Finding the Next Small Thing

Former and current Wigner fellows are developing technologies for imaging, characterizing, and fabricating nanostructures.

ORNL "nanoscopes" are among the tools that may help researchers construct materials as elastic and durable as a butterfly's wing.

Kalinin, a Russian with degrees from Moscow State University and a Ph.D. degree in materials science from the University of Pennsylvania, has authored more than 50 papers, primarily on advanced SPM imaging and manipulation. His primary interest is nanoscale electrical and mechanical phenomena, the functional basis for such systems as high-density ferroelectric non-volatile memories, micro-electrical-mechanical systems, and signal transduction from nerves to muscles in biological systems.

Recently, Kalinin and Alexei Gruverman, a research professor at North Carolina State University, pioneered electromechanical imaging of biological systems. They were able to visualize the spiral shape of a single collagen fibril in human tooth enamel with 5-nm resolution—that is, on the level of a single molecule. Some 200 years later this approach repeats Italian anatomist Luigi Galvani's experiment on a length scale a million times smaller. Galvani's research showed that dead frog muscles would twitch when struck by an electrical spark.

Kalinin and Gruyerman achieved similar resolution using atomic force acoustic microscopy, which employs tiny blasts of sound to probe surface and subsurface structures of delicate biological materials, as exemplified by the wing of a Vanessa Virginiensis (American Lady) butterfly. Their early results provide clues to the complex structure underlying the elasticity and relative durability of the splendidly functional butterfly wing. Combination of acoustic and piezoresponse force microscopies may help scientists relate structure and local properties of a biological system to its functions.

In addition to electromechanical SPM, Kalinin has developed scanning impedance microscopy, in which alternating current is applied across the sample and probe tip, to provide information on frequency-dependent transport in carbon nanotubes and oxide nanowires—crucial information for developing nanoelectronic and molecular devices and sensors. For his development of this technique, Kalinin received the Ross Coffin Purdy Award of the American Ceramic Society.

"Scanning probe microscopy provides the key to understanding electrical, electromechanical, and structural phenomena on the nanoscale," Kalinin says. "I find it challenging to interpret the science behind each SPM image and to make SPM a quantitative tool for probing material properties on nanometer and, ultimately, atomic scales."

Nanomagnetic Materials

The next class of electronic devices will likely combine light-emitting, electron-conducting, and magnetic materials on a single silicon chip, providing multipurpose functionality. These devices will consume remarkably little power because bits of information will be based not on on-off (1 or 0) switches but rather on up-and-down electron spins. If "nanodots" of special materials (e.g., gallium manganese arsenide) could be magnetic at room temperature, devices could be built that would greatly improve battlefield surveillance, urban intelligence gathering, and detection of biological and chemical warfare agents.

Envisioned palm-sized technologies that exploit electron spin as well as electron charge are called spintronic devices. Such devices could outpace today's supercomputers in factoring any number down to its primes to help security agencies rapidly break the encrypted codes of hostile nations and terrorist cells.

At ORNL a leading expert on nanomagnetism and spin-dependent transport in nanostructured materials who could help design next-generation electronic devices is Jian Shen, a Wigner fellow from 1998 to 2000 who received the Presidential Early Career Award in Science and Technology in 2004. He is currently project leader of nanomagnetism and spin-dependent transport research in Condensed Matter Sciences Division. Shen has also been named a research theme leader of the Department of Energy's Center for Nanophase Materials Sciences at ORNL.

Shen and his colleagues have developed novel methods for growing artificially structured materials layer-by-layer, wire-by-wire, and dot-by-dot. The physical properties of these nanostructures can be tuned "beyond nature" by controlling the size, shape, and density of each individual nanostructure.

For example, high-density magnetic data storage devices must have nanometer-sized arrays of magnetic nanodots. Because they are so small, these nanodots usually become magnetic only at very low temperatures.

"We were able to tune the interaction between the nanodots to obtain ferromagnetism well above room temperature," Shen says. "We have also done similar tuning in magnetic semiconductors for spintronic applications."

