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Graphene Leading the Way to Optical Chips

In a joint research project, researchers from the Massachusetts Institute of Technology (MIT), Columbia University, and IBM’s T. J. Watson Research Center have used graphene as a photodetector for enabling an optical chip.

Graphene has tantalized researchers in photodetector applications with its wide spectral range (from the ultraviolet to the infrared), fast optoelectronic response that is the result of high electron mobility, and its lack of a band gap. However, graphene can absorb only a small fraction of incoming light, so its responsivity has been limited.

While this minimal light sensitivity may limit its use in digital camera applications, the MIT, Columbia, and IBM researchers may have engineered a way to use graphene as a photodetector for converting light into electricity for integrated optoelectronic chips.

The research team, which published its work in the journal Nature Photonics (“Chip-integrated ultrafast graphene photodetector with high responsivity”)  developed a method by which they could overcome graphene’s low responsivity to incoming light (measured at between 2 and 3 percent of the light passing through it being converted to electrical current). They turned to creating a bias in the photodetector so that electrons that were disrupted by incoming photons would remain in a higher energy state.

Typically, creating this bias involves maintaining voltage through the photodetector. However, this voltage is a source of noise that compromises the photodetector’s readings. To avoid this noise, the researchers turned to the work of Fengnian Xia and his colleagues at IBM; they produced a bias in a photodetector without the application of a voltage.

This trick is accomplished through an ingenious design in which light is funneled into the photodetector through a channel—or a waveguide—that is capped with a piece of graphene oriented perpendicular to the channel. The graphene has gold electrodes on either side of it, but instead of them being evenly spaced, one of the electrodes is closer to the graphene than the other.

“There’s a mismatch between the energy of electrons in the metal contact and in graphene,” said Dirk Englund, an assistant professor at MIT and the leader of the research team, in a press release. “And this creates an electric field near the electrode.”

So in operation, photons come through the channel and start kicking the electrons up to a higher energy state. These excited electrons are then pulled to the electrodes by the electric field, thereby creating a current—without applying a voltage.

This voltage-free bias boosts the photodetector to the point where it could generate 100 milliamps per watt, a responsivity equal to that of germanium. The researchers believe that with a bit of engineering (i.e., thinner electrodes, and a narrower waveguide), it could be possible to boost these results by a factor of two or perhaps even four.

The impact of chips that use light rather than electricity is clear. They will consume less power and produce less heat. Both of these factors have become ever more critical as chip features get smaller and smaller.

Thomas Mueller, an assistant professor at the Vienna University of Technology’s Photonics Institute, and a co-author of very similar research in the same journal, noted in the press release: “The other thing that I like very much is the integration with a silicon chip, which really shows that, in the end, you’ll be able to integrate graphene into computer chips to realize optical links and things like that.”

Image: MIT/Columbia University/IBM

Top-Down and Bottom-Up Manufacturing Combined in One Technique

An international team of researchers has combined ink-jet printing and self-assembling block copolymers to create a hybrid between top-down and bottom-up manufacturing. This new hybrid approach to building nanostructures promises to overcome the obstacles of fabricating nanostructures out of polymers and other soft materials.

The research, which was published in the journal Nature Nanotechnology ("Hierarchical patterns of three-dimensional block-copolymer films formed by electrohydrodynamic jet printing and self-assembly"),  found that by using self-assembling block copolymers  in combination with ink-jet printing that the resolution for even the best ink jet printers  could be improved from approximately 200 nanometers down to about 15 nm.

Block copolymers are chain like molecules made up of blocks of two types of chemicals. (Imagine a string of Christmas lights having a repeating pattern of 5 green lights and 5 red lights.) Under the right circumstances, such copolymers can fold up to form patterns such as a repeating array of holes.

The ITRS roadmap has long identified block copolymers for use in reducing chip feature sizes. For years block copolymers have demonstrated their usefulness in creating self-assembled photoresists for chip manufacturing.

While the potential for self-assembling block copolymers have long been understood, this work—which is a cooperative effort of researchers from University of Illinois at Urbana-Champaign, the University of Chicago, and Hanyang University in Korea—adds an important twist.

