Gazing at orderly rows of green PacMan-like images on a computer screen, Steve Pennycook can "see" individual columns of silicon atoms and the holes between them. In other materials, he can see defects one millionth the thickness of a human hair.

Pennycook, a scientist in ORNL's Solid State Division, does not have X-ray vision. Instead he has "electron vision"--he uses a powerful electron microscope designed to apply his "Z-contrast imaging" technique to give the sharpest direct images yet of atoms in a solid.

"We now have the ability to use the electron microscope to simply photograph the arrangements of atoms inside materials," says Pennycook. "Our system is more economical than conventional microscopes and provides images that are much easier to interpret."

A 300-kilovolt scanning transmission electron microscope (STEM) built by VG Microscopes of England has been installed at ORNL's Solid State Division. It is being prepared for operation later this year. Earlier tests of it in England indicated that it can see, or resolve, features as small as 1.3 angstroms. It will complement ORNL's 100-kilovolt STEM, which can image atoms 2.2 angstroms apart. A unit of measure, an angstrom is one ten-billionth of a meter and about one millionth of the diameter of a human hair.

Once in operation, the ORNL microscope should have higher resolution than the 1-million-volt Atomic Resolution Microscope at DOE's Lawrence Berkeley Laboratory, a conventional electron microscope that can resolve individual atoms that are only 1.6 angstroms apart. A 1.3 million volt microscope being built by a Japanese company has achieved 1-angstrom resolution, "but the images produced by these conventional instruments generally cannot be directly interpreted by the eye," Pennycook says.

"The key advantage of the Z-contrast images is that we can interpret them by eye," he adds. "In other words, unexpected structures are immediately apparent."

Furthermore, using special computer programs developed for radio astronomy, Pennycook believes the new ORNL microscope "can break the 1-angstrom barrier" and achieve a resolution on the order of 0.8 angstrom.

Using the 300-kilovolt microscope, Pennycook says, ORNL researchers and collaborators should be able to see copper atoms in high-temperature superconducting material made of oxides of yttrium, barium, and copper. With the 100-kilovolt microscope, scientists can see only the wider-spaced columns of yttrium and barium atoms, which have higher atomic numbers (heavier nuclei based on numbers of protons).

"We should also be able to better observe interfaces--the ways that different crystalline grains of materials come together at grain boundaries," Pennycook says. "Examples are metals, ceramics, semiconductors, and superconductors. This information on grain boundaries is important for many applications of high-tech materials. Examples are increasing electron speed in semiconductors and compact electronic devices and improving the ability of high-temperature superconductors to carry electrical current."

The ORNL method for achieving ultrahigh resolution costs about $2 million, roughly 10% of the cost of building and operating a conventional microscope having equivalent resolution. The conventional approach requires a large building and a team of operators to run and maintain the microscope. The ORNL approach requires only one operator and a normal-size room.

Pennycook says that the interpretation of atomic images is improved by his use of a ring-shaped detector, which picks up electrons scattered through large angles (like foul balls in a baseball game) rather than those moving through the central hole (like balls hit "up the middle" in rapid succession).

"The number of electrons scattered by the sample is directly related to the composition," Pennycook says. "Because atoms of higher atomic number, or Z, scatter more electrons, they produce a brighter image. We call this technique

Z-contrast imaging because, although atoms are always white in the image, the heavier an atom is, the whiter it appears."

The 300-kilovolt microscope has higher resolution than the 100-kilovolt one because its higher accelerating voltage produces electrons of a shorter wavelength. The smaller the wavelength and the more tightly the electron beam can be focused, the sharper the image. Pennycook plans to extend the range of materials he images to include more complex semiconducting and superconducting materials and metals and ceramics.--Carolyn Krause


Chemical Analysis on Atomic Level Achieved

Unknown elements and the ways they link up to each other in materials can now be identified at the atomic level using a microscope technique developed at ORNL. This approach, described in the November 9, 1993, issue of Nature, could lead to more effective high-temperature super- conducting materials for energy-saving applications.

Atomic resolution chemical analysis, a major long-term goal of analytical electron microscopy, has recently been achieved in ORNL's Solid State Division by Steve Pennycook and his colleague Nigel Browning. Using Pennycook's successful Z-contrast technique for imaging the atomic-scale structure of materials in a direct manner, scientists now use a scanning transmission electron microscope to analyze the chemical composition of a material at the atomic level. In this way, they can identify atoms of unknown elements and determine the details of their chemical bonding.

