MATERIALS ENGINEERING

For the first time, MIT researchers have shown they can genetically engineer viruses to build both the positively and negatively charged ends of a lithium-ion battery.

The new virus-produced batteries have the same energy capacity and power performance as state-of-the-art rechargeable batteries being considered to power plug-in hybrid cars, and they could also be used to power a range of personal electronic devices, said Angela Belcher, the MIT materials scientist who led the research team.

The new batteries, described in the April 2 online edition of Science, could be manufactured with a cheap and environmentally benign process: The synthesis takes place at and below room temperature and requires no harmful organic solvents, and the materials that go into the battery are non-toxic.

In a traditional lithium-ion battery, lithium ions flow between a negatively charged anode, usually graphite, and the positively charged cathode, usually cobalt oxide or lithium iron phosphate. Three years ago, an MIT team led by Belcher reported that it had engineered viruses that could build an anode by coating themselves with cobalt oxide and gold and self-assembling to form a nanowire.

In the latest work, the team focused on building a highly powerful cathode to pair up with the anode, said Belcher, the Germeshausen Professor of Materials Science and Engineering and Biological Engineering. Cathodes are more difficult to build than anodes because they must be highly conducting to be a fast electrode, however, most candidate materials for cathodes are highly insulating (non-conductive).

To achieve that, the researchers, including MIT Professor Gerbrand Ceder of materials science and Associate Professor Michael Strano of chemical engineering, genetically engineered viruses that first coat themselves with iron phosphate, then grab hold of carbon nanotubes to create a network of highly conductive material.

Because the viruses recognize and bind specifically to certain materials (carbon nanotubes in this case), each iron phosphate nanowire can be electrically "wired" to conducting carbon nanotube networks. Electrons can travel along the carbon nanotube networks, percolating throughout the electrodes to the iron phosphate and transferring energy in a very short time.

The viruses are a common bacteriophage, which infect bacteria but are harmless to humans.

The team found that incorporating carbon nanotubes increases the cathode's conductivity without adding too much weight to the battery. In lab tests, batteries with the new cathode material could be charged and discharged at least 100 times without losing any capacitance. That is fewer charge cycles than currently available lithium-ion batteries, but "we expect them to be able to go much longer," Belcher said.

The prototype is packaged as a typical coin cell battery, but the technology allows for the assembly of very lightweight, flexible and conformable batteries that can take the shape of their container.

Last week, MIT President Susan Hockfield took the prototype battery to a press briefing at the White House where she and U.S. President Barack Obama spoke about the need for federal funding to advance new clean-energy technologies.

Now that the researchers have demonstrated they can wire virus batteries at the nanoscale, they intend to pursue even better batteries using materials with higher voltage and capacitance, such as manganese phosphate and nickel phosphate, said Belcher. Once that next generation is ready, the technology could go into commercial production, she said.

Lead authors of the Science paper are Yun Jung Lee and Hyunjung Yi, graduate students in materials science and engineering. Other authors are Woo-Jae Kim, postdoctoral fellow in chemical engineering; Kisuk Kang, recent MIT PhD recipient in materials science and engineering; and Dong Soo Yun, research engineer in materials science and engineering.

The research was funded by the Army Research Office Institute of the Institute of Collaborative Technologies, and the National Science Foundation through the Materials Research Science and Engineering Centers program.

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Journal reference:

1. Yun Jung Lee, Hyunjung Yi, Woo-Jae Kim, Kisuk Kang, Dong Soo Yun, Michael S. Strano, Gerbrand Ceder, and Angela M. Belcher. Fabricating Genetically Engineered High-Power Lithium Ion Batteries Using Multiple Virus Genes. Science, 2009; DOI: 10.1126/science.1171541

A researcher has developed probes that can help pinpoint the location of tumors and might one day be able to directly attack cancer cells.

Joseph Irudayaraj, a Purdue University associate professor of agricultural and biological engineering, developed the nanoscale, multifunctional probes, which have antibodies on board, to search out and attach to cancer cells.

A paper detailing the technology was released last week in the online version of Angewandte Chemie, an international chemistry journal.

"If we have a tumor, these probes should have the ability to latch on to it," Irudayaraj said. "The probe could carry drugs to target, treat as well as reveal cancer cells."

Scientists have developed probes that use gold nanorods or magnetic particles, but Irudayaraj's nanoprobes use both, making them easier to track with different imaging devices as they move toward cancer cells.

The magnetic particles can be traced through the use of an MRI machine, while the gold nanorods are luminescent and can be traced through microscopy, a more sensitive and precise process. Irudayaraj said an MRI is less precise than optical luminescence in tracking the probes, but has the advantage of being able to track them deeper in tissue, expanding the probes' possible applications.

The probes, which are about 1,000 times smaller than the diameter of a human hair, contain the antibody Herceptin, used in treatment of metastatic breast cancer. The probes would be injected into the body through a saline buffering fluid, and the Herceptin would find and attach to protein markers on the surface of cancer cells.

"When the cancer cell expresses a protein marker that is complementary to Herceptin, then it binds to that marker," Irudayaraj said. "We are advancing the technology to add other drugs that can be delivered by the probes."

Irudayaraj said better tracking of the nanoprobes could allow doctors to pinpoint the location of known tumors and better treat the cancer.

The novel probes were tested in cultured cancer cells. Irudayaraj said the next step would be to run a series of tests in mice models to determine the dose and stability of the probes.

The research was funded through a National Institute of Health grant, as well as by the Purdue Research Foundation. Irudayaraj is head of a biological engineering team that includes postdoctoral researcher Chungang Wang and graduate student Jiji Chen.

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As electronic circuits shrink from finely etched lines in silicon wafers to nearly elusive proportions, researchers at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) and Columbia University are studying how electrons flow through a molecular junction-a nanometer scale circuit element that contacts gold atoms with a single molecule.

Their findings reveal the electrical resistance through this junction can be turned ‘on’ and ‘off’ simply by pushing and pulling the junction-a feature that could be used as a switch in nanoscale electronic devices.

“To design circuit elements at the molecular scale, we need to understand how the intrinsic properties of a molecule or junction are actually connected to its measured resistance,” said Jeff Neaton, Facility Director of the Theory of Nanostructured Materials Facility in the Molecular Foundry, a U.S. Department of Energy User Facility located at Berkeley Lab that provides support to nanoscience researchers around the world. “Knowing where each and every atom is in a single-molecule junction is simply beyond what’s possible with experiments at this stage. For these sub-nanometer scale junctions-just a handful of atoms-theory can be valuable in helping interpret and understand resistance measurements.”

In traditional electronic devices, charge-carrying electrons diffuse through circuits in a well-understood fashion, gaining or losing energy through transactions with impurities or other particles they encounter. Electrons at the nanoscale, however, can travel by a mechanism called quantum tunneling in which, due to the small length scales involved, it becomes possible for a particle to disappear through an energy barrier and suddenly appear on the other side, without expending energy.

Tracking such ‘tunneling’ of electrons through individual molecules in nanoscale devices has proven difficult. “For more than a decade, researchers have been ‘wiring up’ individual molecules and measuring their electrical conductance,” said Neaton. “Forming reliable contacts between nanostructures and ‘alligator clip’ electrical leads is extremely challenging. This made experiments difficult to interpret, and as a result, reported conductance values-in experiment and theory-often varied by an order of magnitude or more. The time was ripe for a quantitative comparison between theory and an experiment with well-defined contacts.”

