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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.

Nanorobots Are Not a Technology; They Are a Prediction

I confess, I don’t get it. Why are people so easily confused by today’s nanoparticles and discussions of futuristic nanorobots?

This confusion reared its ugly head recently in an article discussing nanotechnology in cancer treatments. There is probably no more somber a topic than cancer. If you have not been directly affected by it, chances are you know someone who has. It’s a serious subject.

So the combining of misapprehensions regarding nanorobots and cancer is a harrowing read as I recently discovered in article infuriatingly entitled: “Nanorobots: Novel Technology for Cancer Therapy.”

The basis for much of the mish-mosh found in this blog post seems to be an article that can be found in the IEEE Transactions on Nanotechnology, dating back to June 2003.  

What seems to have escaped the author’s attention is the first sentence of the abstract: "The author presents a new approach WITHIN ADVANCED GRAPHICS SIMULATIONS for the problem of nanoassembly automation and its application for medicine." (Emphasis is mine.) In other words, Cavalcanti was discussing computer-simulated models of nanorobots.

Then the article brings up the use of quantum dots for targeting cancer cells without the slightest hint of a transition after discussing the same capability in nanorobots. But there’s one big difference between the two approaches: one actually exists and the other is only imagined.

What’s perplexing about this blog post is that it presents itself as such a scholarly work—with footnotes and everything—and yet manages to miss the obvious distinction between science fiction and science fact. It’s peppered with enough “coulds” and “woulds” to lend it some plausible deniability. But the overall effect is to give one the impression that nanorobots are a cancer treatment.

Nanotechnology’s role in cancer treatment today is really encouraging in detectiontargeting, and drug delivery.

Isn’t it possible to discuss these breakthroughs in a pseudo-scholarly way and then take the same pseudo approach to discussing the Freitas wing of nanomedicine separately if you’re so enamored with the idea?

I suppose I wouldn’t continue to stamp my feet about this confusion except for the fact that letting it continue has already proven to have dire consequences

Illustration: Guillermo Lobo/iStockphoto

Artificial Retina Impresses, But Is It Nanotechnology?

The Wall Street Journal’s online magazine Market Watch recently ran a story on two new approaches to overcoming degenerative eye diseases with artificial retinas.

The story came across my desk by virtue of the fact that one of the companies goes by the name Nano Retina. The Israel-based company is a joint venture between Rainbow Medical and Zyvex Labs, the latter being well known for its work in nanotechnology and its founder Jim Von Ehr, who has been a strong proponent of molecular mechanosynthesis.

Both Nano Retina’s and its competitor Second Sight’s approaches to providing a solution to disease-caused blindness impressed me. And the Wall Street Journal article expressed intrigue that "the number of blind persons in the U.S. is projected to increase by 70 percent, to 1.6 million by 2020, with a similar rise projected for low vision," according to a 2004 paper prepared by a research group led by a Johns Hopkins University professor, Nathan Congdon. In other words, it's a growth sector, which always pleases investors.

But I wanted to know where the nanotechnology was in the “Nano” Retina. Finding out turned out not to be an easy task. There was the video from the company Web site below, which explained that the implantable device contained “nanoelectrodes.”

The written information on the company did not shine any further light on the subject. The video does seem to claim some pretty amazing capabilities for these nanoelectrodes. Apparently, they “interface with the eye’s bipolar neurons” and restart neural stimulation, allowing for messages to go to the brain.

I have to confess I would like to know more, but I’m sure this is all highly proprietary information for a company that doesn’t expect to start clinical trials until 2013.

Nanotech Webinar Appeals to Wide Audience

Last week, I joined in a webinar entitled “Small is Beautiful: Everyday Applications and Advances in Nanochemistry,” that was hosted by the American Chemical Society in its Joy of Science series.

I was drawn to the webinar by its scheduled speakers: Andrew Maynard, Director of the Risk Science Center at the University of Michigan, and Paul Weiss, Director of California NanoSystems Institute at UCLA. But since I was pressed for time and somewhat weary of PowerPoint slides that discuss the Lycurgus Cup. I decided to cut out early and listen to the archived version found at the link at the top of this post.

Despite the webinar's being hosted by the ACS, it seemed to be very much directed at the layman rather than your typical physical chemist. But that too is somewhat misleading; Weiss very subtly raised important distinctions between terms, such as patterning, control, and—most important—function, that might be lost on someone first being introduced to the subject.

Nearly three-quarters of the hour-long webinar is devoted to Q&A, but in Weiss’ final slide, he had written: Most nanomaterials are not precisely defined. We should not treat them as chemicals.

Of course, if we can’t consider graphene to be graphite we are handing ourselves a lot of toxicology work sorting out how it and other nanomaterials interact with biological systems. As Weiss estimates (arguably) that there are currently 100 000 new nanomaterials, it would take us 10 000 years testing with current methods to determine their risk. A nonsensical task, needless to say.

Weiss hints at a more pragmatic and hierarchical approach, in which a grading system is applied to nanomaterials so those that pose the biggest threat and have the least redeeming value are targeted and those with expected low risk and high benefit are fast-tracked (so to speak).

This proposal sort of scans like the Royal Society’s nanotechnology report from eight years ago. 

But aside from the whole environmental, health, and safety discussion, Weiss had some interesting perspectives on the history of nanotechnology’s development.

