Tuesday, November 1, 2011

Could a computer one day rewire itself? New nanomaterial 'steers' electric currents in multiple dimensions

Scientists at Northwestern University have developed a new nanomaterial that can "steer" electrical currents. The development could lead to a computer that can simply reconfigure its internal wiring and become an entirely different device, based on changing needs.

As electronic devices are built smaller and smaller, the materials from which the circuits are constructed begin to lose their properties and begin to be controlled by quantum mechanical phenomena. Reaching this physical barrier, many scientists have begun building circuits into multiple dimensions, such as stacking components on top of one another.

The Northwestern team has taken a fundamentally different approach. They have made reconfigurable electronic materials: materials that can rearrange themselves to meet different computational needs at different times.

"Our new steering technology allows use to direct current flow through a piece of continuous material," said Bartosz A. Grzybowski, who led the research. "Like redirecting a river, streams of electrons can be steered in multiple directions through a block of the material -- even multiple streams flowing in opposing directions at the same time."

Grzybowski is professor of chemical and biological engineering in the McCormick School of Engineering and Applied Science and professor of chemistry in the Weinberg College of Arts and Sciences.

The Northwestern material combines different aspects of silicon- and polymer-based electronics to create a new classification of electronic materials: nanoparticle-based electronics.

The study, in which the authors report making preliminary electronic components with the hybrid material, will be published online Oct. 16 by the journal Nature Nanotechnology. The research also will be published as the cover story in the November print issue of the journal.

"Besides acting as three-dimensional bridges between existing technologies, the reversible nature of this new material could allow a computer to redirect and adapt its own circuitry to what is required at a specific moment in time," said David A. Walker, an author of the study and a graduate student in Grzybowski's research group.

Imagine a single device that reconfigures itself into a resistor, a rectifier, a diode and a transistor based on signals from a computer. The multi-dimensional circuitry could be reconfigured into new electronic circuits using a varied input sequence of electrical pulses.

The hybrid material is composed of electrically conductive particles, each five nanometers in width, coated with a special positively charged chemical. (A nanometer is a billionth of a meter.) The particles are surrounded by a sea of negatively charged atoms that balance out the positive charges fixed on the particles. By applying an electrical charge across the material, the small negative atoms can be moved and reconfigured, but the relatively larger positive particles are not able to move.

By moving this sea of negative atoms around the material, regions of low and high conductance can be modulated; the result is the creation of a directed path that allows electrons to flow through the material. Old paths can be erased and new paths created by pushing and pulling the sea of negative atoms. More complex electrical components, such as diodes and transistors, can be made when multiple types of nanoparticles are used.

The title of the paper is "Dynamic Internal Gradients Control and Direct Electric Currents Within Nanostructured Materials." In addition to Grzybowski and Walker, other authors are Hideyuki Nakanishi, Paul J. Wesson, Yong Yan, Siowling Soh and Sumanth Swaminathan, from Northwestern, and Kyle J. M. Bishop, a former member of the Grzybowski research group, now with Pennsylvania State University.

Story Source:

The above story is reprinted (with editorial adaptations) from materials provided by Northwestern University, via EurekAlert!, a service of AAAS.

Journal Reference:

Hideyuki Nakanishi, David A. Walker, Kyle J. M. Bishop, Paul J. Wesson, Yong Yan, Siowling Soh, Sumanth Swaminathan, Bartosz A. Grzybowski. Dynamic internal gradients control and direct electric currents within nanostructured materials. Nature Nanotechnology, 2011; DOI: 10.1038/nnano.2011.165

Pendulums and floating film: Two seemingly unrelated phenomena share surprising link

 A coupled line of swinging pendulums apparently has nothing in common with an elastic film that buckles and folds under compression while floating on a liquid, but scientists at the University of Chicago and Tel Aviv University have discovered a deep connection between the two phenomena.

Energy carried in ordinary waves, like those seen on the ocean near a beach, quickly disperses. But the energy in the coupled pendulums and in compressed elastic film concentrates into different kinds of waves, ones with discrete packets of energy called "solitons."

Solitons express themselves in other realms as well. In telecommunications, for example, pulsing light travels as solitons through optical fibers. "What is special about solitons, and this very special way of localizing energy, is that it does not disperse," said Haim Diamant, associate professor of chemistry at Tel Aviv University. "It remains a very well-defined, focused packet."

The connection, which Diamant and UChicago's Thomas Witten report Oct. 14 in the journal Physical Review Letters, is subtle and more easily appreciated in visual form (see graphic). Witten graphed the swing angle of the pendulums as time progresses. Then he made a curve whose angle with the horizontal varies to match the swing angles along the diagonal edge of the graph. The resulting curve takes the same shape as the profile of a folded elastic film floating on a liquid, like the film shown at the bottom of the picture.

