S-t-r-e-t-c-h-i-n-g electrical conductance to the limit

Individual molecules have been used to create electrical components like resistors, transistors and diodes that mimic the properties of familiar semiconductors. But according to Nongjian (NJ) Tao, a researcher at the Biodesign Institute at ASU, unique properties inherent in single molecules also may allow clever designers to produce novel devices whose behavior falls outside the performance observed in conventional electronics.

In research appearing in a recent issue of Nature Nanotechnology, Tao describes a method for mechanically controlling the geometry of a single molecule, situated in a junction between a pair of gold electrodes that form a simple circuit. The manipulations produced over tenfold increase in conductivity.

The unusual, often non-intuitive characteristics of single molecules may eventually be introduced into a broad range of microelectronics, suitable for applications including biological and chemical sensing electronic and mechanical devices.

Delicate molecular manipulations requiring patience and finesse are routine for Tao, whose research at Biodesign's Center for Bioelectronics and Biosensors has included work on molecular diodes, graphene behavior and molecular imaging techniques. Nevertheless, he was surprised at the outcome described in the current paper: "If you have a molecule attached to electrodes, it can stretch like a rubber band," he says. "If it gets longer, most people tend to think that the conductivity will decrease. A longer wire is less conductive than a shorter wire."

Indeed, diminishing conductivity through a molecule is commonly observed when the distance between the electrodes attached to its surface is increased and the molecule becomes elongated. But according to Tao, if you stretch the molecule enough, something unexpected happens: the conductance goes up -- by a huge amount. "We see at least 10 times greater conductivity, simply by pulling the molecule."

As Tao explains, the intriguing result is a byproduct of the laws of quantum mechanics, which dictate the behavior of matter at the tiniest scales: "The conductivity of a single molecule is not simply inversely proportional to length. It depends on the energy level alignment."

In the metal leads of the electrodes, electrons can move about freely but when they come to an interface -- in this case, a molecule that sits in the junction between electrodes -- they have to overcome an energy barrier. The height of this energy barrier is critical to how readily electrons can pass through the molecule. By applying a mechanical force to the molecule, the barrier is lowered, improving conductance.

"Theoretically, people have thought of this as a possibility, but this is a demonstration that it really happens," Tao says. "If you stretch the molecule and geometrically increase the length, it energetically lowers the barrier so electrons can easily go through. If you think in optical terms, it becomes more transparent to electrons."

The reason for this has to do with a property known as force-induced resonant tunneling. This occurs when the molecular energy moves closer to the Fermi level of the electrodes -- that is, toward the region of optimal conductance. Thus, as the molecule is stretched, it causes a decrease in the tunneling energy barrier.

For the experiments, Tao's group used 1,4'-Benzenedithiol, the most widely studied entity for molecular electronics. Further experiments demonstrated that the transport of electrons through the molecule underwent a corresponding decrease as the distance between the electrodes was reduced, causing the molecule's geometry to shift from a stretched condition to a relaxed or squeezed state. "We have to do this thousands of times to be sure the effect is robust and reproducible."

In addition to the discovery's practical importance, the new data show close agreement with theoretical models of molecular conductance, which had often been at variance with experimental values, by orders of magnitude.

Tao stresses that single molecules are compelling candidates for a new types of electronic devices, precisely because they can exhibit very different properties from those observed in conventional semiconductors.

Microelectromechanical systems or MEMS are just one domain where the versatile properties of single molecules are likely to make their mark. These diminutive creations represent a $40 billion a year industry and include such innovations as optical switches, gyroscopes for cars, lab-on-chip biomedical applications and microelectronics for mobile devices.

"In the future, when people design devices using molecules, they will have a new toolbox they can use."

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Journal Reference:

Christopher Bruot, Joshua Hihath, Nongjian Tao. Mechanically controlled molecular orbital alignment in single molecule junctions. Nature Nanotechnology, 2011; DOI: 10.1038/nnano.2011.212

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Disclaimer: Views expressed in this article do not necessarily reflect those of ScienceDaily or its staff.