The son of two chemists who work at the Chinese Academy of Sciences, from which he earned an M.Sc. degree in surface science, Shen obtained a Ph.D. degree in nanomagnetism at the Max-Planck Institut in Germany. At ORNL he is gaining an international reputation for his understanding of the effect of spatial confinement on magnetism and other complex behavior in nanostructured materials.

World Record

In 2004 Maria Varela achieved a world record at ORNL while performing research as a Wigner fellow. Varela made the first spectroscopic identification of a single atom when she detected a lanthanum atom introduced as an isolated impurity in a calcium titanate (CaTiO₃) matrix.

She imaged the lone lanthanum atom using a Z-contrast scanning transmission electron microscope (STEM). (A similar STEM achieved another ORNL world record, also in 2004, of 0.6 angstrom resolution by visualizing lanthanum atoms between silicon nitride grains.) Varela obtained additional information about the isolated lanthanum atom embedded in the solid CaTiO₃ matrix by using electron energy loss spectroscopy.

"The red spectrum in the electron energy loss spectra revealed the presence of a single lanthanum atom within a column of calcium atoms," she says. "We were the first to get a spectroscopic signal from an impurity atom in a solid.

"We used an electron beam to probe the lanthanum atom, exciting its electrons so they jumped to different energy levels. In this way we learned how this atom is bonded to other atoms and how it interacts chemically with its environment. The ultimate experiment in EELS is to identify a single atom by measuring how much energy was lost from the electron beam probing the atom."

Varela published a paper on the world record event in spectroscopy in the January 2004 issue of Physical Review Letters. The paper was one of 15 she has published since moving from Madrid, Spain, to Oak Ridge. In 2004 Steve Pennycook hired her as a staff scientist in his Electron Microscopy Group.

She has been conducting research on both superconductors and manganites, which exhibit colossal magnetoresistance. "When you put these magnetic oxides in a magnetic field," she explains, "their resistance to the flow of electricity might decrease up to a million times. They might someday be used in magnetic sensors or read heads in computer hard drives if scientists can better understand why these complicated materials behave the way they do."

Unusual Properties

In 2006 ORNL will become the world's foremost center for neutron sciences when the Spallation Neutron Source (SNS) goes into operation in conjunction with the upgraded High Flux Isotope Reactor (HFIR). Much of the research at SNS involving neutron scattering will contribute to the understanding of nanomaterials. The Department of Energy's new $65 million Center for Nanophase Materials Sciences, the agency's first, is co-located with SNS.

Mark Lumsden, a native of Canada and former Wigner fellow (1999-2001), is a staff scientist in ORNL's Center for Neutron Scattering. He co-developed the HB3 triple-axis spectrometer at the HFIR. He also helped write a modern data-acquisition software system for controlling neutron scattering instruments, called Spectrometer Instrument Control Environment. He will conduct research at the SNS.

Lumsden performed neutron scattering research to shed light on the previously unknown magnetic properties of potassium vanadate. "This vanadate is particularly interesting because its bulk properties are so unusual," he says. "In particular, when a magnetic field was applied along a certain direction, the magnetic response of this material—that is, its magnetization—showed an unexpected feature. Using neutron scattering at HFIR, we inferred that these unusual properties resulted from a competition between several magnetic interactions."

Nano Sensors

Ming Su was recruited to Oak Ridge to boost research programs in nanosensors and nanobiology. A Wigner fellow since August 2004, Su has been working with Thomas Thundat, leader of the Nanoscale Science and Devices Group in ORNL's Life Sciences Division.

At Northwestern University, Su worked on a doctoral project in which he devised tiny gas sensors from tin oxide, the material found in smoke detectors and carbon monoxide (CO) detectors in people's homes. He obtained several patents on tin oxide sensors that he miniaturized using the "dip pen nanolithography" (DPN) technique. He reported on this development in Applied Physics Letters and Journal of the American Chemical Society.

Su has invented a method of producing a nano-sized solid by writing on a surface with a liquid precursor ink. The precursor ink contains tin chloride, a metal salt, and a surfactant similar to a constituent of cosmetic creams. If the solid is a sensing material (i.e., tin oxide), the composition and sensitivity can be modified easily by doping the material with different metal ions. For instance, doping tin oxide with platinum makes it more sensitive to hydrogen and CO. When the doped oxide is exposed to these substances, its electrical resistance drops.