When Maynard suggested that we are now beginning to see that the macro world that some scientists initially were trying to impose on the nanoscale world—could this be a referral to mechanosynthesis?—were off the mark, Weiss suggested that the field of nanotechnology is actually a throwback.

“At the beginning of the 20th century, they were thinking about atoms and on the single-molecule scale because that was what they were wrapping their hands around with quantum mechanics. We got away from that with the use of ensemble measurements, but as of 30 years ago, when the STM was invented, we’re back again,” explained Weiss.

As I said, I am not really sure who this was all intended for, but it’s all interesting enough that anyone can glean some worthy bits from it.

Nanopillars on Surface of Thin-Film Silicon Could Lead to Better Solar Cells

When you start to discuss the power conversion–efficiency of solar cells, you are bound to ruffle some feathers.

To many it’s an apples-and-oranges debate. You have multijunction solar cells with conversion efficiency rates at 42 percent while dye-sensitized solar cells are now reaching just 10 percent. It's hard to see how they compare—never mind compete.

But what are we really trying to get at with this standard? It would seem that we are trying to sort out the best alternative per kilowatt hour (kWh). Why we don’t set aside the whole energy conversion efficiency debate to focus on kWh figures remains a bit of a puzzle for me.

That said, researchers at the A*STAR Institute of Microelectronics in Singapore tackled the fact that the best thin-film photovoltaics only approach half the energy conversion efficiency of conventional bulk silicon solar cells.

The research, led by Navab Singh and published in the IEEE journal Electron Device Letters, brought about a thin film of silicon with “nanopillars” on the surface to heighten light absorption.

"By investigating a variety of appropriate vertical nanopillar designs, we can enhance the light-trapping and -collection efficiency of thin films to compensate for the efficiency loss caused by reduced material quality and quantity," says Singh.

The trick, of course, is to match—or at least better approach—the energy conversion–efficiency rates of single-crystalline silicon solar cells with thin-film silicon solar cells. Whether this is the particular answer to achieving that remains to be seen, but it seems to be a move in the right direction, unlike the pursuit of ever-higher conversion rates.

Adhesion Capability of Graphene Opens New Application Possibilities

Researchers at the University of Colorado, Boulder, have discovered that graphene possesses unexpected adhesion qualities that could open it to use in new, graphene-based mechanical devices such as gas separation membranes. 

Assistant Professor Scott Bunch of the CU-Boulder mechanical engineering department, along with graduate students Steven Koenig and Narasimha Boddeti and Professor Martin Dunn, published in the August 14 edition of Nature Nanotechnology their discovery that graphene’s flexibility and its strong influence to van der Waals force make it stick to even the smoothest surfaces.

"The real excitement for me is the possibility of creating new applications that exploit the remarkable flexibility and adhesive characteristics of graphene and devising unique experiments that can teach us more about the nanoscale properties of this amazing material," Bunch said.

The experiment ran a so-called “blister test”—essentially, adhesion energy tests—on one to five layers of graphene after it had been placed on a glass substrate. The results showed that the adhesion energies between graphene and the glass substrate were several orders of magnitude higher than those found in typical micromechanical structures (0.45 ± 0.02 J m−2 for monolayer graphene and 0.31 ± 0.03 J m−2 for samples containing two to five graphene sheets).

I understand that these adhesion properties of graphene bode well for membranes for natural gas processing or water purification, and it seems a good line of research to find uses of graphene that would seemingly avoid its inherent weakness of lacking a band gap.

It will be interesting to see what other possible applications people recognize in this new adhesion quality outside of mechanical membrane technologies.

Search Terms Indicate the Expansion of Nanotechnology

Nanowerk, in its most recent Spotlight piece, has highlighted the work of two UC Davis researchers in analyzing the spread of nanotechnology research by using search terms within a scientific database that accesses over 10,000 scientific journals.

Minghua Zhang, a professor in Civil and Environmental Engineering at UC Davis, and Michael L. Grieneisen, a researcher in Zhang's AGIS Lab, have published their findings in the journal Small, in an article entitled "Nanoscience and Nanotechnology: Evolving Definitions and Growing Footprint on the Scientific Landscape."

The Nanowerk article points out that Zhang’s and Grieneisen’s work is based upon a Georgia Tech project called “Refining search terms for nanotechnology” that was published in 2008.

The UC Davis researchers added some carbon nanostructure search terms to the list—graphene, fullerene, buckyball—and excluded a few more terms from the search that may have had a “nano” prefix but weren’t really related to nanotechnology, such as “nanosatellite.” They ran the updated search terms through Web of Science Database (WoS) for the years from 1991 to 2010 and voilà.

The results showed the top five countries in terms of records were:

  • China (20 186)
  • USA (18 472)
  • Japan (6556)
  • Germany (6546)
  • South Korea (5278)

I am really only slightly surprised by these figures. Mainly, I was initially surprised that China was at the top of the list. Then I took a look back at where UC Davis researchers had done their search and where the Georgia Tech folk had decided to focus their queries.

The WoS queries I imagine spread a much wider net than Georgia Tech’s search through “key journals.”

I don’t want to malign anyone here, but 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.



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
Rachel Courtland
Associate Editor, IEEE Spectrum
New York, NY
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