The Z-contrast image itself has chemical sensitivity in that the higher the atomic number, the brighter the image of that atomic column. However, the image cannot by itself reveal the identity of unknown chemical species.

"The Z-contrast technique's great advantage is that it provides a unique image of the atomic structure that can be interpreted directly," Pennycook says. "Even with a completely unexpected structure, the location of the heavier atomic columns can be seen directly."

Because the Z-contrast image uses only those electrons scattered through large angles, the rest of the electrons may be analyzed by an electron spectrometer. It measures the energy the electrons give up to the specimen they pass through.

This technique, called electron energy loss spectroscopy (EELS), can identify unknown chemical species from the fingerprint of energy loss. In addition, the fine structure on the loss peaks in the electron energy spectrum can be used to determine the number and directions of chemical bonds between neighboring atoms.

One of the challenges of microscopy is to determine the oxygen content in grain boundaries--areas where different crystalline grains of a solid come together--of high-temperature superconducting material made of oxides of yttrium, barium, and copper (YBa2Cu3O7).

According to Pennycook, "If there is enough oxygen in the grain boundaries, the material may act as a superconductor, but if there is not enough, it is an insulator." Chemically speaking, the material is superconducting if its formula is YBa2Cu3O7, but it is an insulator if its formula is YBa2Cu3O6. This subtle difference in oxygen content can be determined by the EELS combined with Z-contrast imaging.

ORNL studies have so far revealed the first clear link between structure and oxygen content. Pennycook and Browning have observed one particular type of grain boundary that has no oxygen deficiency.

"This is a particularly exciting observation," he says, "because of the importance of correlating local structure with local superconducting properties. It will greatly assist our scientific understanding of the effect of defects on superconducting properties and may lead to significant technological advances and applications.

For his research Pennycook received the 1992 Materials Research Society Medal and several Department of Energy materials science awards. --Carolyn Krause


First codes run on 512 nodes

Challenge is a key word at ORNL's Center for Computational Sciences. For a year its computer experts struggled to get ORNL's new Intel Paragon XP/S 35 running effectively. ORNL's earlier Paragon machine--the XP/S 5, which has 66 processors, or nodes--had been working soon after the Intel Corporation shipped it to ORNL for testing. But the newer machine was beset by problems largely because of the complications in extending to 512 nodes.

The challenge was recently met by running two large ORNL-developed codes on 512 nodes. Both codes were developed under the Materials Grand Challenge Project of the Partnership in Computational Science (PICS) consortium. The Materials PICS includes ORNL, Brookhaven National Laboratory, and Ames Laboratory.

Grand challenges are huge problems requiring numerous calculations to progress toward a solution. Parallel computers like the Intel Paragon machines have numerous nodes that make calculations simultaneously. They are called supercomputers because if, each node is assigned a small part of a huge problem, the supercomputer can provide solutions in a few hours, whereas previously weeks or even months were required.

The first code to be run on the XP/S 35 was developed by Bill Shelton of the Engineering Physics and Mathematics Division and Malcolm Stock and Yang Wang, both of the Metals and Ceramics Division. This code calculates the physical properties of disordered materials, such as metallic alloys as well as intermetallic and ceramic compounds, on the basis of a fundamental view of the electron "glue" that holds together the atomic nuclei in matter. The code is useful both for understanding physical properties of disordered materials such as electrical resistivity and for predicting the temperatures, pressures, and other conditions under which elements will mix as well as the types of ordered and disordered compounds these elements will form. Such information is critical to the design of new alloy systems and development of new theories.

Another code developed at ORNL under the Materials Grand Challenge Project calculates densities of electrons and surface energies to identify the preferred binding sites for a single germanium atom on a reconstructed silicon surface. The code was developed by Victor Milman, now of the Solid State Division (SSD), and a team of researchers at Cambridge University in England after electron microscope images made by Dave Jesson of SSD suggested that a germanium atom could exchange positions with a silicon atom on a surface. The images were obtained using Steve Pennycook's Z-contrast technique described in the previous article. A colorful visualization of these calculations appears on the front cover of this issue of this issue of the Review.

"Because experimental information on germanium diffusion on the silicon surface is indirect," says Milman,"theoretical modeling is the only way to understand this interaction between germanium and silicon atoms. This large first-principles calculation is possible only due to the availability of the massively parallel computers."

