Voltage increases up to 25 percent observed in closely packed nanowires

Unexpected voltage increases of up to 25 percent in two barely separated nanowires have been observed at Sandia National Laboratories.

Designers of next-generation devices using nanowires to deliver electric currents -- including telephones, handheld computers, batteries and certain solar arrays -- may need to make allowances for such surprise boosts.

"People have been working on nanowires for 20 years," says Sandia lead researcher Mike Lilly. "At first, you study such wires individually or all together, but eventually you want a systematic way of studying the integration of nanowires into nanocircuitry. That's what's happening now. It's important to know how nanowires interact with each other rather than with regular wires."

The work was reported in the December 2011 issue of Nature Nanotechnology.

Though the gallium-arsenide nanowire structures used by Lilly's team are fragile, nanowires in general have very practical characteristics -- they may crack less than their bigger cousins, they're cheaper to produce and they offer better electronic control.

For years, the best available test method required researchers to put a charged piece of material called a gate between two nanowires on a single shelf. The gate, flooded with electrons, acted as a barrier: It maintained the integrity, in effect, of the wires on either side of it by repelling any electrons attempting to escape across it. But the smallest wire separation allowed by the gate was 80 nanometers. Nanowires in future devices will be packed together much more closely, so a much smaller gap was necessary for testing.

The current test design has the brilliance of simplicity. What Lilly and co-workers at McGill University in Montreal envisioned was to put the nanowires one above the other, rather than side by side, by separating them with a few atomic layers of extremely pure, home-grown crystal. This allowed them to test nanowires separated vertically by only 15 nanometers -- about the distance next-generation devices are expected to require. And because each wire sits on its own independent platform, each can be independently fed and controlled by electrical inputs varied by the researchers.

While applications for technical devices interest Lilly, it's the characteristics of nanowires as a problem in one-dimensional (1-D) basic science that fascinates him.

A 1-D wire is not your common, thick-waisted, 3-D household wire, which allows current to move horizontally, vertically, and forward; nor is it your smaller, flattened micron-sized 2-D wires in typical electronic devices that allow electrons to move forward and across but not up and down. In 1-D wires, the electrons can only move in one direction: forward, like prisoners coming to lunch, one behind the other.

"In the long run, our test device will allow us to probe how 1-D conductors are different from 2-D and 3-D conductors," Lilly said. "They are expected to be very different, but there are relatively few experimental techniques that have been used to study the 1-D ground state."

One reason for the difference is the Coulomb force, responsible for what is termed the Coulomb "drag" effect, regardless of whether the force hastens or retards currents. Operating between wires, the force is inversely proportional to the square of the distance; that is, in ordinary microelectronics, the force is practically unnoticeable, but at nanodistances, the force is large enough that electrons in one wire can "feel" the individual electrons moving in another placed nearby.

The drag means that the first wire needs more energy because the Coulomb force creates, in effect, increased resistance. "The amount is very small," said Lilly, "and we can't measure it. What we can measure is the voltage of the other wire."

There are no straightforward answers as to why the Coulomb force creates negative or positive drag, but it does. It was named for 18th century scientist Charles August Coulomb.

What's known is that "enough electrons get knocked along that they provide positive source at one wire end, negative at the other," Lilly said. A voltage builds up in the opposite direction to keep electrons in place," thus increasing drag.

The so-called Fermi sea -- a 3-D concept used to predict the average energy of electrons in metal -- should totally break down in 1-D wires, which instead should form a Luttinger liquid, Lilly said. A Luttinger liquid is a theoretical model that describes the interactions of electrons in a 1-D conductor. To better understand the Luttinger liquid is Lilly's underlying motive for the experiment. (Enrico Fermi was a leading theoretical physicist of the 20th century who played an important role in the development of the atomic bomb. Joaquin Luttinger was a 20th century physicist known for his theories of how electrons interact in one-dimensional metals.)

Having an interest on many levels proved useful because making the test device "took us a very long time," he said. "It's not impossible to do in other labs, but Sandia has crystal-growing capabilities, a microfabrication facility and support for fundamental research from DOE's [the Department of Energy's] Office of Basic Energy Sciences (BES). The BES core program is interested in new science and new discoveries, like the work we're doing in trying to understand what is going on when you're working with very small systems."

Device fabrication was conducted under a user project at the Center for Integrated Nanotechnologies, a DOE Office of Science national user facility jointly run by Sandia and Los Alamos national laboratories. The device design and measurement were completed under the DOE Office of Science BES/Division of Materials Science and Engineering research program.