Through the Molecular Foundry user program, Su Ying Quek, a postdoctoral researcher, worked with Neaton and Latha Venkataraman, an experimental researcher at Columbia University, using a scanning tunneling microscope (STM), which probes changes in current across a material’s surface with a conductive gold tip. Previous work had shown a gold STM tip could be repeatedly be plunged into a gold surface containing a solution of molecules and retracted, until the contact area between the tip and gold surface reduces to a single strand, like a necklace.

When this strand finally breaks, nearby molecules can hop into the gap between strands and contact the gold electrodes, resulting in a sudden change in conductance. Using this technique, Venkataraman and colleagues, including Mark Hybertsen at Brookhaven National Lab, had recently discovered that the conductance of molecules containing amines (a group of molecules related to ammonia) in contact with gold electrodes could be reliably measured.

“We now had a reproducible and consistent data set to benchmark our theory,” said Quek. “Comparing with this data set, we discovered important electron correlation effects previously missing. When we added these, we found-for the first time-quantitative agreement with experimental results.”

Using their new theoretical approach, Quek and Neaton, together with Hybertsen and collaborators Steven G. Louie of University of California Berkeley and Hyoung Joon Choi of Yonsei University in Korea, began to study the conductance of a junction between gold electrodes and bipyridine-a benzene-like ring molecule containing nitrogen. The experimental data showed two stable conductance states, unlike anything seen previously. Working closely with Venkataraman and collaborators, Quek hypothesized the peaks corresponded to two states with different structures within the junction. During the next year, Quek and Neaton meticulously constructed a theory that could describe the conductance of junctions arranged vertically between two gold molecules and sandwiched at angles.

The story that emerged was surprisingly detailed: if bipyridine bonded at an angle, more current could flow compared with when the bipyridine bonded vertically. This suggests the conductance of bipyridine was linked to the molecule’s orientation in the junction, explained Quek. In the STM experiment, as you pull, just after the final strand of gold atoms breaks and snaps back, the vertical gap is not big enough for bipyridine, so it bonds at an angle. As the gap increases, the molecule jumps to a vertical configuration, causing the conductance to plummet abruptly. Eventually, the molecule straightens even more, and the contact breaks. “Once we determined this, we wondered, ‘could you reverse this behavior?’” said Quek.

Teaming with Venkataraman and collaborators, Quek and Neaton demonstrated why pushing the junction to an angle and pulling it straight could repeatedly alter the conductance, creating a mechanical switch with well defined ‘on’ and ‘off’ states. “One of the fascinating things about this experiment is the degree to which it is possible to control the ‘alligator clips’,” said Neaton. “For this particular molecule, bipyridine, experiments can reproducibly and reliably alter these atomic-scale features back and forth to switch the conductance of the junction.”

Quek and Neaton hope to refine and apply their theoretical framework to more complex molecular junctions for study of systems promising for solar energy conversion, such as organic photovoltaics.

“Understanding how electrons move through single-molecule junctions is the first step,” said Neaton. “Organic-inorganic interfaces are everywhere in nanoscience, and developing a better picture of charge transport in hybrid materials systems will certainly lead to the discovery of new and improved electronic devices.”

Portions of this work were supported by the U.S. Department of Energy (DOE) Office of Science, through its Office of Basic Energy Sciences, and by the National Science Foundation through its Nanoscale Science and Engineering Initiative.

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Journal reference:

1. Quek et al. Mechanically controlled binary conductance switching of a single-molecule junction. Nature Nanotechnology, 2009; DOI: 10.1038/nnano.2009.10

Overcoming a critical conductivity challenge to clean energy technologies, Boston College researchers have developed a titanium nanostructure that provides an expanded surface area and demonstrates significantly greater efficiency in the transport of electrons.

The challenge has vexed researchers pursuing solar panels thick enough to absorb sunlight, yet thin enough to collect and transport electrons with minimal energy loss. Similarly, the relatively new science of water splitting requires capturing energy within semiconductor materials and then efficiently transporting charges ultimately used to generate hydrogen.

Boston College Asst. Prof of Chemistry Dunwei Wang and members of his lab found that incorporating two titanium-based semiconductors into a nano-scale structure improved the efficiency of power-collecting efforts by approximately 33 percent, the team reported in the online edition of the Journal of the American Chemical Society.

The team achieved a peak conversion efficiency of 16.7 percent under ultraviolet light, reported Wang and his co-authors, BC graduate students Yongjing Lin and Sa Zhou, post doctoral researcher Xiaohua Liu and undergraduate Stafford Sheehan. That compared to an efficiency of 12 percent from a structure composed only of titanium dioxide (TiO2).

Wang said the efficiency gains within the novel material can serve so-called water-splitting, where semiconductor catalysts have been shown to separate and store hydrogen and oxygen gases.

"The current challenge in splitting water involves how best to capture photons within the semiconductor material and then grab and transport them to produce hydrogen," Wang says. "For practical water splitting, you want to generate oxygen and hydrogen separately. For this, good electrical conductivity is of great importance because it allows you to collect electrons in the oxygen-generation region and transport them to the hydrogen-generation chamber for hydrogen production."

By using two crystalline semiconductors – materials critical to the processes of energy capture and transport – Wang says the researchers discovered a new and successful transfer mechanism in an engineered structure nearly invisible to the human eye.

Titanium dioxide has played a key role in early water-splitting research because of its prowess as a catalyst. However, its light absorption is confined to ultraviolet rays only and the material is also a relatively poor conductor.

Wang and his researchers started by growing a nanostructure made of titanium disilicide (TiSi2), a semiconductor capable of absorbing solar light and a material able to provide a sturdy structure with expanded surface area critical to absorbing photons. Still in need of its catalytic capabilities, titanium dioxide was used to coat the structure, Wang said.

The resulting net-like nanostructure effectively separated charges, collecting the electrons in the titanium disilicide core and transporting them away. The structure transferred positive charges to the titanium dioxide region of the material for chemical reactions. In water-splitting, these charges could potentially be used to generate hydrogen.

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Journal reference:

1. Lin et al. TiO2/TiSi2 Heterostructures for High-Efficiency Photoelectrochemical H2O Splitting. Journal of the American Chemical Society, 2009; 131 (8): 2772 DOI: 10.1021/ja808426h

Just as sonar sends out sound waves to explore the hidden depths of the ocean, electrons can be used by scanning tunnelling microscopes to investigate the well-hidden properties of the atomic lattice of metals.

As researchers from Göttingen, Halle and Jülich now report in the journal Science, they succeeded in making bulk Fermi surfaces visible in this manner. Fermi surfaces determine the most important properties of metals.

"Fermi surfaces give metals their personality, so to speak," explained Prof. Stefan Blügel, Director at the Jülich Institute of Solid State Research. Important properties, such as conductivity, heat capacity and magnetism, are determined by them. On the Fermi surfaces inside the atomic union, high-energy electrons are in motion. Depending on what form the surfaces have and what mobility is assigned to the electrons, they determine the physical properties of metals.

In their latest publication, the researchers report on how they used a scanning tunnelling microscope to direct electrons into a copper sample. As electrons spread out like waves, they pass through the metal and are scattered and reflected at obstacles in the bulk, such as single cobalt atoms. "The overlap between incoming and outgoing waves is so strong," said Dr. Samir Lounis from Forschungszentrum Jülich who turned the theoretical calculations into an experiment, "that they can be measured as spherical patterns on the surface using the scanning tunnelling microscope."

The somewhat deformed rings on the surface allow us to draw direct conclusions on the shape of the Fermi surfaces and the depth of the cobalt atoms, similar to how sonar recognises the ocean floor by means of reflected sound waves. "We hope that more sophisticated methods will make it possible to gain a detailed understanding of deep impurities and interfaces between atomic lattices," explained Lounis. For his simulations of the scanning tunnelling experiment, he used the supercomputer known as JUMP in the Jülich Supercomputing Centre.

