Tuesday, March 27, 2012

Straintronics: Engineers create piezoelectric graphene

 In what became known as the 'Scotch tape technique," researchers first extracted graphene with a piece of adhesive in 2004. Graphene is a single layer of carbon atoms arranged in a honeycomb, hexagonal pattern. It looks like chicken wire.

Graphene is a wonder material. It is one-hundred-times better at conducting electricity than silicon. It is stronger than diamond. And, at just one atom thick, it is so thin as to be essentially a two-dimensional material. Such promising physics have made graphene the most studied substance of the last decade, particularly in nanotechnology. In 2010, the researchers who first isolated it shared the Nobel Prize.

Yet, while graphene is many things, it is not piezoelectric. Piezoelectricity is the property of some materials to produce electric charge when bent, squeezed or twisted. Perhaps more importantly, piezoelectricity is reversible. When an electric field is applied, piezoelectric materials change shape, yielding a remarkable level of engineering control.

Piezoelectrics have found application in countless devices from watches, radios and ultrasound to the push-button starters on propane grills, but these uses all require relatively large, three-dimensional quantities of piezoelectric materials.

Now, in a paper published in the journal ACS Nano, two materials engineers at Stanford have described how they have engineered piezoelectrics into graphene, extending for the first time such fine physical control to the nanoscale.


"The physical deformations we can create are directly proportional to the electrical field applied and this represents a fundamentally new way to control electronics at the nanoscale," said Evan Reed, head of the Materials Computation and Theory Group at Stanford and senior author of the study. "This phenomenon brings new dimension to the concept of 'straintronics' for the way the electrical field strains -- or deforms -- the lattice of carbon, causing it to change shape in predictable ways."

"Piezoelectric graphene could provide an unparalleled degree of electrical, optical or mechanical control for applications ranging from touchscreens to nanoscale transistors," said Mitchell Ong, a post-doctoral scholar in Reed's lab and first author of the paper.

Using a sophisticated modeling application running on high-performance supercomputers, the engineers simulated the deposition of atoms on one side of a graphene lattice -- a process known as doping -- and measured the piezoelectric effect.

They modeled graphene doped with lithium, hydrogen, potassium and fluorine, as well as combinations of hydrogen and fluorine and lithium and fluorine on either side of the lattice. Doping just one side of the graphene, or doping both sides with different atoms, is key to the process as it breaks graphene's perfect physical symmetry, which otherwise cancels the piezoelectric effect.

The results surprised both engineers.

"We thought the piezoelectric effect would be present, but relatively small. Yet, we were able to achieve piezoelectric levels comparable to traditional three-dimensional materials," said Reed. "It was pretty significant."

Designer piezoelectricity

"We were further able to fine tune the effect by pattern doping the graphene -- selectively placing atoms in specific sections and not others," said Ong. "We call it designer piezoelectricity because it allows us to strategically control where, when and how much the graphene is deformed by an applied electrical field with promising implications for engineering."

While the results in creating piezoelectric graphene are encouraging, the researchers believe that their technique might further be used to engineer piezoelectricity in nanotubes and other nanomaterials with applications ranging from electronics, photonics, and energy harvesting to chemical sensing and high-frequency acoustics.

"We're already looking now at new piezoelectric devices based on other 2D and low-dimensional materials hoping they might open new and dramatic possibilities in nanotechnology," said Reed.

The Army High Performance Computing Research Center at Stanford University (http://me.stanford.edu/research/centers/ahpcrc/index.html) and the National Energy Research Scientific Computing Center (NERSC) at the Lawrence Berkeley National Laboratory supported this research.

Listen to Reed and Ong talk about their work with ACS Nano: http://www.stanford.edu/group/evanreed/media/ancac3-0212.mp3

Story Source:

The above story is reprinted from materials provided by Stanford School of Engineering. The original article was written by Andrew Myers, associate director of communications for the Stanford University School of Engineering.

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

Journal Reference:

Mitchell T. Ong, Evan J. Reed. Engineered Piezoelectricity in Graphene. ACS Nano, 2012; 6 (2): 1387 DOI: 10.1021/nn204198g

Novel U. of Colorado 3D super-resolution imaging technology to be developed by Boulder company

The technology was developed by CU-Boulder Professor Rafael Piestun of the electrical and computer engineering department. Super-resolution -- techniques to enhance the resolution of an imaging system beyond the limitations set by the diffraction of light -- is key to the development of next-generation microscopes and other optical instruments. The optical technology combines 3D optics and a unique signal post-processing technique used for quality improvement in image processing.