“The most interesting aspect of this work is the ability to combine ‘top-down’ techniques of jet printing with ‘bottom-up’ processes of self-assembly, in a way that opens up new capabilities in lithography—applicable to soft and hard materials alike,” said John Rogers, a materials science professor at Illinois and one of the authors of the paper, in a press release.

In the work described, the international team turned to the Belgium-based nanoelectronics powerhouse Imec to create chemical patterns over large areas of a substrate with high precision. The research team in Illinois then used ink jet printing to deposit block copolymers on top of these patterns. The block copolymers would self-organize following the patterns laid on the underlying template. The result was that it created patterns that had a greater resolution than the template itself.

While previous work had managed to deposit films on these templates, the result was that the patterns only possessed one characteristic feature size and spacing. With the ultra-precise ink-jet printing tool, it became possible to create multiple dimensions in one layer.

“This invention, to use inkjet printing to deposit different block copolymer films with high spatial resolution over the substrate, is highly enabling in terms of device design and manufacturing in that you can realize different dimension structures all in one layer,” said Paul Nealey, a professor at the University of Chicago and co-author of the paper, in the press release. “Moreover, the different dimension patterns may actually be directed to assemble with either the same or different templates in different regions.”

Image: Serdar Onses/University of Illinois-Urbana

"Piranha Etching" Could Push Nanowire Solar Cells Way Past Theoretical Limits

Researchers at the Eindhoven University of Technology, Delft University of Technology and the company Philips in the Netherlands have developed a method for increasing the conversion efficiency of nanowire-based photovoltaic cells: Give them a good cleaning.  While the researchers have demonstrated an increase to 11.1 percent efficiency with their technique, it is the long-term prospects of the method that appear the most promising with estimates for the conversion efficiency ultimately reaching 65 percent.

The potential for nanowire-based photovoltaics to reach extremely high conversion efficiencies was demonstrated earlier this year when researchers at the Nano-Science Center at the Niels Bohr Institut in Denmark and the Ecole Polytechnique Fédérale de Lausanne in Switzerland suggested that nanowire photovoltaic cells could surpass the Shockley-Queisser limit. The Shockley-Queisser limit is a theory established in 1960 that suggested among other things that that only 33.7 percent of all the sun’s energy could be converted into electricity for solar cells with a single p-n junction. To surpass this limit has been a  Holy Grail quest of sorts for photovoltaics.

Now with this latest research  the Shockley-Queisser limit could be surpassed by a huge margin. The technique, which is described in the journal Nano Letters (“Efficiency Enhancement of InP Nanowire Solar Cells by Surface Cleaning”)  involves a surface cleaning of the nanowires.

The cleaning is a chemical reaction that the researchers have dubbed “piranha etching”. The result of the process is that indium phosphide nanowires come out much smoother and have fewer imperfections.

This cleaning of the surface addresses the Achilles Heel of nanowire photovoltaics. While the surface area of nanowires enables photovoltaics to be produced without using as much costly semiconductor material as ordinary photovoltaics do, the nanowires cell's large surface area is prone to have imperfections that result in energy loss.

The 11.1 percent conversion efficiency achieved by the Netherlands-based researchers does not equal the record of 13.8 percent achieved by an international team of researchers earlier this year. However, that may be of little consequence if the promise of this latest research can be realized.

“By varying the thickness of the nanowires and improving the way the crystals inside them are stacked, we think we should soon be able to approach an efficiency of 20 percent”, says Professor Erik Bakkers, one of the lead researchers, in a press release.

Following this basic approach, the researchers believe that theoretically they could reach a conversion efficiency as high as 65 percent. If that’s true, and it can be produced relatively inexpensively, it'll be a game changer.

Image: Eindhoven University of Technology

Article Makes a Hash out of Nanotechnology and its Impact

Over the years, I thought I had become accustomed to mainstream journalists making a hash out of the subject of nanotechnology. I've even had the misfortune of watching videos starring famed TV physicists making bizarre predictions about the problems that will ensue from the changes brought on by nanotechnology. I thought I had steeled myself so I would not be bothered by these sorts of things anymore, but along came the latest mishmash of half-informed scaremongering.

