Shedding Light On Surfaces: LBL Scientist Studies Optical Properties of Fullerenes

Summer 1993

Also, see the related article Carbon Cages: LBL Scientists Study Fullerenes

By Lynn Yarris, [email protected]

When buckyballs are doped, they lose their perfect symmetry. This distortion results in a nonlinear optical property, which means that exposing the material to one frequency of light causes it to emit a second. Materials with nonlinear optical properties are valued as optical switches in fiberoptic communications systems.

To study the nonlinear optical properties of fullerenes, MSD physicist Yuen-Ron Shen will use two experimental techniques he developed called second-harmonic generation (SHG) and sum-frequency generation (SFG). Together, these two techniques can enable him to characterize surfaces (where most of the chemistry of any substance takes place) and to identify and monitor the activity of specific types of molecules.

"Second-harmonic generation tells us the orientation of the molecules on a surface, and sum-frequency generation tells us the orientation of a particular group of atoms within those molecules," he says.

Both techniques are based on information obtained from the nonlinear reflections of laser beams. In the case of SHG, the incident radiation is visible light; in the case of SFG, the incident radiation is a mix of infrared and visible light. When a laser beam strikes a material, it causes electrons within the material to oscillate. These oscillating electrons reradiate, and this radiation can be grouped into "orders"--first, second, or third--depending upon its frequency and amplitude. Shen works with "second-order radiation," which is readily detectable and is so surface-specific as to be sensitive to submonolayers of molecules on a surface.

Shen plans to attach chromophores to fullerenes. These chromophores contain chains of hydrocarbons that will be "hydrophobic," meaning they will not attach themselves to water. This will provide him with a "highly nonlinear" material that can be spread as a monolayer over the surface of water. SHG and SFG probes will then be used to study the characteristics of the individual molecules that make up the monolayer as the well as the fullerene-based chemistry that takes place at the interface between air and water.

Says Shen, "The most interesting thing about fullerenes is that there are so many ways in which you can change them to give them new properties. We want to learn how to modify them in order to get the specific properties we want."

Initially, Shen will study thin films of pure buckyballs to determine the intrinsic optical nonlinearities that result when quasi-free electrons are distributed around the carbon cage. He also wants to understand why optical properties change when buckyballs are exposed to air.

"Some people think the buckyballs are interacting with oxygen, and some think they are interacting with water, but no one knows the details," says Shen. "Since the contamination can significantly alter the properties of the material, we need to know the answer."