- Researchers image vibrational modes of molecules that were previously observed only in computer simulations.
- They used Tip-Enhanced Raman Spectroscopy to take snapshots of vibrating molecules.
Standard optical microscopy can only image up to a few hundreds of nanometers, which isn’t enough to spot the atomic motion. However, recent advances in optical techniques and electron microscopy have enabled scientists to observe the molecule’s internal structure, at resolutions below 1 nanometer.
Now, researchers at Victoria University of Wellington in New Zealand have achieved angstrom-scale resolution (0.1 nanometer) using an advanced technique called Tip-Enhanced Raman spectroscopy. They have been able to reveal vibrational modes of molecules that were previously observed only in computer simulations.
More specifically, they recorded vibrational spectra within a molecule, obtained pictures of normal modes and atomically parsed the intramolecular charges and vibration-driven currents. The findings provide an ideal model for optics in the atomistic near-field.
Measuring Vibrational Energies
The vibrational motion of any molecule can be formulated as a linear superposition of several vibrational patterns (called normal modes) where all atoms oscillate at the same frequency. These normal modes contain specific energy values.
To measure the vibrational energies, it’s important to analyze how molecules absorb and scatter light. This type of analysis is usually done by infrared spectroscopy and Raman spectroscopy.
In Raman spectroscopy, a visible-light laser is used to excite the molecule’s vibration. The incident photon (generated from the laser) has higher energy than the scattered photon. The energy lost within the process (or Raman shift) accounts for vibrational mode energy.
Raman spectra — a graph of Raman shifts versus intensity of scattered light — shows molecule’s vibration energies, which can also be used as signatures to detect molecules. Usually, vibrational energies range from 25 to 500 millielectronvolts.
Reference: Nature | doi:10.1038/s41586-019-1059-9
However, Raman spectroscopy can only measure energies and general symmetry properties of the motion of a vibrating molecule. That’s why the research team used a rather advanced method called Tip-enhanced Raman spectroscopy (TERS).
TERS is a special approach to surface-enhanced Raman spectroscopy (SERS) in which Raman scattering enhancement only takes place at an extremely sharp point (sharper than nanoscale sizes) coated with gold or copper.
Snapshots Of Vibrating Molecule
In this technique, an atomically sharp metallic tip is precisely placed (with angstrom-scale accuracy) above the given molecule. The laser light is then illuminated on the tip’s apex, which dramatically enhances the Raman scattering at this spot, making it possible to measure the SERS spectrum of the molecule.
Courtesy of researchers
For a specific vibrational mode, the scattered light intensity varies with position: blue color shows low-intensity regions while red shows regions with high intensity (figure b). Such unprecedented resolutions reveal the internal structure of the molecule as well as snapshots of each vibrational mode.
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Researchers performed these experiments in an extremely low temperature (6 K) and ultrahigh vacuum. Also, they used special combinations of molecules and substrate to obtain angstrom-scale resolution and image vibrational modes of the molecules.