It’s a perfect storm of wrongheadedness. It was penned by Ainissa G. Ramirez, Ph.D., a noted author and “science evangelist,” giving it an air of veracity. But that doesn't keep the piece from going wrong right from the outset. You can find the first misstep in the second sentence: “By miniaturizing matter, science fact will look like science fiction.” Okay, once and for all: Nanotechnology has nothing to do with miniaturizing matter. Nanotechnology is not the real-life version of the 1960s sci-fi film “Fantastic Voyage.” We are not shrinking matter.

Ramirez apparently skimmed the wrong articles to mine that nugget of information. The rest of the article, as far as nanotechnology is concerned, scans about right; it includes all the typical references you would expect from someone who skimmed some articles on nanotechnology: gold is red at the nanoscale, using hair to visualize the nanoscale, et cetera.

This is not to say Ramirez does not fudge some other references to nanotechnology for dramatic effect. For example, there's this gem: “Do we want small particles—which we can't imagine let alone see—swimming in our water supply and covering everything around us?”

Swimming? Covering everything around us? Really? The "scholarly paper" she must have been referencing is Michael Crichton’s novel “Prey.” Outside the world of fiction, man-made nanoparticles are not going to cover everything.

While these egregious misstatements of fact got my blood boiling, it’s the main thesis of the article that is perhaps the biggest problem. Ramirez’s argument boils down to the idea that pursuing technology has unintended consequences that, in balance, are bad for us. This is a popular meme among so-called environmentalists. Ramirez suggests that the automobile, while likely considered a really great idea at the time of its invention, brought on obesity because it eliminated an alternate course of history wherein we would have been walking or cycling. I suppose this line of argument appeals to a certain segment of the population that would like us to return to the bucolic times before all the inventions of our modern age. Sigh. Why can’t these technology-for-dummies summaries ever be informed or reasonable?

To address the crossroads that Ramirez believes we are approaching with nanotechnology (where it could potentially be the next thing to blame for our obesity), she suggests public engagement and dialog about its impact. Really? What a novel idea. Too bad it seems to have escaped Dr. Ramirez’s skimming that there has been so much public engagement for years now that research has been looking at whether it has any usefulness. It also doesn’t help matters that the very people that will accept Ramirez's line of thinking are the ones who have boycotted public engagement efforts.

Worse yet, the article was published in the perfect vehicle for wide dissemination: the Huffington Post, which is as mainstream as it gets. So a lot of people are apt to read the article and be misinformed. The Huffington Post is developing a rather poor reputation for its coverage of nanotechnology. And it's a pity because there are lots of brilliant commentators on the subject of nanotechnology’s potential impact—people who could provide well-reasoned and substantiated arguments on the topic.  Ramirez's article, unfortunately, does neither.

Photo: David Monniaux/Wikipedia

Bismuth-filled Carbon Nanotubes Improve CT Scans

Research has shown in recent years that carbon nanotubes can improve fluorescence imaging, replacing the dyes that are typically used. Now researchers at Rice University have demonstrated that carbon nanotubes combined with bismuth can improve the capabilities of computed tomography (CT) scans.

The Rice team has filled single-walled carbon nanotubes with bismuth compounds to make for a more effective contrast agent than is currently available for CT scans.

The research, which was published in the journal of Materials Chemistry B (“Bismuth@US-tubes as a potential contrast agent for X-ray imaging applications”), builds on previous work that examined the use of bismuth as a contrast agent in CT scans and on the Rice team’s own research into using carbon nanotubes as a contrast agent in magnetic resonance imaging (MRI).

According to Rice chemist Lon Wilson, this is the first time that bismuth has been combined with nanotubes for use as a contrast agent for imaging individual cells.

“At some point, we realized no one has ever tracked stem cells, or any other cells that we can find, by CT,” Wilson said in a press release. “CT is much faster, cheaper and more convenient, and the instrumentation is much more widespread (than MRI). So we thought if we put bismuth inside the nanotubes and the nanotubes inside stem cells, we might be able to track them in vivo in real time.”

The new contrast agent starts off as single-walled carbon nanotubes that have been chemically processed to make them into capsules with dimensions of between 20 and 80 nanometers long and about 1.4 nanometers in diameter. These nanocapsules are then are mixed with bismuth chloride to form what the researchers have dubbed Bi@US-tubes.