In a related article in the "Perspectives" section of "Science", the innovative approach is praised. A scanning tunnelling microscope is primarily used to characterise the surface of a sample. Thanks to the theoretical work in Jülich, it can now be used to gain a direct insight into the bulk of solids and to understand interesting effects in the nanoworld.

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Journal reference:

1. Weismann et al. Seeing the Fermi Surface in Real Space by Nanoscale Electron Focusing. Science, 2009; 323 (5918): 1190 DOI: 10.1126/science.1168738

Small is promising when it comes to illuminating tiny tumors or precisely delivering drugs, but many worry about the safety of nano-scale materials. Now a team of scientists has created minuscule flakes of silicon that glow brightly, last long enough to slowly release cancer drugs, then break down into harmless by-products.

"It is the first luminescent nanoparticle that was purposely designed to minimize toxic side effects," said Michael Sailor, a chemistry professor at the University of California, San Diego who led the study.

Many nanoparticles tested in research labs are too poisonous for use in humans.

"This new design meets a growing need for non-toxic alternatives that have a chance to make it into the clinic to treat human patients," Sailor said.

The particles inherently glow, a useful property that is most commonly achieved by including toxic organic chemicals or tiny structures called quantum dots, which can leave potentially harmful heavy metals in their wake.

When the researchers tested their safer nanoparticles in mice, they saw tumors glow for several hours, then dim as the particles broke down. Levels dropped noticeably in a week and were undetectable after four weeks, they report in Nature Materials February 22.

This is the first sudy to image tumors and organs using biodegradable silicon nanoparticles in live animals, the authors say.

The particles begin as thin wafers made porous with an electrical current then smashed to bits with ultrasound. Additional treatment alters the physical structure of the flakes to make them glow red when illuminated with ultraviolet light.

Luminescent particles can reveal tumors too tiny to detect by other means or allow a surgeon to be sure all of a cancerous growth has been removed.

These nanoparticles could also help deliver drugs safely, the researchers report. The cancer drug doxorubicin will stick to the pores and slowly escape as the silicon dissolves.

"The goal is to use the nanoparticles to chaperone the drug directly to the tumor, to release it into the tumor rather than other parts of the body," Sailor said.

Targeted delivery would allow doctors to use smaller doses of the drug. At doses high enough to be effective, when delivered to the whole body, doxorubicin often has toxic side effects.

At about 100 nanometers, these particles are bigger than many designed to deliver drugs, which can be just a few nanometers across – a thousand times smaller than the diameter of a human hair.

Their larger size contributes to both their effectiveness and their safety. Large particles can hold more of a drug. Yet they self-destruct, and the remnants can be filtered away by the kidneys.

Close examination of vulnerable organs like liver, spleen and kidney, which help to remove toxins, revealed no lasting changes in mice treated with the new nanoparticles.

Graduate students Ji-Ho Park and Luo Gu in Sailor's lab; Sangeeta Bhatia, bioengineering professor at the Massachusetts Institute of Technology and graduate student Geoffrey von Malzahn in Bhatia's lab; and Erkki Ruoslahti, professor at the University of California, Santa Barbara all contributed to this work.

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Journal reference:

1. Park et al. Biodegradable luminescent porous silicon nanoparticles for in vivo applications. Nature Materials, 2009; DOI: 10.1038/nmat2398

MARCH 1,2009
An innovative and easily implemented technique in which nanoscale elements precisely assemble themselves over large surfaces could soon open doors to dramatic improvements in the data storage capacity of electronic media, according to scientists at the University of California, Berkeley, and the University of Massachusetts Amherst (UMass Amherst).

"I expect that the new method we developed will transform the microelectronic and storage industries, and open up vistas for entirely new applications," said co-lead investigator Thomas Russell, director of the Materials Research Science and Engineering Center at UMass Amherst, visiting Miller Professor at UC Berkeley's Department of Chemistry, and one of the world's leading experts on the behavior of polymers. "This work could possibly be translated into the production of more energy-efficient photovoltaic cells, for instance."

Russell conceived of this new approach with co-lead investigator Ting Xu, a UC Berkeley assistant professor with joint appointments in the Department of Material Sciences and Engineering and the Department of Chemistry. They describe their work in the Feb. 20 issue of the journal Science.

"The density achievable with the technology we've developed could potentially enable the contents of 250 DVDs to fit onto a surface the size of a quarter," said Xu, who is also a faculty scientist at Lawrence Berkeley National Laboratory.

Xu explained that the molecules in the thin film of block copolymers - two or more chemically dissimilar polymer chains linked together - will self-assemble into an extremely precise, equidistant pattern when spread out on a surface, much like a regiment of disciplined soldiers lining up in formation. For more than a decade, researchers have been trying to exploit this characteristic for use in semiconductor manufacturing, but they have been constrained because the order starts to break down as the size of the area increases.

Once the formation breaks down, the individual domains cannot be read or written to, rendering them useless as a form of data storage.

To overcome this size constraint, Russell and Xu conceived of the elegantly simple solution of layering the film of block copolymers onto the surface of a commercially available sapphire crystal. When the crystal is cut at an angle - a common procedure known as a miscut - and heated to 1,300 to 1,500 degrees Centigrade (2,372 to 2,732 degrees Fahrenheit) for 24 hours, its surface reorganizes into a highly ordered pattern of sawtooth ridges that can then be used to guide the self-assembly of the block polymers.

With this technique, the researchers were able to achieve defect-free arrays of nanoscopic elements with feature sizes as small as 3 nanometers, translating into densities of 10 terabits per square inch. One terabit is equal to 1 trillion bits, or 125 gigabytes.

Because crystals come in a variety of sizes, there are few limitations to how large this block copolymer array can be produced, the researchers said. They also noted that the angle and depth of the sawtooth ridges can be easily varied by changing the temperature at which the crystal is heated to fine tune the desired pattern.

"We can generate nearly perfect arrays over macroscopic surfaces where the density is over 15 times higher than anything achieved before," said Russell. "With that order of density, one could get a high-definition picture on a screen the size of a JumboTron."

"It's one thing to get dozens of soldiers to stand in perfect formation in an area the size of a classroom, each person equidistant from the other, but quite another to get tens of trillions of individuals to do so on the field in a football stadium," Xu added. "Using this crystal surface as a guide is like giving the soldiers a marker so they know where to stand."

Other research teams across the country are engaged in similar efforts to break the size barrier of self-assembled block copolymers, but this new project by the UMass Amherst-UC Berkeley scientists differs in that it does not rely upon advances in lithography to achieve its goals.

In the semiconductor industry, optical lithography is a process in which light passes through a mask with a desired circuit pattern onto a photosensitive material, or photoresist, that undergoes a chemical change. Several steps of chemical treatment are then used to develop the desired pattern for subsequent use.

To keep up with Moore's Law and the demand for increasingly smaller features for semiconductors and microprocessors, industry has turned to nanolithography and the use of ever-shorter wavelengths of light at greater cost.

"The challenge with photolithography is that it is rapidly approaching the resolution limits of light," said Xu. "In our approach, we shifted away from this 'top down' method of producing smaller features and instead utilized advantages of a 'bottom up' approach. The beauty of the method we developed is that it takes from processes already in use in industry, so it will be very easy to incorporate into the production line with little cost."

An added benefit, said Xu, is that "our technique is more environmentally friendly than photolithography, which requires the use of harsh chemicals and acids."

