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Light Absorbing Thin Films Get Even Skinnier

The use of semiconductors in optoelectronic applications has had a bit of thickness problem. For photovoltaics, the need for relatively thick semiconductor thin films has added some significant cost to their manufacture. In other optoelectronic applications, the thick semiconductor materials are getting quite bulky even as the devices they are used in continue to shrink in size.

In two separate research projects, both these issues have been tackled. In one, researchers from North Carolina State University (NCSU) have developed a computer model that indicates that a new design will improve the light absorbing efficiency of semiconductors in thin film solar cells. In the other, a team at the University of Buffalo has developed a strategy that should enhance the light absorbing quality of semiconductors used for nanocavities, which have applications in camera sensors as well as photovoltaics.

The NCSU research, published in the journal Scientific Reports (“Semiconductor Solar Superabsorbers”), used light trapping techniques to determine the intrinsic absorption efficiency for any given semiconductor material. They discovered that in order to maximize the light-absorption efficiency for the material, the light-trapping efficiency had to be equal to the intrinsic absorption of the semiconductor materials.

“We first theoretically predicted the maximum solar light absorption efficiency in given semiconductor materials, and then proposed a design that could be readily fabricated to achieve the predicted maximum,” said Linyou Cao, an assistant professor at NCSU, in a press release. “We developed a new model to do this work, because we felt that existing models were not able to find the upper limit for the solar absorption of real semiconductor materials. And if this works the way we think it will, it would fundamentally solve light-absorption efficiency problems for thin film solar cells.”

When the new design was used in their models, the NCSU researchers report being able to reduce the thickness of the amorphous silicon from 100 nm, which is the current state-of-the-art, down to 10 nm.

Separately, in collaborative research between the University of Buffalo and two Chinese universities, researchers were also able to put thin film semiconductors on a diet by employing an interference effect in the nanocavities that overcomes the limitation between the light absorption and the film’s thickness. Two years ago, researchers at Harvard University were successful at combining thin films of a semiconductor with a gold surface to achieve the effects that the Buffalo researchers now report. However, the drawback in that work was that it needed to use gold, which is fairly expensive.

The Buffalo team's work, which was published in the journal Advanced Materials (“Nanocavity enhancement for ultra-thin film optical absorber”), demonstrated that inexpensive metals like aluminum could be used in nanocavities and still achieve this effect.

“We illustrated a nanocavity, made with aluminum or other whitish metals and alloys that are far less expensive, can be used to increase the amount of light that semiconducting materials absorb,” said Suhua Jiang, associate professor of materials science at Fudan University in China, in a press release.

Nanoporus Graphene Takes Another Step Toward Water Desalination Process

About 18 months ago, I wrote about an MIT project in which computer models demonstrated that graphene could act as a filter in the desalination of water through the reverse osmosis (RO) method. RO is slightly less energy intensive than the predominantly used multi-stage-flash process. The hope was that the nanopores of the graphene material would make the RO method even less energy intensive than current versions by making it easier to push the water through the filter membrane.

The models were promising, but other researchers in the field said at the time it was going to be a long road to translate a computer model to a real product.

“Manufacturing the very precise pore structures that are found in this paper will be difficult to do on a large scale with existing methods,” said Joshua Schrier, an assistant professor of chemistry at Haverford College. However, Schrier also believed that “the predictions are exciting enough that they should motivate chemical engineers to perform more detailed economic analyses of…water desalination with these types of materials.”

It would seem that the MIT researchers agreed it was worth the effort and accepted the challenge to go from computer model to a real device as they announced this week that they had developed a method for creating selective pores in graphene that make it suitable for water desalination.

The MIT group collaborated with a team from Oak Ridge National Laboratory and researchers from Saudi Arabia, a country that has been quite active in trying to use nanotechnology to finding cheaper water desalination processes. They published their results in the journal Nano Letters ("Selective Ionic Transport through Tunable Subnanometer Pores in Single-Layer Graphene Membranes").