Once the Bi@US-tubes are introduced in vivo, they are small enough to diffuse into individual cells where they aggregate into a clump approximately 300 nanometers in diameter. “We think the surfactant used to suspend them in biological media is stripped off when they pass through the cell membrane,” Wilson said in the press release. “The nanotubes are lipophilic, so when they find each other in the cell they stick together.”

“The cells adjust over time to the incorporation of these chunks of carbon and then they go about their business,” Wilson adds in the release.

Bismuth is more effective at diffracting X-rays than the iodine in today's contrast agents, or just about any other material. When it is put in the nanocapsules, the resulting combination can produce high contrasts in very low concentrations.

The surfaces of the nanotubes can also be modified in ways to make them target different kinds of cells and make them more biocompatible.

The next step in the research will be to increase the amount of bismuth in the capsules so as to improve their contrasting capability.

Image: Eladio Rivera/Rice University

Gold Nanoparticles Make Molybdenum Disulfide Extra Special

While graphene has been on a nearly decade long surge of research, its two-dimensional (2D) rival molybdenum sulfide (MoS2) has enjoyed an equal rush of interest over a mere two-and-a-half years. Ever since researchers at Ecole Polytechnique Federale de Lausanne’s (EPFL) Laboratory showed it could be used in replacing silicon in transistors in early 2011, the competitive landscape for 2D materials has gotten a little crowded. Despite the growing competition between 2D materials, MoS2 still holds a special place among the competition because it can be used to further enable graphene or work on its own in the next generation of nanocircuits.

Now researchers Kansas State University have raised the prospects of MoS2 a little bit higher by combining it with gold nanoparticles. The researchers believe that the incorporation of gold nanoparticles with MoS2 will open greater possibilities for the material in diverse applications such as transistors and biochemical sensors.

The research, which was published in the journal NanoLetters ("Controlled, Defect-Guided, Metal-Nanoparticle Incorporation onto MoS2 via Chemical and Microwave Routes: Electrical, Thermal, and Structural Properties"), focused on the surface structure of MoS2. The team decided that MoS2's strong chemical bond with noble metals, like gold, could be an avenue for investigation.

They were not disappointed. They quickly discovered that once a bond had been established between the MoS2 and gold nanostructures, the bond behaved like a highly coupled gate capacitor. Following on this discovery, the Kansas State team was able to further enhance the transistor characteristics of MoS2 by manipulating it with the gold nanostructures.

"The spontaneous, highly capacitive, lattice-driven and thermally-controlled interfacing of noble metals on metal-dichalcogenide layers can be employed to regulate their carrier concentration, pseudo-mobility, transport-barriers and phonon-transport for future devices," Vikas Berry, a professor at Kansas State and a leader of the research, said in a press release (though it does stretch the bounds of credulity to imagine him actually speaking these words aloud without pausing numerous times for breath).

Among the transistor characteristics of MoS2 that the researchers were able to manipulate with the gold was its power requirements. The team also demonstrated a direct route for attaching electrodes to a MoS2 tunneling gate.

"The research will pave the way for atomically fusing layered heterostructures to leverage their capacitive interactions for next-generation electronics and photonics," Berry said. "For example, the gold nanoparticles can help launch 2-D plasmons on ultrathin materials, enabling their interference for plasmonic-logic devices."

In further research, the team intends to create more complex nanoscale structures with MoS2, leading to the building of logic devices and structures.

With MoS2 having the advantage of an inherent band gap—unlike graphene—and the recent flood of research that’s turning up new ways to work with, it may have a slight advantage over graphene at the moment for transistor applications.

Image: Vikas Berry

Dye-based Solar Cells Get Bump in Conversion Efficiency and Lifespan

As I pointed out this week, inexpensive photovoltaics are good, but inexpensive and efficient ones are much better. For years now, the dye-sensitized solar cell (DSSC) has been one of the least expensive photovoltaic devices on the market. But even one of the inventors of the technology conceded just a couple of years ago that there needed to be a big push to improve the energy conversion efficiency of the devices.

