Thursday, March 15, 2012

Solved: Mystery of the nanoscale crop circles

 Almost three years ago a team of scientists at the U.S. Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab) was performing an experiment in which layers of gold mere nanometers (billionths of a meter) thick were being heated on a flat silicon surface and then allowed to cool. They watched in surprise as peculiar features expanded and changed on the screen of their electron microscope, finally settling into circles surrounded by irregular blisters.

The circles varied in diameter up to a few millionths of a meter, and in the center of each was a perfect square. The mysterious patterns were reminiscent of nothing so much as so-called "alien" crop circles.

Until recently the cause of these strange formations remained a mystery. Now theoretical insights have explained what's happening, and the results have been published online by Physical Review Letters.

Eagerly melting alloys

When two solids are combined in just the right proportions, changes in chemical bonding may produce an alloy that melts at a temperature far lower than either can melt by itself. Such an alloy is called eutectic, Greek for "good melting." The eutectic alloy of gold and silicon -- 81 percent gold and 19 percent silicon -- is especially useful in processing nanoscale semiconductors such as nanowires, as well as for device interconnections in integrated circuits; it liquefies at a modest 363° Celsius, far lower than the melting point of either pure gold, 1064°C, or pure silicon, 1414°C.

"Gold-silicon eutectic liquid can safely solder chip layers together or form microscopic conducting wires, by flowing into channels in the substrate without burning up the surroundings," says Berkeley Lab's Junqiao Wu. "It's particularly interesting for processing nanoscale materials and devices." Wu cites the example of silicon nanowires, which can be grown from beads of eutectic liquid that form from droplets of gold. The beads catalyze the deposition of silicon from a chemical vapor and ride atop continually lengthening nanowire whiskers.

Understanding just how and why this happens has been a challenge. Although eutectic alloys are well studied as solids, the liquid state presents more obstacles, which are particularly formidable at the nanoscale because of greatly increased surface tension -- the same surface forces that make it difficult to form ultra-thin films of water, for example, because they pull the water into droplets. At smaller scales the ratio of surface area to bulk increases markedly, and nanoscale structures have been described as virtually "all surface."

These are the conditions that the team led by Wu, who is a faculty scientist in Berkeley Lab's Materials Sciences Division and a professor in the Department of Materials Science and Engineering at the University of California at Berkeley, set out to examine, by creating the thinnest possible films of gold-silicon eutectic alloys. The researchers did so by starting with a substrate of pure silicon, on whose flat surface an extremely thin barrier layer (two nanometers thick) of silicon dioxide had formed. On this surface they laid layers of pure gold, varying the thickness from one trial to the next between just a few nanometers to a hefty 300 nanometers. The silicon dioxide barrier prevented the pure silicon from mixing with the gold.

The next step was to heat the layered sample to 600 °C for several minutes -- not hot enough to melt the gold or silicon but hot enough to cause naturally existing pinholes in the thin silicon dioxide layer to enlarge into small weak spots, through which pure silicon could come in contact with the overlying gold. At the high temperature, silicon atoms quickly diffused out of the substrate and into the gold, forming a layer of eutectic gold-silicon alloy nearly the same thickness as the original gold and spreading in a virtually perfect circle from the central pinhole.

When the circular disk of eutectic alloy got large enough it suddenly broke up, disrupted by the high surface energy of the gold-silicon eutectic liquid. The debris was literally pulled to the edges of the disk, piling up around it to leave a central denuded zone of bare silicon dioxide.

In the center of the denuded zone, a perfect square of gold and silicon remained.

Chemistry and crystallography, not aliens

The researchers' most surprising discovery was that the thinner the original gold layer, the faster the eutectic circles expanded. The reaction rate when the gold layers were only 20 nanometers thick was more than 20 times faster than when the layers were 300 nanometers thick. And while at first glance the dimensions of the gold and silicon squares inside the circular denuded zones seemed variable, there was in fact a strict relation between the size of the square and the size of the circle: the radius of the circle was always the length of the square raised to the power of 3/2.

How did the squares get there in the first place? They originated as weak spots that were the sources of the spreading eutectic gold-silicon circles; when the circular eutectic was ruptured the squares filled with the same eutectic, which remained at the centers of the denuded zones. As they cooled, the gold and silicon within the squares separated, leaving sharply defined edges that were pure silicon; the centers were more roughly outlined squares of pure gold.

