Molecular Architectonics by Takuji Ogawa

Author:Takuji Ogawa
Language: eng
Format: epub
Publisher: Springer International Publishing, Cham

2.5.4 How Rigid Is the Molecule?

Looking at Fig. 6b, one notices a characteristic plateau in the attractive force in the first phase of the removal manipulation. This plateau (or equivalently, the extended range of a near-linear rise in the potential energy) reveals that in this phase the molecule is peeled off the surface. If the molecule was indeed lifted as rigid rod or platelet, as initially surmised, then one would expect a behavior as schematically sketched by the dotted lines in Fig. 6b, namely an initially relatively sharp drop of the attractive force followed by a slow leveling off, similar to what is observed for the final removal of the vertical molecule from the surface.

It is straightforward to understand why peeling results in a plateau in the attractive force. For clarity, we assume that the molecule is as flexible as a scotch tape and bends by 90 at the point where its contact to the surface is being broken. The part of the molecule that has already been removed from the surface contributes only little to the overall force on the tip, since the attractive potential falls off rapidly with distance. The part of the molecule that is still adsorbed on the surface does not contribute at all to the attractive force, because it is still located in the equilibrium adsorption geometry, i.e., in the minimum of its potential energy. Hence, the measured attractive force derives mainly from the bent part of the molecule, connecting the section that has just been removed to the one that is about to be removed. As this “removal front” travels through the molecule for increasing , the measured attractive force will be practically constant, because of the nearly homogeneous structure of the molecule, thus yielding the mentioned plateau. The curve corresponding to this extreme limit of peeling is drawn as a dashed line in Fig. 6b. The curve for the real molecule is located between the two limits of complete peeling and a completely rigid molecule, indicating that the molecule possesses some limited degree of flexibility. In summary, we can say that peeling makes force (and the force gradient) shallow. Note that peeling also limits the attractive force to be overcome and thus allows for the breaking of the bond of a molecule containing tens of atoms to the surface by the bond of a single atom to the tip.

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