Witten and Diamant began collaborating while the latter worked at UChicago's James Franck Institute as a postdoctoral scientist from 1999 to 2002. They have been working together on puzzles emerging from the laboratory of another collaborator, Ka Yee Lee, UChicago professor in chemistry, ever since. Lee's research group studies the complex mixture of lipids and proteins that lines the sacs of the lung. These molecular linings fold and unfold as we inhale and exhale; the folding appears important for normal breathing.

Slowly growing energy

The energy applied to the film's deformation starts out weak, then grows stronger. Once the wrinkling energy in the film grows stronger, it concentrates itself into a fold shaped like the folding curve in the image.

Though the fold appears in a specific place on the film, "the motion resulting from folding extends over a big region. Usually big things are slow," said Witten, the Homer J. Livingston Professor in Physics. "But this is a big thing that is not slow. It's a rapid jerk, and we want to see what enables such rapid, large-scale motion."

Lee and her associates aim to understand breathing mechanics using synthetic films only one layer of molecules thick to simulate the surfactant that lines the microscopic air sacs found in the lungs. Diamant and Witten sought to solve the equation that exactly describes the fold shape of such a film. They knew it would be a difficult task, given that it was a nonlinear equation, one in which simple changes produce complicated effects.

A typical nonlinear problem might absorb decades of work without yielding a solution; this one seemed different. "There were strange hints that told us this problem might be solved exactly," Diamant said. "It's very rare that a nonlinear problem can be solved exactly."

Miracle solution

These hints had appeared in numerical simulations of the folding process generated by Enrique Cerda, a collaborator at the University of Santiago in Chile. "It was a miracle that we found an exact solution, but we had a strong feeling that it existed," Diamant said.

Once Diamant and Witten solved the problem, they realized that the solution resembled the sine-Gordon equation, well-known among mathematicians and physicists, which describes how a coupled line of swinging pendulums concentrate their energy.

Their resulting paper lays out the first example the authors know of in which soliton motion of a dynamical system can help scientists understand material deformation. The materials in this instance involve a thin, rigid layer floating on a fluid surface, a structure commonly found in biological tissues and synthetic coatings.

The finding has still-undetermined technological or biomedical applications, but it offers a way to control the film's deformation, including making the fold stick down into or up out of the water, forming a groove.

"This groove is controllable," Witten said. "You can shape the groove; you can make it come; you can make it go away." One also could control the location of the groove on the film, making it possible to manipulate the film on the scale of a few microns -- a fraction the width of a human hair.

"If there was some other material in the water that was attracted to the surface, we could make it nestle into this shape and we could capture it," Witten explained. "We think that this shape could have some potential that people don't realize."

As a next step, Diamant and Witten wonder if the dynamics of swinging pendulums can tell them anything about the dynamics of a folding elastic film. "All we have described at the moment is this static shape of the fold, but folding too is a dynamic phenomenon," Witten said.

Squeeze the film, and it will begin to fold after a period of time. Witten and Diamant would like to further describe how that process works based on what the swinging pendulums do.

"It seems only natural, but things like that are dangerous and they don't necessarily work," Witten noted. "But we do know that there is a lot known about the solitons that we can potentially harness to understand the folds."


The above story is reprinted (with editorial adaptations) from materials provided by University of Chicago. The original article was written by Steve Koppes.

Journal Reference:

Haim Diamant, Thomas Witten. Compression Induced Folding of a Sheet: An Integrable System. Physical Review Letters, 2011; 107 (16) DOI: 10.1103/PhysRevLett.107.164302

Liquid can turn into solid under high electric field, physicists show in simulations

Physicists have demonstrated in simulations that under the influence of sufficiently high electric fields, liquid droplets of certain materials will undergo solidification, forming crystallites at temperature and pressure conditions that correspond to liquid droplets at field-free conditions. This electric-field-induced phase transformation is termed electrocrystallization.

The study, performed by scientists at the Georgia Institute of Technology, appears online and is scheduled as a feature and cover article in the 42nd issue of Volume 115 of the Journal of Physical Chemistry C.

"We show that with a strong electric field, you can induce a phase transition without altering the thermodynamic parameters," said Uzi Landman, Regents' and Institute Professor in the School of Physics, F.E. Callaway Chair and director of the Center for Computational Materials Science (CCMS) at Georgia Tech.

In these simulations, Landman and Senior Research Scientists David Luedtke and Jianping Gao at the CCMS set out first to explore a phenomenon described by Sir Geoffrey Ingram Taylor in 1964 in the course of his study of the effect of lightning on raindrops, expressed as changes in the shape of liquid drops when passing through an electric field. While liquid drops under field-free conditions are spherical, they alter their shape in response to an applied electric field to become needle-like liquid drops. Instead of the water droplets used in the almost decade-old laboratory experiments of Taylor, the Georgia Tech researchers focused their theoretical study on a 10 nanometer (nm) diameter liquid droplet of formamide, which is a material made of small polar molecules each characterized by a dipole moment that is more than twice as large as that of a water molecule.

