MATERIALS ENGINEERING

Scientists in Washington state are reporting the surprise discovery of the oldest known sample of reactor-produced bomb-grade plutonium, a historic relic from the infancy of America's nuclear weapons program. Their research also represents the first demonstration of how radioactive sodium can be used as a tool in nuclear forensics.

In the new study, Jon Schwantes and colleagues note increased concern about the possibility of terrorists smuggling radioactive materials to make illegal nuclear weapons. As a result, scientists are stepping up efforts to identify and track the source of these radioactive materials using the advanced tools and techniques of a new field called "nuclear archaeology."

The scientists describe efforts to determine the origin of an unknown sample of plutonium (Pu) found in 2004 in a bottle at a waste burial trench at the Hanford nuclear site in Washington. Hanford is the earliest location for U.S. plutonium production for nuclear weapons and now the focus of a massive environmental cleanup effort due to high levels of radioactive waste that remain at the site.

Using multiple pairs of "parent" Pu and "daughter" uranium (U) isotopes, the researchers were able to correct for chemical fractionation that occurred as a result of repackaging in 2004 and determine the age of the sample. Using this technique, they estimated that the Pu in the sample had been separated from U and fission products in 1944, making it the oldest known sample of bomb-grade plutonium produced in a reactor. The only older known samples of Pu-239 were produced by the late Glenn Seaborg and his associates in the beginning of the 1940's when the existence of the element was first confirmed and characterized.

The study identified the Clinton reactor in Oak Ridge, Tenn., as reactor of origin for this material, by comparing reactor burnup modeling results with measurements of minor Pu isotopes. These results were also supported by a series of historical documents tracking the material's movement from Oak Ridge and the processing at Hanford. "Aside from the historical significance of this find, this work provides the public a rare glimpse at a real-world example of the science behind and power of modern-day nuclear forensics," the scientists note.

---

Journal reference:

1. Schwantes et al. Nuclear Archeology in a Bottle: Evidence of Pre-Trinity U.S. Weapons Activities from a Waste Burial Site. Analytical Chemistry, 2009; 81 (4): 1297 DOI: 10.1021/ac802286a

As electronic circuits shrink from finely etched lines in silicon wafers to nearly elusive proportions, researchers at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) and Columbia University are studying how electrons flow through a molecular junction-a nanometer scale circuit element that contacts gold atoms with a single molecule.

Their findings reveal the electrical resistance through this junction can be turned ‘on’ and ‘off’ simply by pushing and pulling the junction-a feature that could be used as a switch in nanoscale electronic devices.

“To design circuit elements at the molecular scale, we need to understand how the intrinsic properties of a molecule or junction are actually connected to its measured resistance,” said Jeff Neaton, Facility Director of the Theory of Nanostructured Materials Facility in the Molecular Foundry, a U.S. Department of Energy User Facility located at Berkeley Lab that provides support to nanoscience researchers around the world. “Knowing where each and every atom is in a single-molecule junction is simply beyond what’s possible with experiments at this stage. For these sub-nanometer scale junctions-just a handful of atoms-theory can be valuable in helping interpret and understand resistance measurements.”

In traditional electronic devices, charge-carrying electrons diffuse through circuits in a well-understood fashion, gaining or losing energy through transactions with impurities or other particles they encounter. Electrons at the nanoscale, however, can travel by a mechanism called quantum tunneling in which, due to the small length scales involved, it becomes possible for a particle to disappear through an energy barrier and suddenly appear on the other side, without expending energy.

Tracking such ‘tunneling’ of electrons through individual molecules in nanoscale devices has proven difficult. “For more than a decade, researchers have been ‘wiring up’ individual molecules and measuring their electrical conductance,” said Neaton. “Forming reliable contacts between nanostructures and ‘alligator clip’ electrical leads is extremely challenging. This made experiments difficult to interpret, and as a result, reported conductance values-in experiment and theory-often varied by an order of magnitude or more. The time was ripe for a quantitative comparison between theory and an experiment with well-defined contacts.”