UC Berkeley and UMass Amherst have filed a joint patent on this technology.

The U.S. Department of Energy and the National Science Foundation helped support this research.

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Journal reference:

1. Park et al. Macroscopic 10-Terabit-per-Square-Inch Arrays from Block Copolymers with Lateral Order. Science, 2009; 323 (5917): 1030 DOI: 10.1126/science.1168108

Just as sonar sends out sound waves to explore the hidden depths of the ocean, electrons can be used by scanning tunnelling microscopes to investigate the well-hidden properties of the atomic lattice of metals.

As researchers from Göttingen, Halle and Jülich now report in the journal Science, they succeeded in making bulk Fermi surfaces visible in this manner. Fermi surfaces determine the most important properties of metals.

"Fermi surfaces give metals their personality, so to speak," explained Prof. Stefan Blügel, Director at the Jülich Institute of Solid State Research. Important properties, such as conductivity, heat capacity and magnetism, are determined by them. On the Fermi surfaces inside the atomic union, high-energy electrons are in motion. Depending on what form the surfaces have and what mobility is assigned to the electrons, they determine the physical properties of metals.

In their latest publication, the researchers report on how they used a scanning tunnelling microscope to direct electrons into a copper sample. As electrons spread out like waves, they pass through the metal and are scattered and reflected at obstacles in the bulk, such as single cobalt atoms. "The overlap between incoming and outgoing waves is so strong," said Dr. Samir Lounis from Forschungszentrum Jülich who turned the theoretical calculations into an experiment, "that they can be measured as spherical patterns on the surface using the scanning tunnelling microscope."

The somewhat deformed rings on the surface allow us to draw direct conclusions on the shape of the Fermi surfaces and the depth of the cobalt atoms, similar to how sonar recognises the ocean floor by means of reflected sound waves. "We hope that more sophisticated methods will make it possible to gain a detailed understanding of deep impurities and interfaces between atomic lattices," explained Lounis. For his simulations of the scanning tunnelling experiment, he used the supercomputer known as JUMP in the Jülich Supercomputing Centre.

In a related article in the "Perspectives" section of "Science", the innovative approach is praised. A scanning tunnelling microscope is primarily used to characterise the surface of a sample. Thanks to the theoretical work in Jülich, it can now be used to gain a direct insight into the bulk of solids and to understand interesting effects in the nanoworld.

---

Journal reference:

1. Weismann et al. Seeing the Fermi Surface in Real Space by Nanoscale Electron Focusing. Science, 2009; 323 (5918): 1190 DOI: 10.1126/science.1168738

Chemistry researchers at the University of Warwick have found that tiny nanoparticles could be twice as likely to stick to the interface of two non mixing liquids than previously believed. This opens up a range of new possibilities for the uses of nanoparticles in living cells, polymer composites, and high-tech foams, gels, and paints. The researchers are also working on ways of further artificially enhancing this new found sticking power.

University of Warwick researchers reviewed molecular simulations of the interaction between a non-charged nanoparticle and an "ideal" liquid-liquid interface. They were surprised to find that very small nanoparticles (of around 1 to 2 nanometres) varied considerably in their simulated ability to stick to such interfaces from what was expected in the standard model.

The researchers found that it took up to 50 percent more energy to dislodge the particles from the liquid-liquid interface for the smallest particle sizes. However as the radius of the particles increased this deviation from the standard model gradually faded out.

The researchers, Dr ir Stefan A. F. Bon and Dr David L. Cheung, believe that previous models failed to take into account the action of "capillary waves" in their depiction of the nanoparticles behaviour at the liquid to liquid interfaces.

Dr ir Stefan A. F. Bon said, " This new understanding on the nano-scale gives us much more flexibility in the design of everything from high-tech composite materials, to the use of quantum dots, cell biochemistry, and the manufacture of new "armored" polymer paint particles."

The researchers are now working on ways to build on this newly found natural stickiness of nanoparticles by designing polymer nanoparticles with opposing hydrophobic and hydrophilic surfaces that will bind even more strongly at oil/water liquid interfaces.

The research was funded by the Engineering and Physical Sciences Research Council (EPSRC)

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Journal reference:

1. Cheung et al. Interaction of Nanoparticles with Ideal Liquid-Liquid Interfaces. Physical Review Letters, 2009; 102 (6): 066103 DOI: 10.1103/PhysRevLett.102.066103

To understand friction on a very small scale, a team of University of Wisconsin-Madison engineers had to think big.

Friction is a force that affects any application where moving parts come into contact; the more surface contact there is, the stronger the force. At the nanoscale — mere billionths of a meter — friction can wreak havoc on tiny devices made from only a small number of atoms or molecules. With their high surface-to-volume ratio, nanomaterials are especially susceptible to the forces of friction.

But researchers have trouble describing friction at such small scales because existing theories are not consistent with how nanomaterials actually behave. Through computer simulations, the group demonstrated that friction at the atomic level behaves similarly to friction generated between large objects. Five hundred years after Leonardo da Vinci discovered the basic friction laws for large objects, the UW-Madison team has shown that similar laws apply at the nanoscale.

The team, which was led by Izabela Szlufarska, an assistant professor of materials science and engineering, and included materials science and engineering graduate student Yifei Mo and mechanical engineering assistant professor Kevin Turner, published its findings in the Feb. 26 issue of the journal Nature.

Current nanoscale friction theories are based on the idea that nanoscale surfaces are smooth, but, in reality, the surfaces resemble a mountain range, where each peak corresponds to an atom or a molecule.

The UW-Madison team performed computer simulations that looked at nanoscale materials as a collection of atoms, monitoring their positions and interactions throughout the entire sliding process. "For the first time, we modeled friction at length scales very similar to experiments, while maintaining atomic resolution and realistic interactions between atoms," say Szlufarska.

The team discovered simple laws of nanoscale friction. They found that friction is proportional to the number of atoms that interact between two nanoscale surfaces. The researchers' simulations showed that, at the nanoscale, materials in contact behave more like large rough objects rubbing against each other, rather than as two perfectly smooth surfaces, as was previously imagined. "When you look at it closely, the surface is made of atoms, so the contact is actually rough," says Szlufarska.

The team's simulation data correlates very well with recorded experimental data — something that previous models have failed to accomplish. Szlufarska hopes to use the simulations as a tool to understand what mechanisms contribute to friction on both the nano- and macroscale.

"Nobody is able to predict friction or design materials with desired friction properties — we measure a lot of friction coefficients for different materials, but it's not really clear how to relate them to the properties of the material," she explains. "The origin of friction is really an open and growing research field."

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Need to store electricity more efficiently? Put it behind bars.

That's essentially the finding of a team of Rice University researchers who have created hybrid carbon nanotube metal oxide arrays as electrode material that may improve the performance of lithium-ion batteries.

With battery technology high on the list of priorities in a world demanding electric cars and gadgets that last longer between charges, such innovations are key to the future. Electrochemical capacitors and fuel cells would also benefit, the researchers said.

The team from Pulickel Ajayan's research group published a paper this week describing the proof-of-concept research in which nanotubes are grown to look – and act – like the coaxial conducting lines used in cables. The coax tubes consist of a manganese oxide shell and a highly conductive nanotube core.

"It's a nice bit of nanoscale engineering," said Ajayan, Rice's Benjamin M. and Mary Greenwood Anderson Professor in Mechanical Engineering and Materials Science.

"We've put in two materials – the nanotube, which is highly electrically conducting and can also absorb lithium, and the manganese oxide, which has very high capacity but poor electrical conductivity," said Arava Leela Mohana Reddy, a Rice postdoc researcher. "But when you combine them, you get something interesting."

