, with the aid of diamonds, but I imagine there will be some hair pulling in the labs before they can physically duplicate the process. So while it all sounds quite intriguing, I am not suggesting by highlighting it in this blog that what we have here is anything beyond a model.

That said, I think I should note that my coverage of graphene, carbon nanotubes, and other nanomaterials in electronic applications is not a implication that these materials will be a replacement for silicon any time soon—as I discovered at least one reader felt I was suggesting in a blog post on graphene earlier this year.

However, the pressures of Moore’s Law require that these materials be looked at intensely to keep pace with the unrelenting doubling of transistors on an IC every two years—band gap or not.

In fact, one of the authors of the article in the Journal of the American Chemical Society, Boris Yakobson, Rice's Karl F. Hasselmann Chair in Engineering and a professor of materials science and mechanical engineering and of chemistry, makes a point of discussing Gordon Moore in the coverage of the research.

“We met in Montréal, when nano was a new kid on the block, and had a good conversation," said Yakobson. "Moore liked to talk about silicon wafers in terms of real estate. Following his metaphor, an upright architecture would increase the density of circuits on a chip—like going from ranch-style houses in Texas to skyscraper condos in Hong Kong.

"This kind of strategy may help sustain Moore's Law for an extra decade," he said.

It will be interesting to see if anyone takes on these calculations and attempts to duplicate the results with physical experiments. But with the “theoretical potential of putting 100 trillion graphene wall field-effect transistors (FETs) on a square-centimeter chip” it would seem to be worth the try.

Metrics for Nanotechnology's Development Are Just Pieces of the Puzzle

I am the type who can easily fall prey to “told-you-so” syndrome. Today is just such an instance.

Last month, I covered research that seemed to indicate that because China was producing so many research papers, they had a kind of lead in nanotech research.

At the time, I cautioned that just because Chinese researchers were publishing lots of research did not necessarily mean much if the research was not being cited by other researchers. I said, “I think it is altogether possible that papers published in journals outside of the top publications might rack up a lot query hits but mean little in terms of actual scientific impact.”

Bingo. I expected sooner or later some kind of evidence or other form of research would validate my point, but I didn’t expect it to come from the Chinese Academy of Sciences (CAS).

According to the CAS article, a “publication bubble” in China is threatening to derail the country’s scientific advances. China has experienced a 14 percent increase in scientific publications from 2005 to 2009. Impressive, but as the article points out:

“But these impressive numbers mask an uncomfortable fact: most of these papers are of low quality or have little impact. Citation per article (CPA) measures the quality and impact of papers. China's CPA is 1.47, the lowest figure among the top 20 publishing countries, according to Elsevier's Scopus citation database.”

To me it’s all a bit of unnecessary worry either way. Whatever metric you want to choose—number of patents, research papers, government investment, etc.—it’s only going to give you part of the picture, a piece of the puzzle, if you will.

It really comes down to how you can put the puzzle together for anyone to make sense of it all. Ultimately, it is a qualitative question, not a quantitative one, as I’ve said before. But people's instinct is to trust a number rather than an expert opinion, often for good reason. Until that changes, we'll continue to see a steady stream of numbers for quantifying the development of nanotech.

What is the Role of the Public in Nanotechnology's Development?

I was very pleased this week to receive an e-mail from Chris Toumey, who, in addition to working at the University of South Carolina NanoCenter, contributes four commentaries per year to Nature Nanotechnology 

It seems Toumey had read one of my pieces (from the description I assume it to be this one) and had sent along a piece he had written for Nature Nanotechnology back in 2008 entitled “Questions and Answers” (subscription required).

The piece describes Toumey’s valiant effort to go through all the basic introductions to nanotechnology and determine which one actually best accomplished what it set out to do.

I won’t go further in describing this piece since you all may not be able to gain access to it, but instead direct you to a more recent piece of Toumey’s, which is more or less on the topic of how science engages the public. It's called “Science in the service of citizens and consumers.”

Science, to my mind, always seemed to be an inquiry on how the world around us operates. If it serves anything, it might be our curiosity, but I am wondering if we might be confusing science with technology when we see it as providing some kind of service to citizens and consumers.