By slicing through the silicon/silicon dioxide/gold layercake and looking sideways at the structures with an electron microscope, the researchers found that the surface squares were the bases of inverted pyramids, resembling teeth penetrating the thin silicon dioxide layer and embedded in the silicon wafer. The squares were square, in fact, because of the silicon's orientation: the substrate had been cut along the crystal plane that defined the base. The four triangular sides of the pyramids lay along the low-energy planes of the crystal lattice and were defined by their intersections.

What began as a puzzling phenomenon reminiscent of "The X Files," if on a considerably smaller scale than the cosmic, the mystery of the "nanoscale crop circles" eventually yielded to careful observation and theoretical analysis -- despite the obstacles posed by high temperatures, nanoscale sizes, instabilities of the liquid state, and extremely rapid time scales.

"We found that the reaction rate in forming small-sized gold-silicon eutectic liquids -- and perhaps in many other eutectics as well -- is dominated by the thickness of the reacting layers," says Wu. "This discovery may provide new routes for the engineering and processing of nanoscale materials."

Story Source:

The above story is reprinted from materials provided by DOE/Lawrence Berkeley National Laboratory.

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

Journal Reference:

Tyler Matthews, Carolyn Sawyer, D. Ogletree, Zuzanna Liliental-Weber, Daryl Chrzan, Junqiao Wu. Large Reaction Rate Enhancement in Formation of Ultrathin AuSi Eutectic Layers. Physical Review Letters, 2012; 108 (9) DOI: 10.1103/PhysRevLett.108.096102

World's smallest radio stations: Two molecules communicate via single photons

 We know since the dawn of modern physics that although events in our everyday life can be described by classical physics, the interaction of light and matter is down deep governed by the laws of quantum mechanics. Despite this century-old wisdom, accessing truly quantum mechanical situations remains nontrivial, fascinating and noteworthy even in the laboratory. Recently, interest in this area has been boosted beyond academic curiosity because of the potential for more efficient and novel forms of information processing.

In one of the most basic proposals, a single atom or molecule acts as a quantum bit that processes signals that have been delivered via single photons. In the past twenty years scientists have shown that single molecules can be detected and single photons can be generated. However, excitation of a molecule with a photon had remained elusive because the probability that a molecule sees and absorbs a photon is very small. As a result, billions of photons per second are usually impinged on a molecule to obtain a signal from it.

One common way to get around this difficulty in atomic physics has been to build a cavity around the atom so that a photon remains trapped for long enough times to yield a favorable interaction probability. Scientists at ETH Zurich and Max Planck Institute for the Science of Light in Erlangen have now shown that one can even interact a flying photon with a single molecule. Among many challenges in the way of performing such an experiment is the realization of a suitable source of single photons, which have the proper frequency and bandwidth. Although one can purchase lasers at different colors and specifications, sources of single photons are not available on the market.

So a team of scientists led by Professor Vahid Sandoghdar made its own. To do this, they took advantage of the fact that when an atom or molecule absorbs a photon it makes a transition to a so-called excited state. After a few nanoseconds (one thousand millionth of a second) this state decays to its initial ground state and emits exactly one photon. In their experiment, the group used two samples containing fluorescent molecules embedded in organic crystals and cooled them to about 1.5 K (-272 °C). Single molecules in each sample were detected by a combination of spectral and spatial selection.

To generate single photons, a single molecule was excited in the “source” sample. When the excited state of the molecule decayed the emitted photons were collected and tightly focused onto the “target” sample at a distance of a few meters. To ensure that a molecule in that sample “sees” the incoming photons, the team had to make sure that they have the same frequency. Furthermore, the precious single photons had to interact with the target molecule in an efficient manner. A molecule is about one nanometer is size (100000 times smaller than the diameter of a human hair) but the focus of a light beam cannot be smaller than a few hundred nanometers.

This usually means that most of the incoming light goes around the molecule, i.e. without them seeing each other. However, if the incoming photons are resonant with the quantum mechanical transition of the molecule, the latter acts as a disk that is comparable to the area of the focused light. In this process the molecule acts as an antenna that grabs the light waves in its vicinity. The results of the study published in Physical Review Letters provide the first example of long-distance communication between two quantum optical antennas in analogy to the 19th century experiments of Hertz and Marconi with radio antennas. In those early efforts, dipolar oscillators were used as transmitting and receiving antennas.

