Researchers image graphene electron clouds, revealing how folds can harm conductivity

 A research team led by University at Buffalo chemists has used synchrotron light sources to observe the electron clouds on the surface of graphene, producing a series of images that reveal how folds and ripples in the remarkable material can harm its conductivity.

The research, scheduled to appear June 28 in Nature Communications, was conducted by UB, the National Institute of Standards and Technology (NIST), the Molecular Foundry at Lawrence Berkeley National Laboratory (Berkeley Lab), and SEMATECH, a global consortium of semiconductor manufacturers.

Graphene, the thinnest and strongest material known to man, consists of a single layer of carbon atoms linked in a honeycomb-like arrangement.

Graphene's special structure makes it incredibly conductive: Under ideal circumstances, when graphene is completely flat, electric charges speed through it without encountering many obstacles, said Sarbajit Banerjee, one of the UB researchers who led the study in Nature Communications.

But conditions are not always optimal.

The new images that Banerjee and his colleagues captured show that when graphene is folded or bent, the electron cloud lining its surface also becomes warped, making it more difficult for an electric charge to travel through.

"When graphene is flat, things just kind of coast along the cloud. They don't have to hop across anything. It's like a superhighway," said Banerjee, an assistant professor of chemistry. "But if you bend it, now there are some obstacles; imagine the difference between a freshly paved highway and one with construction work along the length forcing lane changes.

"When we imaged the electron cloud, you can imagine this big fluffy pillow, and we saw that the pillow is bent here and there," said Banerjee, whose National Science Foundation CAREER award provided the primary funding for the project.

To create the images and understand the factors perturbing the electron cloud, Banerjee and his partners employed two techniques that required use of a synchrotron: scanning transmission X-ray microscopy and near edge X-ray absorption fine structure (NEXAFS), a type of absorption spectroscopy. The experiments were further supported by computer simulations performed on computing clusters at Berkeley Lab.

"Using simulations, we can better understand the measurements our colleagues made using X-rays, and better predict how subtle changes in the structure of graphene affect its electronic properties," said David Prendergast, a staff scientist in the Theory of Nanostructures Facility at the Molecular Foundry at Berkeley Lab. "We saw that regions of graphene were sloped at different angles, like looking down onto the slanted roofs of many houses packed close together."

Besides documenting how folds in graphene distort its electron cloud, the research team discovered that contaminants that cling to graphene during processing linger in valleys where the material is uneven. Such contaminants uniquely distort the electron cloud, changing the strength with which the cloud is bound to the underlying atoms.

Graphene's unusual properties have generated excitement in industries including computing, energy and defense. Scientists say that graphene's electrical conductivity matches that of copper, and that graphene's thermal conductivity is the best of any known material.

But the new, UB-led study suggests that companies hoping to incorporate graphene into products such as conductive inks, ultrafast transistors and solar panels could benefit from more basic research on the nanomaterial. Improved processes for transferring flat sheets of graphene onto commercial products could greatly increase those products' efficiency.

"A lot of people know how to grow graphene, but it's not well understood how to transfer it onto something without it folding onto itself," Banerjee said. "It's very hard to keep straight and flat, and our work is really bringing home the point of why that's so important."

"Graphene is going to be very important in electronics," said PhD candidate Brian Schultz, one of three UB graduate students who were lead authors on the Nature Communications paper. "It's going to be one of the most conductive materials ever found, and it has the capability to be used as an ultrahigh-frequency transistor or as a possible replacement for silicon chips, the backbone of current commercial electronics.

"When graphene was discovered, people were just so excited that it was such a good material that people really wanted to go with it and run as fast as possible," Schultz continued. "But what we're showing is that you really have to do some fundamental research before you understand how to process it and how to get it into electronics."

Other research partners offered the following insight into the significance of the findings:

Dan Fischer, NIST Material Measurement Laboratory, leader, Synchrotron Methods Group: "The NEXAFS results indicating that performance-damaging contaminants cling to graphene during processing highlights the importance of chemically sensitive advanced synchrotron measurement method developments for promoting innovation and industrial competiveness in commercial applications of nanotechnology."Pat Lysaght, SEMATECH Front End Processes, senior member technical staff: "We place a premium on the power of collaboration, and this is a great example of the benefits associated with that philosophy. The unique expertise of each of the four collaborative entities has come together to forge a new understanding of subtle functionalization variations of surface graphene atoms. Our findings represent another important step toward potential industrial applications such as low-cost broadband radio frequency (RF) devices, and correlation of NEXAFS with Raman spectroscopy which may enhance monitoring capabilities for graphene as a replacement for large area organic LED displays."

Synchrotron imaging was conducted at the Canadian Light Source in Saskatchewan in Canada and at the National Synchrotron Light Source (NSLS ) at Brookhaven National Laboratory in New York State. NEXAFS was measured at the NIST soft X-ray beamline of the NSLS.

Story Source:

The above story is reprinted (with editorial adaptations) from materials provided by University at Buffalo, via EurekAlert!, a service of AAAS.

Journal Reference:

Brian J. Schultz, Christopher J. Patridge, Vincent Lee, Cherno Jaye, Patrick S. Lysaght, Casey Smith, Joel Barnett, Daniel A. Fischer, David Prendergast, Sarbajit Banerjee. Imaging local electronic corrugations and doped regions in graphene. Nature Communications, 2011; 2: 372 DOI: 10.1038/ncomms1376

New mass spectrometry technique clouds early European inflation theories

Using a new coupled mass spectrometry technique that employs multiple collectors, researchers in France have shown that it was not an influx of silver from the America's that caused high inflation in Europe from the early 1500's to mid 1600, as some historians have long believed. Their results, published in the Proceedings of the National Academy of Sciences (PNAS) show that the gradual replacement of coins made from Spanish silver to imported Mexican silver, did not occur until nearly fifty years later.

