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A 3D cut away of a four-level  chip.

3D Electronic Nose Demostrates Advantages of Carbon Nanotubes

You’d think computers spend most of their time and energy doing, well, computation. But that’s not the case: about 90 percent of a computer’s execution time and electrical energy is spent transferring data between the processor and the memory banks, says Subhasish Mitra, a computer scientist at Stanford University. Even if Moore’s law continued on indefinitely, computers would still be limited by this memory bottleneck.

This week in the journal Nature, Mitra and collaborators describe a new computer architecture they say addresses this problem—and that Mitra believes will improve both the energy efficiency and speed of computers by a factor of 1000.

The new 3D architecture is based on novel devices including 2 million carbon nanotube transistors and over 1 million resistive RAM cells, all built on top of a layer of silicon using existing fabrication methods and connected by densely packed metal wiring between the layers. As a demonstration, the team built an electronic nose that can sense and identify several common vapors including lemon juice, rubbing alcohol, vodka, wine, and beer.

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3-D structures of atomically thin phosphorus layers buried 5 to 15 nanometers below a silicon surface

Nondestructive Microscopy Technique Offers a Path Toward In-Silicon Quantum Computers

An international team of researchers has developed a nondestructive imaging technique that can peer deep inside of silicon to locate and characterize various structures. While this should be a boon for testing and measuring conventional silicon chips currently used in information processing, it could have its greatest impact by enabling the next generation of devices for quantum information processing.

In research described in the journal Science Advances, researchers from the University of Linz in Austria, University College London, ETH Zurich, and École Polytechnique Fédérale de Lausanne in Switzerland have adapted the well-established microscopy technique known as Scanning Microwave Microscopy (SMM) to identify dopants deep inside the silicon without causing any damage to the material. (Dopants are atoms that are added to a semiconductor to change its electrical and optical properties.)

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Schematic showing a s-CNT-array transistor scaled to a 40-nm-device footprint

Carbon Nanotubes Reduce Transistor Footprint to Forty Nanometers

There have been increasing signs coming from the research community that carbon nanotubes are beginning to step up to the challenge of offering a real alternative to silicon-based complementary metal-oxide semiconductor (CMOS) transistors.

Now, researchers at IBM Thomas J. Watson Research Center have advanced carbon nanotube-based transistors another step toward meeting the demands of the International Technology Roadmap for Semiconductors (ITRS) for the next decade. The IBM researchers have fabricated a p-channel transistor based on carbon nanotubes that takes up less than half the space of leading silicon technologies while operating at a lower voltage.

In research described in the journal Science, the IBM scientists used a carbon nanotube p-channel to reduce the transistor footprint; their transistor contains all components to 40 square nanometers, an ITRS roadmap benchmark for ten years out.

One of the keys to being able to reduce the transistor to such a small size is the use of the carbon nanotube as the channel in place of silicon. The nanotube is only 1 nanometer thick. Such thinness offers a significant advantage in electrostatics, so that it’s possible to reduce the device gate length to 10 nanometers without seeing the device performance adversely affected by short-channel effects. An additional benefit of the nanotubes is that the electrons travel much faster, which contributes to a higher level of device performance.

Another key contributor to the small scale of the transistor is the adoption of end-bonded contacts. The metal contacts in transistors have typically been bonded lengthwise along the main body of the transistor’s semiconductor material, resulting in a long contact along the channel. Two years ago, IBM demonstrated that this “end-bond" configuration made it possible to shrink the contact length of the experimental nanotube transistors from 300 nanometers to just 10 nanometers, without any increases in the contact resistance.

To get this to work, the IBM scientists needed a contact metal for the carbon nanotubes that possessed both sufficient thermal stability and reactivity to carbon. It also had to form end-bonded contacts at a temperature low enough to maintain the device geometries. The scientists found that they could adhere the nanotubes to high-work-function cobalt-molybdenum alloy contacts.

The molybdenum maintains the excellent thermal stability of the alloy, while the cobalt serves as a catalyst, reducing the temperature needed to form metal carbide with nanotubes to 650 degrees Celsius. 

