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Graphene Origami Boxes Exceed Hydrogen Storage Targets

Researchers at the University of Maryland have demonstrated through computer modeling that graphene can be triggered by an electric field to fold itself into a nifty three-dimensional box that can serve as a container for hydrogen storage and then unfold itself.

The way in which the graphene folds up into a box has been dubbed hydrogenation-assisted graphene origami (HAGO) and involves cutting the graphene into a pattern and then functionalizing it by atomically attaching hydrogen to the carbon atoms of the graphene. The electric field that is used does not trigger the graphene to perform its origami but is used to unfold the structure and then repeat the trick.

“First, a suitably functionalized and patterned graphene can spontaneously fold into a 3-D nanostructure.... No external electric field is needed,” explained Teng Li, an associate professor of mechanical engineering at University of Maryland in an e-mail to Nanoclast. “Second, an electric field can cause the polarization of the graphene, effectively reducing the graphene inter-layer adhesion, which causes the folded nanostructure to unfold. Upon turning off the electric field, the graphene folds up into a box spontaneously again. Such a process can be repeated many times.”

In the research, which was published in the journal ACS Nano (“Hydrogenation-Assisted Graphene Origami and Its Application in Programmable Molecular Mass Uptake, Storage, and Release”), the graphene origami boxes demonstrated remarkable hydrogen storage capabilities. The researchers calculate that graphene origami boxes have a hydrogen storage capacity of 9.7 percent by weight, far exceeding targets set by the U.S. Department of Energy (DOE) —5.5 percent by 2017 and 7.5 percent by 2020.

It would seem that nanomaterials are exceeding DOE targets for fuel cells on a pretty regular basis now. However, nanomaterials have a somewhat checkered past with hydrogen storage. At one time, carbon nanotubes were touted as the next big thing in that field, with claims of greater than 50-percent storage capacity. But it is now generally accepted that the figure is really closer to 1-percent. The problem was that the structures of both carbon nanotubes and fullerenes did not remain stable.

This instability has not proven to be a problem with the HAGO boxes. “Much effort has been dedicated in this research to demonstrate the promising feasibility of the HAGO process, including its robustness to possible manufacturing defects and stability at room temperature,” wrote Li. “We will actively pursue collaborations with experimentalists to actually demonstrate.”

Tungsten Diselenide Is New 2-D Optoelectronic Wonder Material

Researchers at the University of WashingtonMassachusetts Institute of Technology (MIT), and Vienna University of Technology (TU) in Austria  have all shown interesting optoelectronics results with a new two-dimensional (2-D) material called tungsten diselenide (WSe2).

The main page of the journal Nature Nanotechnology this week looks almost as though some mistake had been made with three of the top four stories highlighting “2D crystals.” But it was no mistake and foretells of a near future in which new 2-D materials are developed that will show either improved or complementary properties to that of graphene, the granddad of 2-D materials.

Tungsten diselenide belongs to a larger group of transition metal dichalcogenides that also includes molybdenum disulfide (MoS2). This family consists of materials that combine one of 15 transition metals with one of three members of the chalcogen family: sulfur, selenium, or tellurium. With only a few of these transition metals having been experimented upon, it’s likely we should see more coming down the pike.

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Light on Boron Nitride Creates Tunable Ripples

Researchers at the University of California San Diego (UCSD) have discovered that light can cause a ripple effect on the two-dimensional (2-D) material, hexagonal boron nitride, that can be maintained long enough for the waves to be usable for practical applications. This discovery could lead to the transmission of information in computer chips, better management of heat flow in nanoscale devices, or the creation higher resolution images than is possible with light, the researchers say.

The waves in this rippling effect that the researchers observed are called phonon polaritons. Polaritons are the quasiparticles produced when any type of photon strikes an object. Specifically, phonon polaritons occur when an infrared photon strikes a material.

The UCSD researchers discovered that phonon polaritons are far smaller than light waves and can be tuned to particular frequencies and amplitudes by varying the number of layers of the boron nitride. This is the feature that makes it conceivable to use them for producing images with a higher resolution than is possible with light, say the scientists.

The research, which was published in the journal Science (“Tunable Phonon Polaritons in Atomically Thin van der Waals Crystals of Boron Nitride”), involved focusing a laser on to the tip of an atomic force microscope as it scanned across the boron nitride. The AFM was taking measurements as infrared light from the laser struck the material.  An interference patterns were created as the phonon polariton waves traveled to the edge of the material and then reflected back.

"A wave on the surface of water is the closest analogy," said Dimitri Basov, professor of physics at the University of California, San Diego, who led the project, in a press release. "You throw a stone and you launch concentric waves that move outward. This is similar. Atoms are moving. The triggering event is illumination with light."

Basov added: "You can bounce these waves off edges. You can bounce them off defects. You can play all sorts of cool tricks with them. And of course, you can design the wavelength and amplitude of these oscillations in a way that suits your purpose."

The discovery was somewhat of a surprise. It stemmed from continuing research Basov and his colleagues have been conducting with 2-D materials, such as 2012 work in which they were experimenting with focusing infrared light on the surface of graphene to control the ripples of electrons that this caused across the surface of the material.