The process the researchers developed for creating these nanopores in the graphene involves a two-step process. First the graphene is bombarded with gallium ions. This process breaks the carbon bonds in the hexagonal array of carbon atoms that makes up graphene. The next step involves etching the resulting material with an oxidizing solution that reacts strongly with the disrupted carbon bonds. This creates holes in the material in every spot where the gallium ions disrupted the carbon. The researchers were able to control the pore size by timing how long the graphene is kept in the oxidizing solution.

“We’ve developed the first membrane that consists of a high density of subnanometer-scale pores in an atomically thin, single sheet of graphene,” said Sean O’Hern, a graduate student who led the research along with MIT professor of mechanical engineering Rohit Karnik, in a press release.

The pores per square centimeter are well suited for filtration applications, with measurements indicating that there are 5 trillion pores per square centimeter. “To better understand how small and dense these graphene pores are, if our graphene membrane were to be magnified about a million times, the pores would be less than 1 millimeter in size, spaced about 4 millimeters apart, and span over 38 square miles, an area roughly half the size of Boston,” O’Hern explained in the release.

Of course, this is just an initial step in developing graphene as a filter medium for these purposes. To scale the graphene filter up to the size that would be needed for water filtration remains a challenge. However, O’Hern believes that initial applications in biofiltration of molecules, such as the removal of unreacted reagents from DNA, could be a more attainable application in the short term.

Nanopillars Could be Key to Efficient Thermoelectric Materials

Researchers at the University of Colorado Boulder have developed a model of a thermoelectric material that can insulate itself from heat transfers while still allowing electricity to flow. Such a combination is the key to a material's ability to efficiently converting heat to electricity. The trick is to place an array of nanopillars on top of the thermoelectric material, in this case a silicon-based thin film.

The basic principle of thermoelectric materials is that the difference in temperature between one side of the material and the other generates electricity. So, if you applied a voltage on one side of the material , one side would become hot while the other would cool. The problem has been that if the material used is good at insulating the heat exchanging between the two sides, it ends up not being very good for conducting electricity.

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Graphene Films Promise Secure Wireless Networks

What people who are increasingly demanding graphene commercialization avenues often miss is that a good portion of the research into the “wonder material” remains just figuring out what it can do.

In the continuing research to characterize graphene’s properties, researchers at Queen Mary University of London and the Cambridge Graphene Centre investigated its ability to absorb electromagnetic radiation from across the radio frequency spectrum. Before this research, these specific properties of graphene had never been examined.

The research, which was published in the journal Scientific Reports ("Experimental demonstration of a transparent graphene millimetre wave absorber with 28% fractional bandwidth at 140 GHz"), demonstrated that graphene is capable of absorbing 90 percent of electromagnetic energy across a high bandwidth. 

The UK researchers built their graphene-based broadband absorbers from several layers of graphene sheets on a quartz substrate that is backed by a grounding plate. The graphene films are first grown on silicon wafers through chemical vapor deposition. A repeated etch-and-transfer process further fabricates the graphene films to eliminate the polymer residue.

“The transparent material could be added as a coating to car windows or buildings to stop radio waves from travelling through the structure,” said Yang Hao, co-author of the study and Professor of Antennas and Electromagnetics at Queen Mary’s School of Electronic Engineering and Computer Science, in a press release. “This, in turn, could be used to improve secure wireless network environments, for example.”

Of course, this begs the question that if the graphene film blocks radio waves from coming into the car, how do the radio waves from the mobile phone connect to outside antennas?

Hao explained to me in an e-mail that the application scenario described in the press release would require that the mobile signals be communicated via an in-car radio system and bluetooth wireless links to the outside world.

When asked whether the radio waves that are blocked from the inside of the car would be able to affect the external antenna, Hao said that  for the in-car radio system, the antenna can be placed on the roof which is usually used for satellite navigation and mobile communications. He added: "I don't feel that for this case, the outside antenna performance would be comprised.”

Illustration: Randi Klett; Images: iStockphoto; Wikipedia

Graphene-based Microbattery Ushers in New Age for Biotelemetry

There's no denying that building the world’s smallest battery is a notable achievement. But while they may lay the groundwork for future battery technologies, today such microbatteries are mostly laboratory curiosities.

