Shining light on molecular behaviour

A team of North Carolina State University physicists has discovered a new method for measuring the molecular properties of materials.

The technique — called Gradient-Field-Raman (GFR) spectroscopy — measures the behaviour 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 North Carolina State, has found molecules reacting in a manner that differs significantly from advanced spectroscopy techniques for studying the vibrations of molecules or solids. The new GFR spectroscopy takes advantage of these differences.

‘Using Gradient-Field-Raman spectroscopy, we can look at nanostructures of all sorts: semiconductors, biological materials and nanofabricated structures,’ said Hallen. ‘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 United States are currently engaged in a major nanotechnology research push. Their aim is to develop the ability to build new materials at the molecular level.

Potential new materials include structures stronger than steel along with lighter, minuscule transistors and memory chips, DNA-based structures, quantum wires and laser emitters.

GFR spectroscopy is said to be similar in principle to Raman spectroscopy, but with resolution measured in nanometers rather than in millimetres.

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.

When Hallen and his colleagues used 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 found 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,’ said Hallen. ‘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