Two techniques unite to provide molecular detail

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Two techniques unite to provide molecular detail

Raman spectroscopy souped up with scanning tunnelling
microscopy hones in on individual atoms and bonds.

By Mark Peplow
Nature News
June 5, 2012

Prepare to take flight across the surface of a molecule.
An unprecedented window on the nanoscale world lets you
feel the heft of the atoms beneath and test the strength
of the chemical bonds that hold them together.

This vision is now a reality, thanks to a system reported
in this week's Nature1 that combines the best features of
two imaging techniques: Raman spectroscopy and the
scanning tunnelling microscope (STM). �It enables you to
look at the guts of a molecule,� says Joanna Atkin, a
spectroscopist at the University of Colorado Boulder, who
co-wrote a News and Views commentary to accompany the
work2.

Raman spectroscopy uses laser light to make sample
molecules vibrate in various ways: atoms wag or twist,
for example, and chemical bonds stretch back and forth.
The photons that make the molecule dance lose a certain
amount of energy depending on the movement, and have a
lower frequency when they scatter away. Different groups
of atoms vibrate in unique ways, so the changes in the
scattered light constitute a characteristic fingerprint
that can identify a molecule.

But the technique is no good for studying very small
samples. �Only one in a million photons drives one of
these vibrational modes and gets shifted,� explains
Atkin. The conventional Raman signal from a single
molecule would be much too weak to discern.

Quantum leap

In recent years, however, researchers have found ways to
use Raman spectroscopy on ever-smaller targets, with the
help of the incredibly sharp metal tip of an STM. When
the tip hovers less than one nanometre above a surface,
electrons can quantum-tunnel across the gap; the
intensity of the resulting electric current depends on
the position and character of the atoms...

Continues at:

http://www.nature.com/news/two-techniques-unite-to-provide-molecular-detail-1.13140

Jai Maharaj, Jyotishi
Om Shanti

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