L

50 nm

Tip Displacement

Tip Displacement

Fig. 6. (A) Schematic representation of a gold substrate and gold-coated AFM tip, modified with a thin film of poly(vinylferrocene) (PVF). The tip and film were mounted inside a glass fluid cell in an atomic force microscope and held under potential control. Also shown are typical force curves for (B) the interaction between neutral polymer films and (C) interactions between oxidized polymer films.

Force spectroscopy measurements between neutral polymer films (Fig: 6(B)) show significantly higher adhesion than do measurements between oxidized polymer films (Fig. 6(C)). Statistical analysis of over 100 consecutive force curves for each type yielded an average adhesion force of 12.2 ± 0.3 nN for the neutral polymer films and 3.2 ± 0.4 nN for the oxidized polymer films. The large difference in adhesion force was ascribed primarily to differences in solvation energies and electrostatic interactions. The unique ability to control the degree of adhesion at an AFM tip was exploited in the construction of a novel nanoscale mechanical switch [34]. By carefully closing the scan range of a continuous force plot between neutral polymer films, the AFM tip could be positioned into the region of the force plot immediately prior to where the tip pulls off the sample surface. By stepping the electrochemical potential to oxidize the polymer films, the drastic decrease in adhesion causes the tip to be released from the sample surface. This experiment serves as a model for a unique way to effect a mechanical change by formation of an electrochemically reversible bond. Switches of this type could ultimately be designed to activate under the response of a single electron.

The ability to remotely control the surface chemistry of an AFM tip, as we have done, represents a convenient method for studying potential-dependent chemical properties of molecular redox films and evidences the utility of redox molecules in nanotechnology.

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