That would be the ability to hold a lot of juice and transmit it efficiently. The researchers expect the number of charge/discharge cycles such batteries can handle will be greatly enhanced, even with a larger capacity.

"Although the combination of these materials has been studied as a composite electrode by several research groups, it's the coaxial cable design of these materials that offers improved performance as electrodes for lithium batteries," said Ajayan.

"At this point, we're trying to engineer and modify the structures to get the best performance," said Manikoth Shaijumon, also a Rice postdoc. The microscopic nanotubes, only a few nanometers across, can be bundled into any number of configurations. Future batteries may be thin and flexible. "And the whole idea can be transferred to a large scale as well. It is very manufacturable," Shaijumon said.

The hybrid nanocables grown in a Rice-developed process could also eliminate the need for binders, materials used in current batteries that hold the elements together but hinder their conductivity.

The paper was written by Reddy, Shaijumon, doctoral student Sanketh Gowda and Ajayan. It appears in the online version of the American Chemical Society's Nano Letters.

The project is supported by funding from the Hartley Family Foundation.

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Journal reference:

1. Reddy et al. Coaxial MnO2/ Carbon Nanotube Array Electrodes for High-Performance Lithium Batteries. Nano Letters, 2009; 090202085144070 DOI: 10.1021/nl803081j

Imagine unloading a pile of bricks onto the ground and watching the bricks assemble themselves into a level, straight wall in only a few minutes. While merely a fantasy for builders in the everyday world, these types of self-assembled structures are a reality for those who build materials in the nanoworld.

Michael C. Tringides, a senior physicist at the U.S. Department of Energy's Ames Laboratory, has shown that nanoscale "straight wall" lead islands on silicon are spontaneously and quickly created by unusually mobile atoms.

Several years ago, Tringides' research group was the first to observe that lead atoms deposited on a silicon surface at low temperatures self-organize into uniform-height island nanostructures. The laws of quantum mechanics – specifically, Quantum Size Effects – determine why lead atoms stack up to create uniform islands while other nanostructure systems organize into islands that vary in height.

How the lead-on-silicon islands organized into uniform-height islands remained a mystery until Tringides' team made the surprising discovery that when lead atoms move along the surface of a silicon substrate, the lead atoms exhibit a liquid-like motion instead of the typical random-type diffusion observed in other systems. The liquid-like motion of atoms was observed using scanning tunneling microscopy at Ames Lab and low energy electron microscopy performed by collaborators in Hong Kong.

"One big surprise was that the atoms were moving a lot at such a low temperature: 150 degrees Kelvin or minus 123 degrees Celsius," said Tringides. "The other surprise was that the atoms weren't moving randomly like individual atoms as we would expect. In this particular case, it seemed like the whole layer of lead atoms was moving like a liquid.

Fluid-like motion of the lead atoms explains why the layer moves so easily and forms uniform islands so quickly.

"When applying nanotechnology, it's very important to be able to make nanostructures of the same dimension using a method that others can easily replicate," said Tringides. "And, it's important that the growth process is fast."

Tringides' work succeeds in terms of uniformity and speed. The lead islands self-organize on silicon in only two to three minutes. Also, better understanding of how the lead islands grow will help researchers see if other systems show the same liquid-like behavior at low temperatures.

With such promising findings in hand, Tringides' team, which includes associate scientist Myron Hupalo and graduate students Steven Binz and Jizhou Chen, further investigated the possible use of these unusual lead islands on silicon as templates to study typical atomic processes, such as adsorption, nucleation and atom bonding. These processes are important in the study of reactivity and catalysis.

During those experiments, Tringides' group made another unexpected discovery. Normally atomic processes depend on an element's chemical nature, but the group found that when it came to lead islands, quantum mechanics had another surprise in store: The atomic processes depend dramatically on whether the island height is odd or even rather than its chemical nature. Tringides' group made this intriguing observation in a large lead island that had formed over a step on the original silicon surface. The top of the large island was flat as expected.

"But, the part of the island sitting on the higher terrace of silicon was four layers high, and the other part of the island sitting on the lower terrace was five layers," said Tringides.

The group studied nucleation on this unusual island by adding a very small amount of lead to its surface, creating many new small islands on top of the large island. Examination revealed that the density of the new islands was 60 times higher on the four-layer part of the island than on the five-layer part even though the two parts of the island were connected, suggesting that atom bonding is easier on the four-layer islands.

"The island was made up of the same element, lead, throughout," said Tringides. "So, we would expect the two parts of the island to communicate with each other, and atoms should be able to easily move from left to right and right to left among both halves of the island, so the density of the new small islands should have been the same in both parts."

Instead, the two halves of the island behaved like two separate islands. The four-layer section of the island has similar characteristics to independent four-layer islands, and the five-layer section behaved like other five-layer islands.

"For the purpose of growing materials, the two-part island indicates that we may not have to change the element to create variation in material properties," said Tringides. "Instead, we may be able to just change the height of the island."

"This is promising because it's easier to change the geometry of an island than to go out and find a new, exotic material," he added.

Tringides plans further experiments using gas adsorption to test the relationship between material reactivity and island height.

The Department of Energy's Office of Science, Basic Energy Sciences Office funded the work.

Lawrence Livermore National Laboratory scientists for the first time have validated the idea of using extremely short and intense X-ray pulses to capture images of objects such as proteins before the X-rays destroy the sample.

At the same time, the team also established a speed record of 25 femtoseconds for flash imaging.

The new method will be applicable to atomic-resolution imaging of complex biomolecules when even more powerful X-ray lasers, currently under construction, are available. The technique will allow scientists to gain insight into the fields of materials science, plasma physics, biology and medicine.

Using the free-electron laser at Deutsches Elektronen-Synchrotron (DESY) in Hamburg, Livermore scientists, as part of an international collaboration led by LLNL's Henry Chapman and Janos Hajdu of Uppsala University, were able to record a single diffraction pattern of a nanostructured object before the laser destroyed the sample. A Livermore-developed computer algorithm was then used to recreate an image of the object based on the recorded diffraction pattern. This "lensless" imaging technique could be applied to atomic-resolution imaging because it is not limited by the need to build a high-resolution lens. The flash images could resolve features 50 nanometers in size, which is about 10 times smaller than what is achievable with an optical microscope.

Theory predicts that a single diffraction pattern may be recorded from a large macromolecule, a virus or a cell with an ultra-short and extremely bright X-ray pulse before the sample explodes and turns into a plasma. This means that scientists could better understand the structure of macromolecular proteins without crystallizing them and thus allow rapid study of all classes of proteins.

Livermore scientists, along with colleagues at Uppsala University in Sweden, DESY, Technische Universität Berlin, the Center for Biophotonics Science and Technology at UC Davis, Stanford Synchrotron Radiation Laboratory, and private firm Spiller X-ray Optics of Livermore, conducted the first experimental demonstration of this theory.

Computer simulations based on four different models suggest that a near-atomic resolution structure could be achieved by well-thought out choice of pulse length and intensity of X-ray wavelength before the sample is stripped of its electrons and is destroyed. However, up until now, there had been no experimental verification of the technique.

The experimental demonstration of "flash diffractive imaging" uses the first soft X-ray FEL (free electron laser) in the world located at the FLASH facility at DESY. FLASH generates high-power soft X-ray pulses by the principle of self-amplification of spontaneous emission. The pulses are 10 million times brighter than today's brightest X-ray sources, synchrotrons. In addition, this experiment showed that it only takes a 25-femtosecond pulse duration to capture the image.

