An international team of researchers under the umbrella of the EU-funded Graphene Flagship have taken a significant step in thermal infrared (IR) photodetctors with the development of the most sensitive uncooled graphene-based thermal detector yet fabricated. These new photodetectors, known as bolometers, are so sensitive that they can register the presence of a scant few nanowatts of radiation. That level of radiation is about a thousandth of what would be given off by a hand waving in front of the detector.
In the research described in the journal Nature Communications, scientists from the University of Cambridge, UK; the Institute of Photonic Sciences (ICFO), Spain; the University of Ioannina, Greece; and from Nokia and Emberion found that the combination of graphene and pyroelectric materials—which generate a voltage when they are heated or cooled—yields a unique synergy that boosts the performance of thermal photodetectors.
Researchers at the University of Manchester in the UK have built photosensors and programmable logic memory devices with inkjet printers using biocompatible, water-based 2D crystal inks that are highly concentrated for improved electrical performance.
The Manchester research, described in the journal Nature Nanotechnology, represents a significant development over current ink formulations that are typically toxic and difficult to process. While this new formulation is easier to produce, biocompatibility may be its most attractive feature, opening up potential applications in medical devices.
The Swedish company Smoltek AB, a spinout from Chalmers University, sees chip packaging as the new frontier in nanoelectronics. It has been positioning itself at the forefront of this new movement over the last five years with its development of a variation on chemical vapor deposition (CVD) technology it has dubbed SMOLTEK TigerTM.
To many, the sell-by date on the carbon nanotube-based non-volatile random access memory (NRAM) developed by Nanterohas long since passed. IEEE Spectrumcharacterized the technology as a “loser” nearly a decade ago after several of the company’s launch dates came and went with hardly a whimper.
The technology’s promise was that it could lend itself to easy mass production because it relies on a group of nanotubes deposited randomly on a substrate rather than individual nanotubes precisely placed. By eliminating the need for individual placement, Nantero hoped to sidestep the main bugbear of nanotubes in electronics: purity. It turns out purity could not be sidestepped to the degree originally believed, and a decade-and-half of disappointment ensued.
But a new analyst report published by BCC Research asserts that NRAM’s ship may have finally come in. The upshot: It (and a host of other non-volatile memory approaches) may be poised to dislodge flash from its long-held throne.
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.
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.
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.
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.
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.
“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.
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.