Authors: Alexander Sperl, Jorg Kroger, and Richard Berndt
Journal: Journal of the American Chemical Society
Affiliation: Institut for Experimentelle und Angewandte Physik, Christian-Albrechts-Universitat zu Kiel, Germany, and Institut for Physik, Technische Universitat Ilmenau, Germany
Sometimes a scientific article is published that is so awesome, the authors do not even have to try and justify the potential applications of their work. These authors, however, offer some examples of how demetalation of phthalocyanine could be used as molecular switches for electronics. The really interesting aspect of this paper is the description of the experimental procedure of removing the central metal atom, in this case lead (Pb), from a single phthalocyanine molecule.
The authors use a Scanning Tunnelling Microscope (STM) as their “atomic crane.” A STM is apparatus that contains a metal probe with a single atom on the tip, they are most useful for imaging extremely small objects (and have recently imaged a single molecule). When a potential is applied to this tip and it is positioned above a metal surface of a different potential, electrons will tunnel from the surface to the tip (or vice versa) producing a current, which is translated into a contour map. By imaging a surface, then physically bashing into an atom and applying an electrical potential, STM’s can remove single atoms on a surface. By applying the opposite potential, STM’s can then drop the aformentioned single atom onto a different spot, a technique which has been used (famously) before by IBM to create some interesting pictures.
For this study, the authors deposited one monolayer of Pb-phthalocyanine on a monolayer of lead atoms, which was itself deposited on a single crystal of silver. The authors scanned the surface, lowered the tip of the STM to one of the Pb atoms, applied a voltage of 2 volts, and noticed that the Pb atom had attached itself to the tip of their probe. A follow-up scan, using the Pb-modified probe proved that a single lead atom was removed. This process could be carried out multiple times in succession. The authors also proved that the probe did not simply invert the Pb-phthalocyanine complex.
Selective Removal of Pb Atoms: (a) A cartoon of the process (b) An array of Pb-phthalocyanine complexes, the white dots are the Pb atoms (c) Image after the removal of one Pb atom (d) image after the successive removal of three Pb atoms. (e) Another array of Pb-phthalocyanine before and (f) after removal of a single Pd atom. The color scale on the left corresponds to height where black = 0 picometers and white = 280 picometers
Future work will focus on the selective replacement of one metal atom with another, which will be an awesome result as well. I’m imagining a STM image of a phthalocyanine array that corresponds to the transition metal portion of the periodic table, that’s a picture I’d like to hang on my wall.