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Graphene and Carbon Nanotubes: Two Great Materials Even Better Together

James Tour at Rice University has a history of finding links between carbon nanotubes and graphene, which are often regarded merely as rivals for a host of electronic applications. A few years back, Tour developed a process for “unzipping” carbon nanotubes so that they transformed into graphene. Now Tour and his colleagues have used that unzipping technique to develop a method in which carbon nanotubes are used as a kind of reinforcing rebar for graphene, protecting it during the manufacturing process.

Why is this useful? Because to produce high-quality graphene for electronic applications (a promising one is replacing indium tin oxide as a transparent conductor in displays for controlling pixels), a manufacturing process known as chemical vapor deposition (CVD) is used, and CVD has an Achilles heel: while it is possible to grow large sheets of graphene on a copper substrate in a furnace, when you try to remove the graphene sheets from the copper, you find that it is difficult to do so without breaking the graphene. A reinforcement polymer is usually laid over the graphene to keep it from breaking during its removal, but this polymer leaves impurities.

Solving this manufacturing issue has led to some intriguing new methods, including work late last year out of the National University of Singapore, where researchers developed a method in which the copper is sandwiched between a silicon layer and graphene layer so that when the copper is etched away the graphene and silicon remain attached to each other.

The Rice University researchers have taken another approach. First, they coat the copper with both single-walled and multi-walled carbon nanotubes and then heat and cool the material. These carbon nanotubes serve as carbon source without the need of adding any other carbon to the process. When heated, the carbon nanotubes both decompose into graphene and—voilà!—unzip to form covalent junctions with the new graphene layer. The researchers have dubbed the resulting material "rebar graphene."

In a paper published in the journal ACS Nano, the Rice team describes how the rebar graphene could be transferred onto target substrates without needing a polymer coating due to the reinforcement effect. The resulting rebar graphene also exhibits better electrical conductivity than graphene produced through other CVD processes.

"Normally you grow graphene on a metal, but you can’t just dissolve away the metal," Tour said in a press release. “You put a polymer on top of the graphene to reinforce it, and then dissolve the metal. Then you have polymer stuck to the graphene. When you dissolve the polymer, you’re left with residues, trace impurities that limit graphene’s effectiveness for high-speed electronics and biological devices. By taking away the polymer support step, we greatly expand the potential for this material."

Tour believes that this rebar graphene could be a competitive alternative for the replacement of indium tin oxide in displays, an application that could potentially add flexibility to them. Before that happens, however, the researchers will have to show that their new manufacturing process can scale up enough and lower production costs to make it truly competitive with ITO.

Molecular Gears Turn Under Pressure

In collaborative research between Georgia Tech and the University of Toledo, both computer modeling and experiments have demonstrated that when pressure is applied to a superlattice structure the bonds that link the nanoparticles making up the structure behave as though they were gear-like, molecular-scale machines.

Superlattices form when an almost atomically thin layer of one material is laid down over another in an alternating pattern, creating numerous interfaces. IBM has used graphene-based superlattices to build experimental photodetectors.The researchers in this latest resarch believe that this type of superlattice could prove useful for developing molecular-scale switching, sensing and even energy absorption applications.

The appearance of gear-like movement in the structure is the result of the hydrogen bonds that connect organic molecules surrounding clusters of silver nanoparticles in the superlattice. Under pressure these bonds move like a hinge keeping the nanostructure from breaking.

While this hinge-like movement was unexpected, the pressure itself also had an unexpected consequence of softening the superlattice so that that subsequent compression of the structure could be done with less force. The researchers discovered that after the structure had been compressed by about six percent of its volume, the pressure required for additional compression suddenly dropped significantly. This drop occurred as the nanocrystal components rotated, layer-by-layer, in opposite directions.

“As we squeeze on this material, it gets softer and softer and suddenly experiences a dramatic change," said Uzi Landman, a professor in the School of Physics at the Georgia Institute of Technology, in a press release. "When we look at the orientation of the microscopic structure of the crystal in the region of this transition, we see that something very unusual happens. The structures start to rotate with respect to one another, creating a molecular machine with some of the smallest moving elements ever observed."