Through the Molecular Foundry user program, Su Ying Quek, a postdoctoral researcher, worked with Neaton and Latha Venkataraman, an experimental researcher at Columbia University, using a scanning tunneling microscope (STM), which probes changes in current across a material’s surface with a conductive gold tip. Previous work had shown a gold STM tip could be repeatedly be plunged into a gold surface containing a solution of molecules and retracted, until the contact area between the tip and gold surface reduces to a single strand, like a necklace.

When this strand finally breaks, nearby molecules can hop into the gap between strands and contact the gold electrodes, resulting in a sudden change in conductance. Using this technique, Venkataraman and colleagues, including Mark Hybertsen at Brookhaven National Lab, had recently discovered that the conductance of molecules containing amines (a group of molecules related to ammonia) in contact with gold electrodes could be reliably measured.

“We now had a reproducible and consistent data set to benchmark our theory,” said Quek. “Comparing with this data set, we discovered important electron correlation effects previously missing. When we added these, we found-for the first time-quantitative agreement with experimental results.”

Using their new theoretical approach, Quek and Neaton, together with Hybertsen and collaborators Steven G. Louie of University of California Berkeley and Hyoung Joon Choi of Yonsei University in Korea, began to study the conductance of a junction between gold electrodes and bipyridine-a benzene-like ring molecule containing nitrogen. The experimental data showed two stable conductance states, unlike anything seen previously. Working closely with Venkataraman and collaborators, Quek hypothesized the peaks corresponded to two states with different structures within the junction. During the next year, Quek and Neaton meticulously constructed a theory that could describe the conductance of junctions arranged vertically between two gold molecules and sandwiched at angles.

The story that emerged was surprisingly detailed: if bipyridine bonded at an angle, more current could flow compared with when the bipyridine bonded vertically. This suggests the conductance of bipyridine was linked to the molecule’s orientation in the junction, explained Quek. In the STM experiment, as you pull, just after the final strand of gold atoms breaks and snaps back, the vertical gap is not big enough for bipyridine, so it bonds at an angle. As the gap increases, the molecule jumps to a vertical configuration, causing the conductance to plummet abruptly. Eventually, the molecule straightens even more, and the contact breaks. “Once we determined this, we wondered, ‘could you reverse this behavior?’” said Quek.

Teaming with Venkataraman and collaborators, Quek and Neaton demonstrated why pushing the junction to an angle and pulling it straight could repeatedly alter the conductance, creating a mechanical switch with well defined ‘on’ and ‘off’ states. “One of the fascinating things about this experiment is the degree to which it is possible to control the ‘alligator clips’,” said Neaton. “For this particular molecule, bipyridine, experiments can reproducibly and reliably alter these atomic-scale features back and forth to switch the conductance of the junction.”

Quek and Neaton hope to refine and apply their theoretical framework to more complex molecular junctions for study of systems promising for solar energy conversion, such as organic photovoltaics.

“Understanding how electrons move through single-molecule junctions is the first step,” said Neaton. “Organic-inorganic interfaces are everywhere in nanoscience, and developing a better picture of charge transport in hybrid materials systems will certainly lead to the discovery of new and improved electronic devices.”

Portions of this work were supported by the U.S. Department of Energy (DOE) Office of Science, through its Office of Basic Energy Sciences, and by the National Science Foundation through its Nanoscale Science and Engineering Initiative.

---

Journal reference:

1. Quek et al. Mechanically controlled binary conductance switching of a single-molecule junction. Nature Nanotechnology, 2009; DOI: 10.1038/nnano.2009.10

Scientists have found that graphite behaves like a semiconductor in ultrafast time scales. The results are of fundamental importance for future electronic devices based on carbon, in which high electrical fields or frequencies are processed.

Nanomaterials like carbon possess unique properties, which have led to first applications in novel electronic devices and sensors. These materials are based on ordered, atomically thin layers of carbon atoms, for example in the form of a single layer as so-called “graphene”, or rolled-up in carbon nanotubes. The electronic properties of such structures are closely related to those of graphite, which consists of a stack of graphene sheets.