With the use of molecular dynamics simulations developed at the CCMS, which allow scientists to track the evolution of materials systems with ultra-high resolution in space and time, the physicists explored the response of the formamide nano-droplet to an applied electric field of variable strength. Influenced by a field of less than 0.5V/nm, the spherical droplet elongated only slightly. However, when the strength of the field was raised to a critical value close to 0.5 V/nm, the simulated droplet was found to undergo a shape transition resulting in a needle-like liquid droplet with its long axis -- oriented along the direction of the applied field -- measuring about 12 times larger than the perpendicular (cross-sectional) small axis of the needle-like droplet. The value of the critical field found in the simulations agrees well with the prediction obtained almost half a decade ago by Taylor from general macroscopic considerations.

Past the shape transition further increase of the applied electric field yielded a slow, gradual increase of the aspect ratio between the long and short axes of the needle-like droplet, with the formamide molecules exhibiting liquid diffusional motions.

"Here came the Eureka moment," said Landman. "When the field strength in the simulations was ramped up even further, reaching a value close to 1.5V/nm, the liquid needle underwent a solidification phase transition, exhibited by freezing of the diffusional motion, and culminating in the formation of a formamide single crystal characterized by a structure that differs from that of the x-ray crystallographic one determined years ago under zero-field conditions. Now, who ordered that?" he added.

Further analysis has shown that the crystallization transition involved arrangement of the molecules into a particular spatial ordered lattice, which optimizes the interactions between the positive and negative ends of the dipoles of neighboring molecules, resulting in minimization of the free energy of the resulting rigid crystalline needle. When the electric field applied to the droplet was subsequently decreased, the crystalline needle remelted and at zero-field the liquid droplet reverted to a spherical shape. The field reversal process was found to exhibit a hysteresis.

Analysis of the microscopic structural changes that underlie the response of the droplet to the applied field revealed that accompanying the shape transition at 0.5 V/nm is a sharp increase in the degree of reorientation of the molecular electric dipoles, which after the transition lie preferentially along the direction of the applied electric field and coincide with the long axis of the needle-­­like liquid droplet. The directional dipole reorientation, which is essentially complete subsequent to the higher field electrocrystallization transition, breaks the symmetry and transforms the droplet into a field-induced ferroelectric state where it possesses a large net electric dipole, in contrast to its unpolarized state at zero-field conditions.

Along with the large-scale atomistic computer simulations, researchers formulated and evaluated an analytical free-energy model, which describes the balance between the polarization, interfacial tension and dielectric saturation contributions. This model was shown to yield results in agreement with the computer simulation experiments, thus providing a theoretical framework for understanding the response of dielectric droplets to applied fields.

"This investigation unveiled fascinating properties of a large group of materials under the influence of applied fields," Landman said. "Here the field-induced shape and crystallization transitions occurred because formamide, like water and many other materials, is characterized by a relatively large electric dipole moment. The study demonstrated the ability to employ external fields to direct and control the shape, the aggregation phase (that is, solid or liquid) and the properties of certain materials."

Along with the fundamental interest in understanding the microscopic origins of materials behavior, this may lead to development of applications of field-induced materials control in diverse areas, ranging from targeted drug delivery, nanoencapsulation, printing of nanostructures and surface patterning, to aerosol science, electrospray propulsion and environmental science.

This research was supported by a grant from the U.S. Air Force Office of Scientific Research.

Story Source:

The above story is reprinted (with editorial adaptations ) from materials provided by Georgia Institute of Technology.

Journal Reference:

William D. Luedtke, Jianping Gao, Uzi Landman. Dielectric Nanodroplets: Structure, Stability, Thermodynamics, Shape Transitions and Electrocrystallization in Applied Electric Fields. The Journal of Physical Chemistry C, 2011; 110901150249036 DOI: 10.1021/jp206673j

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

Watching motion of electrons in molecules during chemical reactions

 A research group led by ETH Zurich has now, for the first time, visualized the motion of electrons during a chemical reaction. The new findings in the experiment are of fundamental importance for photochemistry and could also assist the design of more efficient solar cells.

In 1999, Ahmed Zewail was awarded the nobel prize in chemistry for his studies of chemical reactions using ultrashort laser pulses. Zewail was able to watch the motion of atoms and thus visualize transition states on the molecular level. Watching the dynamics of single electrons was still considered a dream at that time. Thanks to the latest developments in laser technology and intense research in the field of attosecond spectroscopy (1 attosecond = 10-18 s) the research has developed fast. For the first time, Prof. Hans Jakob Wörner from the Laboratory of Physical Chemistry at ETH Zurich, together with colleagues from Canada and France, was able to record electronic motion during a complete chemical reaction. The experiment is described in the latest issue of Science.