Anyway, the main point of the piece seems to be that we should focus on what the public actually wants to know instead of what scientists believe they should know.  

It makes sense, of course, but the problem is that typically, what the public really wants to know is whether Britney Spears will remarry. People's sometimes ugly confusion that derives from this fundamental urge includes all sorts of misapprehensions about the world around them, as I have indicated in the past.

On the issue of citizens, consumers, or whatever other term you would like to use to identify the public, and their role in science, I put myself squarely in the “cynic” category. I am hard pressed to imagine how science can best be guided by an uninformed public, or, worse, one informed by scare mongers and half-truths. 

But I am no social scientist, and determining a toxicology paradigm for nanoparticles may actually benefit from the input of someone who can tell you the comings and goings of the latest Hollywood starlet. Who knows?

Colloidal Quantum Dot Solar Cells Improve Energy Conversion Efficiency

Back at the end of June this year, I covered work that Edward H. Sargent and his research team at the University of Toronto conducted in making solar cells from colloidal quantum dots (CQDs) more efficient.

At that time, the solar power conversion efficiency for the device they described in their Nature Photonics article was 4.2 percent.

Now the Sargent team, along with researchers from King Abdullah University of Science & Technology (KAUST) and Pennsylvania State University (Penn State), has bumped that number up to 6 percent, creating what is claimed to be “the most efficient colloidal quantum dot (CQD) solar cell ever.”

This time, the research was published in the journal Nature Materials and showed that quantum dots could be more densely populated on a surface by using inorganic ligands in the place of organic molecules, allowing the quantum dots to be closer together.

“We wrapped a single layer of atoms around each particle. This allowed us to pack well-passivated quantum dots into a dense solid,” explained Dr. Jiang Tang, the first author of the paper, who conducted the research while a post-doctoral fellow in the Edward S. Rogers Department of Electrical and Computer Engineering at U of T.

As I mentioned in my initial piece on this line of research back in June, the Saudi Arabian government has been financing Sargent’s work in this area to the tune of US $10 million since 2008.

In this latest phase of the research, it appears KAUST was involved in the research by contributing the microscopy and visualization aspects. In addition, it seems that the licensing deal on this research is going to be shared by the University of Toronto and KAUST.

“The world—and the marketplace—need solar innovations that break the existing compromise between performance and cost. Through the partnership between U of T, MaRS Innovations, and KAUST, we are poised to translate exciting research into tangible innovations that can be commercialized,” said Sargent. 

If Sargent’s previous prediction proves correct, that these CQD materials in photovoltaics will be in building materials, mobile devices, and automobile parts in the next five years, there may some time yet before the licensing agreement will mean much. Meanwhile, inexpensive alternatives, namely dye-sensitized solar cells, are reaching 10 percent conversion efficiency now and appear poised to enter new markets. 

Memristors Go Biological

It seems that ever since the riddle of the elusive memristor was solved, it has been on a track for commercial development. This probably is in no small part due to the fact that research was done at HP and not at the University of Wherever.

As a result, we seem to have been on a commercialization watch ever since.  

It’s been just three years since the memristor was identified, so if statistical norms of commercialization are in place we can expect another four years of waiting before we see this material in our Smartphones. In fact, this timeline is pretty close to HP’s expectations of 2014 as a target date for incorporation into electronic devices.

During this time, researchers have not been and will not be sitting on their hands while engineers work out scalability and yields. One of the issues that researchers have been investigating is whether biomaterials like proteins will exhibit the same memristive phenomena found in materials such as metal oxides, chalcogenides, amorphous silicon, carbon, and polymer-nanoparticle composites.

In a Spotlight piece over at Nanowerk: “Researchers in Singapore have now demonstrated that proteins indeed can be used to fabricate bipolar memristive nanodevices. This work provides direct proof that natural biomaterials, especially redox proteins, could be used to fabricate solid-state devices with transport junctions and can be the core component in the development of bioelectronic devices.”