Despite intensive research in the past, the fundamental behavior of electrons in this material are not fully understood and still controversially debated.

Markus Breusing, Claus Ropers und Thomas Elsaesser, three scientists from the Max-Born-Institute in Berlin, have now investigated the behavior of electrons in thin graphite films in real time.

As they now report in Physical Review Letters,* they recorded the dynamics of electrons with an unprecedented temporal resolution of only 10 femtoseconds (one femtosecond is a millionth of a billionth of a second). Electrons were excited to high energy states with ultrashort laser pulses, and their return to equilibrium was observed. The individual steps of this process are temporally resolved, and the momentary distribution of electrons in the material is identified. Within 30 femtoseconds, electrons form a hot gas with temperatures of 2500 °C, which cools down to about 200 °C in only 500 femtoseconds. The energy dissipated in this process is transferred to the crystal lattice. After this process, the electrons slowly return to their initial states.

For the first time, the study shows conclusively that, on ultrashort time scales, graphite behaves like a semiconductor, such as silicon or gallium arsenide, and not like a metal.

The observed dynamics have significant consequences for electrical transport, such as currents flowing through the material at high frequencies. The results are of fundamental importance for future electronic devices based on carbon, in which high electrical fields or frequencies are processed.

*Volume 102, Issue 08, 086809/1-4, 2009

---

Microscopic particles of carbon known as buckyballs may be able to keep the nation's water pipes clear in the same way clot-busting drugs prevent arteries from clogging up.

Engineers at Duke University have found that buckyballs hinder the ability of bacteria and other microorganisms to accumulate on the membranes used to filter water in treatment plants. This attribute leads the researchers to believe that coating pipes and membranes with these nanoparticles may prove to be an effective strategy for addressing one of the major problems and costs of treating water.

"Just as plaque can build up inside arteries and reduce the flow of blood, bacteria and other microorganisms can over time attach and accumulate on water treatment membranes and along water pipes," said So-Ryong Chae, post-doctoral fellow in Duke's environmental and civil engineering department. The results of his experiments were published March 5, 2009 in the Journal of Membrane Sciences.

"As the bacteria build up on these surfaces, they attract other organic matter, creating a biofilm that slowly builds up over time," Chae continued, "The results of our experiments in the laboratory indicate that buckyballs may be able to prevent this clogging, known as biofouling. The only other options to address biofouling are digging up the pipes and replacing the membranes, which can be expensive and inconvenient."

A buckyball, or C60, is one shape within the family of tiny carbon shapes known as fullerenes. They are named after Richard Buckminster Fuller, the inventor of the geodesic dome, since their shape resembles his famous structure.

"Biofouling is viewed as one of the biggest costs associated with membrane-based water treatment systems," said Claudia Gunsch, assistant professor of civil engineering at Duke's Pratt School of Engineering and senior member of the research team. "These membranes have very small pores, so they can get stopped up quickly. If we could increase the time between membrane replacements by 50 percent, for example, that would be a huge cost savings."

According to Chae, the addition of buckyballs to treatment membranes had a two-fold effect. First, treated membranes showed less bacterial attachment than non-treated membranes. After three days, the membranes treated with buckyballs had on average 20 colony forming units, the method by which bacterial colonies are counted.

"In contrast, the number of bacterial colonies on the untreated membrane was too numerous to count," Chae said.

Chae also found that the presence of the buckyballs inhibited respiration, or the ability of the bacteria to use oxygen to fuel its activities.

"As the concentration of buckyballs increased, so did the inhibition of respiration," Chae said. "This respiratory inhibition and anti-attachment suggests that this nanoparticle may be useful as an anti-fouling agent to prevent the biofouling of membranes or other surfaces."

Gunsch said the mechanisms involved are not well-understood.

Both Gunsch and Chae believe that since buckyballs are one of the most widely used nanoparticles, additional research is needed to determine if they have any detrimental effects on the environment or to humans. This is one of many issues being studied at Duke's Center for Environmental Implications of Nanotechnology.

