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Schematic diagram showing the structure of a carbon nanotube transistor

Scientists: Carbon Nanotubes Would Outperform Silicon Transistors at the Same Scale

The end appears nigh for scaling down silicon-based complementary metal-oxide semiconductor (CMOS) transistors, with some experts seeing the cutoff date as early as 2020

While carbon nanotubes (CNTs) have long been among the nanomaterials investigated to serve as replacement for silicon in CMOS field-effect transistors (FETs) in a postsilicon future, they have always been bogged down by some frustrating technical problems. But, with some of the main technical showstoppers having been largely addressed—like sorting between metallic and semiconducting carbon nanotubes—the stage has been set for CNTs to start making their presence felt a bit more urgently in the chip industry.

Peking University scientists in China have now developed carbon-nanotube field-effect transistors (CNT FETs) having a critical dimension—the gate length of just 5 nanometers—that would outperform silicon-based CMOS FETs at the same scale. The researchers claim in the journal Science that this marks the first time that carbon-nanotube CMOS FETs under 10 nanometers have been reported.

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Super-thin graphene-based health monitor is mechanically invisible

Graphene Temporary Tattoo Tracks Vital Signs

A graphene health sensor that goes on the skin like a temporary tattoo takes measurements with the same precision as bulky medical equipment. The graphene tattoos, presented in December at the International Electron Devices Meeting in San Francisco, are the thinnest epidermal electronics ever made. They can measure electrical signals from the heart, muscles, and brain, as well as skin temperature and hydration.

Researchers at the University of Texas at Austin who are developing the sensors hope to develop them for consumer cosmetic use. They also hope the ultrathin sensors will provide a more comfortable replacement for existing medical equipment.

Today, if your doctor wants to monitor your heart rate over an extended period of time to help diagnose some cardiac irregularity, you’ll be sent home with a bulky EKG monitoring harness to wear for 24 hours. The Texas researchers hope to make a system that can take measurements of the same quality or better, but that’s unobtrusive. Deji Akinwande, an electrical engineer who specializes in 2D materials, is collaborating on the project with Nanshu Lu, who works on epidermal electronics.

Materials scientists have for years sung the praises of graphene’s electrical properties and mechanical toughness. What’s been underappreciated, says Akinwande, is that this single-atom-thick stuff is mechanically invisible. When it goes on the skin, it doesn’t just stay flat—it conforms to the microscale ridges and roughness of the epidermis. “You don’t feel it because it’s so compliant,” says Akinwande.

The Texas researchers start by growing single-layer graphene on a sheet of copper. The 2D carbon sheet is then coated with a stretchy support polymer, and the copper is etched off. Next, the polymer-graphene sheet is placed on temporary tattoo paper, the graphene is carved to make electrodes with stretchy spiral-shaped connections between them, and the excess graphene is removed. Now the sensor is ready to be applied by placing it on the skin and wetting the back of the paper.

In their proof-of-concept work, the researchers used the graphene tattoos to take five kinds of measurements, and compared the data with results from conventional sensors. The graphene electrodes can pick up changes in electrical resistance caused by electrical activity in the tissue underneath. When worn on the chest, the graphene sensor detected faint fluctuations that were not visible on an EKG taken by an adjacent, conventional electrode. The sensor readouts for electroencephalography (EEG) and electromyography (EMG, which can be used to register electrical signals from muscles and is being incorporated into next-generation prosthetic arms and legs) were also of good quality. And the sensors could measure skin temperature and hydration, something cosmetics companies are interested in, says Akinwande.

Graphene’s conformity to the skin might be what enables the high-quality measurements. Air gaps between the skin and the relatively large, rigid electrodes used in conventional medical devices degrade these instruments’ signal quality. Newer sensors that stick to the skin and stretch and wrinkle with it have fewer airgaps, but because they’re still a few micrometers thick, and use gold electrodes hundreds of nanometers thick, they can lose contact with the skin when it wrinkles. The graphene in the Texas researchers’ device is 0.3-nm thick. Most of the tattoo’s bulk comes from the 463-nm-thick polymer support.

The next step is to add an antenna to the design so that signals can be beamed off the device to a phone or computer, says Akinwande.