There has been a question whether the diffraction pattern recorded under these circumstances could be reconstructed to obtain undamaged sample information.

"These results could become a standardized method," Chapman said. "This imaging could be applied at the cellular, sub-cellular and down on to single molecule scale."

Other Livermore authors include Anton Barty, Michael Bogan, Sebastien Boutet, Matthias Frank, Stefan Hau-Riege, Stefano Marchesini, Bruce Woods, Saša Bajt, Henry Benner, Richard London, Richard Lee and Abraham Szoke.

The work was funded by a Laboratory Directed Research and Development strategic initiative proposal for "biological imaging with fourth-generation light sources."

The research appears in the Nov. 12 online edition of Nature Physics. It will appear on the cover of the December hard copy issue of Nature Physics.

This research was funded in part by the National Science Foundation's Center for Biophotonics Science and Technology (CBST) headquartered at the UC Davis Medical Center in Sacramento. CBST is a multi-institutional research center established in 2002 by LLNL and UC Davis researchers and is dedicated to the development and application of photonic instrumentation and methods to address important problems in the biomedical technology sector.

Founded in 1952, Lawrence Livermore National Laboratory is a national security laboratory, with a mission to ensure national security and apply science and technology to the important issues of our time. Lawrence Livermore National Laboratory is managed by the University of California for the U.S. Department of Energy's National Nuclear Security Administration.

A new imaging technique developed by researchers at the University of Illinois overcomes the limit of diffraction and can reveal the atomic structure of a single nanocrystal with a resolution of less than one angstrom (less than one hundred-millionth of a centimeter).

Optical and electronic properties of small assemblages of atoms called quantum dots depend upon their electronic structure – not just what's on the surface, but also what's inside. While scientists can calculate the electronic structure, they need to know where the atoms are positioned in order to do so accurately.

Getting this information, however, has proved to be a challenge for nanocrystals like quantum dots. Mapping out the positions of atoms requires clues provided by the diffraction pattern. But quantum dots are so small, the clues provided by diffraction alone are not enough.

By combining two sources of information – images and diffraction patterns taken with the same electron microscope – researchers at the U. of I. can achieve sub-angstrom resolution of structures that were not possible before.

"We show that for cadmium-sulfide nanocrystals, the improved image resolution allows a determination of their atomic structures," said Jian-Min (Jim) Zuo, a professor of materials science and engineering at the U. of I., and corresponding author of a paper that describes the high-resolution imaging system in the February issue of Nature Physics.

Images from electron microscopy can resolve individual atoms in a nanocrystal, but the atoms soon suffer radiation damage, which limits the length of observations. Patterns from X-ray diffraction can be used to determine the structure of large crystals, but not for nanocrystals, which are too small and don't diffract well.

To achieve sub-angstrom resolution, Zuo and colleagues developed a reiterative algorithm that processes and combines shape information from the low-resolution image and structure information from the high-resolution diffraction pattern. Both the image and the diffraction pattern are taken with the same transmission-electron microscope.

"The low-resolution image provides the starting point by supplying missing information in the central beam and supplying essential marks for aligning the diffraction pattern," said Zuo, who also is a researcher at the university's Frederick Seitz Materials Research Laboratory. "Our phase-retrieval algorithm then reconstructs the image."

To demonstrate the technique, the researchers took a new look at cadmium-sulfide quantum dots.

"We chose cadmium-sulfide quantum dots because of their size-dependent optical and electronic properties, and the importance of atomic structure on these properties," Zuo said. "Cadmium-sulfide quantum dots have potential applications in solar energy conversion and in medical imaging."

Using the reiterative algorithm, the smallest separation between the cadmium and sulfide atomic columns was measured at 0.84 angstroms, the researchers report.

"Since low-resolution images can be obtained from different sources, our technique is general and can be applied to non-periodic structures, such as interfaces and local defects," Zuo said. "Our technique also provides a basis for imaging the three-dimensional structure of single nanoparticles."

With Zuo, co-authors of the paper are former doctoral student and lead author Weijie Huang (now at Dow Chemical Co.), U. of I. professor of materials science and engineering Moonsub Shim, former postdoctoral research associate Bin Jiang (now at FEI Co.), and former doctoral student Kwan-Wook Kwon (now at LAM Research).

The U.S. Department of Energy, the American Chemical Society and the National Science Foundation funded the work.

In the world of nanomaterials, scientists and engineers can create new structures with tiny building blocks as small as one billionth of a meter.

But in order to construct new materials and devices, researchers first need to understand how these tiny units interact with each other.

One such building block is graphite oxide, which is often used to make graphene — a hotly studied material that scientists believe could be used to produce low-cost carbon-based transparent and flexible electronics. Like graphene, graphite oxide is essentially a sheet that is only one atom thick, but can be as wide as tens of micrometers.

Jiaxing Huang, assistant professor of materials science and engineering at Northwestern University, and his research group at the McCormick School of Engineering and Applied Science set out to investigate how these graphite-oxide sheets assemble. Their results, published as the cover article in the Jan. 26 issue of the Journal of the American Chemical Society, surprised them.

“We were very curious how these extremely thin two-dimensional sheets interact with each other,” Huang says. “This knowledge can also help to prepare better graphene thin films.”

Huang and his group studied the sheets by putting them onto a water surface — a process called Langmuir-Blodgett assembly, which makes the sheets stay flat and allows scientists to move them around.

The effect reminded the researchers of water lilies on a pond, and Huang asked his sister to help to create a Chinese water painting similar to that of Claude Monet’s series of paintings “Water Lilies” to demonstrate the idea. The artwork was chosen as one of the first illustrated covers for the 130-year-old journal.

Researchers used a barrier to push the sheets together to see how they would interact and then “fished” the interacting sheets off the water surface using glass slides or silicon wafers. Huang and his colleagues expected to see that individual sheets had stacked one upon the other, like a shuffled deck of cards. Instead they found that the edges of the graphite oxide sheets rumpled as they were pushed together.

“This was quite a surprise for us,” Huang says. “Now we understand that electrostatic repulsion is the dominant interaction when these sheets are pushed together in this edge-to-edge geometry. This prevents graphite oxide layers from overlapping with each other.”

When squeezed even further, the sheets eventually formed an interlocking structure that becomes a continuous membrane.

This film — consisting of flat, non-overlapping single layers tiling over large areas — has been very difficult to achieve by conventional thin-film processing techniques such as drop casting or spraying.

This breakthrough could have two immediate technological impacts. “Because we can keep them close to each other and still keep them flat, it provides high coverage of the surface with the single layers — which in turn will translate into high successful yield in graphene device fabrication,” Huang says. “On the other hand, the continuous graphite oxide monolayer can be made into a transparent conductor after conversion to graphene.”

Now, after studying how they interact edge-to-edge, Huang hopes to study face-to-face contact of the graphene-based materials. Stacking graphene sheets directly on top of each other will form graphite and lose the advantages of the single-atom-thick graphene materials. But Huang hopes to find a way to stack graphene without making graphite, which could create functional materials for energy-related applications such as electrodes for batteries, ultracapacitors and fuel cells.

“If we are good at making these tiny building blocks and if we can control how they assemble, we will create a lot of wonderful new things,” Huang says.

In addition to Huang, co-authors of the paper include National Science Foundation graduate research fellow Laura Cote and postdoctoral fellow Franklin Kim, both of whom, according to Huang, “did a wonderful job” to create the high-quality graphite oxide sheets used in the experiment.

Researchers “tune” graphene’s properties by growing it on different surfaces.

