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Graphene and Ammonia Combine for Novel Ferroelectric Tunnel Junction

The next generation of high-speed and high-capacity random-access memory (RAM) has seen a number of competitive approaches take center stage of late, from spin-transfer torque (STT) Magnetic RAM to resistive RAM.

Now, researchers at the University of Nebraska at Lincoln have shown a way forward that could offer a general improvement to RAM. They’ve made improvements to the ferroelectric tunnel junction by combining graphene with ammonia so that it is capable of switching on and off the flow of electrons more completely. The result is a distinct improvement in the reliability of RAM devices and the ability to read data without having to rewrite it.

“This is one of the most important differences between previous technology that has already been commercialized and this emergent ferroelectric technology,” said Alexei Gruverman, a physics professor who co-authored the study, in a press release.

The researchers, who published their findings in the journal Nature Communications, tackled the ferroelectric tunnel junction (FTJ), which consists of a layer of material so thin that electrons can tunnel through it. This ferroelectric layer is positioned between two electrodes so that when an electric field is applied to them, it can reverse the direction of the junction’s polarization. This reversal of polarization serves to change the alignment of positive and negative charges, which in turn is used to represent zero and one in binary computing.

The new wrinkle the Nebraska researchers brought to this setup was making the electrodes out of graphene for the first time. While graphene is primarily useful in electronic applications for its conductivity, the Nebraska-Lincoln team was more interested in its ability to accommodate just about any molecule. In this case, they used an ammonia molecule, which sat between the electrodes and the ferroelectric layer.

Typically, the polarity of a junction determines its resistance to the tunneling current; in one direction, the current is allowed to flow and in the other direction the current is greatly reduced. The researchers determined that the graphene-ammonia combination made the difference between these “on” and “off” states even more dramatic.

Ferroelectric materials do have the advantage of being non-volatile, in that they can maintain their polarization even in the absence of an external power source and thereby keep their stored information. Nonetheless, the space between the positive and negative charges is so small in these junctions that it becomes difficult to maintain the polarization.

“In all memory devices, there is a gradual relaxation, or decrease, of this polarization,” says Gruverman in the press release. “The thinner the ferroelectric layer is, the more difficult it is to keep these polarization charges separate, as there is a stronger driving force in the material that tries to get rid of it.”

This most recent research promises a way to eliminate that weakness for ferrolectric materials.

A Single Nanoparticle Enables Two Medical Imaging Techniques

Researchers at the Massachusetts Institute of Technology (MIT) have developed a nanoparticle that enables both magnetic resonance imaging (MRI) as well as fluorescent imaging in living animals.  The researchers believe that a single nanoparticle capable of performing these two functions should be able to help track specific molecules through the body, monitor a tumor’s environment, and determine whether drugs have reached their intended target.

In research published in the journal Nature Communications, the MIT team combined an MRI contrasting agent called nitroxide and a fluorescent molecule called Cy5.5 to produce a nanostructure called a branched bottlebrush polymer. The ratio of the two materials in the nanoparticle is 99 percent nitroxide and 1 percent Cy5.5.

This combination enables both MRIs and fluorescent imaging because of the interesting way these materials interact with each other. The nitroxides are reactive molecules in which a nitrogen atom is bound to an oxygen atom with one unpaired electron. Typically, the nitroxides suppress the Cy5.5’s fluorescence, except when the nitroxides are in the presence of molecule from which they can grab an electron, which, in the case of this study, was a vitamin C molecule. Once the free electrons in the nitroxides bind with the free electrons from another molecule, the MRI signal switches off and the Cy5.5 fluoresces.

In the study, which used mice as subjects, the researchers found that the nanoparticles headed towards the liver, where nanoparticles typically end up. It was there that the nanoparticles came in contact with vitamin C (which is produced by the mouse's liver), the MRI signals were turned off, and the Cy5.5 began to fluoresce. Vitamin C ultimately finds its way to a mouse’s brain, and so the researchers were able to detect fluorescence there. Meanwhile, in areas where the concentration of vitamin C was low, the MRI contrast proved to be strong.

The researchers continue to work on enhancing the signal differences that occur when the nanoparticles encounter a target molecule, such as vitamin C. They have also experimented with adding up to three different drugs to the nanostructure. The aim of having these drugs hitch a ride on the nanostructure is to give diagnosticians the ability to track whether they reach their intended target. A fair amount of research is currently being done in this area, wherein nanoparticles behave as both diagnostic tools and therapeutic devices—a capability that has led to the nanoparticles being dubbed “theranostic” nanoparticles.

“That’s the advantage of our platform—we can mix and match and add almost anything we want,” said Jeremiah Johnson, an assistant professor of chemistry at MIT and senior author of the study, in a press release.