Black and white microscope image of molybdenum disulfide transistors shows vertical bars at top crossing a horizontal bar and vertical bars at bottom intersecting the spaces between them.

Molybdenum-Disulfide 2D Transistors Go Ballistic

Molybdenum disulfide, a two dimensional semiconductor that’s just 3 atoms thick, has had a big year. In October, a group of researchers made a 1-nanometer transistor from the material, showing that even if silicon transistors stop shrinking, the new material might provide a path forward. In December, at the IEEE International Electron Devices Meeting in San Francisco, researchers presented work they say shows that molybdenum disulfide not only makes for superlative single transistors, but can be made into complex circuits using realistic manufacturing methods.

At the meeting, a group from Stanford showed that transistors made from large sheets of MoS2 can be used to make transistors with 10-nanometer-long, gate having electronic properties that approach the material’s theoretical limits. The devices displayed traits close to ballistic conduction, a state of very low electrical resistance that allows the unimpeded flow of charge over relatively long distances—a phenomenon that should lead to speedy circuits. Separately, a team from MIT demonstrated complex circuit elements made from MoS2 transistors.

Most of the work on molybdenum disulfide so far has been what Stanford electrical engineer Eric Pop calls “Powerpoint devices.” These one-off devices, made by hand in the lab, have terrific performance that looks great in a slide. This step is an important one, says Pop, but the 2D material is now maturing.

The Stanford lab’s transistors are not as small as the record-breaking ones demonstrated in October. What’s significant, says group leader Pop, is that these latest transistors maintained similar performance even though they were made using more industrial-type techniques. Instead of using Scotch tape to peel off a layer of molybdenum disulfide from a rock of the material, then carefully placing it down and crafting one transistor at a time, Pop’s grad student started by growing a large sheet of the material on a wafer of silicon.

In a transistor, a gate electrode switches the semiconductor channel between conducting and insulating states. In the Stanford device, the tricky part was coming up with an easy way to make a small gate atop the molybdenum disulfide without harming it, says Pop. That is, until his student, Christopher English, realized they could harness the power of rust. English chose a somewhat unusual material, aluminum, to serve as the gate electrode. He deposited a 20-nanometer finger of aluminum on the molybdenum, then allowed it to oxidize and shrink down to a smaller size. The gate ends up being about 10 nanometers.

At these relatively small dimensions, the molybdenum disulfide transistors approach their ultimate electrical limit, a state called ballistic conduction. When a device is small enough (or at low enough temperature), electrons will travel through the conducting medium without scattering because of collisions with the atoms that make up the material. Transistors operating ballistically should switch very fast and enable high-performance processors. Pop estimates that about 1 in 5 electrons moves though the rusty transistors ballistically. By further improving the quality of the material (or making the transistors smaller), he expects that ratio to improve. The important thing, he says, is the way they achieved this: using methods that could translate to larger scales. “We have all the ingredients we need to scale this up,” says Pop.

Zippy nanoscale transistors are great on their own, but they’re useful only if you can build them into circuits.  Researchers from MIT demonstrated just that by constructing working registers and latches. They managed the feat, says electrical engineer Dina El-Damak, by creating computer-aided design software tailored to MoS2. This sort of software is common in the silicon world and enables designers to come up with new circuits relatively easily. (El-Damak worked on the molybdenum disulfide project at MIT and is now a professor at the University of Southern California in Los Angeles.)

Since molybdenum disulfide is so new, not many circuit designers have worked with the material. So far, most work has been done by trial and error, one device at a time. The MIT group can create an informed circuit design, using their computer models to simulate the best and worst cases, based on the material’s known properties and the performance of previous devices, says El-Damak. Then the group fabricates the design that seems most likely to work, tests its performance, and feeds the results back into the program. “By doing this, we have more confidence in scaling up this technology,” she says.

Both Pop and El-Damak say molybdenum disulfide is unlikely to be a direct replacement for silicon. The material will either be used to build complementary systems on top of silicon chips, or it will be used on its own in flexible, transparent electronics. It’s also possible that some other 2D semiconductor will end up being a better option. Molybdenum disulfide is a few steps ahead because researchers have worked with it more than, say, tungsten selenide, and know how to grow the material over large areas.