In this case, they were working with the insulator hexagonal boron nitride, which when combined with the conductor graphene, could make complex circuits.

In their similar work with graphene, the researchers were able to generate these phonon polaritons as well, but the waves would dissipate quickly. When using the boron nitride, the waves could be maintained for a long period of time.

"Because these materials are insulators, there is no electronic dissipation. So these waves travel further," Basov said. "We didn't expect them to be long-lived, but we are pleased that they are. It's becoming kind of practical."

Optical Tweezers Can Now Manipulate Matter on a Nanoscale

Optical tweezers—more formally known as “single-beam gradient force traps"—have been a key instrument in manipulating matter in biology and quantum optic applications since Bell Labs described an instrument in 1986. The problem for the tool’s use in nanoscale applications is that it couldn’t really manipulate particles smaller than a few hundred nanometers.

Now researchers at the ICFO-The Institute of Photonic Sciences near Barcelona, Spain have taken optical tweezers to a new level by employing plasmonics to make it possible for the instrument to manipulate objects in three dimensions .

Plasmonics takes advantage of the surface plasmons that are generated when photons hit a metal structure. The surface plasmons are essentially oscillations in the density of electron fields. Plasmonics has a number of applications ranging from the transmitting of data on computer chips to producing high-resolution lithography.

In this application, the plasmonics confine light to a very small dimension that accentuate the capabilities of nano tweezers, which involves focusing a laser light into a small spot. This focused laser light serves as an attractive force to small particles that become “trapped” in the beam of light. The plasmonic structure further confines the light from the laser.

In the IFCO work, which was published in the journal Nature Nanotechnology (“Three-dimensional manipulation with scanning near-field optical nanotweezers“), the researchers were able to attach a plasmonic device that took the shape of bowtie aperture to the extremity of an optical fiber.

The result of this addition has allowed the researchers to manipulate particles in three dimensions as small as a few tens of nanometers using a low, non-invasive laser intensity. The researchers were able to move a nanoparticle over tens of micrometres over a period of several minutes.

While this new tool will no doubt excite the imagination of molecular manufacturing adherents, who are looking to build macroscale products from nanoscale objects piece by piece, its initial applications are more likely to be in medical research to better understand the biological mechanisms that lead to disease.

Graphene Flakes Bring Higher Efficiencies to Polymer Solar Cells

The hunt by researchers for applications of graphene in photovoltaics has been, for the most part, limited to serving as a replacement to indium tin oxide (ITO), which is used as the transparent electrodes in organic solar cells. That started to change last year when researchers started to look at the potential of graphene in the conversion and conduction layers of solar cells.

Now researchers at the University of Cincinnati are experimenting with adding a small amount of graphene flakes to polymer-blend bulk-heterojunction (BHJ) solar cells and are finding that it improves the conversion efficiency of the cells significantly. The semiconducting part of a BHJ solar cells is made from two different materials—an electron donor and an acceptor. Light forms excitons at their interface, which separate into holes and the electrons producing a voltage. In the work performed by the Cincinnati researchers, they have discovered that they can increase the ratio of electron donors to electron acceptors to boost the energy absorbed by the cell.

“Because graphene is pure carbon, its charge conductivity is very high,” said Fei Yu, a University of Cincinnati doctoral student, in a press release. “We want to maximize the energy being absorbed by the solar cell, so we are increasing the ratio of the donor to acceptor and we’re using a very low fraction of graphene to achieve that.”

In research, which was presented 3 March at the American Physical Society Meeting in Denver (“Graphene-Based Polymer Bulk Heterojunction Solar Cells”), Yu reported a three-fold increase in energy conversion efficiency over typical polymer-blend BHJ solar cells.

“The increased performance, although well below the highest efficiency achieved in organic photovoltaic devices, is nevertheless significant in indicating that pristine graphene can be used as a charge transporter,” said Yu in the release.

The next step for the Cincinnati researchers will be to characterize the physics of the device, its film morphology and how to control and optimize the distribution of the graphene flakes to achieve better performance.

At present, no conversion efficiencies have been provided in the release or the abstract of the research. To give it some context, one of the highest reported conversion efficiencies for a polymer solar cell was as high as 10.6 percent for cells with more than one p-n junction. And those with a single junction have reached nearly 9 percent, with the expectation that they could exceed 10 percent in commercial products.

Improved conversion efficiency is just one of the issues that needs to be addressed in polymer solar cells. Reducing the cost of the materials that make up the modules (namely the ITO) is another for which graphene is being heavily researched. The next thing is to make the polymer solar cells survive in outdoor environments. With graphene’s nearly mythical strength,  maybe it could help out there too.

Nanocatalysts for Fuel Cells Exceed Targets Set for 2017

Researchers at the U.S. Department of Energy (DOE) Lawrence Berkeley National Laboratory and Argonne National Laboratory (ANL) have developed bimetallic nanocatalysts that are an order of magnitude higher in activity than the 2017 target set for fuel cells by the U.S. Department of Energy (DOE).

This development marks a very long effort by researchers around the world to find some way to apply nanomaterials to fuel cells

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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

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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
 
Contributor
Rachel Courtland
Associate Editor, IEEE Spectrum
New York, NY
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