"We need to figure out how resistant these coatings will be to long-term use," Gunsch said. "If they can indeed prevent fouling, they will last longer. If they slough off over time, we need to know what the effects will be."

The current experiments in the laboratory were conducted with Escherichia coli K12, a strain of the bacteria that is widely used in laboratory experiments.

"We focused on a quite specific microorganism, so the next stage of our research will to see if these nanoparticles will have the same effects on bacteria commonly found in the environment or those in mixed microbial communities," Chae said. "We also plan to build a small-scale version of a treatment plant in the lab to conduct these tests."

The research was supported by the Office of Naval Research, National Science Foundation and the Korea Research Foundation. Other Duke members of the team were Shuyi Wang, Zachary Hendren and Mark R. Wiesner. Yoshimasa Watanabe, Hokkaido University, Japan, was also part of the team.

---

Researchers at Uppsala University have managed for the first time to measure magnetic properties in new materials quantitatively with the help of electron microscopy – with unparalleled precision. The secret behind the breakthrough is a successful elaboration of electron microscope technology.

The findings, published in the scientific journal Physical Review Letters, means that the energy loss entailed in all electromagnetic materials can ultimately be minimized.

Apace with the miniaturization of electronic components, new methods are needed to analyze the properties of materials down to the atomic level. In 2006 a scientific article showed that it is possible to use a transmission electron microscope to study the magnetic properties of a material, using a technique called “Electron Magnetic Circular Dichroism,” (EMCD). As different materials are combined, often in thin atomic monolayer films, exciting new magnetic properties are created.

This is an interesting research field that is used in hard drives, for example. Today scientists are primarily studying magnetic properties with the aid of an extremely expensive synchrotron light source, whereas EMCD affords a cheaper and considerably more detailed study of the magnetic properties of each layer down to one nanometer.

Until now it has only been shown that EMCD works qualitatively. The Uppsala University researchers have further elaborated the technology to enable it to measure the magnetic forces of the sample quantitatively as well.

“This means we can put a number on the magnetic strength of the sample, which is key to understanding how various materials interact,” says Klaus Leifer, professor of experimental physics at the Department of Engineering Sciences.

By combining practical experiments and theoretical calculations, the method of measuring the EMCD signal has now been optimized and the computer processing of the experimental data further developed. The article is the result of collaborative work involving researchers in materials theory (Professor Olle Eriksson), physical materials synthesis (Professor Björgvin Hjörvarsson), and experimental physics.

These findings are important for our ability to analyze the magnetic properties of a material using equipment that is standard in most electron microscopy laboratories today.

“The technology will also enhance our knowledge of the energy losses that occur in magnetic components in generators and transformers,” says Klaus Leifer.

---

Journal reference:

1. Hans Lidbaum, Ján Rusz, Andreas Liebig, Björgvin Hjörvarsson, Peter M. Oppeneer, Ernesto Coronel, Olle Eriksson, and Klaus Leifer. Quantitative Magnetic Information from Reciprocal Space Maps in Transmission Electron Microscopy. Physical Review Letters, 2009; 102 (3): 037201 DOI: 10.1103/PhysRevLett.102.037201

Overcoming a critical conductivity challenge to clean energy technologies, Boston College researchers have developed a titanium nanostructure that provides an expanded surface area and demonstrates significantly greater efficiency in the transport of electrons.

The challenge has vexed researchers pursuing solar panels thick enough to absorb sunlight, yet thin enough to collect and transport electrons with minimal energy loss. Similarly, the relatively new science of water splitting requires capturing energy within semiconductor materials and then efficiently transporting charges ultimately used to generate hydrogen.

Boston College Asst. Prof of Chemistry Dunwei Wang and members of his lab found that incorporating two titanium-based semiconductors into a nano-scale structure improved the efficiency of power-collecting efforts by approximately 33 percent, the team reported in the online edition of the Journal of the American Chemical Society.

The team achieved a peak conversion efficiency of 16.7 percent under ultraviolet light, reported Wang and his co-authors, BC graduate students Yongjing Lin and Sa Zhou, post doctoral researcher Xiaohua Liu and undergraduate Stafford Sheehan. That compared to an efficiency of 12 percent from a structure composed only of titanium dioxide (TiO2).

