'Nanowiggles:' Scientists discover graphene nanomaterials with tunable functionality in electronics

ScienceDaily (Jan. 4, 2012) — Electronics are getting smaller and smaller, flirting with new devices at the atomic scale. However, many scientists predict that the shrinking of our technology is reaching an end. Without an alternative to silicon-based technologies, the miniaturization of our electronics will stop. One promising alternative is graphene -- the thinnest material known to man. Pure graphene is not a semiconductor, but it can be altered to display exceptional electrical behavior. Finding the best graphene-based nanomaterials could usher in a new era of nanoelectronics, optics, and spintronics (an emerging technology that uses the spin of electrons to store and process information in exceptionally small electronics).

Scientists at Rensselaer Polytechnic Institute have used the capabilities of one of the world's most powerful university-based supercomputers, the Rensselaer Center for Nanotechnology Innovations (CCNI), to uncover the properties of a promising form of graphene, known as graphene nanowiggles. What they found was that graphitic nanoribbons can be segmented into several different surface structures called nanowiggles. Each of these structures produces highly different magnetic and conductive properties. The findings provide a blueprint that scientists can use to literally pick and choose a graphene nanostructure that is tuned and customized for a different task or device. The work provides an important base of knowledge on these highly useful nanomaterials.

The findings were published in the journal Physical Review Letters in a paper titled "Emergence of Atypical Properties in Assembled Graphene Nanoribbons."

"Graphene nanomaterials have plenty of nice properties, but to date it has been very difficult to build defect-free graphene nanostructures. So these hard-to-reproduce nanostructures created a near insurmountable barrier between innovation and the market," said Vincent Meunier, the Gail and Jeffrey L. Kodosky '70 Constellation Professor of Physics, Information Technology, and Entrepreneurship at Rensselaer. "The advantage of graphene nanowiggles is that they can easily and quickly be produced very long and clean."

Nanowiggles were only recently discovered by a group led by scientists at EMPA, Switzerland. These particular nanoribbons are formed using a bottom-up approach, since they are chemically assembled atom by atom. This represents a very different approach to the standard graphene material design process that takes an existing material and attempts to cut it into a new structure. The process often creates a material that is not perfectly straight, but has small zigzags on its edges.

Meunier and his research team saw the potential of this new material. The nanowiggles could be easily manufactured and modified to display exceptional electrical conductive properties. Meunier and his team immediately set to work to dissect the nanowiggles to better understand possible future applications.

"What we found in our analysis of the nanowiggles' properties was even more surprising than previously thought," Meunier said.

The scientists used computational analysis to study several different nanowiggle structures. The structures are named based on the shape of their edges and include armchair, armchair/zigzag, zigzag, and zigzag/armchair. All of the nanoribbon-edge structures have a wiggly appearance like a caterpillar inching across a leaf. Meunier named the four structures nanowiggles and each wiggle produced exceptionally different properties.

They found that the different nanowiggles produced highly varied band gaps. A band gap determines the levels of electrical conductivity of a solid material. They also found that different nanowiggles exhibited up to five highly varied magnetic properties. With this knowledge, scientists will be able to tune the bandgap and magnetic properties of a nanostructure based on their application, according to Meunier.

Meunier would like the research to inform the design of new and better devices. "We have created a roadmap that can allow for nanomaterials to be easily built and customized for applications from photovoltaics to semiconductors and, importantly, spintronics," he said.

By using CCNI, Meunier was able to complete these sophisticated calculations in a few months.

"Without CCNI, these calculations would still be continuing a year later and we would not yet have made this exciting discovery. Clearly this research is an excellent example illustrating the key role of CCNI in predictive fundamental science," he said.

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The above story is reprinted from materials provided by Rensselaer Polytechnic Institute.

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Journal Reference:

Eduardo Costa Gir?o, Liangbo Liang, Eduardo Cruz-Silva, Antônio Filho, Vincent Meunier. Emergence of Atypical Properties in Assembled Graphene Nanoribbons. Physical Review Letters, 2011; 107 (13) DOI: 10.1103/PhysRevLett.107.135501

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Scientists play ping-pong with single electrons

ScienceDaily (Sep. 22, 2011) — Scientists at Cambridge University have shown an amazing degree of control over the most fundamental aspect of an electronic circuit, how electrons move from one place to another.

Researchers from the University's Cavendish Laboratory have moved an individual electron along a wire, batting it back and forth over sixty times, rather like the ball in a game of ping-pong. The research findings, published September 22 in the journal Nature, may have applications in quantum computing, transferring a quantum 'bit' between processor and memory, for example.

Imagine you are at a party and you want to get to the other side of a crowded room to talk to someone. As you walk you have to weave around people who are walking, dancing or just standing in the way. You may also have to stop and greet friends along the way and by the time you reach the person you wanted to talk to you have forgotten what you were going to say. Wouldn't it be nice to be lifted up above the crowd, and pushed directly to your destination?

In a similar way, electrons carrying a current along a wire do not go directly from one end to the other but instead follow a complicated zigzag path. This is a problem if the electron is carrying information, as it tends to 'forget' it, or, more scientifically, the quantum state loses coherence.

In this work, a single electron can be trapped in a small well (called a quantum dot), just inside the surface of a piece of Gallium Arsenide (GaAs). A channel leads to another, empty, dot 4 microns (millionths of a metre) away. The channel is higher in energy than the surrounding electrons. A very short burst of sound (just a few billionths of a second long) is then sent along the surface, past the dot. The accompanying wave of electrical potential picks up the electron, which then surfs along the channel to the other dot, where it is captured. A burst of sound sent from the other direction returns the electron to the starting dot where the process can be repeated. The electron goes back and forth like a ping-pong ball. Rallies of up to 60 shots have been achieved before anything goes wrong.

"The movement of electrons by our 'surface acoustic wave' can also be likened to peristalsis in the esophagus, where food is propelled from the mouth to the stomach by a wave of muscle contraction," explains Rob McNeil, the PhD student who did most of the work, helped by postdoc Masaya Kataoka, both at the University of Cambridge's Department of Physics, the Cavendish Laboratory.

"This is an enabling technology for quantum computers," Chris Ford, team leader of the research from the Semiconductor Physics Group in the Cavendish, says. "There is a lot of work going on worldwide to make this new type of computer, which may solve certain complex problems much faster than classical computers. However, little effort has yet been put into connecting up different components, such as processor and memory. Although our experiments do not yet show that electrons 'remember' their quantum state, this is likely to be the case. This would make the method of transfer a candidate for moving quantum bits of information (qubits) around a quantum circuit, in a quantum computer. Indeed, our theorist, Crispin Barnes, proposed using this mechanism to make a whole quantum computer a long time ago, and this is an important step towards that goal."

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The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by University of Cambridge. The original story is licensed under a Creative Commons license.

Journal Reference:

R. P. G. McNeil, M. Kataoka, C. J. B. Ford, C. H. W. Barnes, D. Anderson, G. A. C. Jones, I. Farrer, D. A. Ritchie. On-demand single-electron transfer between distant quantum dots. Nature, 2011; 477 (7365): 439 DOI: 10.1038/nature10444

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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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The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by Duke University.

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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