Researchers at Rensselaer Polytechnic Institute have discovered a new method for controlling the nature of graphene, bringing academia and industry potentially one step closer to realizing the mass production of graphene-based nanoelectronics.

Graphene, a one-atom-thick sheet of carbon, was discovered in 2004 and is considered a potential heir to copper and silicon as the fundamental building blocks of nanoelectronics.

With help from an underlying substrate, researchers for the first time have demonstrated the ability to control the nature of graphene. Saroj Nayak, an associate professor in Rensselaer’s Department of Physics, Applied Physics, and Astronomy, along with Philip Shemella, a postdoctoral research associate in the same department, have determined that the chemistry of the surface on which graphene is deposited plays a key role in shaping the material’s conductive properties. The results are based on large-scale quantum mechanical simulations.

Results show that when deposited on a surface treated with oxygen, graphene exhibits semiconductor properties. When deposited on a material treated with hydrogen, however, graphene exhibits metallic properties.

“Depending on the chemistry of the surface, we can control the nature of the graphene to be metallic or semiconductor,” Nayak said. “Essentially, we are ‘tuning’ the electrical properties of material to suit our needs.”

Conventionally, whenever a batch of graphene nanostructures is produced, some of the graphene is metallic, while the rest is semiconductor. It would be nearly impossible to separate the two on a large scale, Nayak said, yet realizing new graphene devices would require that they be comprised solely of metallic or semiconductor graphene. The new method for “tuning” the nature of graphene is a key step to making this possible, he said.

Graphene’s excellent conductive properties make it attractive to researchers. Even at room temperature, electrons pass effortlessly, near the speed of light and with little resistance. This means a graphene interconnect would likely stay much cooler than a copper interconnect of the same size. Cooler is better, as heat produced by interconnects can have negative effects on both a computer chip’s speed and performance.

Results of the study were published this week in the paper “Electronic structure and band-gap modulation of graphene via substrate surface chemistry” in Applied Physics Letters, and are featured on the cover of the journal’s January 19 issue.

Large-scale quantum simulations for the study were run on Rensselaer’s supercomputing system, the Computational Center for Nanotechnology Innovations (CCNI).

Researchers received funding for the project from the New York State Interconnect Focus Center at Rensselaer.

University of Maryland physicists have shown that in graphene the intrinsic limit to the mobility, a measure of how well a material conducts electricity, is higher than any other known material at room temperature. Graphene, a single-atom-thick sheet of graphite, is a new material which combines aspects of semiconductors and metals.

Their results, published online in the journal Nature Nanotechnology, indicate that graphene holds great promise for replacing conventional semiconductor materials such as silicon in applications ranging from high-speed computer chips to biochemical sensors.

A team of researchers led by physics professor Michael S. Fuhrer of the university's Center for Nanophysics and Advanced Materials, and the Maryland NanoCenter said the findings are the first measurement of the effect of thermal vibrations on the conduction of electrons in graphene, and show that thermal vibrations have an extraordinarily small effect on the electrons in graphene.

In any material, the energy associated with the temperature of the material causes the atoms of the material to vibrate in place. As electrons travel through the material, they can bounce off these vibrating atoms, giving rise to electrical resistance. This electrical resistance is "intrinsic" to the material: it cannot be eliminated unless the material is cooled to absolute zero temperature, and hence sets the upper limit to how well a material can conduct electricity.

In graphene, the vibrating atoms at room temperature produce a resistivity of about 1.0 microOhm-cm (resistivity is a specific measure of resistance; the resistance of a piece material is its resistivity times its length and divided by its cross-sectional area). This is about 35 percent less than the resistivity of copper, the lowest resistivity material known at room temperature.

"Other extrinsic sources in today's fairly dirty graphene samples add some extra resistivity to graphene," explained Fuhrer, "so the overall resistivity isn't quite as low as copper's at room temperature yet. However, graphene has far fewer electrons than copper, so in graphene the electrical current is carried by only a few electrons moving much faster than the electrons in copper."

In semiconductors, a different measure, mobility, is used to quantify how fast electrons move. The limit to mobility of electrons in graphene is set by thermal vibration of the atoms and is about 200,000 cm2/Vs at room temperature, compared to about 1,400 cm²/Vs in silicon, and 77,000 cm²/Vs in indium antimonide, the highest mobility conventional semiconductor known.

"Interestingly, in semiconducting carbon nanotubes, which may be thought of as graphene rolled into a cylinder, we've shown that the mobility at room temperature is over 100,000 cm²/Vs" said Fuhrer (T. Dürkop, S. A. Getty, Enrique Cobas, and M. S. Fuhrer, Nano Letters 4, 35 (2004)).

Mobility determines the speed at which an electronic device (for instance, a field-effect transistor, which forms the basis of modern computer chips) can turn on and off. The very high mobility makes graphene promising for applications in which transistors much switch extremely fast, such as in processing extremely high frequency signals.

Mobility can also be expressed as the conductivity of a material per electronic charge carrier, and so high mobility is also advantageous for chemical or bio-chemical sensing applications in which a charge signal from, for instance, a molecule adsorbed on the device, is translated into an electrical signal by changing the conductivity of the device.

Graphene is therefore a very promising material for chemical and bio-chemical sensing applications. The low resitivity and extremely thin nature of graphene also promises applications in thin, mechanically tough, electrically conducting, transparent films. Such films are sorely needed in a variety of electronics applications from touch screens to photovoltaic cells.

Fuhrer and co-workers showed that although the room temperature limit of mobility in graphene is as high as 200,000 cm²/Vs, in present-day samples the actual mobility is lower, around 10,000 cm²/Vs, leaving significant room for improvement. Because graphene is only one atom thick, current samples must sit on a substrate, in this case silicon dioxide.

Trapped electrical charges in the silicon dioxide (a sort of atomic-scale dirt) can affect the electrons in graphene and reduce the mobility. Also, vibrations of the silicon dioxide atoms themselves can also have an effect on the graphene which is stronger than the effect of graphene's own atomic vibrations. This so-called "remote interfacial phonon scattering" effect is only a small correction to the mobility in a silicon transistor, but because the phonons in graphene itself are so ineffective at scattering electrons, this effect becomes very important in graphene.

"We believe that this work points out the importance of these extrinsic effects, and creates a roadmap for finding better substrates for future graphene devices in order to reduce the effects of charged impurity scattering and remote interfacial phonon scattering." Fuhrer said.

"Intrinsic and Extrinsic Performance Limits of Graphene Devices on SiO₂," J. H. Chen, C. Jang, S. Xiao, M. Ishigami, M. S. Fuhrer (will appear in Nature Nanotechnology online March 23, 2008)

A game of billiards may never get smaller than this.

Physicists at UC Riverside have demonstrated that graphene -- a one-atom thick sheet of carbon atoms arranged in hexagonal rings -- can act as an atomic-scale billiard table, with electric charges acting as billiard balls.

The finding underscores graphene's potential for serving as an excellent electronic material, such as silicon, that can be used to develop new kinds of transistors based on quantum physics. Because they encounter no obstacles, the electrons in graphene roam freely across the sheet of carbon, conducting electric charge with extremely low resistance.

The research team, led by Chun Ning (Jeanie) Lau, found that the electrons in graphene are reflected back by the only obstacle they meet: graphene's boundaries.

"These electrons meet no other obstacles and behave like quantum billiard balls," said Lau, an assistant professor who joined UCR's Department of Physics and Astronomy in 2004. "They display properties that resemble both particles and waves."

Lau observed that when the electrons are reflected from one of the boundaries of graphene, the original and reflected components of the electron can interfere with each other, the way outgoing ripples in a pond might interfere with ripples reflected back from the banks.