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S-t-r-e-t-c-h-i-n-g electrical conductance to the limit

Individual molecules have been used to create electrical components like resistors, transistors and diodes that mimic the properties of familiar semiconductors. But according to Nongjian (NJ) Tao, a researcher at the Biodesign Institute at ASU, unique properties inherent in single molecules also may allow clever designers to produce novel devices whose behavior falls outside the performance observed in conventional electronics.

In research appearing in a recent issue of Nature Nanotechnology, Tao describes a method for mechanically controlling the geometry of a single molecule, situated in a junction between a pair of gold electrodes that form a simple circuit. The manipulations produced over tenfold increase in conductivity.

The unusual, often non-intuitive characteristics of single molecules may eventually be introduced into a broad range of microelectronics, suitable for applications including biological and chemical sensing electronic and mechanical devices.

Delicate molecular manipulations requiring patience and finesse are routine for Tao, whose research at Biodesign's Center for Bioelectronics and Biosensors has included work on molecular diodes, graphene behavior and molecular imaging techniques. Nevertheless, he was surprised at the outcome described in the current paper: "If you have a molecule attached to electrodes, it can stretch like a rubber band," he says. "If it gets longer, most people tend to think that the conductivity will decrease. A longer wire is less conductive than a shorter wire."

Indeed, diminishing conductivity through a molecule is commonly observed when the distance between the electrodes attached to its surface is increased and the molecule becomes elongated. But according to Tao, if you stretch the molecule enough, something unexpected happens: the conductance goes up -- by a huge amount. "We see at least 10 times greater conductivity, simply by pulling the molecule."

As Tao explains, the intriguing result is a byproduct of the laws of quantum mechanics, which dictate the behavior of matter at the tiniest scales: "The conductivity of a single molecule is not simply inversely proportional to length. It depends on the energy level alignment."

In the metal leads of the electrodes, electrons can move about freely but when they come to an interface -- in this case, a molecule that sits in the junction between electrodes -- they have to overcome an energy barrier. The height of this energy barrier is critical to how readily electrons can pass through the molecule. By applying a mechanical force to the molecule, the barrier is lowered, improving conductance.

"Theoretically, people have thought of this as a possibility, but this is a demonstration that it really happens," Tao says. "If you stretch the molecule and geometrically increase the length, it energetically lowers the barrier so electrons can easily go through. If you think in optical terms, it becomes more transparent to electrons."

The reason for this has to do with a property known as force-induced resonant tunneling. This occurs when the molecular energy moves closer to the Fermi level of the electrodes -- that is, toward the region of optimal conductance. Thus, as the molecule is stretched, it causes a decrease in the tunneling energy barrier.

For the experiments, Tao's group used 1,4'-Benzenedithiol, the most widely studied entity for molecular electronics. Further experiments demonstrated that the transport of electrons through the molecule underwent a corresponding decrease as the distance between the electrodes was reduced, causing the molecule's geometry to shift from a stretched condition to a relaxed or squeezed state. "We have to do this thousands of times to be sure the effect is robust and reproducible."

In addition to the discovery's practical importance, the new data show close agreement with theoretical models of molecular conductance, which had often been at variance with experimental values, by orders of magnitude.

Tao stresses that single molecules are compelling candidates for a new types of electronic devices, precisely because they can exhibit very different properties from those observed in conventional semiconductors.

Microelectromechanical systems or MEMS are just one domain where the versatile properties of single molecules are likely to make their mark. These diminutive creations represent a $40 billion a year industry and include such innovations as optical switches, gyroscopes for cars, lab-on-chip biomedical applications and microelectronics for mobile devices.

"In the future, when people design devices using molecules, they will have a new toolbox they can use."

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and Google +1:

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Story Source:

The above story is reprinted from materials provided by Arizona State University.

Note: Materials may be edited for content and length. For further information, please contact the source cited above.

Journal Reference:

Christopher Bruot, Joshua Hihath, Nongjian Tao. Mechanically controlled molecular orbital alignment in single molecule junctions. Nature Nanotechnology, 2011; DOI: 10.1038/nnano.2011.212

Note: If no author is given, the source is cited instead.

Disclaimer: Views expressed in this article do not necessarily reflect those of ScienceDaily or its staff.