Since the nanotubes are end-bonded to these contacts, the contacts present a substantial transport barrier for the injection of electrons to the nanotube channel. As a result, the transistors are p-channel only. 

“It is challenging to form end-bonded contacts to nanotubes using low-work-function metals for n-channel operation,” explained Qing Cao, Research Staff Member at IBM T.J. Watson Research Center and co-author of the Science paper, in an e-mail interview with IEEE Spectrum. “However, we have already developed some processes to effectively dope the nanotube channel, so that n-channel device operation can be realized even with high-work-function end contacts.”

While there is a work around for achieving n-channel device operation through doping, the device structure does have an out-of-the-box benefit of having a top-gated structure. The top-gated device structure, which is used in today’s silicon transistors, allows easier formation of complex device-to-device interconnects than bottom-gated devices do. This allows for higher device integration density.

In addition to the nanotube channel being end-bonded to cobalt-molybdenum alloy contacts, an ultrathin layer of high-k oxide is sitting on top of the nanotube as the gate dielectric with a metal top gate.

Cao concedes that there are still several manufacturability issues that need to be solved before high-performance nanotube logic transistors become a commercial technology.

“The major challenge at the present stage is device variability,” says Cao. “Ultimately, we want to integrate billions of nanotube transistors into functional circuits. To do this, we need good consistency from one transistor to the next so they can all work together at the same voltage.”

While the purity of semiconducting nanotubes has been significantly improved over the past few years—to the point where IBM has recorded electrically verified purity above 99.999 percent—the process needs to be further standardized and stabilized for reliable large volume production.

“Incorporation of nanotubes into current CMOS processes in a semi-production line has been realized at IBM, but better engineering control is still needed to minimize the amount of impurities in our nanotube source,” adds Cao.

According to Cao, the IBM researchers know that the randomness of fixed charges largely accounts for the variation. But they still don’t fully understand where the charges come from. “Are they mainly from dangling bonds at the oxide surface, from damage to the oxide during the fabrication process, or from residue left by the nanotube solution?” 

In terms of engineering, Cao believes that they first need to establish better control over the nanotube source and the deposition process. “The current nanotube solution isn’t really electronic grade, so we may introduce charges on the oxide during the nanotube deposition process,” he says. “Another aspect is to find a better passivation for the nanotube transistors using high-quality, freshly grown dielectrics with no free surface near the nanotube.”

Schematic of the experimental geometry for investigation of the prototype FEDW device.

Ferroelectric Domain Wall Memory Shows Its Promise

Researchers at the University of New South Wales (UNSW) in Australia have taken a significant step in the development of so-called ferroelectric domain wall (FEDW) memories. These results could lead to a non-volatile memory with a higher storage density than traditional memory devices and be the realization of the unfulfilled promise presented by magnetic domain wall memory, also known as “racetrack memory”.

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Are high-tech headphones the killer app for graphene?

Montreal Startup Ora Releases Novel Graphene-Based Headphones

Graphene has long been touted as a miracle material that would deliver everything from tiny, ultralow-power transistors to the vastly long and ultrastrong cable [PDF] needed for a space elevator. And yet, 13 years of graphene development, and R&D expenditures well in the tens of billions of dollars have so far yielded just a handful of niche products. The most notable by far is a line of tennis racquets in which relatively small amounts of graphene are used to stiffen parts of the frame.

Ora Sound, a Montreal-based startup, hopes to change all that. On 20 June, it unveiled a Kickstarter campaign for a new audiophile-grade headphone that uses cones, also known as membranes, made of a form of graphene. “To the best of our knowledge, we are the first company to find a significant, commercially viable application for graphene,” says Ora cofounder Ari Pinkas, noting that the cones in the headphones are 95 percent graphene.

During an interview and demonstration in the IEEE Spectrum offices, Pinkas and Robert-Eric Gaskell, another of the company’s cofounders, explained graphene’s allure to audiophiles. “Graphene has the ideal properties for a membrane,” Gaskell says. “It’s incredibly stiff, very lightweight—a rare combination—and it’s well damped,” which means it tends to quell spurious vibrations. By those metrics, graphene soundly beats all the usual choices: mylar, paper, aluminum, or even beryllium, Gaskell adds.