 In the video below you can see the structure of the superlattice, which consists of clusters with cores of 44 silver atoms each. Thirty molecules of an organic material known as mercaptobenzoic acid (p-MBA) protect the silver clusters. The organic molecules are attached to the silver by sulfur atoms. Again, the parts that move, the “hinges”, are the hydrogen bonds that attach the organic molecules.

The research, which was published in the journal Nature Materials ("Hydrogen-bonded structure and mechanical chiral response of a silver nanoparticle superlattice"), described the production of the superlattice structure through self assembly. In a solution, clusters of the silver nanoparticles and organic molecules assemble themselves into the larger superlattice, guided by the hydrogen bonds, which can only form between the p-MBA molecules at certain angles.

As if producing some of the tiniest mechanical objects ever weren't enough, the researchers believe there work also represents the largest solid ever mapped in detail using a combined X-ray and computational techniques.

First Wafer-Scale, Single-Crystal, Monolayer Graphene Made in Bulk

Samsung’s Advanced Institute of Technology has been at the forefront of applying graphene to electronics, developing such things as zippy new types of transistors.

Despite these efforts, for graphene to succeed at reaching commercial applications in digital electronics a way of producing high-quality monolayer graphene in bulk needs to be developed.

In this week's issue of the journal Science (“Wafer-Scale Growth of Single-Crystal Monolayer Graphene on Reusable Hydrogen-Terminated Germanium”), Samsung along with researchers at South Korea’s Sungkyunkwan University claim to have produced the first wafer-scale growth of wrinkle-free single-crystal monolayer graphene on a silicon wafer.

While there has been another successful demonstration of producing single-crystal monolayer graphene, that method is too costly and cannot be readily adapted to bulk-scale production.

The South Korean researchers overcame this production limitation in a process that used chemical vapor deposition (CVD) method for growing the graphene on the surface of a germanium coated silicon wafer coated. However, in this new method the researchers were able to transfer the graphene from the germanium without resorting to the damaging wet chemical etching process typically used. Instead, the researchers deposited a thin film layer of gold atop the graphene using a thermal evaporator. By then attaching the gold/graphene/germanium combination to a thermal release tape and applying a bit of pressure, the graphene and germanium substrate were easily separated .

Research late last year demonstrated that graphene loses none of its attractive electronic properties, such as high electron mobility, when paired with silicon. So it would appear that Samsung's new technique could help usher graphene into digital electronic applications, where high-quality material is essential.

But the hurdles are high in this field. To win a place in electronics, graphene has to be both cheaper and better than silicon and a host of alternative materials that have years of a developmental head start. For instance, in flexible electronics, which is one of the applications cited as a potential use for Samsung’s new graphene, the material not only has to prove itself better than the specially formulated plastics that currently rule the roost, but also needs to outperform carbon nanotubes, as well.

But you don' t have to wait for graphene to take over the semiconductor scene to see it used in electronics. Lower quality graphene should be just fine for thermal management systems and supercapacitors for portable electronics that would not just power the devices but serve as a high-frequency filter.

Nanomaterials Improve Both the Anode and Cathode of Li-ion Batteries

Lithium-ion (Li-ion) batteries have been the subject of intense research aimed at manufacturing them with nanomaterials that will let them better meet the demands of everything from laptops and mobile devices to all-electric vehicles (EVs).

A large portion of the research has been focused on developing nanomaterials for the anode of the Li-ion that will replace graphite. But there has definitely been a shift toward the cathode, as evidenced by research over the last few years.

Now a research team out of the University of Southern California (USC) has taken a novel approach to the improvement of Li-ion batteries with nanomaterials: tackling both the cathode and the anode simultaneously.

For the anode, the USC researchers developed an inexpensive method for producing porous silicon nanoparticles through ball milling and stain etching. For the cathode, they developed a method of coating sulfur powder with graphene oxide to improve performance.