In the current experiment, two single molecules mimic that scenario at optical frequencies and via a nonclassical optical channel, namely a single-photon stream. This opens many doors for further exciting experiments in which single photons act as carriers of quantum information to be processed by single emitters.

The experimental work was performed at ETH Zurich before the group of Prof. Sandoghdar moved to the newly founded Max Planck Institute for the Science of Light in Erlangen in 2011.

Share this story on Facebook, Twitter, and Google:

Other social bookmarking and sharing tools:

Story Source:

The above story is reprinted from materials provided by ETH Zürich, via AlphaGalileo.

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

Journal Reference:

Y. L. A. Rezus, S. G. Walt, R. Lettow, A. Renn, G. Zumofen, S. Götzinger, and V. Sandoghdar. Single-Photon Spectroscopy of a Single Molecule. Physical Review Letters, 108, 093601; Feb 27, 2012

Note: If no author is given, the source is cited instead.

Disclaimer: Views expressed in this article do not necessarily reflect those of ScienceDaily or its staff.

Toppling Raman shift in supercritical carbon dioxide: Vibrational mix shines new light on carbon sequestration measurements

Just as a wine glass vibrates and sometimes breaks when a diva sings the right note, carbon dioxide vibrates when light or heat serenades it. When it does, carbon dioxide exhibits a vibrational puzzle known as Fermi resonance. Now, researchers studying geologic carbon storage have learned a bit more about the nature of carbon dioxide.

The results provide clues to the nature of the Fermi resonance in other molecules, and will help researchers better understand details in chemical reactions. The team of researchers from the Department of Energy's Pacific Northwest National Laboratory report their findings in the February 28 issue of the journal in Physical Chemistry Chemical Physics.

"We're happy to be able to say something new about something so old," said PNNL chemist and author Charles Windisch, Jr. "We figured out how the different carbon dioxide molecules are vibrating at some of the Fermi resonance frequencies. And, of course, we can calibrate our data with more accuracy now."

"Even to this day, people mark Raman spectra incorrectly," said PNNL computational chemist Vassiliki-Alexandra Glezakou. "It helps to know what we are looking at, if we are going to use certain bands as guidelines to understand molecular interactions."

Carbon Dioxide Conundrum

The PNNL researchers did not set out to study again a phenomenon that dates back to the 1930s. Instead, they wanted to investigate what happens when carbon dioxide is stored underground as part of a national research effort to reduce carbon emissions from power generation. To do so, researchers plan to inject carbon dioxide in an unusual form of the gas that behaves like a liquid due to being under high pressure, called supercritical. To follow supercritical carbon dioxide in chemical reactions, researchers often use a technique called Raman spectroscopy.

Raman spectroscopy is a way of capturing a molecule's vibration. Simple molecules can vibrate in well-defined modes such as stretching and bending, which correspond to peak frequencies on a graph. These peaks are as unique and reproducible as a fingerprint.

The number and position of these peaks in a spectrum can be predicted by quantum mechanics, but Fermi resonances result in unanticipated peaks due to a combination of two different vibrations, such as stretching and bending. First recognized in carbon dioxide and explained by Enrico Fermi in 1931, scientists agree that the Fermi peaks are the result of the mixing of the two vibrational modes, but they often label one of them as 'stretch' and the other as 'bend'. This labeling became a problem when PNNL researchers observed a 'flip' in the Raman spectrum of supercritical carbon dioxide.

Shift or Flip?

To follow reactions, researchers often use different versions of elements called isotopes. Normally, carbon dioxide contains carbon plus the isotope oxygen-16, the most common form of oxygen. By using a heavier isotope of oxygen with its own fingerprint, oxygen-18, PNNL researchers can track the fate of carbon dioxide when it reacts with minerals, particularly when there are other sources of oxygen present such as water.

In the Raman spectra of the lighter supercritical carbon dioxide, the pair of Fermi peaks included a weaker one at a lower frequency and a stronger one at higher frequency. When they replaced all of the oxygens with the heavier isotope, however, the peaks seemed to flip, with the stronger one appearing at a lower frequency instead.