The researchers also believe that this platform should allow detection of oxygen radicals inside a patient’s tumor—a biomarker that can indicate how aggressive the tumor is.

“We think we may be able to reveal information about the tumor environment with these kinds of probes, if we can get them there,” said Johnson. “Someday you might be able to inject this in a patient and obtain real-time biochemical information about disease sites and also healthy tissues, which is not always straightforward.”

Topological Insulators Move Closer to Practical Applications

Nearly a decade ago, theorists predicted the upside-down world of topological insulators, which were supposed to possess the peculiar property of being insulators on the inside but conductors on the outside.  While the theories were experimentally confirmed just a couple of years later, making practical devices out of the material has remained a largely unsolved challenge.

Now researchers at Purdue University claim they have found evidence that is the “smoking gun” proving that topological insulators are indeed a path towards realizing practical quantum computers as well as “spintronic” devices that are far more powerful than today’s number crunchers.

The “smoking gun” in this case comes in the form of something called a half-integer quantum Hall effect on the surface of a topological insulator. This is an effect that is seen in graphene in which there is a no resistance plateau at a zero magnetic field.

“This is unambiguous smoking-gun evidence to confirm theoretical predictions for the conduction of electrons in these materials,” said Purdue doctoral student Yang Xu, lead author of the paper detailing the discovery, in a press release.

The research, which appears in the journal Nature Physics, demonstrated for the first time that a three-dimensional material’s electrical resistance did not have to be dependent on the thickness of the material.

The researchers discovered further evidence that the conduction of electrons were topologically protected in these materials, which means their surfaces are guaranteed to be robust conductors. In the experiments, the researchers would slice off thin layers from the surface of the material and after each thinning of the material, the surface maintained its conductance without the slightest change.

“For the thinnest samples, such topological conduction properties were even observed at room temperature, paving the way for practical applications,” Xu said in the release.

This conduction on the surface of topological insulators is important for spintronic devices. The unique applicability of topological insulators to spintronic devices comes from the fact that the conducting electrons on the surface have no mass and are automatically “spin polarized,” leading to the unique half-integer quantum Hall effect that was observed in this research.

This past summer, researchers at Penn State and Cornell University provided one of the first promising indications that it might actually be possible to derive practical applications such as spintronic devices from these topological insulator materials.

In addition to spintronics, researchers believe that topological insulators, when combined with superconductors, could lead to a practical quantum computer

“One of the main problems with prototype quantum computers developed so far is that they are prone to errors,” said Yong P. Chen, a Purdue associate professor, in the press release. “But if topologically protected, there is a mechanism to fundamentally suppress those errors, leading to a robust way to do quantum computing.”

Chen added: "This experimental system provides an excellent platform to pursue a plethora of exotic physics and novel device applications predicted for topological insulators.”

A Drop of Water and a Push Transfers 2-D Material Between Substrates

The production of molybdenum disulfide (MoS2) films has been getting better of late. Recently researchers at Rice University found a way to orient the two-dimensional (2-D) material on its side so that the maximum amount of edge is exposed—a good thing for fuel cell catalysts and the supercapcitor electrodes.

Now researchers at North Carolina State University (NCSU) have developed a method for transferring these MoS2 films onto any substrate without causing any damage to the 2-D material  in the process. The researchers believe that this comparitively easy process could lead to the material’s use in flexible electronics for computing, photonic systems, and more.

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Nanofilm Serves as Artificial Retina

Israel-based Nano Retina made some noise a few years back with its development of so-called “nanoelectrodes” that “interface with the eye’s bipolar neurons” and restart neural stimulation, allowing for messages to go to the brain. The nanoelectrodes served as a kind of artificial retina.

Since then it doesn’t appear that Nano Retina has had much more to report on the development of its implantable device, at least from what its website reveals. But it other Israeli researchers do, in a paper published in the journal Nano Letters. The team from Tel Aviv University, the Hebrew University of Jerusalem Centers for Nanoscience and Nanotechnology and Newcastle University combined semiconductor nanorods and carbon nanotubes to create a wireless, light-sensitive, flexible film that could potentially act in the place of a damaged retina.

The researchers used a plasma polymerized acrylic acid midlayer to make covalent bonds with the semiconductor nanorods directly onto neuro-adhesive, three-dimensional carbon nanotube surfaces.

The researchers have tested the device in a chick’s retina that in normal conditions would not have responded to light. These tests demonstrated that the flexible film absorbed light, which then triggered neuronal activity in the chick.

The researchers claim that this film is more durable, flexible and efficient, as well as better able to stimulate neurons, compared to other competitive devices. While there have been a number of approaches to creating artificial retinas to address diseases of the retina, such as macular degeneration, the stumbling block has largely been getting the device to fit inside the eye itself.