Wang said the efficiency gains within the novel material can serve so-called water-splitting, where semiconductor catalysts have been shown to separate and store hydrogen and oxygen gases.

"The current challenge in splitting water involves how best to capture photons within the semiconductor material and then grab and transport them to produce hydrogen," Wang says. "For practical water splitting, you want to generate oxygen and hydrogen separately. For this, good electrical conductivity is of great importance because it allows you to collect electrons in the oxygen-generation region and transport them to the hydrogen-generation chamber for hydrogen production."

By using two crystalline semiconductors – materials critical to the processes of energy capture and transport – Wang says the researchers discovered a new and successful transfer mechanism in an engineered structure nearly invisible to the human eye.

Titanium dioxide has played a key role in early water-splitting research because of its prowess as a catalyst. However, its light absorption is confined to ultraviolet rays only and the material is also a relatively poor conductor.

Wang and his researchers started by growing a nanostructure made of titanium disilicide (TiSi2), a semiconductor capable of absorbing solar light and a material able to provide a sturdy structure with expanded surface area critical to absorbing photons. Still in need of its catalytic capabilities, titanium dioxide was used to coat the structure, Wang said.

The resulting net-like nanostructure effectively separated charges, collecting the electrons in the titanium disilicide core and transporting them away. The structure transferred positive charges to the titanium dioxide region of the material for chemical reactions. In water-splitting, these charges could potentially be used to generate hydrogen.

---

Journal reference:

1. Lin et al. TiO2/TiSi2 Heterostructures for High-Efficiency Photoelectrochemical H2O Splitting. Journal of the American Chemical Society, 2009; 131 (8): 2772 DOI: 10.1021/ja808426h

Just as sonar sends out sound waves to explore the hidden depths of the ocean, electrons can be used by scanning tunnelling microscopes to investigate the well-hidden properties of the atomic lattice of metals.

As researchers from Göttingen, Halle and Jülich now report in the journal Science, they succeeded in making bulk Fermi surfaces visible in this manner. Fermi surfaces determine the most important properties of metals.

"Fermi surfaces give metals their personality, so to speak," explained Prof. Stefan Blügel, Director at the Jülich Institute of Solid State Research. Important properties, such as conductivity, heat capacity and magnetism, are determined by them. On the Fermi surfaces inside the atomic union, high-energy electrons are in motion. Depending on what form the surfaces have and what mobility is assigned to the electrons, they determine the physical properties of metals.

In their latest publication, the researchers report on how they used a scanning tunnelling microscope to direct electrons into a copper sample. As electrons spread out like waves, they pass through the metal and are scattered and reflected at obstacles in the bulk, such as single cobalt atoms. "The overlap between incoming and outgoing waves is so strong," said Dr. Samir Lounis from Forschungszentrum Jülich who turned the theoretical calculations into an experiment, "that they can be measured as spherical patterns on the surface using the scanning tunnelling microscope."

The somewhat deformed rings on the surface allow us to draw direct conclusions on the shape of the Fermi surfaces and the depth of the cobalt atoms, similar to how sonar recognises the ocean floor by means of reflected sound waves. "We hope that more sophisticated methods will make it possible to gain a detailed understanding of deep impurities and interfaces between atomic lattices," explained Lounis. For his simulations of the scanning tunnelling experiment, he used the supercomputer known as JUMP in the Jülich Supercomputing Centre.

In a related article in the "Perspectives" section of "Science", the innovative approach is praised. A scanning tunnelling microscope is primarily used to characterise the surface of a sample. Thanks to the theoretical work in Jülich, it can now be used to gain a direct insight into the bulk of solids and to understand interesting effects in the nanoworld.

---

Journal reference:

1. Weismann et al. Seeing the Fermi Surface in Real Space by Nanoscale Electron Focusing. Science, 2009; 323 (5918): 1190 DOI: 10.1126/science.1168738