"Previous work on memristors were based on man-made inorganic/organic materials, so we asked the question whether it is possible to demonstrate memristors based on natural materials," Xiaodong Chen, an assistant professor in the School of Materials Science & Engineering at Nanyang University, tells Nanowerk in the article cited above. "Many activities in life exhibit memory behavior, and substantial research has focused on biomolecules serving as computing elements, hence, natural biomaterials may have potential to be exploited as electronic memristors."

The research, which was published in the Wiley journal Small, used the protein ferritin in combination on-wire lithography to fashion memristors.

"The programmable resistive switches were due to the electrochemical processes in the active centre of ferritin" explains Chen. "In addition, we demonstrated that such ferritin-based nanodevices with reversible resistance can be used for nonvolatile memory based on write-read-erase cycle tests."

Gold Nanoparticles Self Assemble into Very Large 2D Superlattice

One could argue that the industry that has driven nanotechnology’s development most over the past 20 years has been the semiconductor industry.

So when researchers at Rensselaer Polytechnic Institute discovered a method for creating a 2D layer of gold nanoparticles that self assembled themselves into a superlattice, they obviously thought of semiconductors.

“Thinking about semiconductors, this discovery could offer new solutions for scaling down the features of today's most advanced 32-nm computer chips to have features in the range of less than 20 nm, or even less than 10 nm," says Sang-Kee Eah, assistant professor in the Department of Physics, Applied Physics, and Astronomy at Rensselaer.

The research, which was published in the Journal of Materials Chemistry, discovered that when the gold nanoparticles were infused with liquid toluene a monolayer of gold would form on the surface of the liquid where it met the air. They moved the layer onto a silicon wafer and evaporated the water.

The video below provides some nice visuals on the monolayer.

Carbon Nanotubes Braided into Macroscale Wires Challenge Copper

Just last week I covered research that seemed to have put the final kibosh on the possibility of replacing copper in logic or memory devices with carbon nanotubes (CNTs).

While the NIST researchers responsible for that report still maintained that carbon nanotubes could be useful in for “interconnects for flexible electronic displays or photovoltaics,” it didn’t sound particularly encouraging for using CNTs in place of copper.

But this week I came across research that demonstrated that CNTs look to be a promising replacement for copper in wires for conducting electricity, such as in power transmission cables.  

Researchers at Rice University published their work in the Nature journal Scientific Reports. While they concede that making “electrically conducting cables from macroscopic aggregates of carbon nanotubes, to replace metallic wires, is still a dream”, it seems “the conductivity variation as a function of temperature for the cables is five times smaller than that for copper.” 

The reason for replacing metal wires with CNT-based wires centers primarily around weight factors that may be important in weight-sensitive applications, such as airplanes and automobiles. Also, “the conductivity-to-weight ratio (called specific conductivity) beats metals, including copper and silver, and is second only to the metal with highest specific conductivity, sodium.”

Impressive, but what about reliability issues that were brought to the fore in recent NIST research? Apparently, the CNT-based wires did not show any signs of degradation during the demonstration in which the wire powered a light bulb for days on end. 

Interesting research, especially since it showed the reliability of CNT wires braided into the macroscale, but it’s not entirely clear at this point where this research is headed as far as applications are concerned. A “dream” is still where we are with this one.

 

 

Optoelectronics Appear as Promising Application for Graphene

Seeing near daily reports on the latest research on graphene, some observers are becoming weary and asking when the wonder material will find its way into commercial products.

Much graphene research has been dedicated to overcoming its liabilities—namely, its lack of a band gap—but we are now beginning to see some research emerging that plays to its strength in potential applications and more or less avoids its weaknesses, such as in recent work applying it to membranes for natural gas and water purification.

Another application area in which graphene has demonstrated some real promise is in optoelectronics, such as with its ability to function as a “mode-locked” laser despite lacking a band gap.

Now the scientists who won the Nobel Prize for Physics for discovering graphene have just published a paper in the journal Nature Communications, in which they demonstrate that combining graphene with plasmonic nanostructures can increase the previous efficiency of graphene-based photodectors by 20 times.