This is why solutions such as Nano Retina’s nanoelectrodes have been so attractive. It would seem, based Nano Retina’s reticence since its initial announcement, that the engineering issues are significant even with a nanoscale solution. Whether this latest research can find a way around them remains to be seen.

Each Nanopore in a Material Serves as a Battery

We have seen batteries get so small that their anodes consist of a single nanowire. In that instance, the research was not really aimed at creating a new battery design but instead at demonstrating that the researchers could use a liquid electrolyte in the vacuum of a transmission electron microscope (TEM).

In contrast, researchers at the University of Maryland have their focus on building a new type of battery that is based on tiny nanopores in a ceramic material. The researchers have developed a method for introducing an electrolyte into the nanopores so that each cavity acts as an individual battery cell and all of them are joined in parallel.

Commenting on their research, published in the journal Nature Nanotechnology, the Maryland researchers note that: “a single nanopore structure that embeds all components of an electrochemical storage device could bring about the ultimate miniaturization in energy storage.”

In the video below, one of the authors of the paper, Chanyuan Liu, explains that the battery they developed in the lab can undergo one thousands charge/discharge cycles. It can be fully charged in 12 minutes, she adds in the press release.

Eleanor Gillette, another member of the group, says in the video that the aim is to develop the manufacturing technology to make larger nanopore  structures possible. Liu says that they have already identified ways to increase the power of the batteries by ten times.

The entire design of the battery involves each of its nanobattery components being composed of an anode, a cathode, and a liquid electrolyte confined within the nanopores of anodic aluminium oxide, which is an advanced ceramic material. Each nanoelectrode includes an outer ruthenium nanotube current collector and an inner nanotube of vanadium pentoxide storage material. These together form a symmetric full nanopore storage cell with anode and cathode separated by an electrolyte region. The vanadium pentoxide is treated with lithium at one end to serve as the anode, with pristine vanadium pentoxide at the other end serving as the cathode.

The researchers believe that the key to the success of the design is the uniformity in shape and size of the nanopores, which allows for a dense packing of the nanopores into the ceramic material.

Graphene-based Supercapacitors Take Another Crack at All-electric Vehicles

Researchers at the Queensland University of Technology (QUT) in Australia have developed a supercapacitor featuring graphene carbon nanotube films. They’re confident that their creation could dramatically boost the power and range of all-electric vehicles that now rely on lithium-ion (Li-ion) batteries for propulsion.

In research that was published in both the Journal of Power Sources and Nanotechnology, the Australian researchers used graphene films as the electrodes and carbon nanotube films as current collectors. The result was devices demonstrating energy densities ranging from  8 to 14 watt-hours per kilogram, and power densities between 250 and 450 kilowatts per kilogram.

The hope has been that someone could make graphene electrodes for supercapacitors that would boost their energy density into the range of chemical-based batteries. The supercapacitors currently on the market have on average an energy density around 28 Wh/kg, whereas a Li-ion battery holds about 200Wh/kg. That’s a big gap to fill.

The research in the field thus far has indicated that graphene’s achievable surface area in real devices—the factor that determines how many ions a supercapacitor electrode can store, and therefore its energy density—is not any better than traditional activated carbon. In fact, it may not be much better than a used cigarette butt.

Though graphene may not help increase supercapacitors’ energy density, its usefulness in this application may lie in the fact that its natural high conductivity will allow superconductors to operate at higher frequencies than those that are currently on the market. Another likely benefit that graphene will yield comes from the fact that it can be structured and scaled down, unlike other supercapacitor materials.

It is this ability to be molded into various shapes that the Australian researchers hope to exploit; they suggest that the supercapacitor films they have developed could be used to line parts of a car’s chassis to offer a quick energy boost to all-electric vehicles using Li-ion batteries.

"Vehicles need an extra energy spurt for acceleration, and this is where supercapacitors come in. They hold a limited amount of charge, but they are able to deliver it very quickly, making them the perfect complement to mass-storage batteries," said Marco Notarianni, a QUT researcher, in a press release.

Notarianni added: "Supercapacitors offer a high power output in a short time, meaning a faster acceleration rate of the car and a charging time of just a few minutes, compared to several hours for a standard electric car battery."

Graphene-based supercapacitors have been a bit of a disappointment to those who envisioned them completely replacing lithium-ion (Li-ion) batteries for powering all-electric vehicles.  This latest complementary role suggested by the Australian researchers may be a way to see them put to some use.

However, it’s not clear that the energy density numbers achieved during this latest round of research have given us any reason to think that a graphene-based supercapacitor will be the route leading to an all-electric vehicle that operates solely on supercapacitors.

Nanodiamond Production Technique Opens Up Electronic Applications

With news just last week that nanodiamonds could aid in the development of new methods for drug delivery and cancer therapeutics, the prospects for successful cancer treatment got a shot in the arm.

