(Press-News.org) Tightening or relaxing the tension on a drumhead will change the way the drum sounds. The same goes for drumheads made from graphene, only instead of changing the sound, stretching graphene has a profound effect on the material's electrical properties. Researchers working at the National Institute of Standards and Technology (NIST) and the University of Maryland have shown that subjecting graphene to mechanical strain can mimic the effects of magnetic fields and create a quantum dot, an exotic type of semiconductor with a wide range of potential uses in electronic devices.
The results were reported in the June 22, 2012, issue of Science.*
Graphene is a single layer of carbon atoms arranged in a honeycomb lattice. Able to conduct electricity with little resistance at room temperature, graphene is a prime candidate for applications ranging from flexible displays to high speed transistors.
However, the same lack of electrical resistance that makes graphene attractive for some uses also makes it ill-suited for digital computing applications. Graphene conducts electricity so well because it doesn't have a band gap—an energetic threshold beneath which the material won't conduct electricity. This means that graphene can't be turned "off," and computers need "on" and "off" signals to transmit and process information.
Because substrates slow the speed of electrons moving through graphene, Nikolai Klimov, a University of Maryland postdoctoral researcher working at NIST, suspended the graphene over shallow holes in a substrate of silicon dioxide—essentially making a set of graphene drumheads. To measure the graphene's properties, the team used a unique scanning probe microscope designed and built at NIST.
When they began to probe the drumheads, they found that the graphene rose up to meet the tip of the microscope— a result of the van der Waals force, a weak electrical force that creates attraction between objects that are very close to each other.
"While our instrument was telling us that the graphene was shaped like a bubble clamped at the edges, the simulations run by our colleagues at the University of Maryland showed that we were only detecting the graphene's highest point," says NIST scientist Nikolai Zhitenev. "Their calculations showed that the shape was actually more like the shape you would get if you poke into the surface of an inflated balloon, like a teepee or circus tent."
The researchers discovered that they could tune the strain in the drumhead using the conducting plate upon which the graphene and substrate were mounted to create a countervailing attraction and pull the drumhead down. In this way, they could pull the graphene into or out of the hole below it.
And their measurements showed that changing the degree of strain changed the material's electrical properties.
For instance, the group observed that when they pulled the graphene membrane into the tent-like shape, the region at the apex acted just like a quantum dot, a type of semiconductor in which electrons are confined to a small region of space.
Creating semiconducting regions like quantum dots in graphene by modifying its shape might give scientists the best of both worlds: high speed and the band gap crucial to computing and other applications.
According to Zhitenev, the electrons flow through graphene by following the segments of the hexagons. Stretching the hexagons lowers the energy near the apex of the tent-like shape and causes the electrons to move in closed, clover-shaped orbits—mimicking nearly exactly how the electrons would move in a vertically varied magnetic field.
"This behavior is really quite remarkable," says Zhitenev. "There is a little bit of electron leakage, but we found that if we complemented the pseudomagnetic field with an actual magnetic field, there was no leakage whatsoever."
"Normally, to make a graphene quantum dot, you would have to cut out a nanosize piece of graphene," says NIST Fellow Joseph Stroscio. "Our work shows that you can achieve the same thing with strain-induced pseudomagnetic fields. It's a great result, and a significant step toward developing future graphene-based devices."
### The work was a collaborative effort with the University of Maryland, College Park, and the Korea Research Institute of Standards and Science.
*N. Klimov, S. Jung, T. Li, C. Wright, S. Solares, D. Newell, N. Zhitenev, and J. Stroscio. Electromechanical Properties of Graphene Drumheads. Science. June 22, 2012.
Researchers tune the strain in graphene drumheads to create quantum dots
2012-06-22
ELSE PRESS RELEASES FROM THIS DATE:
Genomics and African queens
2012-06-22
Researchers have started to unveil the genetic heritage of Ethiopian populations, who are among the most diverse in the world, and lie at the gateway from Africa. They found that the genomes of some Ethiopian populations bear striking similarities to those of populations in Israel and Syria, a potential genetic legacy of the Queen of Sheba and her companions.
The team detected mixing between some Ethiopians and non-African populations dating to approximately 3,000 years ago. The origin and date of this genomic admixture, along with previous linguistic studies, is consistent ...
Enzyme offers new therapeutic target for cancer drugs
2012-06-22
Researchers at the University of California, San Diego School of Medicine have uncovered a new signal transduction pathway specifically devoted to the regulation of alternative RNA splicing, a process that allows a single gene to produce or code multiple types of protein variants. The discovery, published in the June 27, 2012 issue of Molecular Cell, suggests the new pathway might be a fruitful target for new cancer drugs.
