Using a phenomenon known as "circular dichroism," researchers successfully characterized the surface of a sample of oxidized tungsten metal and demonstrated the potential of their beamline and experimental station for future studies of magnetic properties at surfaces and interfaces.
The successful team included Charles Fadley, a physicist with the Laboratory's Materials Sciences Division and a professor of physics at UC Davis; ALS scientist Zahid Hussain; former Lab director Dave Shirley, now a professor at Penn State University; and graduate students and postdoctoral associates from both UC Davis and UC Berkeley.
Surface science has always been a important field of study because most chemistry takes place either on the surfaces of materials, or at the interface where two surfaces meet. It will take on even greater importance in the future, however, with the coming of "nanotechnology," which makes use of devices whose dimensions are measured in billionths of a meter (nanometers).
Two of the best techniques for studying surfaces and interfaces are photoelectron spectroscopy (PES) and photoelectron diffraction (PED). PES/PED starts with a beam of photons striking the surface of a sample. Electrons in the constituent atoms absorb the incoming energy and are ejected from the sample as photoelectrons. These photoelectrons are emitted at energies that can be measured to identify each type of emitting atom and determine how many there are and what their chemical or magnetic state is (PES). Photoelectrons emitted from the inner shells of an atom also behave as outgoing waves. These waves can be scattered by nearby atoms to produce diffraction patterns that can be analyzed to locate the positions of the atoms (PED).
"Surfaces are both wonderful and terrible to study because they have to be prepared very carefully and special techniques have to be used to avoid looking at the atoms in the bulk of the material," says Fadley, one of the country's foremost practitioners of PES/PED. "Most techniques that use photons to probe solids are really bulk probes. The beauty of photoelectron spectroscopy and diffraction is that only effects that come from within the first 5 to 10 layers of atoms are measured."
Through the combination of high resolution in both energy and photoelectron emission angles (the best currently possible), Fadley and his research team can obtain detailed information on surface atoms and their positions. By treating the outgoing photoelectron wave as a reference wave and analyzing the interference patterns created with waves scattered from other atoms, it is possible to make a photoelectron hologram. With enough holographic data, true 3-D images of individual atoms can be produced. This can be done selectively for individual atoms even when bound in a surface molecule like an oxide--a major advantage for developing the next generation of nanoscale semiconductor or magnetic storage devices.
The surface science experimental station that has been set up by Fadley, Hussain, and their students is located on ALS beamline 9.3.2, a bend- magnet beamline that produces photons between 30 and 1500 electron volts in energy. It features a state-of-the-art PES/PED system, with in-situ sample preparation and characterization equipment, designed to capitalize on the high photon fluxes and variable light polarization that the ALS can deliver.
To this station, Fadley and his colleagues will soon add a scanning tunneling microscope (STM), an instrument that can be used to map the atomic topography of a surface. Linking PES/PED and STM capabilities within a single ultrahigh vacuum system will provide highly complementary information on the changes that atomic structures undergo during surface chemical reactions.
Another special feature allows the researchers to vary the polarization of their x-ray light from linear to circular. Circularly polarized light can be used to measure the degree of "circular dichroism" displayed by the material. Circular dichroism is the phenomenon whereby the amount of light a material absorbs (or the number of photoelectrons it ejects) depends upon the direction (clockwise or counterclockwise) in which the light is polarized. It is especially useful in the study of magnetic materials because it distinguishes photoelectrons on the basis of their up or down "spin" orientation.
"In a magnetically-ordered environment, photoelectrons with up and down spins may not have the same energy nor scatter with equal strength from the neighboring atoms," says Fadley. "We can use these differences to measure the kind of magnetism in the vicinity of each type of atom in a sample, even for the non-magnetic ones that may have magnetic atoms nearby."
To test these capabilities, the team, working with visitor Hiroshi Daimon of The Photon Factory in Japan, carried out the first circular dichroism measurements to be performed at the ALS. Using a sample of tungsten over which was grown a thin oxide film, they showed that strong circular dichroism effects arise even in the non-magnetic tungsten atoms which are present in both the oxide and metal surfaces. This demonstrated that circular dichroism in PED can be used to study non-magnetic as well as magnetic materials.
"It was no small feat to put together all the components we needed to make this study work, and to have all these components perform so well," Fadley says. "I don't think it would have been possible anywhere but at the ALS."