The Stanford and MIT research demonstrates important progress in this field, says Deji Akinwande, an electrical engineer at the University of Texas at Austin who co-chaired the IEDM session on 1D and 2D devices. People who work in industry are always asking when these materials will be made into useful circuits, and now it’s happening, he says. “Industry is starting to take this more seriously, now that it’s no longer just the grad student in the basement working on it,” he says.

to the left, a blown-out copper interconnect, to the right, a graphene-girded interconnect

Graphene-Girded Interconnects Could Enable Next-Gen Chips

Most of the celebration and hand wringing over Moore’s law focuses on the ever-shrinking silicon transistor. But increasingly researchers are focusing on another critical part of the infrastructure: the copper wires that connect individual transistors into complex circuits.

At the IEEE International Electron Devices Meeting in San Francisco in December, researchers described the coming problems for copper interconnects, and debated ways of getting around them. One approach studied by a group led by Stanford electrical engineer H.-S. Philip Wong, is to bolster copper with graphene. Wong’s group found that the nanomaterial can alleviate a major problem facing copper, called electron migration.

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Silver nanowire films conduct electricity well enough to form functioning circuits without applying high heat

Nanowires Offer Low-Cost Printed Electronics

The use of smart packaging often comes down to cost. If a company is considering including a radio frequency identification (RFID) tag in the packaging of one of its products, the decision may well depend on whether the tag will cut into profits more than it adds value. 

One of the obstacles to reducing the costs associated with smart packaging is the need to heat up the nanoparticles so they melt together and achieve good conductivity. Because of this, you can’t print the circuits on substrates made from inexpensive plastics or paper because the heat can destroy these less robust materials.

Now researchers at Duke University say they may have developed a solution for this after an exhaustive look at a range of silver-based nanostructures that are used to print the circuits for smart packing. They discovered that silver nanowires are able to achieve the desired level of conductivity for printed circuits without needing to be heated to the point where they would harm the less expensive substrates.

“The nanowires had a 4,000 times higher conductivity than the more commonly used silver nanoparticles that you would find in printed antennas for RFID tags,” said Benjamin Wiley, assistant professor of chemistry at Duke, in a press release. “So if you use nanowires, then you don’t have to heat the printed circuits up to such high temperature and you can use cheaper plastics or paper.”

In research described in the journal ACS Applied Materials and Interfaces,  the Duke team compared the silver nanowires with silver nanoflakes and silver nanoparticles in the form of films. The results showed that the silver nanowires were able to achieve the tremendous conductivity Wiley describes when heated to only 70 degrees Celsius.

“There is really nothing else I can think of besides these silver nanowires that you can just print and it’s simply conductive, without any post-processing,” Wiley added.

The advantage the silver nanowires have over the nanoparticles and flakes is the individual nanostructures provided by the nanowires offers a better way for electrons to move along without being interrupted. When the electrons have to jump from one nanostructure to another, as they do with the nanoparticle and nanoflakes, they do not flow as easily. For this reason, the researchers were not surprised that the nanowires outperformed the other materials in conductivity, but the extent of the difference was a bit unexpected.

“The resistivity of the long silver nanowire films is several orders of magnitude lower than silver nanoparticles and only 10 times greater than pure silver,” said Ian Stewart, a recent graduate student in Wiley’s lab and first author of the paper.

In continuing research, the team is looking at using aerosol jets to print circuits using the silver nanowire inks. In the future, they will also be looking to see if they can reduce costs further by coating copper nanowires with silver and seeing if they get comparable conductivity.

Conceptual rendering of a spin-filtering graphene junction

Graphene Enables Spin Filtering at Room Temperatures for First Time

Graphene has had an unusual history in the world of spintronics where the intrinsic spin of an electron is used to encode information rather than its charge. At first, graphene was dismissed in these applications because when it is laid out flat, the spin of the electrons is unaffected and their direction remains random rather than patterned. But a host of research projects changed this idea when results indicated a great deal of possible uses for graphene in spintronic applications.