View the original article here

Gold nanowires in engineered patches enhance electrical signaling and contraction

ScienceDaily (Sep. 26, 2011) — A team of physicians, engineers and materials scientists at Children's Hospital Boston and the Massachusetts Institute of Technology have used nanotechnology and tiny gold wires to engineer cardiac patches, with cells all beating in time, that could someday help heart attack patients.

As reported online by Nature Nanotechnology on September 25, the addition of gold wires to the engineered heart tissue make it electrically conductive, potentially improving on existing cardiac patches. Such patches are starting to go into clinical trials for heart patients.

"If you don't have the gold nanowires, and you stimulate the cardiac patch with an electrode, the cells will beat only right where you're stimulating," says senior investigator Daniel Kohane, MD, PhD, of the Laboratory for Biomaterials and Drug Delivery at Children's Hospital Boston. "With the nanowires, you see a lot of cells contracting together, even when the stimulation is far away. That shows the tissue is conducting."

After incubation, the patches studded with the gold nanowires were thicker and their heart muscle cells better organized. When stimulated with an electrical current, the cells produced a measurable spike in voltage, and electrical communication between adjacent bundles of cardiac cells was markedly improved. In contrast, only a negligible current passed through patches lacking the wires, and cells beat only in isolated clusters.

Kohane thinks the nanowire technology could be applied to the engineering of any electrically excitable tissue, including tissue in the brain and spinal cord. Gold was chosen as a material because it's a conductive material, easy to fabricate, scientists have a lot of experience with it, and it is tolerated by the body.

The wires average 30 nanometers thick and 2-3 microns long, just barely visible to the naked eye.

Since testing has so far been done only in cell cultures, the team plans to do further experiments to see how well the cardiac patches function in live animal models, and to get a better understanding of how exactly the nanowires are enhancing electrical signaling and contraction.

Kohane believes the gold fibers help because they're long enough to cross the scaffolding material that holds the cells and may act as a barrier to electrical conduction. In addition, the experiments showed enhanced production of troponin I, a protein involved in muscle calcium binding and contraction, and connexin-43, a protein involved in electrical coupling between cells that is believed to play a critical role in the development of the heart's architecture and in the synchronized contraction of the heart.

The study was funded by the National Institutes of Health and the American Heart Association. The paper's co-first authors were Tal Dvir, PhD, and Brian Timko, PhD, both of the Department of Chemical Engineering, Massachusetts Institute of Technology, and the Laboratory for Biomaterials and Drug Delivery at Children's Hospital Boston.

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The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by Children's Hospital Boston, via EurekAlert!, a service of AAAS.

Journal Reference:

Tal Dvir, Brian P. Timko, Mark D. Brigham, Shreesh R. Naik, Sandeep S. Karajanagi, Oren Levy, Hongwei Jin, Kevin K. Parker, Robert Langer & Daniel S. Kohane. Nanowired three-dimensional cardiac patches. Nature Nanotechnology, 25 September 2011 DOI: 1038/nnano.2011.160

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Disclaimer: This article is not intended to provide medical advice, diagnosis or treatment. Views expressed here do not necessarily reflect those of ScienceDaily or its staff.

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How graphene's electrical properties can be tuned

ScienceDaily (Sep. 26, 2011) — An accidental discovery in a physicist's laboratory at the University of California, Riverside provides a unique route for tuning the electrical properties of graphene, nature's thinnest elastic material. This route holds great promise for replacing silicon with graphene in the microchip industry.

The researchers found that stacking up three layers of graphene, like pancakes, significantly modifies the material's electrical properties. When they fabricated trilayer graphene in the lab and measured its conductance, they found, to their surprise, that depending on how the layers were stacked some of the trilayer graphene devices were conducting while others were insulating.

"What we stumbled upon is a simple and convenient 'knob' for tuning graphene sheets' electrical properties," said Jeanie Lau, an associate professor of physics and astronomy, whose lab made the serendipitous finding.

Study results appeared online Sept. 25 in Nature Physics.

Graphene is a one-atom thick sheet of carbon atoms arranged in hexagonal rings. Bearing excellent material properties, such as high current-carrying capacity and thermal conductivity, this "wonder material" is ideally suited for creating components for semiconductor circuits and computers.