The work required the crystal-growing expertise of Sandia researcher John Reno, the fabrication and measurement skills of McGill doctoral student Dominique Laroche and elements of previous work by Sandia researcher Jerry Simmons.

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The above story is reprinted from materials provided by DOE/Sandia National Laboratories.

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Journal Reference:

D. Laroche, G. Gervais, M. P. Lilly, J. L. Reno. Positive and negative Coulomb drag in vertically integrated one-dimensional quantum wires. Nature Nanotechnology, 2011; 6 (12): 793 DOI: 10.1038/NNANO.2011.182

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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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Scientists observe how superconducting nanowires lose resistance-free state

ScienceDaily (Sep. 22, 2011) — Even with today's invisibility cloaks, people can't walk through walls. But, when paired together, millions of electrons can. The electrons perform this trick, called macroscopic quantum tunneling, when they pair up and move into a region of space that is normally off-limits under the laws of classical mechanics. The problem is that as millions of electrons collectively move through a superconducting nanowire, they use energy and give off heat.

The heat can build, transforming sections of the wire into a non-superconducting state. The process, called a phase slip, adds resistance to an electrical system and has implications for designing new nano-scale superconductors.

Now, scientists have observed individual phase slips in aluminum nanowires and characterized the nature and temperature at which they occur. This information could help scientists remove phase slips from nano-scale systems, which could lead to more reliable nanowires and more efficient nano-electronics, said Duke physicist Albert Chang.

The results appeared online Sept. 21 in Physical Review Letters.

The macroscopic quantum tunneling effect was first observed in a system called a Josephson junction. This device has a thin insulating layer connecting two superconductors, which are several nanometers wide and have a three-dimensional shape.

To study the tunneling and phase slips in a simpler system, however, Chang and his colleagues used individual, one-dimensional nanowires made of aluminum. The new observations are "arguably the first convincing demonstration of tunneling of millions of electrons in one-dimensional superconducting nanowires," said Chang, who led the study.

In the experiment, the wires ranged in length from 1.5 to 10 micrometers, with widths from five to 10 nanometers. Chang cooled the wires to a temperature close to absolute zero, roughly 1 degree Kelvin or -458 degrees Fahrenheit.

At this temperature, a metal's crystal lattice vibrates in a way that allows electrons to overcome their negative repulsion of one other. The electrons make pairs and electric current flows essentially resistance-free, forming a superconductor.

The electron pairs move together in a path in a quantum-mechanical space, which resembles the curled cord of an old phone. On their way around the path, all of the electrons have to scale a barrier or a wall. Moving past this wall collectively keeps the electrons paired and the superconducting current stable.

But, the collective effort takes energy and gives off heat. With successive scaling attempts, the heat builds, causing a section of the wire to experience a phase slip from a superconducting to a non-superconducting state.

To pinpoint precisely how phase slips happen, Chang varied the temperatures and amount of current run through the aluminum nanowires.

The experiments show that at higher temperatures, roughly 1.5 degrees Kelvin and close to the critical temperature where the wires naturally become non-superconducting, the electrons have enough energy to move over the wall that keeps the electrons paired and the superconducting current stable.

In contrast, the electrons in the nanowires cooled to less than 1 degree Kelvin do not have the energy to scale the wall. Instead, the electrons tunnel, or go through the wall together, all at once, said Duke physicist Gleb Finkelstein, one of Chang's collaborators.

The experiments also show that at the relatively higher temperatures, individual jumps over the wall don't create enough heat to cause a breakdown in superconductivity. But multiple jumps do.

At the lowest temperatures, however, the paired electrons only need to experience one successful attempt at the wall, either over or through it, to create enough heat to slip in phase and break the superconducting state.

Studying the electrons' behavior at specific temperatures provides scientists with information to build ultra-thin superconducting wires that might not have phase slips. Chang said the improved wires could soon play a role in ultra-miniaturized electrical components for ultra-miniaturized electronics, such as the quantum bit, used in a quantum computer.

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Journal Reference:

Peng Li, Phillip Wu, Yuriy Bomze, Ivan Borzenets, Gleb Finkelstein, A. Chang. Switching Currents Limited by Single Phase Slips in One-Dimensional Superconducting Al Nanowires. Physical Review Letters, 2011; 107 (13) DOI: 10.1103/PhysRevLett.107.137004

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