Reports detailing the research projects were published in the journal Nano Letters, but in separate papers. The work on the silicon anode ("Large-Scale Fabrication, 3D Tomography, and Lithium-Ion Battery Application of Porous Silicon") was originally published online in November; details about the work on the cathode ("Solution Ionic Strength Engineering As a Generic Strategy to Coat Graphene Oxide (GO) on Various Functional Particles and Its Application in High-Performance Lithium–Sulfur (Li–S) Batteries") made the journal in December.

The silicon anode the research team developed demonstrated a stable capacity above 1100 milliamp-hours per gram (mAh/g) for 600 extended cycles, making it nearly three times as powerful and durable as a typical commercial anode. Meanwhile, the team's graphene oxide coating improved the sulfur cathode's capacity to 800 mAh/g for 1000 charge-discharge cycles, which is more than five times the capacity of commercial cathodes.

While the capacity numbers are good, the cycle numbers appear low—at least compared to the 6000 cycles reported for nanostructured silicon in anodes just two years ago. And early last year, researchers at Argonne National Laboratory were able to achieve a specific anode capacity of 1250 mAh/g and maintain it for 5000 charge-discharge cycles.

The USC researchers note that they also developed a silicon nanowire-based anode that had high capacity and was extremely durable, but the production method was prohibitively expensive. The researchers believe this new approach, which builds on their previous research, strikes an attractive balance between performance and cost-effective production.

"Our method of producing nanoporous silicon anodes is low-cost and scalable for mass production in industrial manufacturing, which makes silicon a promising anode material for the next generation of lithium-ion batteries," said Chongwu Zhou, an electrical engineering professor at USC's Viterbi School of Engineering, in a press release. "We believe it is the most promising approach to applying silicon anodes in lithium-ion batteries to improve capacity and performance."

Interestingly, the team’s use of graphene for the cathode hints at its effectiveness as an anode material. So-called decorated graphene, where nanoparticles are scattered around the surface of the one-atom-thick sheet, is relatively easy to produce cheaply and effectively. What's more, it not only has high storage capacity (or energy density), but also high power density. This is due to graphene’s inherent high conductivity.

Whether nanostructured silicon or graphene will win the day as an anode material remains to be seen; we can rest assured we will see higher capacity cathodes, which is the source of the battery’s energy.

Polymer Nanofibers Could Beat the Heat in Chips

Back in 2001, Patrick P. Gelsinger, at the time the chief technology officer at Intel Corp., suggested that if the trend towards hotter and hotter chips continued—which thankfully it did not—microprocessors would generate heat equivalent to the temperature found on the surface of the sun within a decade (the time that we currently inhabit).

In order to overcome this thermal management issue, researchers have, for the past decade, been researching and using different nanomaterials and nanostructures to beat the heat.

Now researchers at the Georgia Institute of Technology have developed a polymer with aligned arrays of nanofibers that acts as a thermal interface for electronics. This alignment, they report, makes it 20 times better at conducting heat than the original polymer.

The research, which was published in the journal Nature Nanotechnology (“High thermal conductivity of chain-oriented amorphous polythiophene”), produced the new polymer from a conjugated polymer, polythiophene, in which aligned polymer chains in nanofibers facilitate the transfer of phonons—but without the brittleness associated with crystalline structures. Phonons are vibrations in a crystal lattice that carry heat.

The researchers believe that the resulting polymer could be used to draw heat away from electronic devices in servers, automobiles, high-brightness LEDs and some mobile devices.

“Thermal management schemes can get more complicated as devices get smaller,” said Baratunde Cola, an assistant professor at the Georgia Institute of Technology, in a press release. “A material like this, which could also offer higher reliability, could be attractive for addressing thermal management issues. This material could ultimately allow us to design electronic systems in different ways.”

While most thermal management systems for computers have relied on fans as a solution, the real hurdle has been getting heat from the chip to the heat sink. This particular issue has been addressed by solder, which is liable to fail after a few thousand cycles of heat-induced expansion and contraction.  This new polymer, which adheres well to devices, would be fabricated on heat sinks and heat spreaders, replacing the solder.