Signal transduction in the cell involves kinases and phosphatases, enzymes that transfer or remove phosphates in protein molecules in a cascade or ...
McGill researchers discover the cause of an inherited form of epilepsy
2012-06-22
Researchers at McGill University have discovered the cause of an inherited form of epilepsy. The disease, known as double-cortex syndrome, primarily affects females and arises from mutations on a gene located on the X chromosome. Drs. Susanne Bechstedt and Gary Brouhard of the Department of Biology have used a highly advanced microscope to discover how these mutations cause a malformation of the human brain. The results of their study are published in the journal Developmental Cell.
When the brain develops in the uterus, new brain cells are born deep within the brain, ...
Climate drilling in the Arctic Circle
2012-06-22
During the past 2.8 million years extreme warm periods occurred in the Arctic at irregular intervals. Analytical results from the longest sediment core that has ever been drilled in the terrestrial areas of the Arctic have shown temperatures that were previously considered impossible for the Arctic Circle. In addition, a notable correlation of the warm periods in the Arctic with large melting events in Antarctica points to previously unknown interactions between the Polar Regions. These are the findings of an international research team led by Professor Martin Melles of ...
New deglaciation data opens door for earlier First Americans migration
2012-06-22
CORVALLIS, Ore. - A new study of lake sediment cores from Sanak Island in the western Gulf of Alaska suggests that deglaciation there from the last Ice Age took place as much as1,500 to 2,000 years earlier than previously thought, opening the door for earlier coastal migration models for the Americas.
The Sanak Island Biocomplexity Project, funded by the National Science Foundation, also concluded that the maximum thickness of the ice sheet in the Sanak Island region during the last glacial maximum was 70 meters – or about half that previously projected – suggesting ...
Stanford-led study explains how stress can boost immune system
2012-06-22
STANFORD, Calif. — A study spearheaded by a Stanford University School of Medicine scientist has tracked the trajectories of key immune cells in response to short-term stress and traced, in great detail, how hormones triggered by such stress enhance immune readiness. The study, conducted in rats, adds weight to evidence that immune responsiveness is heightened, rather than suppressed as many believe, by the so-called "fight-or-flight" response.
The study's findings provide a thorough overview of how a triad of stress hormones affects the main cell subpopulations of the ...
UCLA study uncovers new tools for targeting genes linked to autism
2012-06-22
UCLA researchers have combined two tools – gene expression and the use of peripheral blood -- to expand scientists' arsenal of methods for pinpointing genes that play a role in autism. Published in the June 21 online edition of the American Journal of Human Genetics, the findings could help scientists zero in on genes that offer future therapeutic targets for the disorder.
"Technological advances now allow us to rapidly sequence the genome and uncover dozens of rare mutations," explained principal investigator Dr. Daniel Geschwind, the Gordon and Virginia MacDonald Distinguished ...
Top predators key to extinctions as planet warms
2012-06-22
New Haven, Conn.—Global warming may cause more extinctions than predicted if scientists fail to account for interactions among species in their models, Yale and UConn researchers argue in Science.
"Currently, most models predicting the effects of climate change treat species separately and focus only on climatic and environmental drivers," said Phoebe Zarnetske, the study's primary author and a postdoctoral fellow at the Yale School of Forestry & Environmental Studies. "But we know that species don't exist in a vacuum. They interact with each other in ways that deeply ...
University of Exeter research uncovers rice blast infection mechanism
2012-06-22
Scientists at the University of Exeter have made a new discovery that they hope might lead to effective control of rice blast disease. Rice blast is the most serious disease of cultivated rice and affects all the rice-growing regions of the world, causing losses of up to 30% of the global rice harvest.
Yasin Dagdas and colleagues studied the rice blast fungus, which develops a pressurised infection cell, called an appressorium to rupture the rice leaf cuticle. The appressorium generates extreme pressure, estimated to be 40 times that of a car tyre. Dagdas and colleagues, ...
Telehealth can reduce deaths and emergency hospital care, but estimated cost savings are modest
2012-06-22
Research: Effect of telehealth on use of secondary care and mortality: findings from the Whole System Demonstrator cluster randomised trial
Editorial: Telehealth for long term conditions
For people with long term conditions, telehealth can reduce deaths and help patients avoid the need for emergency hospital care, finds a study published on bmj.com today.
However, the estimated scale of hospital cost savings is modest and may not be sufficient to offset the cost of the technology, say the authors.
Telehealth uses technology to help people with health problems live ...