The research, led by Anne-Marie Desaulty, sought to answer once and for all the question of why the whole of Europe experienced a dramatic, inexplicable rise in overall prices, shortly after the discovery of the new world.

Until now, researchers have had to rely on the results of mass spectrometry analysis of lead and copper found in coins to trace its origins, because the results obtained from doing so on silver couldn’t be trusted. Unfortunately, due to the difficulty of reading isotope results for lead, and the fact that copper was used at later dates to re-mint coins, no real conclusions could be drawn from the results of such tests. Now however, using the new technique, the team was able to discern that silver from Mexico didn’t begin appearing in Spanish coins until the inflationary period was over; though it did become the principal source of silver in such coins thereafter.

In the past, mass spectrometry tests on silver were fraught with difficulty due to the ratio of its two stable isotopes, silver-107 and 109; making them extremely difficult to measure. New advances in mass spectrometry devices however, coupled with multiple collectors, has made the process more sensitive; sensitive enough so that the results of such tests can now be trusted; and those findings suggest that it was not the sudden importation of Mexican silver as a means of minting Spanish coins that led to the inflation, because there simply wasn’t enough of it present in coins during the period in question.

Unfortunately though, because the study was able to rule out the influx of Mexican silver as a cause for the inflation, a new gap in knowledge has been left behind, which will send scholars and researchers back to the drawing boards to explain why in fact, prices in Europe rose as they did, and why it happened for so long.

More information: Isotopic Ag–Cu–Pb record of silver circulation through 16th–18th century Spain, PNAS, Published online before print May 23, 2011, doi: 10.1073/pnas.1018210108

Abstract
Estimating global fluxes of precious metals is key to understanding early monetary systems. This work adds silver (Ag) to the metals (Pb and Cu) used so far to trace the provenance of coinage through variations in isotopic abundances. Silver, copper, and lead isotopes were measured in 91 coins from the East Mediterranean Antiquity and Roman world, medieval western Europe, 16th–18th century Spain, Mexico, and the Andes and show a great potential for provenance studies. Pre-1492 European silver can be distinguished from Mexican and Andean metal. European silver dominated Spanish coinage until Philip III, but had, 80 y later after the reign of Philip V, been flushed from the monetary mass and replaced by Mexican silver.

 

Physicists create clouds of impenetrable gases that bounce off each other

When one cloud of gas meets another, they normally pass right through each other. But now, MIT physicists have created clouds of ultracold gases that bounce off each other like bowling balls, even though they are a million times thinner than air -- the first time that such impenetrable gases have been observed.

While this experiment involved clouds of lithium atoms, cooled to near absolute zero, the findings could also help explain the behavior of similar systems such as neutron stars, high-temperature superconductors, and quark-gluon plasma, the hot soup of elementary particles that formed immediately after the Big Bang. A paper describing the work will appear in the April 14 issue of Nature.

The researchers, led by MIT assistant professor of physics Martin Zwierlein, carried out their experiment with an isotope of lithium that belongs to a class of particles called fermions. All building blocks of matter -- electrons, protons, neutrons and quarks -- are fermions.

Different states of fermionic matter are distinguished by their mobility. For example, electrons can be mobile, as in a metal; immobile, as in an insulator; or flow without resistance, as in a superconductor. However, for many types of material, including high-temperature superconductors, it is not known what circumstances induce fermions to form a given state of matter. This is especially true of materials with strongly interacting fermions, meaning they are more likely to collide with each other (also called scattering).

In this study, the researchers set out to model strongly interacting systems, using lithium gas atoms to stand in for electrons. By tuning the lithium atoms' energy states with a magnetic field, they made the atoms interact with each other as strongly as the laws of nature allow, meaning that they scatter every time they encounter another atom.

To eliminate any effects of heat energy, the researchers cooled the gas to about 50 billionths of one Kelvin, close to absolute zero (-273 degrees Celsius). They used magnetic forces to separate the gas into two clouds, labeled "spin up" and "spin down, then made the clouds collide in a trap formed by laser light. Instead of passing through each other, as gases would normally do, the clouds repelled in dramatic fashion.

"When we saw that these ultra dilute puffs of gas bounce off each other, we were completely amazed," says graduate student Ariel Sommer, lead author of the Nature paper.

The gas clouds did eventually diffuse into each other, but in several cases it took an entire second or more -- an extremely long time for events occurring at microscopic scales.

The research, conducted at the MIT-Harvard Center for Ultracold Atoms, is part of a program aimed at using ultracold atoms as easily controllable model systems to study the properties of complex materials, such as high-temperature superconductors and novel magnetic materials that have applications in data storage and improving energy efficiency.

In future work, the researchers plan to confine the lithium gases to two-dimensions, which will allow them to simulate the two-dimensional state in which electrons exist in high-temperature superconductors.

Their work can also be used to model the behavior of other strongly interacting systems, such as high-density neutron stars, which are only a few tens of kilometers in diameter but more massive than our sun.

Another substance that interacts as strongly as the atoms in the ultracold lithium gas clouds created at MIT is quark-gluon plasma, which existed at the universe's formation and has been recreated in particle colliders by colliding atomic nuclei at energies corresponding to a trillion degrees.

Story Source:

The above story is reprinted (with editorial adaptations ) from materials provided by Massachusetts Institute of Technology. The original article was written by Anne Trafton.

Journal Reference:

Ariel Sommer, Mark Ku, Giacomo Roati, Martin W. Zwierlein. Universal spin transport in a strongly interacting Fermi gas. Nature, 2011; 472 (7342): 201 DOI: 10.1038/nature09989