Manipulation of Organic Molecules by AFM

The adsorption of organic molecules on patterned surfaces has become a subject of intensive study motivated by the prospect of hybrid molecular electronic devices. The operation of such devices is ultimately governed by the electronic properties of single or little clusters of molecules. Investigating and modifying the arrangement of organic aggregates on different surfaces is therefore a primary goal in molecular electronics. Whereas organic molecules on metals have been repeatedly observed by STM, only few groups observed their disposal on insulating surfaces by AFM. Figure 1 shows the first ordered arrangement of organic molecules observed on a nanostructured insulator by our group. The substrate is a KBr crystal, where rectangular pits, acting as molecular traps, were previously created by electron irradiation [1].

The probing tip can be used not only to image, but also to manipulate molecules on a surface. One of the first examples of nanomanipulation was given by Jung et al., who displaced porphyrin molecules across a copper surface with an STM tip [2]. We are currently studying the same molecules on copper surfaces by AFM. In some cases porphyrin molecules slightly move while scanning (Figure 2). In other cases the displacement is dramatic and “molecular trajectories” can be recognized. In both cases, AFM allows to determine the mechanical forces acting between tip and molecules, which is not possible by STM.

Figure 1: First experimental observation of organic molecules (subphthalocyanines) trapped in rectangular holes.

Figure 2:Porphyrin molecules on a copper surface investigated by AFM. Note that some molecules changed their position in the two frames.

Contact:

Enrico Gnecco	Ernst Meyer
Institute of Physics University of Basel Switzerland

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