At first, it was not clear how the two sets of Fermi peaks related to each other -- whether the peaks were really a mirror image or if the stronger oxygen-16 peak somehow morphed into a weaker peak when heavy oxygen-18 was introduced. Typically, a heavier isotope will shift peaks to lower frequencies, although different modes are not necessarily affected by the same amount.

The researchers needed to unambiguously identify the peaks and to figure out how much bending and stretching modes contributed to each one. To do so, the team decided to simulate the carbon dioxide molecules with different oxygen isotopes on a computer and see if they could recreate the Raman spectra they saw in their experiments.

To the Computer

Using computing resources at EMSL, DOE's Environmental Molecular Sciences Laboratory at PNNL, Glezakou simulated carbon dioxide in supercritical conditions similar to those in the experiment. The molecules were "made" with either oxygen-16 or oxygen-18.

They analyzed the motion of the molecules to produce computational spectra that echoed the real spectra. In this way, the team was able to determine the percent of bending and stretching modes expected in each peak.

The results showed that with oxygen-16, the stronger peak at the higher frequency is due mostly to the stretching mode, while the weaker peak at the lower frequency is due mostly to the bending mode.

Oxygen-18, however, told a different story. The results with heavy carbon dioxide showed unequivocally that the light- and heavy-oxygen peaks were not exactly mirror images of each other. Carbon dioxide is mostly a linear molecule, so the bending motion is much less affected than the stretch when the oxygen-16 is replaced by its heavier isotope. As a result, the composition of the peaks does not remain the same.

"The heavier oxygen doesn't just shift the peaks. It changes their identity," said Glezakou. "And the bigger effects are on the stretching, because the peak with the most stretching has the biggest frequency shift."

Windisch added that the experimental results matched the computational ones nicely, in spite of the difficulty. "Our colleague Paul Martin here at PNNL had to build equipment so we could do these experiments at the pressures we needed. Not easy," he said.

Having nailed down the vibrational pedigree of these carbon dioxide molecules, they plan to use these results to understand better other reactions between carbon dioxide and a variety of minerals.

This work was supported by the U.S. Department of Energy, Office of Fossil Energy.

Story Source:

The above story is reprinted from materials provided by Pacific Northwest National Laboratory, via Newswise.

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

Journal Reference:

Charles F. Windisch, Vassiliki-Alexandra Glezakou, Paul F. Martin, B. Peter McGrail, Herbert T. Schaef. Raman spectrum of supercritical C18O2 and re-evaluation of the Fermi resonance. Physical Chemistry Chemical Physics, 2012; 14 (8): 2560 DOI: 10.1039/C1CP22349F

That caffeine in your drink -- is it really 'natural?'

 That caffeine in your tea, energy drink or other beverage -- is it really natural? Scientists are reporting successful use for the first time of a simpler and faster method for answering that question. Their report appears in the American Chemical Society (ACS) journal Analytical Chemistry.

Maik A. Jochmann, Ph.D., and colleagues point to the growing consumer preference for foods and beverages that contain only natural ingredients. Coffee, tea, colas, energy drinks and other caffeine-containing drinks are the most popular beverages in the world. Food regulatory agencies require that caffeine be listed on package labels, but do not require an indication of whether the caffeine is from natural or synthetic sources. The scientists set out to develop a faster, simpler method for categorizing caffeine's origins.

In the study, they describe use of a technique called stable-isotope analysis to differentiate between natural and synthetic caffeine. The test makes use of differences in the kinds of carbon isotopes -- slight variations of the same element -- found in caffeine made by plants and caffeine made in labs with petroleum-derived molecular building blocks. Their analysis, which takes as little as 15 minutes, found four products that contained synthetic caffeine, despite a "natural" label.

The authors acknowledge funding from the German Federal Ministry of Economics and Technology and the German Research Foundation.

Share this story on Facebook, Twitter, and Google:

Other social bookmarking and sharing tools:

Story Source:

The above story is reprinted from materials provided by American Chemical Society.

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

Journal Reference:

Lijun Zhang, Dorothea M. Kujawinski, Eugen Federherr, Torsten C. Schmidt, Maik A. Jochmann. Caffeine in Your Drink: Natural or Synthetic? Analytical Chemistry, 2012; : 120306094119006 DOI: 10.1021/ac203197d