A team of North Carolina State University physicists has discovered a new method for measuring the molecular properties of materials, which could assist in the development of a wide variety of cutting-edge nanostructure technologies.
The technique -- called Gradient-Field-Raman (GFR) spectroscopy -- measures the behavior of molecules, at a scale of one-billionth of a meter, by reflecting light off the material being studied.
Dr. Hans D. Hallen, assistant professor of physics at NC State, has found molecules reacting differently to the light than would be expected using the previously most advanced spectroscopy technique for studying the vibrations of molecules or solids. The new GFR spectroscopy takes advantage of these differences.
Hallen, along with former students Eric Ayars and Catherine Jahncke, is publishing those results in the Nov. 6 edition of the physics journal Physical Review Letters.
"Using Gradient-Field-Raman spectroscopy, we can look at nanostructures of all sorts: semiconductors, biological materials and nanofabricated structures," Hallen said. "If you have something small, nanometers in size, and want to know how it fits together, this is the way to do it."
Scientists and engineers across the nation are currently engaged in a major nanotechnology research push. Their goal is to develop the ability to build new materials at the molecular level. Potential new materials include structures stronger than steel but much lighter, minuscule transistors and memory chips, DNA-based structures, quantum wires and laser emitters.
Science policy experts say nanotechnology advances could result in a science and technology revolution. But first, they say, we have to understand the principles of structures at such tiny scales.
The work of Hallen and his colleagues is an important step in that direction.
GFR spectroscopy is similar in principle to Raman spectroscopy, but with resolution measured in nanometers (or one-billionth of a meter) rather than in millimeters. Raman spectroscopy was developed in the 1920s and refined in the 1970s to study materials at a microscopic scale (at one-millionth of a meter). With both Raman and GFR spectroscopy, light directed at a sample is reflected from the sample at a different frequency than the light's initial frequency. The frequency difference, caused by the coupling of the light photons with bonds in the molecule or solid, indicates the vibration and rotation of the molecules being studied.
In normal Raman spectroscopy, the coupling between the light and the molecule is brought about by a change in polarizability as the molecule vibrates along a bond. But when Hallen and his colleagues used a new instrument -- called a near-field scanning optical microscope -- to get a closer look, they discovered vibration patterns that couldn't be explained using the rules associated with normal Raman spectroscopy.
They then discovered that the coupling between the light and molecule in GFR spectroscopy is moderated by a strong electric field gradient that shifts the potential energy of the atoms as they move during the vibration.
"This helps you pick apart the various vibrations at the surface of a sample a little bit better than you could before," Hallen said. "You can get a good, almost three-dimensional, picture of the vibration modes."
More information about Hallen's research is on the Web at www.physics.ncsu.edu/optics.
-- potter --
"Electric Field Gradient Effects in Raman Spectroscopy"
Authors: E.J. Ayars and H.D. Hallen, North Carolina State University, and C.L. Jahncke, St. Lawrence University.
Published: Nov. 6, 2000, in Physical Review Letters
Abstract:
Raman spectra of materials subject to strong electric field gradients, such as those present near a metal surface, can show significantly altered selection rules. We describe a new mechanism by which the field gradients can produce Raman-like lines. We develop a theoretical model for the “gradient-field Raman” effect, discuss selection rules, and compare to other mechanisms that produce Raman-like lines in the presence of strong field gradients. The mechanism can explain the origin and intensity of some Raman modes observed in SERS and through a near-field optical microscope (MSOM-Raman).
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