Researchers use Raman spectroscopy and STM to allow chemical mapping of molecules to 1nm resolution

Posted on June 7, 2013
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When a weak light beam of green color illuminates the molecule alone, the molecule is visible but lack of structural details (owing to the optical diffraction limit). However, when positioned under a tip, a much more intense and localized red-shifted light, produced by the plasmonic field, is acting on the molecule. The combination of both beams projects the vibrational fingerprints of the molecule into the emitting beam, chemically resolving the inner structure of the molecule with sub-nm resolution. Credit: Dong Xie and Rongting Zhou.

A team of researchers working at China’s University of Science and Technology has succeeded in developing a chemical mapping technique capable of revealing the constituent atoms of a single molecule. In their paper published in the journal Nature, the team describes how they combined Raman spectroscopy with a scanning tunneling microscope (STM) to allow for chemical mapping of a molecule to a resolution of less than 1nm.

Raman spectroscopy is where chemists shine a laser on a small group of molecules and then measure the light as it’s bounced back. The photons from the light source cause the molecules to vibrate and to interact with the bonds that hold molecules together causing a shift in their frequency—the scattering that results is unique for each type of molecule and thus allows for the method to be used as a means of identifying molecule types.

A STM is a device that allows for creating images of materials at the atomic level—one of its unique features is the very tiny metal tip used at the point of scanning. In this new effort the researchers combined Raman spectroscopy with STM to allow for unprecedented levels of molecular mapping.

Prior research has shown that when a STM tip is placed within nanometers of certain metals, plasmonic excitation occurs that when combined with Raman scattering can allow for mapping molecules to within 10nm. In this new research, the team has found that if the frequency of the plasmonic excitation is adjusted to match the molecular vibrations caused by photons from the laser light, the Raman signal is increased sharply, resulting in an ability to map the molecule being studied to less than 1nm.

Top left: experimental map of an isolated porphyrin molecule for a given vibration frequency revealing the four-lobe pattern. Bottom left: theoretical calculation of the same molecular vibration showing its fingerprint. On the right: molecular structure of the porphyrin used in the experiment. Credit: Guoyan Wang and Yan Liang.

Top left: experimental map of an isolated porphyrin molecule for a given vibration frequency revealing the four-lobe pattern. Bottom left: theoretical calculation of the same molecular vibration showing its fingerprint. On the right: molecular structure of the porphyrin used in the experiment. Credit: Guoyan Wang and Yan Liang.

Read more at: Phys.org