While medical applications for nanodiamonds got a boost, it left the fortunes of nanodiamonds in electronics-related applications a bit out in the cold. The wait was not long, however, with research out of Purdue University that demonstrated a pulsed laser could be used to create synthetic nanodiamond films and patterns on the surface of graphite. This development should have an impact on potential applications for nanodiamonds including biosensors, quantum computing, fuel cells and next-generation computer chips.

"The biggest advantage is that you can selectively deposit nanodiamond on rigid surfaces without the high temperatures and pressures normally needed to produce synthetic diamond," said Gary Cheng, an associate professor of industrial engineering at Purdue University, in a press release. "We do this at room temperature and without a high temperature and pressure chamber, so this process could significantly lower the cost of making diamond. In addition, we realize a direct writing technique that could selectively write nanodiamond in designed patterns."

In research published in the Nature journal Scientific Reports, the Purdue team started with a multilayered film containing a layer of graphite and covered with a glass sheet. They then exposed this layered structure to an ultra-fast pulsing laser that instantaneously transformed the graphite into an ionized plasma that generates a downward pressure. The graphite plasma is prevented from escaping by the glass cover of the multilayered film where, trapped, it quickly solidifies into diamond.

"These are super-small diamonds and the coating is super-strong, so it could be used for high-temperature sensors," Cheng said in the release.

Strength was always the aim of the research. In fact, the technique was originally developed to find a way to strengthen metals. It was only serendipitously that they discovered that it produced this nanodiamond film.

With this development, the researchers believe nanodiamonds could begin to have an impact in electronics. For instance, nanodiamonds have been suggested as a way to get quantum computers to operate at room temperature as opposed to near absolute zero. This would be accomplished by replacing the ions used in some quantum computers with nitrogen-vacancy centers in diamonds.

Nanodiamonds could also lead to next-generation computer chips based on optical transitors in which photons replace electrons. In this scenario, nanodiamonds would replace the special dye molecules that are used in today’s optical transistors. The use of these dyes requires special cooling, which eliminates them from practical use. However, researchers at the Institute of Photonics Sciences (ICFO) in Barcelona demonstrated last year showed that nanodiamond operating at room temperature could be used in an ultrafast optical switch, replacing the dye molecules.

New Method for Producing Molybdenum Disulfide Provides Two Potential Applications

Molybdenum disulfide (MoS2) started out as the next big thing after graphene in electronic applications. But that excitement started to wane somewhat when it was revealed that MoS2 contained traps—impurities or dislocations that can trap an electron or hole and hold it until a pair is completed—that limit its electronic properties.

While researchers continue to work on removing those traps in order to improve its electronic properties, others have been looking at uses for MoS2 outside of digital electronics in applications including photovoltaics and wearable electronics.

In this ever-expanding application universe, researchers at Rice University have found a method for manipulating MoS2 so that it could serve both as an improved catalyst for fuel cells and as the electrodes of supercapacitors.

In research published in the journal Advanced Materials, the Rice team, led by James Tour, developed a simple method for producing flexible films made from MoS2 that orients the material on its sides. In other words, they made the material in such a way that the maxiumum amount of its edges are exposed.

The researchers showed that when oriented in this manner, the MoS2 can serve as an effective catalyst in the hydrogen evolution reaction (HER), a process used in fuel cells to pull hydrogen from water.

“So much of chemistry occurs at the edges of materials,” said Tour in a press release. “A two-dimensional material is like a sheet of paper: a large plane with very little edge. But our material is highly porous. What we see in the images are short, 5- to 6-nanometer planes and a lot of edge, as though the material had bore holes drilled all the way through.”

Tour added: “Its performance as a HER generator is as good as any molybdenum disulfide structure that has ever been seen, and it’s really easy to make.”

Other research has attempted to take advantage of MoS2 as a catalyst for fuel cells by standing them up on their sides. The Rice team took a different approach. First, they grew a porous molybdenum oxide film onto a molybdenum substrate through room-temperature anodization, an electrochemical process for thickening metal parts by adding a natural oxide layer.

The researchers then exposed the film to sulfur vapor at 300 °C (572 °F) for one hour. The result was molybdenum disulfide that had a flexible, nano-porous sponge-like structure.

Since the key to catalysts and to the electrodes in supercapacitors is surface area, the researchers immediately realized that the material would fit the bill for both applications. The Rice team developed a supercapacitor using the material and found the device retained 90 percent of its capacity after 10,000 charge-discharge cycles and 83 percent after 20,000 cycles.

Tour believes that this method of exploiting anodization could serve as a platform for a range of applications and devices.

“We see anodization as a route to materials for multiple platforms in the next generation of alternative energy devices,” Tour said. “These could be fuel cells, supercapacitors and batteries. And we’ve demonstrated two of those three are possible with this new material.”

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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
New York, NY
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