The research team irradiated nitrogen dioxide molecules (NO2) with a very short ultraviolet pulse. Subsequently, the molecule takes up the energy from the pulse which sets the electrons in motion. The electrons start rearranging themselves, which causes the electron cloud to oscillate between two different shapes for a very short time, before the molecule starts to vibrate and eventually decomposes into nitric oxide and an oxygen atom.

Conical intersections

Nitrogen dioxide has model character with respect to understanding electronic motion. In the NO2 molecule, two states of the electrons can have the same energy for a particular geometry -- commonly described as conical intersection. The conical intersection is very important for photochemistry and frequently occurs in natural chemical processes induced by light. The conical intersection works like a dip-switch. For example, if the retina of a human eye is irradiated by light, the electrons start moving, and the molecules of the retina (retinal) change their shape, which finally converts the information of light to electrical information for the human brain. The special aspect about conical intersections is that the motion of electrons is transferred to a motion of the atoms very efficiently.

Snapshot of an electron

In an earlier article, Hans Jakob Wörner has already published how attosecond spectroscopy can be used for watching the motion of electrons. The first weak ultraviolet pulse sets the electrons in motion. The second strong infrared pulse then removes an electron from the molecule, accelerates it and drives it back to the molecule. As a result, an attosecond light pulse is emitted, which carries a snapshot of the electron distribution in the molecule. Wörner illustrates the principle of attosecond spectroscopy: "The experiment can be compared to photographs, which, for example, image a bullet shot through an apple. The bullet would be too fast for the shutter of a camera, resulting in a blurred image. Therefore, the shutter is left open and the picture is illuminated with light flashes, which are faster than the bullet. That's how we get our snap-shot."

From the experiment to solar cells

When the electron returns to the molecule, it releases energy in the form of light. In the experiment, Wörner and his colleagues measured the light of the electrons and were therefore able to deduce detailed information on the electron distribution and its evolution with time. This information reveals details of chemical reaction mechanisms that were not accessible to most of previous experimental techniques. The experiment on NO2 helps understanding fundamental processes in molecules and is an ideal extension of computer simulations of photochemical processes: "What makes our experiment so important is that it verifies theoretical models," says Wörner. The immense interest in photochemical processes is not surprising, as this area of research aims at improving solar cells and making artificial photosynthesis possible.

The above story is reprinted (with editorial adaptations ) from materials provided by ETH Zürich.

Journal Reference:

H. J. Worner, J. B. Bertrand, B. Fabre, J. Higuet, H. Ruf, A. Dubrouil, S. Patchkovskii, M. Spanner, Y. Mairesse, V. Blanchet, E. Mevel, E. Constant, P. B. Corkum, D. M. Villeneuve. Conical Intersection Dynamics in NO2 Probed by Homodyne High-Harmonic Spectroscopy. Science, 2011; 334 (6053): 208 DOI: 10.1126/science.1208664

Scientists demonstrate the power of optical forces in blood cell identification

U.S. Naval Research Laboratory researchers Dr. Sean J. Hart, Dr. Colin G. Hebert and Mr. Alex Terray have developed a laser-based analysis method that can detect optical pressure differences between populations or classes of blood cells that does not rely on prior knowledge, antibodies, or fluorescent labels for discrimination.

"Biological analysis systems that rely on labels can be costly, labor intensive and depend upon prior knowledge of the target in question," says Dr. Hart, NRL Chemistry Division. "Using whole blood, which is composed of a variety of cell types, we have demonstrated the power of optical forces to separate different blood components."

When a laser beam impinges on a biological particle, a force is generated due to the scattering and refraction of photons. The resulting force is called optical pressure and can be used to physically move a biological cell, suspended in water, several millimeters.

Using this , scientists are able to exploit the inherent differences in optical pressure, which arise from variations in particle size, shape, refractive index, or morphology, as a means of separating and characterizing particles.

As an initial step toward developing a system for label-free sorting and characterization of blood components, the optical pressures of purified human components, including lymphocytes, monocytes, granulocytes, and erythrocytes, have been determined. Significant differences exist between the cell types, indicating the potential for separations based on these 'optical pressures.'

"While additional research is required, this is an important step toward the development of a system for the label-free optical fractionation of and components based on intrinsic characteristics," adds Dr. Hart.

In general, the throughput for optical-based sorting has been relatively low, on the order of tens of cells per second. However, with an increase in both fluid flow and , the throughput could be increased significantly, exceeding 100 particles per second in some favorable cases.

Such a system could be used in the future for antibody-free detection of blood-borne pathogens for the prevention of sepsis and other diseases as well as the detection of biological threat agents.

Provided by Naval Research Laboratory (news : web)