“Polymers aren’t typically thought of for these applications because they normally degrade at such a low temperature,” Cola said in the release. “But these conjugated polymers are already used in solar cells and electronic devices, and can also work as thermal materials. We are taking advantage of the fact that they have a higher thermal stability because the bonding is stronger than in typical polymers.”

The polymer is produced in an electrochemical polymerization process in which an alumina template with tiny pores is covered with an electrolyte containing monomer precursors. An electrical potential is applied to the template that causes electrodes at the base of each to attract the monomers in the electrolyte, forming the hollow nanofibers. The nanofibers are cross-linked through electropolymerization and the template is then removed. The resulting material is applied to the electronic devices using water or some other solvent that spreads the fibers through capillary action and van der Waal forces, the repulsive or attractive forces between molecules.

While the researchers don’t fully understand this fabrication technique theoretically, they are confident that it could be scaled up for commercialization. “There are some challenges with our solution, but the process is inherently scalable in a fashion similar to electroplating,” said Cola in the release. “This material is well known for its other applications, but ours is a different use.”

Gold Nanoparticles and Near-infrared Light Kill Cancer Cells With Heat

Nanoparticles have been suggested as a way to kill cancer cells in a multitude of ways. Recent research has suggested a method for surrounding gold nanoparticles with nanobubbles that would rip open small pores in cancer cell membranes. This would allow drugs present outside the cells to get in. Another cancer killing treatment is tricking lymphoma cells into eating gold nanoparticles. Once ingested, the nanoparticles make it impossible for the cancer cells to eat anything else, dooming them to death by starvation.

You may have noticed the recurring use of gold nanoparticles in cancer research. Following that tradition, researchers at ETH Zurich in Switzerland have demonstrated that gold nanoparticles, in combination with near-infrared light, can turn up the heat on cancer cells enough to kill tumors.

While gold nanoparticles are well tolerated by the human body, they are not too good at absorbing long-wavelength red light, which is able to penetrate human tissue better than short-wavelength blue light. The nanoparticles that are effective at this are known as plasmonic nanoparticles. Plasmonics is a field in which free electrons in a metal can be excited by the electric component of light so that there are collective oscillations in the material with heat generation being one effect.

The ETH Zurich researchers knew that if they molded the gold nanoparticles into a particular shape, such as a rod or a shell, they could give it the plasmonic property for absorbing near-infrared light it otherwise lacked. The problem with this approach is that is complex and expensive.

In research published the journal Advanced Functional Materials ("Photothermal Killing of Cancer Cells by the Controlled Plasmonic Coupling of Silica-Coated Au/Fe2O3 Nanoaggregates"), the Swiss researchers devised a way to make sphere-shaped gold nanoparticles aggregate into a light-absorbing design. To do this, each particle was coated with a silicon dioxide layer that serves as a buffer between the individual spheres in the aggregate. Maintaining precise spacing between each nanoparticle makes the configuration absorb the near-infrared light.

“The silicon dioxide shell has another advantage”, explains Georgios Sotiriou, lead author on the study, in a press release. “It prevents the particles from deforming when they heat up.” This is a critical feature since the particles' near-infrared light absorbing qualities are dependent upon them maintaining their spherical shape.

That’s great, but how do they reach the cancer cells so they kill the tumor when they heat up? The trick the researchers developed was adding superparamagnetic iron oxide particles in with the gold particles; the iron makes the nanoparticles steerable via magnetic fields so that they accumulate next to the cancer cells.

While issues still need to be addressed such as how the particles are removed from the body, the nanoparticles do lend themselves to being used as a contrast agent in magnetic resonance imaging.

“You could even couple the particles with temperature-responsive drug carriers, which would then allow the drug release if a certain temperature were exceeded,” explains Sotiriou in the release.

Living Materials Respond to the Environment and Communicate With Each Other

Nearly four decades ago, researchers managed to coax the bacterium Escherichia coli (E. coli) into producing a protein. Now researchers at MIT have devised a way to combine a living E. coli cell with inanimate building blocks, like gold nanoparticles and quantum dots, to create a hybrid “living material.”