The offers a major opportunity to provide multifunctional 3D super-resolution imaging capability to thousands of cellular, and biophysics laboratories in the United States and around the world. The Double Helix technology platform is applicable to a variety of scientific, industrial and consumer applications, including microscopy, metrology and computational digital photography, said Piestun.

Piestun also is the director of Computational Optical Sensing and Imaging, a National Science Foundation-funded program for education and research training.

"We are looking forward to bringing this leading-edge technology to the market, initially in microscopy and later to more markets including metrology and digital optics, a stronghold of the Boulder entrepreneurial community," said Double Helix founding partner Leslie Kimerling.

"We are excited to see this company launch with our broad fundamental patents," said Ted Weverka, a licensing manager at the CU Technology Transfer Office. "The cost savings and superior will give Double Helix a strong lead."

Provided by University of Colorado at Boulder (news : web)

New microfluidic chip can generate microbubbles to break open cells for biochemical analysis

Currently there is a wide range of methods to disintegrate or lyse cell membranes and to release the contained within. However, most of these methods can cause denaturation of proteins or interfere with subsequent assaying. Ow and co-workers explored the possibility of using ultrasound in microfluidics to lyse cells. They applied short bursts of ultrasound with periods of rest to prevent the proteins from overheating as a result of dissipation of .

When the rapid changes of pressure generated with ultrasound are applied to a liquid, small bubbles are formed which oscillate in size and generate a cyclic shear stress. These rapidly oscillating bubbles generate a mini shockwave when they implode, which can be strong enough to cause the to rupture. The researchers generated microbubbles in the meandering microfluidic channel by introducing a gas via a separate inlet to generate a gas–liquid interface and subsequently applying ultrasound to the system.

As a proof of principle, the researchers tested the performance of their microfluidic device on genetically engineered bacteria and yeast that express the green fluorescent protein. The researchers found that the bacteria are completely disintegrated after only 0.4 seconds of ultrasound exposure (see image). The concentration of DNA released from yeast cells reached a plateau after only one second exposure (which contained six bursts of ultrasound each of 0.154 seconds), indicating that most cells are successfully lysed. Importantly the temperature of the sample was shown not to rise above 3.3 °C. “The large surface to volume ratio of the environment means that the small amount of heat that is generated rapidly diffuses away,” says Ow.

The researchers have proposed many ideas for applications. “In collaboration with another institute, we are developing a rapid and sensitive label-free optical method for on-chip detection of bioanalytes from lysed cells,” says Ow. “We also want to modify the device to break more difficult-to-lyse endospores, and to develop a rapid on-chip detection device to counter the threats of bioterrorism.”

More information: Research article in Lab on a chip

Provided by Agency for Science, Technology and Research (A*STAR)

Butterfly molecule may aid quest for nuclear clean-up technology

 Scientists have produced a previously unseen uranium molecule, in a move that could improve clean-up of nuclear waste.

The distinctive butterfly-shaped compound is similar to radioactive molecules that scientists had proposed to be key components of nuclear waste.

However, these were thought too unstable to exist for long.

Researchers have shown the compound to be robust, which implies that molecules with a similar structure may be present in radioactive waste.

Better clean-up

University scientists, who carried out the study, say their findings suggest the molecule may play a role in forming clusters of radioactive material in waste.

These are difficult to separate during clean-up.

Improving treatment processes for nuclear waste, including targeting this type of molecule, could help the nuclear industry move towards cleaner power generation.

Ideally, all the radioactive materials from spent fuel can be recovered and made safe or used again.

This would reduce the amount of waste and curb risks to the environment.

Distinctive shape

The Edinburgh team worked in collaboration with scientists in the United States and Canada to verify the structure of the uranium compound.

They made the molecule by reacting a common uranium compound with a nitrogen and carbon-based material.

Scientists used chemical and mathematical analyses to confirm the structure of the molecule's distinctive butterfly shape.

The study, funded by the Engineering and Physical Sciences Research Council, the EaStCHEM partnership and the University of Edinburgh, was published in Nature Chemistry.

"We have made a molecule that, in theory, should not exist, because its bridge-shaped structure suggests it would quickly react with other chemicals. This discovery that this particular form of uranium is so stable could help optimise processes to recycle valuable radioactive materials and so help manage the UK's nuclear legacy," said Professor Polly Arnold of the School of Chemistry.

Story Source:

The above story is reprinted from materials provided by University of Edinburgh.

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

Journal Reference:

Polly L. Arnold, Guy M. Jones, Samuel O. Odoh, Georg Schreckenbach, Nicola Magnani, Jason B. Love. Strongly coupled binuclear uranium–oxo complexes from uranyl oxo rearrangement and reductive silylation. Nature Chemistry, 2012; 4 (3): 221 DOI: 10.1038/nchem.1270