Such calculations combined with information from Pennycook's Z-contrast electron microscope images (see previous highlights) may lead to an understanding at the atomic level of how to grow light-conducting silicon-germanium films. Such "optically active" silicon-germanium films would revolutionize the manufacture of advanced semiconductor devices that incorporate light-wave guides. These devices will increase the speed of information flow because light signals are faster than electrical currents. Wave guides for compact disk players and other uses are currently made of gallium arsenide. Because gallium arsenide is more expensive than silicon and involves the use of the toxic metal arsenic, silicon-germanium films could better meet the need of the computer, communications, and entertainment industries for faster and cheaper semiconductor devices.

These materials challenges will more likely be met if ORNL succeeds at the challenges of operating a new class of parallel machines with an increasing number of nodes. ORNL in 1994 will be testing an XP/S 75 (1024 nodes) and an XP/S 150 (2048 nodes). Won't it be grand to run ORNL codes on more and more nodes?--Carolyn Krause


From Waste to Warheads:
New Uses for Neutron Interrogation Technology

For the past decade, researchers at ORNL's Waste Examination and Assay Facility (WEAF) have been using neutron interrogation technology to determine or verify the contents of drums of radioactive waste. A spin-off of this process, known as the Pulsed Interrogation Neutron Gamma system, or PING, is demonstrating the versatility of neutron interrogation as an analytical tool.

In recent years, PING has been adapted by ORNL researchers for potential use in airport security systems and is now being billed as a tool for discriminating between chemical and conventional warheads; verifying the absence of high explosives in nuclear warheads; and monitoring the sulfur, energy, and trace-element content of coal.

"Components of the PING system have been used for waste characterization work at the site for years," says Fred Schultz, a researcher in ORNL's Waste Management and Remedial Actions Division, who developed PING, along with George Vourvopoulos of Western Kentucky University. "Now we're applying waste assay techniques to entirely new fields."

The PING system can nondestructively analyze a sealed container, such as a warhead or artillery shell, and detect common constituents of both conventional explosives and chemical warfare agents. As a result, the system promises to provide a fast and reliable method of differentiating between chemical and conventional weapons. This capability gives PING the potential to be used for such activities as monitoring arms control agreements and enforcing provisions of treaties that limit conventional or chemical military capabilities, such as the current treaty between the United Nations and Iraq.

The theory behind the PING system is that materials can be distinguished from each other by determining their chemical profiles, that is, which elements are present and their relative concentrations.

PING is capable of detecting carbon, nitrogen, and oxygen, components of conventional explosives, as well as sulfur, chlorine, fluorine, and phosphorus, common constituents of chemical weapons. The system's ability not only to detect the presence of these elements but also to determine their proportions makes it possible for PING to discriminate between various classes of conventional or chemical weapons, such as nerve gas and mustard gas.

The system uses a pulsed neutron generator to bombard samples with bursts of neutrons. The elemental and, under certain circumstances, chemical, composition of the samples is determined by an analysis of several waves of gamma rays resulting from collisions between neutrons and atoms in the sample. These collisions cause atoms of different elements to emit gamma rays of characteristic energies, allowing researchers to determine which elements are present in the sample.

Samples are bombarded with two kinds of neutrons, fast neutrons and thermalized neutrons. The neutron generator produces "fast," or high-energy neutrons in pulses of about 10 microseconds with gaps of approximately 90 microseconds between pulses. At the end of each pulse, some of these neutrons pass through a neutron moderator, which slows them down, creating "thermalized" neutrons.

Within the first few microseconds after a neutron pulse begins, interactions between fast neutrons and carbon and oxygen atoms, if any are present, produce gamma rays of 4.44 and 6.13 million electron volts (MeV), respectively. In the gaps between pulses, the thermalized neutrons interact with nitrogen, sulfur, and chlorine atoms to produce characteristic gamma rays of 10.828, 5.42, and 6.11 MeV, respectively.

After every few hundred pulses, a gap of several microseconds allows the detection of gamma rays from oxygen, fluorine, and phosphorus nuclei (at energies of 6.13, 1.357, and 1.779 MeV, respectively), which have become radioactive through interactions with fast neutrons. This longer gap is necessary for the detection of fluorine and phosphorus because they emit relatively low-energy gamma rays, which are obscured by high-energy gamma rays emitted by other elements during the system's normal cycle.

The neutron generator currently used with the PING system requires 24 minutes to produce enough gamma rays to analyze a sample. A larger generator could cut this time to two minutes or less. Schultz and other ORNL researchers are working with commercial firms to develop a generator for use with the system. They are also working with a group of French researchers to borrow a suitable generator.