What this would translate into in terms of today’s optoelectronic data transfer rates is a boost of anywhere from 10 up to 100 times the speed of today’s systems. The key to the improved efficiency seems to be “efficient field concentration in the area of a p–n junction,” according to the abstract of the paper.

Those who are losing patience with graphene always being talked about but never seen used will need to hang on a bit longer, however. While application possibilities that draw on the materials’ strength are beginning to be investigated, it’s still a long and arduous road to a commercial product that could take years and may have little to do with science and technology.

Graphene Pushes Flash Memory to New Heights

Nanotechnology has a somewhat infamous relationship with flash memory. It has usually taken on the role as its adversary, such as in the case of Nantero or IBM’s Millipede project, and walked away with less than encouraging results.

So I was interested to see that researchers were using graphene as a platform for flash memory that appears to outperform other flash memory structures. If you can’t beat ’em, join ’em.

Researchers from UCLA, IBM's T.J. Watson Research Center, Samsung Electronics, Aerospace Corporation, and the University of Queensland, led by Kang Wang, recently published in ACS Nano an article entitled “Graphene Flash Memory.” The article demonstrates that graphene may have what it takes to outperform current flash memory technology.

As I suggested in my post from last week, researchers are not breaking their backs trying to overcome graphene’s lack of band gap as much now as they are looking for ways to exploit its intrinsic strengths.

In this case, the researchers were trying to take advantage of graphene’s high density of states, high work function, and atomic thinness.

In the Nanowerk article cited above, one of the researchers, Emil B. Song, a Ph.D. student in the Device Research Laboratory at UCLA's Electrical Engineering Department, explains.

“These unique properties provide improvements for flash memory in memory window, retention time, and cell-to-cell interference, respectively. The enhanced memory window and retention time increases the fidelity of information stored in the memory device. The reduction of cell-to-cell interference offers a solution to achieve higher-density-storage memory devices." 

According to the article, Song and his colleagues believe that by using graphene’s unique characteristics that “flash memory can be further scaled beyond the 20nm node, which is what polysilicon can achieve, and result in even greater storage-density.”

NIST Reveals Reliability Problems with Carbon Nanotubes in Electronics

Poor old carbon nanotubes. CNTs have long been heralded as the new wonder material, especially in electronics applications where their charge-carrier mobility was able to outperform silicon—according to some estimates by a factor of 10—but researchers have struggled to find a satisfactory proposal for getting them into some kind of ordered array

While researchers have continued for the last 20 years to push CNTs beyond a single transistor or attempted to use their propensity for forming a rat’s nest as a strength rather than a weakness, they have faced the unexpected problem over the last decade of their toxicological issues

First, the research hasn’t progressed quite as hoped. Then, environmental, health, and safety concerns presented an entirely new challenge. But—as though those two weren’t enough—along comes a new wonder material: graphene.

As I said, poor old CNTs. So it should come as no surprise in the tale of woe that has followed CNTs that NIST should report CNTs have a major reliability issue in electronics.

The research was presented in a paper at the recent IEEE Nano 2011 in Portland, Oregon. From the NIST Web site:

“…NIST researchers fabricated and tested numerous nanotube interconnects between metal electrodes. NIST test results, described at a conference this week, show that nanotubes can sustain extremely high current densities (tens to hundreds of times larger than that in a typical semiconductor circuit) for several hours but slowly degrade under constant current. Of greater concern, the metal electrodes fail—the edges recede and clump—when currents rise above a certain threshold. The circuits failed in about 40 hours.”

One of the authors of the paper, Mark Strus, a NIST postdoctoral researcher, suggested that while this research may spell the end for CNTs as “the replacement for copper in logic or memory devices,” there still remained the possibility of using the material for “interconnects for flexible electronic displays or photovoltaics.”

That is, of course, when just looking at CNTs’ use as an interconnect. The field of research for CNTs has become so broad over the past 20 years that they are being tested for use in fields as divergent as electrodes in lithium-ion batteries to improving medical imaging.

We haven’t yet reached the point of singing CNTs swan song.

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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
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