The latest research comes from a team at the Naval Research Laboratory (NRL) in which a layer of graphene is placed between layers of nickel and iron. The layering creates the first thin-film junction device capable of spin filtering at room temperature. The results could be a boon for next-generation magnetic random access memory (MRAM) in which spin-polarized pulses flip a magnetic bit from 0 to 1 and back again.

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An illustration shows the basic nanowire building block.

Self-Assembly Process Strikes Perfect Balance for Making Atoms-Wide Nanowires

A team of researchers from Stanford University and the US Department of Energy’s SLAC National Accelerator Laboratory have developed a self-assembly process that uses diamondoids to create nanowires with a solid, 3-atom wide copper-sulfur crystalline core—the smallest possible.

The resulting nanowires possess superior electrical properties due to the lack of defects present in the solid crystalline core. Perhaps more impotantly, the self-assembly process for making the nanowires could lead to new kinds of optoelectronic devices and superconducting materials.

“Achieving a 'solid core' of a three atom cross section is ideal,” says Nicholas Melosh, an associate professor at SLAC and Stanford, in an e-mail interview with IEEE Spectrum. “It’s small enough to exhibit unique functionality, yet it can tolerate single defects or strains since there is still a pathway for the electrons to flow.”

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CNT-enabled carbon fiber composites offers a new age of unheard of functionalities

Carbon Nanotubes Make Aerospace Composites Conductive

We have all been witness to the proliferation of carbon fiber adding lightweight strength to sporting goods like bicycles and tennis racquets. That application of carbon fiber reinforced polymers (CFRPs) has been no more popular than in the aerospace industry, where every gram counts. 

What many of us may not have understood about CFRPs is something called “polymer sizing.” This is a coating that is applied to the surface of the carbon fibers to make them easier to handle and to improve the adhesion between the fibers and the polymer matrix in which they’re embedded.

Now researchers at the University of Surrey’s Advanced Technology Institute (ATI), the University of Bristol’s Advanced Composite Centre for Innovation and Science (ACCIS), and aerospace company Bombardier have collaborated on the development of a carbon nanotube–enabled material set to replace the polymer sizing.  The reinforced polymers produced with this new material have enhanced electrical and thermal conductivity, opening up new functional possibilities. It will be possible, say the British researchers, to embed gadgets such as sensors and energy harvesters directly into the material.

In research described in the journal Scientific Reports, the researchers were able to grow the carbon nanotubes (CNTs) on the carbon fibers in a chemical vapor deposition (CVD) process.

Professor Ravi Silva, Director of ATI and head of the University of Surrey’s Nanoelectronics Centre (NEC), told IEEE Spectrum: 

We addressed a current challenge with chemical vapor deposition–grown carbon nanotubes on carbon fiber, by utilizing a metallic interlayer, which we have shown in previous work by our group to minimize degradation to the underlying substrate...The low temperature photo-thermal CVD (PT-CVD) growth process we have adopted is highly suited for large area, high quality carbon nanotube growth on temperature sensitive substrates. This means that the substrates do not degrade in the growth of CNT.

While this work does not represent the first time carbon nanotubes have been incorporated into polymer composites, this work does stake claim to being the first to replace polymer sizing.

Silva notes that, even without a polymer sizing layer, the nanotubes improved the mechanical integrity of the carbon fiber fabric. This was remarkable, he says, because carbon fibers without sizing are inherently difficult to manipulate and make process of incorporating them into a composite difficult.

Silva also pointed out that the incorporation of nanotubes within the carbon fiber polymers did not result in high void fractions, and therefore maintained the sheets’ mechanical integrity.

What’s more, the carbon nanotube–modified fiber composites could have electronic gadgets baked right into their structures or be endowed  with self-healing capabilities.

The collaborators in this research, who have jointly protected the intellectual property, say the next challenge for them is to scale the technology for production using a roll-to-roll system.

Silva added: “We have in mind the optimization of growth of CNTs for composite applications, the scale up of the technology and the optimization for the various applications. We are looking to progress the technology in a number of different fields and will be happy to work with partners in agreed fields of research.”

The chip is tested in the lab. The electrodes detect electrical changes that occur when the cancer cells are captured.