Her lab detected the "electronic interference" by measuring graphene's electrical conductivity at extremely low (0.26 Kelvin) temperatures. She explained that at such low temperatures the quantum properties of electrons can be studied more easily.

"We found that the electrons in graphene can display wave-like properties, which could lead to interesting applications such as ballistic transistors, which is a new type of transistor, as well as resonant cavities for electrons," Lau said. She explained that a resonant cavity is a chamber, like a kitchen microwave, in which waves can bounce back and forth.

In their experiments, Lau and her colleagues first peeled off a single sheet of graphene from graphite, a layered structure consisting of rings of six carbon atoms arranged in stacked horizontal sheets. Next, the researchers attached nanoscale electrodes to the graphene sheet, which they then refrigerated in a cooling device. Finally, they measured the electrical conductivity of the graphene sheet.

Graphene, first isolated experimentally less than three years ago, is a two-dimensional honeycomb lattice of carbon atoms, and, structurally, is related to carbon nanotubes (tiny hollow tubes formed by rolling up sheets of graphene) and buckyballs (hollow carbon molecules that form a closed cage).

Scientifically, it has become a new model system for condensed-matter physics, the branch of physics that deals with the physical properties of solid materials. Graphene enables table-top experimental tests of a number of phenomena in physics involving quantum mechanics and relativity.

Bearing excellent material properties, such as high current-carrying capacity and thermal conductivity, graphene ideally is suited for creating components for semiconductor circuits and computers. Its planar geometry allows the fabrication of electronic devices and the tailoring of a variety of electrical properties. Because it is only one-atom thick, it can potentially be used to make ultra-small devices and further miniaturize electronics.

Study results appear in Science on September 14.

Lau, whose research focuses on nanowires, carbon nanotubes, graphene and other organic molecules, was joined in the research by UCR's Feng Miao, Sithara Wijeratne, Wenzhong Bao, Yong Zhang and Ulas C. Coskun. The research was performed at UCR. Currently, Zhang is at Southwest University, China; Coskun is at Duke University, N.C.

UCR startup funds and the UCR Center for Nanoscale Science and Engineering supported the research.

Graphene, a single-atom-thick sheet of carbon, holds remarkable promise for future nanoelectronics applications. Whether graphene actually cuts it in industry, however, depends upon how graphene is cut, say researchers at the University of Illinois.

Graphene consists of a hexagonal lattice of carbon atoms. While scientists have predicted that the orientation of atoms along the edges of the lattice would affect the material's electronic properties, the prediction had not been proven experimentally.

Now, researchers at the U. of I. say they have proof.

"Our experimental results show, without a doubt, that the crystallographic orientation of the graphene edges significantly influences the electronic properties," said Joseph Lyding, a professor electrical and computer engineering. "To utilize nanometer-size pieces of graphene in future nanoelectronics, atomically precise control of the geometry of these structures will be required."

Lyding and graduate student Kyle Ritter (now at Micron Technology Inc. in Boise, Idaho) report their findings in a paper accepted for publication in Nature Materials online Feb. 15.

To carry out their work, the researchers developed a method for cutting and depositing nanometer-size bits of graphene on atomically clean semiconductor surfaces like silicon.

Then they used a scanning tunneling microscope to probe the electronic structure of the graphene with atomic-scale resolution.

"From this emerged a clear picture that edges with so-called zigzag orientation exhibited a strong edge state, whereas edges with armchair orientation did not," said Lyding, who also is affiliated with the university's Beckman Institute and the Micro and Nanotechnology Laboratory.

"We found that pieces of graphene smaller than about 10 nanometers with predominately zigzag edges exhibited metallic behavior rather than the semiconducting behavior expected from size alone," Lyding said. "This has major implications in that semiconducting behavior is mandatory for transistor fabrication."

Unlike carbon nanotubes, graphene is a flat sheet, and therefore compatible with conventional fabrication processes used by today's chipmakers. But, based on the researchers' experimental results, controlled engineering of the graphene edge structure will be required for obtaining uniform performance among graphene-based nanoelectronic devices.

"Even a tiny section of zigzag orientation on a 5-nanometer piece of graphene will change the material from a semiconductor into a metal," Lyding said. "And a transistor based on that, will not work. Period."

The Office of Naval Research and the National Science Foundation funded the work.

Nearly 2,000 years ago, the discovery of paper revolutionized human communication. Now researchers at Northwestern University have fabricated a new type of paper that they hope will create a revolution of its own -- and while it won't replace your notepad, this remarkably stiff and strong yet lightweight material should find use in a wide variety of applications.

In a paper to be published July 26 in the journal Nature, researchers led by Rod Ruoff, John Evans Professor of Nanoengineering in the Robert R. McCormick School of Engineering and Applied Science, report on the development of graphene oxide.

Ruoff's research team was the first to develop graphene-based composite materials, which was reported in Nature last year. Graphene -- a sheet of carbon only one atom thick -- has the potential to serve as the basis of an entirely new class of materials.

"The mechanical, thermal, optical and electrical properties of graphene are exceptional," says Ruoff. "For example, the stiffness and strength of these graphene-like sheets should be superior to all other materials, with the possible exception of diamond."

To form the graphene oxide paper, the group oxidized graphite to create graphite oxide, which falls apart in water to yield well-dispersed graphene oxide sheets. After filtering the water, the team was able to fabricate pieces of graphene oxide 'paper' more than five inches in diameter and with thicknesses from about one to 100 microns, in which the individual micron-sized graphene oxide sheets are stacked on top of each other.

"I have little doubt that very large-area sheets of this paper-material could be made in the future," Ruoff notes.

In addition to their superior mechanical properties as individual sheets, the graphene oxide layers stack well, which could be key to the development of other materials.

"You can imagine that these microscale sheets may be stacked together and chemically linked, allowing us to further optimize the mechanical properties of the resulting macroscale object," Ruoff says. "This combination of excellent mechanical properties and chemical tunability should make graphene-based paper an exciting material."

Of further interest are the electrical properties of the graphene oxide paper in comparison to graphene sheets. "When we oxidize the graphene sheets to create graphene oxide, the material goes from being an electrical conductor to an electrical insulator," Ruoff says. "This is an important step and in the future it will be possible to tune the material as a conductor, semiconductor or insulator. One will be able to control the electrical properties without sacrificing exceptional mechanical properties."

Ruoff sees a wide variety of application for graphene oxide paper, including membranes with controlled permeability, and for batteries or supercapacitors for energy applications. Graphene oxide paper could also be infused to create hybrid materials containing polymers, ceramics or metals, where such composites would perform much better than existing materials as components in, for example, airplanes, cars, buildings and sporting goods products.

The development of this paper-like material is the latest of several recent advancements by Ruoff's team in launching the new field of graphene-based materials. In a paper in the July issue of Nano Letters, the group reported that graphene sheets could be embedded into glass films to make them electrically conductive. These transparent thin films could find applications in solar cells or a variety of transparent electronics such as electronic paper and flexible color screens. The processing of these films may provide a cheaper alternative to the widely used indium tin oxide coatings that are typically used as the transparent conductive film.

In addition to Ruoff, other authors on the Nature paper are Dmitry Dikin, Sasha Stankovich, Eric Zimney, Richard Piner, Geoffrey Dommett, Guennadi Evmenenko and SonBinh Nguyen. For the Nano Letters paper, this group of researchers was also joined by coauthors Supinda Watcharotone, and from the National Cheng Kung University in Taiwan, Shang-En Wu, Shu-Fang Chen and Chuan-Pu Liu.