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Posted: Jun 05, 2013

Seeing inside a molecule using light

(Nanowerk News) An international team, with the participation of the Donostia International Physics Center (DIPC) and the Centre of Materials Physics (CSIC-University of the Basque Country [UPV/EHU]), has managed to resolve and identify, with a hitherto unprecedented resolution, a single organic molecule using light. The prestigious Nature journal published and highlighted the research work ("Chemical mapping of a single molecule by plasmon-enhanced Raman scattering"), which opens doors to possible technological applications in photochemistry and nanotechnology.
Research work led by researchers at the University of Science and Technology of China (USTC), and in which Javier Aizpurua, researcher at the DIPC and the CSIC-UPV/EHU participated, has managed to resolve and identify for the first time a single organic molecule with a subnanometric-range resolution, using light. In the words of Javier Aizpurua: “We have been able to look “inside” a single molecule and identify what kind it is, simply by using light”. This unprecedented result, has earned its publication in the prestigious journal Nature, where he has been highlighted for its relevance.
Visible light is an electromagnetic wave the length of which is between 400 nanometres (nm), (blue) and 750 nm (red). Due to what is known as the resolution limit of diffraction, it has been impossible using light to directly resolve or photograph objects with a size less than half of the wavelength of light, i.e. less than 200 nm. In order to overcome this limitation, over recent years specialists in Nanophotonics have used metal particles that act as minute optical antennae, concentrating and enhancing the visible light spectrum on a nanometric scale. However, even this technique has had its limitations and difficulties when trying to resolve nanometric objects.
The optical resolution achieved in this research, hitherto never obtained, has been possible thanks to the combined use of the scanning tunneling microscope (STM) technique under ultra-high vacuum and low temperature conditions, with the Tip-enhanced Raman spectroscopy (TERS) technique, which dramatically enhances the field that acts on the molecule located in the cavity of the tip of the microscope.
The USTC single-molecule optoelectronics group lead by Dr. Zhenchao Dong combines these two techniques together in a smart way to enable "photographing" organic molecules for the first time at a subnanometric scale. The exquisite tuning of the collective oscillation of electrons at the tip of the microscope, the so-called plasmons, with the vibration of the molecule, enables generating an optical signal with a resolution below that of a nanometre.
When the microscope tip is scanned over the molecule, the Raman signal emitted at each point enables identifying the vibrational signature of the molecule in such a way that, apart from looking “inside” the molecule, it is simultaneously possible to identify the chemical structure of molecule. Javier Aizpurua explained that "it is like peering “inside” the molecule and taking its fingerprints".
This level of resolution has only been possible to date using electrons as the probes, but in this research it is the photons of visible light that manage to achieve the miracle of identifying a molecule, going beyond all limits of optical diffraction hitherto known.
The results of this work open the doors to the direct identification of molecules when their concentration is very small, managing to identify even a single isolated molecule. This ability gives rise to a wide range of possible technological applications, such as in biosensor ones for the analysis of molecular chains, in health and safety for detecting dangerous substances, and in public health for the control of food quality, amongst others.
Source: Basque Research
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