Su has demonstrated that DPN can modify coatings on the microcantilever sensors invented by Thundat. "DPN will make it easier to coat each microcantilever in a large array of these tiny 'diving boards' with many different nano-spots, creating an electronic nose," Su explains. "This sensor array can detect and identify different gases in the air."

In a nanobiology application, Su used gold nanoparticle–labeled DNAs to amplify the mass change on the cantilever so it bends more. In this way, a target DNA strand could be more easily identified.

Nanotube–Polymer Composites

Like straw in clay bricks, carbon nanotubes have an enormous potential as a reinforcing phase in polymer composites. Because of their strength, resistance to fracture, and elasticity, carbon nanotubes used to reinforce polymers can give the composites better mechanical properties than those of current carbon- fiber–reinforced composites. In addition, the unique electrical, thermal, and optical properties of carbon nanotubes provide polymer composites with multiple functionalities, permitting automatic alteration of their properties, depending on the environmental conditions.

Many issues remain before researchers can achieve the predicted mechanical property improvements for nanotube-polymer composites. One of many possible applications of these composites is the storage of electrical energy or hydrogen. One challenge is that the orientation of the nanotubes can radically affect the composite's properties.

In work conducted with Ilia Ivanov and Dave Geohegan, former Wigner Fellow Michael Lance and Chun-Hway Hsueh developed a computer model to predict how carbon nanotubes can reorient while under an applied stress.

"If you had a bunch of cocktail wieners in a vat of mashed potatoes and you squished the mashed potatoes, the cocktail wieners would rotate till they pointed perpendicular to the direction of compression," Lance explains. "We predicted that stiff nanotubes would act in a similar way in a compliant polymer matrix."

The new model will help researchers determine how the properties of nanotube composites change under load and suggest new ways to orient nanotubes. The research will be published in the April 2005 issue of the Journal of Materials Research.

Nanoparticles, Antibodies, and Bacteria

Adam Rondinone, a Wigner fellow from 2001 to 2003, has been involved in nanoscience and neutron science projects since he was a doctoral student at Georgia Tech. "I studied magnetic nanoparticles using neutron scattering at HFIR for my Ph.D. thesis research," he says.

Now a staff researcher in ORNL's Chemical Sciences Division, Rondinone is leading the development of a radioactive nanoparticle that can be attached to an antibody, potentially for the treatment of non-Hodgkins lymphoma, a type of cancer.

The nanoparticle, which will be linked to an antibody that targets lymph node tumors, can withstand various chemical environments in the body without degrading. Steve Kennel and Saed Mirzadeh of the Life Sciences Division are working with Rondinone on attaching the nanoparticles to a special antibody, a project funded internally by ORNL's Laboratory Directed Research and Development Program.

The current treatment for non-Hodgkins lymphoma approved by the Food and Drug Administration (FDA) uses an organic chelator that holds a radioactive metal, a beta emitter called yttrium-90, which can escape to the bone and destroy its marrow. The serious potential side effect limits the allowable dose of the radioactive yttrium.

"We think our approach of making a metal oxide nanoparticle with mostly natural, nonradioactive yttrium oxide and only a little radioactive yttrium-90 is safer," Rondinone says. "Encasing the radioactive yttrium metal in an yttrium oxide nanoparticle will prevent the metal from escaping because it is confined in a ceramic particle, thus eliminating the serious side effect. Our nanoparticle will not dissolve and will eventually be flushed from the body. We believe our treatment for lymphoma and other non-solid tumors could be more effective than the current approach."

Rondinone also works with Tommy Phelps in ORNL's Environmental Sciences Division on "training" certain bacteria to churn out magnetic nanoparticles of a specific size. These magnetite-synthesizing bacteria discovered by Phelps persistently grow nanoparticles measuring 40 to 50 nm. The researchers seek to alter the chemical environment of these bacteria so that they grow smaller magnetic nanoparticles of a desired size, say, for computer memories.

Working at levels almost beyond the imagination, ORNL researchers are embarking upon the discovery of "the next small thing."

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