Developing a battery that's no bigger than a grain of rice—and that's actually useful in the real world—is quite another kind of achievement. Researchers at Pacific Northwest National Laboratory (PNNL) have done just that, creating a battery based on graphene that has successfully been used in monitoring the movements of salmon through rivers.

The microbattery is being heralded as a breakthrough in biotelemetry and should give researchers never before insights into the movements and the early stages of life of the fish.

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Nanotechnology Helps 3-D TV Make a Comeback Without Glasses

At this year’s Consumer Electronics Show (CES), it became clear that the much-ballyhooed age of 3-D TV was coming to a quiet and uncelebrated end. One of the suggested causes of its demise was the cost of the 3D glasses. If you wanted to invite a group over to watch the big sporting event, you had better have a lot of extra pairs on hand, which might cost you a small fortune.

Eliminating the glasses from the experience has been proposed from the first moment 3-D TVs were introduced to the marketplace. In 2010, Toshiba and Nintendo shared their plans to bring glasses-free 3-D to portable devices.

There have been a number of approaches proposed for accomplishing the feat. Now researchers at the University of Central Florida (UCF) are leveraging nanomanufacturing techniques to do the job.

Jayan Thomas, an assistant professor at UCF’s NanoScience Technology Center, has received a US $400 000 grant from the National Science Foundation to pursue the use of nanoprinting techniques for turning polymers into displays whose images appear in 3-D to the naked eye. The kind of 3-D displays Thomas envisions conjure images of the holograms used to display messages in the Star Wars movies.

“The TV screen should be like a table top,” Thomas said. “People would sit around and watch the TV from all angles, like sitting around a table. Therefore, the images should be like real-world objects. If you watch a football game on this 3-D TV, you would feel like it is happening right in front of you. A holographic 3-D TV is a feasible direction to accomplish this without the need of glasses.”

The nanomanufacturing techniques Thomas uses are similar to the printing process he developed for creating nanomaterials to be used in supercapacitors—a process that we covered last year. That technique involved printing polymer nanostructures on a substrate that served as a scaffold upon which electrode material made of manganese dioxide is deposited. That technique is a variation on the simple spin-on nanoprinting (SNAP) technique.

With these nanomanufacturing techniques, Thomas has developed a polymer composite that improves the process of making the 3-D images in the first place. When you are watching 3-D television, what you are really seeing is two perspectives of an image, so it is actually not very close to a real world object.  The 3-D glasses help to provide a 3-D appearance of the image. 

"Our technology uses multiple cameras positioned above and around an object to photograph it from multiple perspectives," explains Thomas. "We are then doing a couple of new things; we need to make the recording process so fast that the human eye will not see the images refreshing from the multiple perspectives. This requires new materials options—a new plastic type display on which to play what are ultimately holographic images."

Whether this technique proves to be any more successful than those offered by MIT and other research groups, remains to be seen. In any case, we may not yet have seen the end of 3D TV, as long as it doesn't require glasses.

Illustration: Randi Klett; Photos: iStockphotos

Nanomotors Could Churn Inside of Cancer Cells to Mush

Researchers at Penn State University have placed nanoparticles inside living human cells and been able to direct the movement of the particles through the use of both ultrasonic waves and magnetic forces. While similar demonstrations have been conducted in a test tube (in vitro), this marks the first time that this kind of work has succeeded inside a live human cell.

Once the nanoparticle motors start moving about inside them, the researchers observed that the cells begin to react in ways not previously observed.

"As these nanomotors move around and bump into structures inside the cells, the live cells show internal mechanical responses that no one has seen before," Tom Mallouk, a professor of materials chemistry and physics at Penn State, said in a press release.

(A video showing how the nanomotors move around inside the cells is below.)

While the over-exuberant press release likened the work to the 1960s sci-fi movie “Fantastic Vogage”, it does not involve anything remotely close to plot details of that movie. (It’s time to put aside this misleading comparison once and for all.)  However, this research does invite comparisons to a proposal out of Stanford University back in 2012 in which small antennas on a microchip could receive magnetic fields that propel the chip through the blood stream.

