Graphene quantized-electron bubble discovery at UC Berkeley to benefit electronic devices

(July 29, 2010) – Graphene, a sheet of pure carbon and a possible replacement for silicon-based semiconductors, has been found to have a unique property that could make it even more suitable for future electronic devices. When subjected to a 3-point stretch, graphene sprouts nanobubbles with electrons moving as if subjected to a strong magnetic field. The discovery was made by physicists at the University of California, Berkeley, and the Lawrence Berkeley National Laboratory (LBNL).

Figure. A scanning tunneling microscope image of a single layer of graphene on platinum with four nanobubbles at the graphene-platinum border and one in the patch interior. The inset shows a high-resolution image of a graphene nanobubble and its distorted honeycomb lattice due to strain in the bubble. (Source: Crommie lab, UC Berkeley)

Specifically, the electrons within each nanobubble segregate into quantized energy levels instead of occupying energy bands, as in unstrained graphene. The energy levels are identical to those that an electron would occupy if it were moving in circles in a very strong magnetic field, as high as 300 tesla, which is bigger than any laboratory can produce except in brief explosions, said Michael Crommie, professor of physics at UC Berkeley and a faculty researcher at LBNL. Magnetic resonance imagers use magnets less than 10 tesla, while the Earth’s magnetic field at ground level is 31 microtesla.

"This gives us a new handle on how to control how electrons move in graphene, and thus to control graphene’s electronic properties, through strain," Crommie said. "By controlling where the electrons bunch up and at what energy, you could cause them to move more easily or less easily through graphene, in effect, controlling their conductivity, optical or microwave properties. Control of electron movement is the most essential part of any electronic device."

Crommie and colleagues report the discovery in the July 30 issue of the journal Science

Aside from the engineering implications of the discovery, Crommie is eager to use this unusual property of graphene to explore how electrons behave in fields that until now have been unobtainable in the laboratory.

"When you crank up a magnetic field you start seeing very interesting behavior because the electrons spin in tiny circles," he said. "This effect gives us a new way to induce this behavior, even in the absence of an actual magnetic field."

Among the unusual behaviors observed of electrons in strong magnetic fields are the quantum Hall effect and the fractional quantum Hall effect, where at low temperatures electrons also fall into quantized energy levels.

The new effect was discovered by accident when a UC Berkeley postdoctoral researcher and several students in Crommie’s lab grew graphene on the surface of a platinum crystal. Graphene is a one atom-thick sheet of carbon atoms arranged in a hexagonal pattern, like chicken wire. When grown on platinum, the carbon atoms do not perfectly line up with the metal surface’s triangular crystal structure, which creates a strain pattern in the graphene as if it were being pulled from three different directions.

The strain produces small, raised triangular graphene bubbles 4 to 10 nanometers across in which the electrons occupy discrete energy levels rather than the broad, continuous range of energies allowed by the band structure of unstrained graphene. This new electronic behavior was detected spectroscopically by scanning tunneling microscopy. These so-called Landau levels are reminiscent of the quantized energy levels of electrons in the simple Bohr model of the atom, Crommie said.

The appearance of a pseudomagnetic field in response to strain in graphene was first predicted for carbon nanotubes in 1997 by Charles Kane and Eugene Mele of the University of Pennsylvania. Nanotubes are a rolled up form of graphene. Within the last year, however, Francisco Guinea of the Instituto de Ciencia de Materiales de Madrid in Spain, Mikhael Katsnelson of Radboud University of Nijmegen, the Netherlands, and A. K. Geim of the University of Manchester, England predicted what they termed a pseudo quantum Hall effect in strained graphene. This is the very quantization that Crommie’s research group has experimentally observed. Boston University physicist Antonio Castro Neto, who was visiting Crommie’s laboratory at the time of the discovery, immediately recognized the implications of the data, and subsequent experiments confirmed that it reflected the pseudo quantum Hall effect predicted earlier.

"Theorists often latch onto an idea and explore it theoretically even before the experiments are done, and sometimes they come up with predictions that seem a little crazy at first. What is so exciting now is that we have data that shows these ideas are not so crazy," Crommie said. "The observation of these giant pseudomagnetic fields opens the door to room-temperature ‘straintronics,’ the idea of using mechanical deformations in graphene to engineer its behavior for different electronic device applications."
Crommie noted that the "pseudomagnetic fields" inside the nanobubbles are so high that the energy levels are separated by hundreds of millivolts, much higher than room temperature. Thus, thermal noise would not interfere with this effect in graphene even at room temperature. The nanobubble experiments performed in Crommie’s laboratory, however, were performed at very low temperature.

Normally, electrons moving in a magnetic field circle around the field lines. Within the strained nanobubbles, the electrons move in circles in the plane of the graphene sheet, as if a strong magnetic field has been applied perpendicular to the sheet even when there is no actual magnetic field. Apparently, Crommie said, the pseudomagnetic field only affects moving electrons and not other properties of the electron, such as spin, that are affected by real magnetic fields.

Other authors of the report, in addition to Crommie, Castro Neto and Guinea, are Sarah Burke, now a professor at the University of British Columbia; Niv Levy, now a postdoctoral researcher at the National Institute of Technology and Standards; and graduate student Kacey L. Meaker, undergraduate Melissa Panlasigui and physics professor Alex Zettl of UC Berkeley. The research was funded through the U.S. Department of Energy Office of Science and the U.S. Office of Naval Research.

Follow Small Times on Twitter.com by clicking www.twitter.com/smalltimes. Or join our Facebook group

Read more articles on graphene:

Bulk graphene oxide without the toxic gases

A new paper from the lab of Rice University chemist James Tour demonstrates an environmentally friendly way to make bulk quantities of graphene oxide (GO), an insulating version of single-atom-thick graphene expected to find use in all kinds of material and electronic applications.

Scooping quantum dots out of graphane

Carving up graphane sheets creates spaces of pure graphene with properties of semiconducting quantum dots, results that point to possible future work in nanoelectronics, say researchers at Rice U.

Clearing a fabrication hurdle with graphene

Researchers from Lawrence Berkeley National Labs say they have taken a big step forward in addressing one of the major challenges in graphene: figuring out an economical, high-quality and production-worthy way of making it.

IBM resets bar for fastest graphene transistor

Researchers at IBM say they have once again set the mark for graphene-based transistors, setting a new mark of 100GHz cutoff frequency (100 billion cycles/sec), the highest achieved so far for any graphene device, and using process technology compatible with advanced silicon device fabrication.

Cornell touts "mess-free" graphene growth

Researchers at Cornell say they’ve come up with a way to make graphene devices more simply, by growing the material directly onto a silicon wafer.

More…

POST A COMMENT

Easily post a comment below using your Linkedin, Twitter, Google or Facebook account. Comments won't automatically be posted to your social media accounts unless you select to share.

Leave a Reply

Your email address will not be published. Required fields are marked *

You may use these HTML tags and attributes: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <strike> <strong>

NEW PRODUCTS

Dynaloy unveils safer cleaners
11/19/2014In response to evolving industry trends and customer preferences for products with better environmental, health, and safety (EHS) profiles, Dynaloy LLC is launching three...
Entegris' VaporSorb filter line protects advanced yield production
10/21/2014Entegris, Inc. today announced a new product for its VaporSorb line of airborne molecular contamination (AMC) filters. ...
Next-generation nanoimprint lithography technology
10/21/2014EV Group (EVG) today introduced its SmartNIL large-area nanoimprint lithography (NIL) process....