Want To Know A Molecule Inside Out? : Plasmon-enhanced Raman scattering shows the way.

A research team working at China’s University of Science and Technology has devised an innovative technique to image a single molecule and even identify its constituent atoms. Not just that, the researchers claim they can also find out the strengths of the bonds holding the atoms in the molecule.



The technique combines Scanning Tunneling Microscopy (STM) with Raman spectroscopy (named after the Indian scientist C.V. Raman, who won the Nobel Prize for Physics in 1930 for his Raman effect) to achieve molecular imaging resolutions less than 1 nm (0.2 nm to be precise).

A Scanning Tunneling Microscope (STM) is a device to create images of matter at the atomic level. It uses a very sharp conducting tip at the point of scanning. When a suitable voltage is applied to the tip, it draws the electrons out of the atoms present on the surface. These electrons cause a flow of current which varies with the distance of the tip from the surface. This measurement is used to recreate the image of the surface.

One can take a look at the following video to understand the working of STM :

But STM alone is unable to identify the constituent atoms of the molecule.

In Raman spectroscopy, a fine laser beam hits the surface, causing the molecules to get excited and vibrate or rotate in characteristic ways, even when they are firmly attached to their locations. This takes up some energy of the incident photon which scatters with a reduced frequency. The magnitude of the frequency-shift is different for different substances and detection of these frequency-shifts can help in identifying the substance, provided the sample is taken in sufficiently high quantity.

So, what to do if we need to excite only a single molecule? The answer comes from this new research.

The setup is placed at –200◦C under high vacuum. A super-sharp metallic tip of the STM is positioned about a nanometre above the surface of a metal cavity that holds the molecule to be analysed. This causes the formation of plasmons (a cloud of excited electrons).


A fine beam of laser is now focused in the metal cavity exciting the sample molecule. If the frequency of the plasmons is adjusted to match the oscillations caused by the laser photons then there is a huge gain in the Raman scattering signal. This enables the imaging of different regions of the molecule at spatial resolutions superior to one nanometre. The results can further be used to determine the bond strengths of the various bonds in the molecule.


The researchers believe that their work will bring improvements in fields like Bio-imaging and DNA sequencing. They are quick to add that their work is still in its very early stages and a lot of fine tuning needs to be done before the practical applications start emerging.

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