While new manufacturing techniques have recently been proposed that should further reduce the manufacturing costs associated with producing DSSCs, a bit of a bump up in conversion efficiency would perhaps be a more welcome development.

To meet this need, researchers at the KTH Royal Institute of Technology in Sweden have developed a method for making DSSCs that are not only are more efficient but longer lasting. The foundation of the improvement is a new, quasi-liquid, polymer-based electrolyte that increases the solar cells' voltage and current and lowers resistance between electrodes.

DSSCs are essentially a photochemical system in which a photo-sensitized anode and an electrolyte form a semiconductor. In their commercial incarnation, today's DSSCs consist of a porous layer of titanium dioxde (TiO2) nanoparticles that have been covered with a molecular dye that absorbs sunlight, and a platinum-based catalyst. The TiO2 , which is immersed in an electrolyte solution that acts as a conductor, is the device's anode; the platinum, which sits atop the electrolyte, is the cathode.

A more efficient DSSC would use a material like acetonitrile for the electrolyte. However, this material does not lend itself to the production of a stable solar cell that could be commercially marketed. Instead, a low-volatility solvent is typically used, but this comes at the price of being more viscous and impeding the flow of ions.

The novel quasi-liquid electrolyte that the KTH researchers have developed delivers the best of all worlds: overcoming the viscosity problem, improving the flow of electrons, and doing so at a much lower volatility than can be achieved with acetonitrile.

“We now have clear evidence that by adding [a special] ion-conducting polymer to the solar cell’s cobalt redox electrolyte, the transport of oxidized electrolytes is greatly enhanced,” said James Gardner, a professor of photoelectrochemistry at KTH, in a press release. “The fast transport increases solar cell efficiency by 20 percent.”

With conversion efficiencies for DSSCs already having already reached the 10 percent mark, this would boost efficiency to around 12 percent.

These are impressive numbers, but perhaps a more beneficial characteristic—at least for the economics of DSSCs—would be a longer lifespan. With this new electrolyte, DSSCs could have a longer lifespan, making it possible to amortize the cost of the initial installation over a greater period of time. It's not clear how much more life could be added to the DSSCs, but every bit counts.

Photo: David Callahan

 

Nanoparticle May Drive Down Price of Photovoltaics

There is still a school of thought in the world photovoltaics that says if you can make solar cells cheaply, it will result in widespread use of solar power. Despite that fervent belief, photovoltaics really benefit from achieving a balance between cheap manufacturing costs and high energy-conversion efficiency.

Unfortunately, it’s a struggle to strike that balance with current technology. If you want to use an inexpensive spray-on photovoltaic material made from nanoparticles, you might get to around 1 percent conversion efficiency—a figure so low that it’s not clear whether the amount of electricity generated is worth the effort. However, if you raise that figure to 10 percent, you could change the game.

While the energy conversion numbers are still not in yet, researchers at the University of Alberta in Canada believe they have created one of the cheapest nanoparticles yet developed for photovoltaics. It's so cheap to make because it's based on two of the most abundant elements: phosphorous and zinc.

In addition to their low price compared to elements like cadmium, phosphorus and zinc don't bog manufacturers down with the restrictions that come with lead-based nanoparticles.

The research, which was published in the journal ACS Nano (“Solution-Processed Zinc Phosphide (α-Zn3P2) Colloidal Semiconducting Nanocrystals for Thin Film Photovoltaic Applications”), took years to complete. But all the hard work may pay huge dividends since material seems to lend itself to a variety of manufacturing processes, including roll-to-roll printing or spray coating.

“Nanoparticle-based ‘inks’ could be used to literally paint or print solar cells or precise compositions,” said Jillian Buriak, a professor at the University of Alberta, in a press release. In fact, it is this spray coating method that Buriak and her colleagues are experimenting with to determine the energy conversion efficiency levels.

As we await those final numbers, the team has applied for a provisional patent and has already secured some funding to scale up the process for manufacturing. Whether the energy conversion efficiency of the nanoparticles can reach somewhere around the 10-percent mark may determine whether there’s a market to ramp up for.

