Scanning Tunnelling Microscopy: Imaging the Inside of Complex Molecules
- A comparison between the usual image produced by a scanning tunnelling microscope with a blank tip (left) and that of a microscope with a xenon atom as a sensor (right). Only the new method shows the atomic structure of a copper phthalocyanine (CuPc) molecule beside a layer of perylenetetracarboxylic acid dianhydride (PTCDA). Source: Forschungszentrum Jülich
- With the aid of individual atoms as contrast media, even weak bonds between molecules can be imaged. Hydrogen bridge bonds within a layer of perylenetetracarboxylic acid dianhydride (PTCDA) are shown here as an example. Source: Forschungszentrum Jülich
Scanning tunnelling microscopes are among the most important and most widely used tools for visualizing structures at the atomic level. In the past, however, it was virtually impossible to use them to penetrate inside complex molecules. Jülich researchers have now cleared another hurdle in order to overcome this limitation. Using individual atoms between the tip of the microscope and the sample as a sort of contrast medium, they image the inner structure of the molecule as well as the intermolecular forces. The method was presented in the Journal of the American Chemical Society.
Biomolecules, proteins or organic semiconductors for future electronic components - many of the materials at the heart of the nanosciences proved difficult to investigate with conventional scanning tunnelling microscopes in the past. These microscopes trace the surface of a sample using a fine metal tip that is often tapered to a single atom, and in doing so, they determine the strength of an electrical current. However, this "tunnelling current" only detects the external electron shell. As these shells stretch the length of the entire molecule in the case of many complex molecules, the conventional use of such microscopes does not provide any indications of the deeper atomic molecular structure.
The Jülich group headed by Dr. Ruslan Temirov and Prof. Stefan Tautz from the Peter Grünberg Institute have been investigating methods of expanding the potential of scanning tunnelling microscopy for a number of years. In their article, they describe how different types of atoms or molecules can be used as signal transducers with different characteristics. The atoms are attached to the tip of the microscope. With small displacements they react extremely sensitively to the contours of molecules, which in turn influences the measurable tunnelling current. In this way, conventional, industrially manufactured scanning tunnelling microscopes can be used to produce images of the order of atomnside complex molecules and even to visualize intermolecular forces, such as hydrogen bridge bonds.
Georgy Kichin, Christian Weiss, Christian Wagner, F.
Stefan Tautz, Ruslan Temirov: Single Molecule and Single Atom Sensors for Atomic Resolution Imaging of Chemically Complex Surfaces, J. Am. Chem. Soc., 3 October 2011 (online), DOI: 10.1021/ja204624g
Looking Inside Molecules