The MIT researchers have managed to get the bacterium to produce a biofilm that is able to attach to different nanoparticles, resulting in a hybrid living material that responds to its environment, produces complex biological molecules, and spans multiple length scales. The properties that these living materials take on include the ability to conduct electricity and emit light.

The research, which was published in the journal Nature Materials ("Synthesis and patterning of tunable multiscale materials with engineered cells"), is expected to serve as a demonstration of the new approach's potential to produce larger devices such as solar cells, self-healing materials, or diagnostic sensors.

“Our idea is to put the living and the nonliving worlds together to make hybrid materials that have living cells in them and are functional,” said Timothy Lu, an assistant professor of electrical engineering and biological engineering at MIT, in a press release. “It’s an interesting way of thinking about materials synthesis, which is very different from what people do now, which is usually a top-down approach.”

The researchers chose to use E. coli since it naturally produces a biofilm. This biofilm contains something dubbed “curli fibers,” which are amyloid proteins that help the bacteria attach to surfaces. The researchers modify the repeating protein chain of the curli fibers (called CsgA) by adding protein fragments called peptides. By modifying the curli fibers that attach to different substances, it’s possible to create a range of hybrid materials. They can create gold nanowires that behave like a conducting film or tiny crystals that exhibit quantum mechanical properties.

Perhaps the most significant property of these hybrid materials is that they can communicate with each other. “It’s a really simple system but what happens over time is you get curli that’s increasingly labeled by gold particles. It shows that indeed you can make cells that talk to each other and they can change the composition of the material over time,” Lu said in the release. “Ultimately, we hope to emulate how natural systems, like bone, form. No one tells bone what to do, but it generates a material in response to environmental signals,” said Lu.

Early application ideas for the hybrid materials include batteries and solar cells, but the researchers are also investigating the potential of coating the biofilms with enzymes to catalyze the breakdown of cellulose, which could be useful for converting agricultural waste to biofuel.

Rhenium Disulfide: A New 3-D Material That Has the Electronic Properties of a 2-D Material

Earlier this year, “borophene” elbowed its way into the increasingly crowded world of two-dimensional (2-D) materials.  With rough counts estimating that there are over 100 layered materials that could potentially be stripped down to a monolayer to form a 2-D material, we are likely to see more of these in the coming years.

Researchers at the U.S Department of Energy (DOE) Berkeley Lab have added a new twist to the quest for 2-D materials by creating a 3-D material that behaves like a 2-D monolayer.  The material is called rhenium disulfide (ReS2) and unlike 2-D materials, such as molybdenum disulfide (MoS2), it maintains its direct bandgap even in a 3-D version. The direct bandgap means that unlike silicon it can emit and absorb light easily, making materials that have this property attractive for optoelectronic applications.

“Typically the monolayers in a semiconducting transition metal dichalcogenides, such as molybdenum disulfide, are relatively strongly coupled, but isolated monolayers show large changes in electronic structure and lattice vibration energies,” said Sefaattin Tongay, lead author of the research, in a press release. “The result is that in bulk these materials are indirect gap semiconductors and in the monolayer they are direct gap.”

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Carbon Nanotube Circuits Are Back in the Running as a Viable Material for Flexible Electronics

There was a time, not so long ago, when carbon nanotubes (CNTs) were the “wonder material” that everyone was talking about—of course, that was before graphene hit the scene.

But even before graphene, researchers had begun to doubt whether CNTs were actually well suited for electronics applications. There are two stubborn obstacles that stand in the way of applying carbon nanotubes to electronics: it’s tricky to get them to go where you want them and it's difficult to create CNTs that are homogeneous enough to ensure stable electrical responses.

In spite of these hurdles, one researcher, Zhenan Bao of Stanford University, has remained focused on applying CNTs to electronic applications. Bao addressed the homogeneity issue a few years back by developing a new sorting process for the CNTs that ensured conducting and semiconducting CNTs were separated.

Now Bao, along with Yi Cui an assistant professor at Stanford, has developed a process by which CNT circuits can tolerate power fluctuations in flexible electronic devices in much the same way that silicon circuitry manages the fluctuations.

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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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