The new nanomotor research, which was published in the 10 February international edition of the journal Angewandte Chemie ("Acoustic Propulsion of Nanorod Motors Inside Living Cells"), used HeLa cells, a type of human cervical cancer cells. The HeLa cells ingest the nanoparticles. Once a high level of ultrasonic waves is focused on the nanoparticles, they begin to move about. If the level is too low, the nanoparticle will not react to the ultrasonic waves.

If a cancer cell were to ingest these nanoparticles, they could be moved around fast enough so that they acted as a sort of high-tech food processor, making a homogenized mix of the cell’s contents.

But Mallouk also sees a more refined role for them. “We might be able to use nanomotors to treat cancer and other diseases by mechanically manipulating cells from the inside,” said Mallouk in the release. “Nanomotors could perform intracellular surgery and deliver drugs noninvasively to living tissues."

It is through the combination of the ultrasonic waves and the magnetic forces that the researchers have been able to make the nanoparticles move autonomously from each other. The ultrasonic waves manage to move the nanoparticles forward or spin them around. But the magnetic forces are used to actually steer them.

"Autonomous motion might help nanomotors selectively destroy the cells that engulf them," Mallouk said. "If you want these motors to seek out and destroy cancer cells, for example, it's better to have them move independently. You don't want a whole mass of them going in one direction."

Image: Mallouk Lab/Penn State University

Graphene Sandwich Enables Clear Images of Biomolecules

Electron microscopy and spectroscopy are great tools for peering into matter on the molecular scale. But they’re not terribly effective if that matter happens to be biological.

Researchers at the University of Illinois in Chicago (UIC) may have changed that with a novel use of graphene in which a biomolecule is sandwiched between two sheets of graphene, making it possible to create much higher resolution images than in previous methods.

“We found a way to encapsulate a liquid sample in two very thin layers of graphene — sheets of carbon that are only one atom thick,” said Canhui Wang, UIC graduate student in physics and first author of the study, in a press release.

In the research, which was published in the journal Advanced Materials (“High-Resolution Electron Microscopy and Spectroscopy of Ferritin in Biocompatible Graphene Liquid Cells and Graphene Sandwiches”), the molecule ferritin was imaged. Ferritin is an iron-storage protein, and itself has been proposed for developing magnetic nanoparticles to enable forward osmosis for water purification.

Prior to this research, if you wanted to image a biomolecule like ferritin with an electron microscope, you would need to use a container known as a “liquid stage” that is wedged between two thick windows of silicon nitrate to protect the sample from the vacuum.

Robert Klie, the senior investigator on the study, likened the difference between the liquid stage approach and the graphene sandwich as “…the difference between looking through Saran Wrap and thick crystal.”

The better resolution produced by the graphene sandwich is not just because of graphene’s superior transparency. The graphene sandwich also provides better protection from the electron beam that is fired at a sample during microscopy. Wang says that some have calculated that to visualize a sample requires 10 times the amount of radiation one would be exposed to from a 10-megaton hydrogen bomb when standing just 30 meters away.

To mitigate the deleterious effect of the electron beam, most electron microscopes use a low-energy beam that results in a fuzzy picture that has to be corrected by imaging algorithms. Because of graphene’s high thermal and electrical conductivity the material removes both the heat and electrons generated by the beam as it passes through the sample. So higher energy electron beams can be used on the sample, resulting in higher resolution images.

In their experiments with the ferritin, the researchers were able to image for the first time iron oxide in the core of ferritin changing its electric charge, leading to the release of iron. The imaging of this process could lead to a better understanding of some human disorders.

“Defects in ferritin are associated with many diseases and disorders, but it has not been well understood how a dysfunctional ferritin works towards triggering life-threatening diseases in the brain and other parts of the human body,” said Tolou Shokuhfar, assistant professor of mechanical engineering-engineering mechanics at Michigan Technological University and adjunct professor of physics at UIC, in a press release.