Carbon Nanotubes Make New Approach to Microfluidics More Effective

Circulating tumor cells (CTC) are key early indicators of metastasis, which is the process by which cancer cells move from one organ group in the body to another. Once cancer spreads, the prognosis is generally not good. So, early identification of CTCs can help prevent them from creating new colonies of malignant cells.

Researchers at Worcester Polytechnic Institute (WPI) in Massachusetts have developed a new approach to microfluidics to detect CTCs in blood. The WPI researchers believe that their technique could form the basis of a simple lab test for quick detection of early signs of metastasis and help physicians select treatments targeted at the specific cancer cells identified.

Current microfluidic techniques used in tumor cell isolation have been dependent on flow rate and require off-chip post-processing. The WPI researchers’ technique employs static isolation of tumor cells from the blood by fractionation of the blood into small droplets.

In research described in the journal Nanotechnology, the WPI researchers were able to create a chip design in which antibodies are attached to an array of carbon nanotubes at the bottom of a tiny well in the chip. The chips have an array of these tiny wells, each about three millimeters across.

When the blood droplets are put into the well, the heavier cancer cells drop to the bottom where they become attached to the antibodies.  Each of the wells holds a specific antibody that will bind to one type of cancer cell. The chip’s electrodes detect electrical changes that occur when the cancer cells are captured by the antibodies.

Using an array of antibodies makes it possible to identify several different types of cancer cells within a single blood sample. To put that in perspective, the researchers could fill 170 wells with just 0.85 millileter of blood. The chips were able to capture between one and a thousand cells per device, equating to an efficiency of between 62 and 100 percent.

You can see a video that offers a demonstration of how the chip works below.

The advantages of this technique over traditional microfluidic methods are numerous and significant. But let’s just focus on the advantages derived from the use of carbon nanotubes.

First, the nanotube-based microarrays include both detection and capture technology, unlike traditional microfluidics, which only capture. Second, the nanotube microarray allows for a wide variety of antibodies so that it can attract and identify different types of cells that may need to be fought in different ways.

Another one of the advantages of this approach over other microfluidics is that it can capture exosomes, which are produced by cancer cells and carry the same markers.

“These highly elusive 3-nanometer structures are too small to be captured with other types of liquid biopsy devices, such as microfluidics, due to shear forces that can potentially destroy them,” said Balaji Panchapakesan, associate professor of mechanical engineering at WPI and director of the Small Systems Laboratory, in a press release. “Our chip is currently the only device that can potentially capture circulating tumor cells and exosomes directly on the chip, which should increase its ability to detect metastasis.”  Panchapakesan adds that this is important because research is showing that tiny proteins excreted with exosomes can actually suppress cancer drug delivery and hinder treatment.

Panchapakesan believes the technology is ready for commercialization, but his team just needs more data on patients (delineated by stage of cancer) to move the technology along further in its development.

In an e-mail interview with IEEE Spectrum, Panchapakesan added: “If there is any equipment that needs to be developed more, [it’ll probably be] the automation and robotic handling of the entire system from drop deposition to microscopy. But really we just need patients, patients, patients.”

ultrafast black phosphorous switch

Black Phosphorus Enables Plasmonic Logic

Much research has been dedicated to exploiting the waves and oscillations of electrons that are produced on the surface of a metallic structure when photons of light strike it. These waves of electrons are called either surface plasmons when referring to the oscillations in charge alone, or surface plasmon polaritons when referring to both the charge oscillations and the electromagnetic wave. The field developed around exploiting this phenomenon has become known as plasmonics.

Plasmonics has made all sorts of interesting things possible, such as confining wavelengths of light to design smaller photonic devices. However, there has been no good way to turn the waves “on” and “off” rapidly.

Now researchers at the University of Regensburg in Germany, in collaboration with colleagues from Istituto Nanoscienze–CNR and Scuola Normale Superiore in Pisa, Italy, have demonstrated the ability to selectively choose between an “on” state, where surface polaritons can be excited and propagate across the sample, and an “off” state, where no polaritons are present.

So what is the trick to achieving these “on/off” states? Don’t use a metal at all. Instead, employ the two-dimensional material du jour: black phosphorus.

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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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