The problem is making it in sheets large enough to fashion into cones. So-called “pristine” graphene exists as flakes, perhaps 10 micrometers across, and a single atom thick. To make larger, strong sheets of graphene, researchers attach oxygen atoms to the flakes, and then other elements to the oxygen atoms to cross-link the flakes and hold them together strongly in what materials scientists call a laminate structure. The intellectual property behind Ora’s advance came from figuring out how to make these structures suitably thick and in the proper shape to function as speaker cones, Gaskell says. In short, he explains, the breakthrough was, “being able to manufacture” in large numbers, “and in any geometery we want.”

Much of the R&D work that led to Ora’s process was done at nearby McGill University, by professor Thomas Szkopek of the Electrical and Computer Engineering department. Szkopek worked with Peter Gaskell, Robert-Eric’s younger brother. Ora is also making use of patents that arose from work done on graphene by the Nguyen Group at Northwestern University, in Evanston, Ill.

Robert-Eric Gaskell and Pinkas arrived at Spectrum with a preproduction model of their headphones, as well as some other headphones for the sake of comparison. The Ora prototype is clearly superior to the comparison models, but that’s not much of a surprise. The other units sell for about $100, while the Ora headphones will have a suggested retail of $499 when they are in production, Pinkas says. (On Ora’s Kickstarter page the headphones are offered as a bonus for pledges ranging from $199 to $350.)

Even given that higher anticipated cost, the headphones do not disappoint. In the 20 minutes or so I had to audition Ora’s preproduction model, I listened to an assortment of classical and jazz standards and I came away impressed. The sound is precise, with fine details sharply rendered. To my surprise, I was reminded of planar-magnetic type headphones that are now surging in popularity in the upper reaches of the audiophile headphone market. Bass is smooth and tight. Overall, the unit holds up quite well against closed-back models in the $400 to $500 range I’ve listened to from Grado, Bowers & Wilkins, and Audeze.

Consumer headphones may be just the beginning for Ora. Pinkas tells me that the firm is in negotiations now with an assortment of major smartphone, tablet, smarthome-speaker, hearing-aid, and augmented-reality companies that are considering the possibility of incorporating the graphene-cone speaker into their offerings. For these companies, the advantage of the graphene speaker is not only its high efficiency (linked to graphene’s light weight) but also its very high thermal coefficient, which lets it disperse heat rapidly from the speaker coil within the tight confines of a compact consumer product. Gaskell claims that Ora’s graphene cone weighs only one-third as much as a comparable mylar one, which translates into an increase in battery life of up to 70 percent.

Ora is not the only organization investigating graphene’s potential in audio. As Spectrum has reported, researchers at the Korea Advanced Institute of Science and Technology and the University of Exeter in the UK have recently announced breakthroughs in the use of graphene to produce speakers that have features unavailable with conventional units. The audio speaker, little changed since its invention in the Victorian era, seems poised at last for a major upgrade.

View of the in-plane atomic lattice of a single CrI3 layer.

Single-Layer 2D Magnets Are Here

Scientists from the University of Washington and MIT have demonstrated that a monolayer of a two-dimensional material can exhibit an intrinsic magnetism

This might seem like the latest in a series of seemingly similar announcements on 2D-material magnetism.However, this latest work differs from the others we’ve covered in that it demonstrates ferrimagnetism in a single layer of a 2D material without the use of an accompanying substrate to provide the magnetism.

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Experimental set-up and device structure.

Magnetocapacitance Turned Upside Down Offers a New Tool in Spintronics

Two years ago, research out of Brown University offered up a new approach to increasing the electric storage capacity in magnetic tunneling junctions—which among other things form the backbone of read heads in giant magnetoresistance (GMR) hard disk drives.

Now the same Brown researchers in collaboration with Japanese scientists have found a way to invert the effect and lower the capacitance of these junctions. The results could lead to the development magnetic sensors for “spintronic” applications including computer hard drives and next-generation random access memory (RAM) chips.