Illustration: Celia Gorman; Image: iStockphoto

Graphene Nanoribbons Get Electrons to Behave Like Photons

An international team of researchers has developed a novel way to produce graphene nanoribbons that enables electrons to travel though it without resistance at room temperature, a property known as ballistic transport. The ballistic transport properties measured by the researchers exceeds previous theoretical limits for graphene by a factor of ten, opening up the potential in the opinion of the researchers for graphene to usher in a new era in electronics despite the material's lack of a band gap. 

The team of researchers, including those at Georgia Institute of Technology along with others from Leibniz Universität Hannover in Germany, the Centre National de la Recherche Scientifique (CNRS) in France and Oak Ridge National Laboratory in the United States, has developed a process for producing graphene nanoribbons on a silicon wafer that results in such high electron mobility that the electrons behave more like photons do when in an optical fiber.

The lack of inherent band gap in graphene—its inability to effectively stop and start the flow of electrons—has remained a major stumbling block in developing it for use in electronics. Of course, there have been various approaches to engineering a band gap into graphene, such as applying a strong electrical field to bilayer graphene. However, Walt de Heer, a Regent's professor in the School of Physics at the Georgia Institute of Technology, believes that we should stop trying to get graphene to behave like silicon, and instead design a new type of electronics that exploits graphene’s unique properties. This is along the lines of what Samsung proposed 18 months ago by developing new switches to exploit graphene’s electron mobility.

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Graphene Circuit Competes Head-to-Head With Silicon Technology

IBM has built on their previous graphene research and developed what is being reported as the best graphene-based integrated circuit (IC) built to date, with 10 000 times better performance than previously reported efforts.

This graphene-based IC serves as a radio frequency receiver that performs signal amplification, filtering and mixing. In tests, the IBM team was able to use the circuit to send text messages (in this case, “IBM”) without any distortion.

“This is the first time that someone has shown graphene devices and circuits to perform modern wireless communication functions comparable to silicon technology,” IBM Research director of physical sciences Supratik Guha said in a release.

The IC, which is fully described in the journal Nature Communications (“Graphene radio frequency receiver integrated circuit”), overcomes major problems previously encountered with graphene-based ICs that cause the transistor performance to degrade.

The key to overcoming this issue was a new manufacturing method. Simply put, the graphene is added late in the process to prevent it from being damaged during other manufacturing steps.

Despite the improved manufacturing method for the IC, the IBM researchers still depended on a costly method for producing the graphene that was used. They believe that if a high-quality graphene could be produced in a roll-to-roll process, the IC would become easier and cheaper to produce.

This latest circuit builds on the first integrated circuit built from graphene—developed by IBM in 2011—that was a broadband radio-frequency mixer, a fundamental component of radios that processes signals by finding the difference between two high-frequency wavelengths.

While others have judged the odds that graphene will yield benefits in electronic applications as slim to none because it lacks an inherent band gap, IBM has stayed on a steady course to test those assumptions. In early 2010, Big Blue researchers engineered a band gap into graphene large enough to pursue the use of graphene in infrared (IR) and terahertz (THz) detectors and emitters. Then a year later, IBM followed up with a graphene transistor capable of operating at 100 gigahertz that has the same gate length as silicon chips with speeds of 40 GHz.  Of course, a transistor on its own can’t do much of anything, so about six months later, IBM reported building the first integrated graphene circuit that was the precursor to this latest version.

In describing the impact of the research, Shu-Jen Han of IBM Research said in an IBM blog:

“Our demonstration has the potential to improve today’s wireless devices’ communication speed, and lead the way toward carbon-based electronics device and circuit applications beyond what is possible with today’s silicon chips. Integrating graphene radio frequency (RF) devices into today’s low-cost silicon technology could also be a way to enable pervasive wireless communications allowing such things as smart sensors and RFID tags to send data signals at significant distances.”

With IBM's apparent relentless pursuit of an IC for a radio frequency receiver, it would seem that seeing these devices in our telephones at some point in the future could be a realistic prospect.

Photo: IBM Research - Zurich

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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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