The Promise of New Materials
The wavelengths and energies of ALS photons match the primary atomic resonances of the most important elements on the periodic table for making advanced materials, i.e, carbon, oxygen, copper, silicon, manganese, etc. What this means is that ALS photons are readily absorbed by the atoms of these elements. When atoms absorb photons, they eject electrons, with the energy and number of electrons ejected dependent upon the frequency and brightness of the light. Analysis of absorption spectra, or of the associated photoelectron emission spectra, yields critical information about the atoms' electronic configurations. Consequently, in the super-heated field of materials research today, the ALS is very much a hot spot, drawing scientists to its light from around the world.
For example, Anders Nilsson, a physicist with Uppsala University in Sweden, leads a research group that is using undulator light from Beamline 8.0.1 to study the chemical bonds that play a key role in catalysis. Catalysts are materials that initiate, speed-up or slow-down chemical reactions without themselves undergoing any changes. All of the critical action in the catalytic process takes place at the interface where the surfaces of the catalyst and the reacting material meet. Understanding this process is vital, because almost all manufacturing processes start with catalysis. With the high brightness of ALS undulator light and a technique called x-ray emission spectroscopy (XES) in which the absorption of an x-ray photon causes the emission of a fluorescence x-ray, Nilsson and his colleagues can actually observe what takes place between critical chemical bonds during catalysis and other surface reactions on an atom-by-atom basis. Says Nilsson, "XES applied to surfaces is extremely demanding owing to the considerable inefficiencies associated with the creation and detection of fluorescence x-rays. It is only recently that the field has begun to be exploited thanks to the appearance of third-generation synchrotron radiation sources such as the ALS."
Closer to home, across the country, is Harald Ade, a physicist at North Carolina State University. A major thrust of his research group is the study of polymers, the general name given to a class of materials that includes plastics. Polymers are valued for their resistance to corrosion and their high strength-to-weight ratioplastics can be stronger than steel in tensile strength. The strength and other properties of any polymer are largely determined by its microstructurethe concentration, size, distribution, and chemical composition of its constituent particles. The minute size and chemical complexity of these particles, however, make the study of polymer microstructures a daunting task. Working at Beamline 7.0.1, an undulator that serves several experimental stations collectively known as "The Spectromicroscopy Facility," Ade and his colleagues can analyze polymeric samples and determine the chemical nature (speciation) of their constituents. Ade will soon be overseeing construction of an x-ray microscope at a bend magnet beamline which will increase the ALS' capacity for spectromicroscopy experiments. It will be used, along with Beamline 7.0.1, to map the distribution of chemical species throughout the microdomains of a polymer.
"The undulator beamline will let us identify all the different chemical species and their distribution in our samples rather rapidly and at high energy resolution, while the new bend magnet beamline will let us take advantage of the ALS' high brightness," says Ade. "It is the collection of complementary x-ray microscopes that makes the ALS such an exciting facility to use." Steve Kevan, a physicist at the University of Oregon, is using The Spectromicroscopy Facility for, among other studies, investigating the electronic, structural, and dynamic properties of natural and synthetic two-dimensional material structures. When things fabricated from materials become so small they are essentially only two, one, or even zero-dimensional objects (surfaces, lines or dots), the properties of those materials become size-dependent. Understanding the effects of this "reduced dimensionality" could play a critical role in the development of nanotechnologydevices less than a millionth of a meter in size.
"The ALS is optimized to produce partially coherent soft x-rays in a wavelength range corresponding to the important length scales in many reduced-dimensionality phenomena," Kevan says. "This enables elastic and inelastic scattering experiments of various types which provides an incisive picture of material properties." Nanotechnology will spearhead the advance of computers into the next century. A critical question to be answered is how small can a magnetic storage device be?
Says Joachim Stohr, a physicist at IBM's Almaden Research Center, "Finding
magnetic materials that can store information in an area only 2000 square nanometers (too
small to be seen with the naked eye), and materials that can read the minuscule magnetic
signal from such a tiny spot poses major challenges." Magnetic materials absorb light
from a beam whose electric-field component rotates around the direction in which the beam
is traveling, a phenomenon known as "circular polarization." Stohr and his
collaborators want to use "circularly polarized" ALS light to gain knowledge
that will be crucial to making future magnetic storage devices. These devices, such as the
hard-drive in a personal computer, consist of a disk that holds the information and a set
of read/write heads that extract the information. In the future, disks and read/write
heads will probably be constructed out of magnetic and nonmagnetic metals deposited in
layers only a few atoms thick.
For this to happen, says Stohr, "We need to know not only the overall response of these layered structures to different magnetic fields, but also the magnetism, chemical bonding, and geometrical arrangement of the atomic elements in each layer."
The "extraordinarily bright circularly polarized beams" generated at the ALS, he says, makes it possible to probe these layers and get the needed information. Among the most intriguing of materials for generating magnetic fields are the superconductorsmaterials that lose all electrical resistance when cooled below a critical temperature (symbolized Tc). When a material goes superconducting, it can generate a colossal magnetic field with little energy. Superconductivity enthusiasts dream of maglev trains, hover cars, and miniature supercomputers. For years it looked as if such things would never be more than dreams, since superconductors could only be made from metals chilled to liquid helium temperatures. A tremendous breakthrough was scored in 1986 with the discovery of a new class of ceramic oxide materialsthe so-called "high-temperature" or high-Tc superconductorsthat can sustain superconductivity with inexpensive liquid nitrogen. Advancing to the ultimate goal of room-temperature superconductors could, without exaggeration, alter the progress of civilization.
Achieving that goal is more likely with a better fundamental understanding of the superconductivity in these ceramic oxides (the phenomenon appears to be different from metal superconductors). Stanford University's Zhi-Xun Shen is using photons from Beamline 10.0, the ALS' newest undulator light, to search for answers. High-Tc superconductors are characterized by strong interactions between electronic charge and spin, and the geometry of their atomic structure. The secret appears to lie in the atomic planes of copper and oxygen atoms, two of the materials' basic compositional elements. The tunability of light from Beamline 10.0 (meaning the degree to which its wavelengths can be varied) is a tremendous asset for probing the mysteries within these planes.
"I use ALS photons to excite electrons in solids and then analyze their energy and momentum," says Shen. "The ALS is an excellent source for this type of experiment because, among other things, it provides a wide photon range with very good resolution." Eli Rotenberg, who oversees use of The Spectromicroscopy Facility, says a critical factor in the growing demand for time on ALS beamlines by materials scientists has been the quick turn-around for results.
"With the high flux of photons we provide, researchers can obtain the same volume of data in minutes that could take hours or days at other facilities," he says. "It can be overwhelming to deal with that much data at once, so we have developed a suite of online visualization and analysis tools to help researchers stay on top of the data."