"We've proven the principles of the system," says Schultz. "We've collected gamma-ray data on each of the elements separately; now we have to do it all at once with either real or simulated explosives. When we get a larger neutron generator, we can begin doing this kind of work."

If all goes well with the new neutron generator, the PING system may soon add arms control verification to its diverse list of capabilities.--Jim Pearce


New Probe Detects Trace Pollutants in Groundwater

To determine whether groundwater has been cleaned up or whether a pollution problem is developing, methods for spotting trace amounts of groundwater contamination at waste sites are needed. Such detection technology may be field tested soon in Oak Ridge.

One highly sensitive technique for detecting and identifying trace contaminants in groundwater is being developed at ORNL. The technique has the potential of being a million times more sensitive than current detection methods.

The high-tech probe, which combines a computer, laser, optical fibers, and a power supply, is called a spectroelectrochemical sensor. It is being developed by Eric Wachter, John Storey, Bob Shelton, and Tye Barber, all of the Measurement Systems Research Group (under Richard Gammage) in the Health Sciences Research Division.

Field tests of this groundwater monitoring technique were conducted in September 1993 at the Paducah Gaseous Diffusion Plant, where concentrations of the groundwater contaminant trichloroethylene (TCE) are already well documented. TCE is a potential carcinogen commonly used for removing grease from metals and for dry cleaning clothes. According to Wachter, the technique enabled direct detection of TCE in groundwater at levels as low as 150 parts per million.

Besides TCE, the technique could be used to detect other common groundwater pollutants, such as other chlorinated hydrocarbons, aromatic hydrocarbons, cyanide, nitrates, sulfates, and transuranic elements. It might also be adapted for detection and measurement of trace levels of drugs in body fluids, contaminants in food and water, and nicotine from secondhand smoke.

According to Wachter, with this new technique, virtually any groundwater contaminant can be broken down into a substance that is easy to analyze. This transformation is accomplished by passing an electrical current through a metal electrode in the probe, which is immersed in contaminated groundwater.

Contaminants in contact with the electrode will undergo a chemical change, called an electrochemical reaction. The products of this reaction can be detected and their concentrations measured using spectroscopy--thus the name spectroelectrochemical sensor.

Determining the identity and concentration of the electrochemical product indicates which and how much of a contaminant is present.

This information is obtained by shining a laser light down an optical fiber onto the electrochemical product on the metal surface. Interaction of the light with the product's molecules results in emission of light known as surface-enhanced Raman scattering (SERS). The frequencies and intensities of the SERS light indicate the identity and concentration of the target material.

In the ORNL sensor, the SERS light is carried by optical fibers back to an optical spectrometer on the ground. A computer takes data on the light, identifies the detected groundwater contaminant, and calculates its concentration.

In the electrochemical reaction in the ORNL sensor, TCE is broken down into silver chloride and an organic derivative, which emit SERS light when excited by the laser. However, Wachter says, ORNL researchers hope to improve the detection sensitivity for TCE by chemically converting the contaminant's breakdown products to colored products that emit even more light when excited by a laser. To accomplish this goal, they are collaborating with Mike Angel of Lawrence Livermore Laboratory (who will become a faculty member at the University of South Carolina in the fall of 1993). He has used chemical reagents to make colored products from TCE, which should exhibit "surface-enhanced resonance Raman scattering."

"This approach," says Wachter, "may work better on TCE and some other contaminants that do not interact strongly with metal surfaces. It should greatly increase our detection sensitivity and the range of contaminants that can be detected."

The ORNL group will work with Angel to develop ways to immobilize chemicals for making colored products on the metal surface and to replenish their supplies continuously. Angel is expected to work at ORNL during the summer of 1994.

Wachter says that the current optical probe for groundwater is about 1 inch in diameter but the goal is 0.5 inch. He adds that the equipment aboveground--personal computer, optical spectrometer, and helium-neon or diode lasers--take up a "few cubic feet and could easily fit into the back of a van."

SERS was discovered by Martin Fleishmann of cold fusion fame and later explained by Rick Van Dyne of Northwestern University. A mechanism for SERS involves the surface plasmon, whose existence was predicted by Rufus Ritchie of ORNL's Health Sciences Research Division.

The work is sponsored by the Department of Energy, Office of Research and Development, Waste Management Operations Research and Development Division.--Carolyn Krause


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