Photo: University of Alberta

Researchers Publish Cookbook for Carbon Nanotubes

Back before graphene became the favored child of the nanomaterial family, carbon nanotubes held the mantle of the “wonder material” that would replace silicon. But a succession of problems with applying carbon nanotubes to electronics led to it losing favor.

Putting them where you wanted them and connecting them was exceedingly difficult. But a perhaps more stubborn obstacle has been the difficulty of controlling their purity and quality. While all sorts of ingenious methods have been developed over the years for working around these problems, just accepting that we could never produce a set of pure carbon nanotubes and proceeding from there didn’t seem to be a satisfactory solution.

Now researchers at the University of Southern California (USC) claim to have developed a method for producing carbon nanotubes with specific and predictable atomic structures.

“We are solving a fundamental problem of the carbon nanotube,” Chongwu Zhou, a USC professor and corresponding author of the paper, said in a press release. “To be able to control the atomic structure, or chirality, of nanotubes has basically been our dream.”

The researchers, who published their work in the journal Nano Letters (“Chirality-Dependent Vapor-Phase Epitaxial Growth and Termination of Single-Wall Carbon Nanotubes”), found that if they used chirality-pure, short carbon nanotubes as “seeds,” they could essentially clone duplicates using vapor-phase epitaxial growth.

The group actually developed this growth technique last year; the latest wrinkle reported in the paper is a set of recipes for building carbon nanotubes with specific atomic structures. And having a recipe means at least one thing: the process can be repeated if you follow the instructions.

“We identify the mechanisms required for mass amplification of nanotubes,” said co-lead author Jia Liu in a press release.

Bilu Liu, another of the authors, added: “Previously it was very difficult to control the chirality, or atomic structure, of nanotubes, particularly when using metal nanoparticles. The structures may look quite similar, but the properties are very different. In this paper we decode the atomic structure of nanotubes and show how to control precisely that atomic structure.”

Zhou says that the next step will be to scale up the process.  He adds in the release: “Our method can revolutionize the field and significantly push forward the real applications of nanotube in many fields.”

Whether this new development can bring carbon nanotube research back into favor for electronic applications, after years of focus and attention being lavished on graphene, remains to be seen. Working in carbon nanotubes' favor—that is, if this work can be scaled up—is the fact that researchers looking to endow electronics with graphene's amazing characteristics are not having such an easy time of it .

Image: Chongwu Zhou and Jia Liu

For First Time Graphene and Metal Make Super Strong Composite

One of the characteristics of graphene that is often mentioned but seldom exploited is its strength compared to other materials. Its tensile strength has been measured at 130 GigaPascals, making it 200 times as strong as steel.

Now researchers at the Korean Advanced Institute of Science and Technology (KAIST) have put graphene’s tensile strength to work by using it in a composite consisting of copper and nickel. The graphene makes the copper 500 times as strong as it would be on its own and the nickel 180 times as strong.

This work is a significant breakthrough, since previous attempts to use graphene in a metal composite have not resulted in increased strength in the doped material. In the KAIST research, which was published in the journal Nature Communications ("Strengthening effect of single-atomic-layer graphene in metal–graphene nanolayered composites"), chemical vapor deposition (CVD) was used to grow a single layer of graphene on a metallic deposited substrate, and then another metal layer was deposited. These steps were repeated, resulting in a multilayer metal-graphene composite material.

According to the researchers, this work represents the first time a metal-graphene multilayer composite material has been successfully produced that exploits graphene's extraordinary strength.

“The result is astounding as 0.00004% in weight of graphene increased the strength of the materials by hundreds of times,” said Professor Seung Min Han in a press release. “Improvements based on this success, especially enabling mass production with roll-to-roll process or metal sintering process, in the production of automobile and spacecraft lightweight, ultra-high strength parts may become possible.”

If the process can be duplicated on an industrial scale, it would indeed be a possible way to make automobiles and aircraft lighter and therefore more fuel efficient. We’ve already seen how a nanocoating on aircraft that reduces fuel consumption by just 2 percent resulted in a whopping US $22 million in savings per year for just one airline.

This graphene-metal composite, according to Han, can also be used as a coating in nuclear reactor construction or other structural applications.

Image: iStockphoto

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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.

 
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Dexter Johnson
Madrid, Spain
 
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Rachel Courtland
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