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Ferrimagnetic material

Ferrimagnetism in a Two-Layer Material Opens New Doors in Computing

Scientists in Thomas Jung’s research groups at the Paul Scherrer Institute (PSI) and at the University of Basel in Switzerland have fabricated the first two-dimensional ferrimagnetic material that consists of only two layers of material.

Two-dimensional magnetic structures have been hotly pursued in the research community because the magnetic properties of single molecules in these structures can be indivdually addressed and modified. This is especially important in spintronics, where the aim is to use the spins of electrons to encode information. 

In a three-dimensional magnetic material, it is difficult to determine, or change, the spin of an electron when there are lots of other layers of material on top or below it. Developing a 2D material that exhibits ferrimagnetism promises a much more effective way to perform spintronics for things like data storage or to use electrons’ spins as quantum bits in quantum computing.

With that in mind, researchers at Lawrence Berkeley Laboratory demonstrated last month that multi-layered examples of the 2D material chromium germanium telluride did have an intrinsic ferrimagnetism.  However, these multilayered crystals (whose layers are held together by van der Waal forces) do not provide the advantages that ferrimagnetism would give in a monolayer.

Jung and his colleagues did not, strictly speaking, make a monolayer ferrimagnet either—the existence of which is impossible according to Heisenberg’s model of magnetic systems. Instead, in research described in the journal Nature Communications, they fabricated a monolayer comprising molecules called porphyrins. These organic structures, used in biochemistry and sensing applications, are about one nanometer in size. Right in the middle of each porphyrin molecule sits a magnetic atom—like iron. The molecules assemble themselves on top of a gold monolayer, forming a checkerboard pattern in which the spin of the magnetic atoms at the center of the molecules alternate between up and down.

Jung explained in an interview with IEEE Spectrum that without the gold surface—which is not magnetic but highly conductive—the system wouldn't be ferrimagnetic.

The gold’s conduction electrons couple themselves to the magnetic atoms at the center of the molecules, a phenomenon called the Kondo effect. Since the gold is a conductor but not magnetic it serves just as a sea of electrons below the magnetic molecules. The electrons in the gold surface are attracted to the opposite spins of the magnetic molecules above. This attracting of opposite spins leads to the kind of coupling in which one neighboring molecule knows about the situation of the next. This is critical for a magnetic system.

“Without the gold the magnetic molecules wouldn't know what the neighbor is doing; it wouldn't be magnetic at all,” says Jung. “Once we put the gold into this system the electrons below either of the spins recognize what is there and they tell their neighbor spins so that the neighboring atom knows from the electrons how it should behave.”

Jung is quick to caution that this is still fundamental research and should not really be considered a technology—say, a new data storage system—quite yet. In order for something like a data storage device to be developed out of this material, some kind of sensor would need to be developed to read the surface of the magnetic molecules. For the research described in the paper, a scanning tunneling microscope was able to go to one or the other of these spin centers and detect the Kondo effect locally.

In continuing research, Jung and his colleagues are looking at similar molecular architectures. But the metal atom at the center of their molecule will be manganese or cobalt instead of iron.

Jung adds: “In material science, you like to play with the ‘Legos’ and see if by combining different pieces we get something stronger with even more exciting properties.”

A transparent electronic skin for tactile sensing.

Nanogenerator Gets More Flexible and Transparent

Just last week, a research team in South Korea  devised a way to improve the electrical output of the triboelectric nanogenerators (TENGs) developed by researchers at the Georgia Institute of Technology. 

Not to be outdone, a team of scientists at Georgia Tech, led by Zhong Lin (Z.L.) Wang, have improved the capabilities of their TENGs technology by making them far more flexible. In the process, the team has given the devices a new name: skin-like triboelectric nanogenerators, or STENGs. These stretchy generators should provide another flexible power source for the increasing number of flexible electronics.

In research described in the journal Science Advances, the Georgia Tech researchers combined a hybrid material made up of an elastomer and an ionic hydrogel that can harvest energy from movement and provide tactile sensing. The flexibility and tactile sensing suggests that the material could be used to make self-powered electronic skin or self-powered soft robots.

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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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