Because of the planar and chicken wire-like structure of graphene, its sheets lend themselves well to stacking in what is called 'Bernal stacking,' the stacking fashion of graphene sheets.

In a Bernal-stacked bilayer, one corner of the hexagons of the second sheet is located above the center of the hexagons of the bottom sheet. In Bernal-stacked trilayer (ABA), the top (third) sheet is exactly on top of the lowest sheet. In rhombohedral-stacked (ABC) trilayer, the top sheet is shifted by the distance of an atom, so that the top (third) sheet and the lowest sheet form a Bernal stacking as well.

"The most stable form of trilayer graphene is ABA, which behaves like a metal," Lau explained. "Amazingly, if we simply shift the entire topmost layer by the distance of a single atom, the trilayer -- now with ABC or rhombohedral stacking -- becomes insulating. Why this happens is not clear as yet. It could be induced by electronic interactions. We eagerly await an explanation from theorists!"

Her lab used Raman spectroscopy to examine the graphene devices' stacking orders. Next the lab plans to investigate the nature of the insulating state in ABC-stacked graphene. In this kind of stacked graphene, they also plan to study the band gap -- a range in energy, critical for digital applications, in which no electrons can exist.

"The presence of the gap in ABC-stacked graphene that arises, we believe, from enhanced electronic interactions is interesting since it is not expected from theoretical calculations," Lau said. "Understanding this gap is particularly important for the major challenge of band gap engineering in graphene electronics."

Besides graphene, Lau studies nanowires and carbon nanotubes. Her research has helped physicists gain fundamental understanding of how atoms and electrons behave when they are ruled by quantum mechanics. Her lab studies novel electrical properties that arise from the quantum confinement of atoms and charges to nanoscale systems. Her research team has shown that graphene can act as an atomic-scale billiard table, with electric charges acting as billiard balls.

Her other research interests include superconductivity, thermal management and electronic transport in nanostructures, and engineering new classes of nanoscale devices.

An educational component of Lau's research effort is the active involvement of high school, undergraduate, and graduate students, especially minority and women, in her cutting-edge research, taking advantage of the ethnic diversity of UCR's student population and local communities. She is a founding faculty member of the UCR Undergraduate Research Journal. She also organized a "Women in Physics" lunch group that provides a friendly platform for female students, postdocs and faculty members to interact.

After receiving her bachelor's degree in physics from the University of Chicago in 1994, Lau proceeded to Harvard University from where she received her master's and doctoral degrees in physics in 1997 and 2001, respectively. She joined UCR in 2004, after an appointment as a research associate in the Hewlett-Packard Laboratory.

Lau's awards and honors include a Presidential Early Career Award for Scientists and Engineers, 2009; a National Science Foundation CAREER Award, 2008; the Richter Fellowship for Undergraduate Research, 1992; a David W. Grainger Senior Scholarship, 1993; and a Robert T. Poe Faculty Development Grant from the Chinese-American Faculty Association of Southern California, 2007. She has published more than 60 research articles in peer-reviewed journals.

Lau, a member of UCR's Center for Nanoscale Science and Engineering, was joined in the research by W. Bao (the first author of the research paper), L. Jing, J. Velasco Jr., Y. Lee, G. Liu, D. Tran and M. Bockrath at UCR; B. Stanley at Caltech; M. Aykol and S. B. Cronin at the University of Southern California; D. Smirnov at the National High Magnetic Field Laboratory, Fla.; M. Koshino at Tohoku University, Japan; and E. McCann at Lancaster University, United Kingdom.

The research was funded by grants from the National Science Foundation, Office of Naval Research, and the Focus Center for Functional Engineered Nano Architectonics.

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The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by University of California - Riverside.

Journal Reference:

W. Bao, L. Jing, J. Velasco, Y. Lee, G. Liu, D. Tran, B. Standley, M. Aykol, S. B. Cronin, D. Smirnov, M. Koshino, E. McCann, M. Bockrath, C. N. Lau. Stacking-dependent band gap and quantum transport in trilayer graphene. Nature Physics, 2011; DOI: 10.1038/nphys2103

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