Scientists in Germany have plucked a metal ion from the middle of a phthalocyanine molecule on a silver surface. The simple method of removal, which employs a scanning tunnelling microscope (STM), could be used to make cheaper molecular storage devices.

Phthalocyanines are molecular rings of 16 alternating carbon and nitrogen atoms that can incorporate a range of metal atoms at their centres. They can be intensely coloured and are used as dyes. In addition, when integrated into extended macromolecular structures on surfaces, they can be used in data storage or as sensors.

The chemical and electronic properties of phthalocyanine complexes vary depending on the metal ion at their centre, but removing and replacing these in a simple, controlled way has remained a challenge.

Earlier this year, Richard Berndt from the University of Kiel and colleagues successfully inserted metal ions into phthalocyanine molecules on a surface using the tip of an STM.¹ Now, the same team has successfully done the reverse, removing lead ions from phthalocyanine molecules using the same approach.²

An STM comprises a metal probe with one atom at the tip. When the tip is placed above a metal surface that has a different potential to that of the STM, electrons can tunnel from the metal surface to the tip, producing a current. This can be converted into a contour map and then an image. Once the surface atoms have been mapped, the STM can remove a single atom from a surface, by colliding with it and applying an electrical potential to make it stick. By applying the opposite potential, the atom can be dropped into another position on the surface.

Alexander Sperl, who carried out the work, explains that in lead-phthalocyanine complexes, the lead is only weakly bonded to the four nitrogens in the ring. By approaching the lead with the STM tip, therefore, a stronger bond between the tip apex atom and the lead can be formed.

'When the tip is retracted the less strong bond - the bond between the lead and the phthalocyanine - breaks and demetalation occurs,' Sperl says. 'Together with the metalation work, we have shown that it is feasible to controllably remove metal atoms from metalphthalocyanines as well as putting another metal atom inside. Therefore, we have a tool box for the manipulation of such molecules and can tune the properties of such molecules directly on the surface,' he adds.

'This is one of the first examples of a truly interfaced organised system in which the local molecular properties of each unit can be remotely controlled by an external action,' says Davide Bonifazi, an expert on supramolecular chemistry at interfaces using STM, from the University of Namur, Belgium. 'This approach could be further extended in new patterned materials that might find use in molecular-based storage devices,' he adds.

Mike Brown

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