Detection of Discrete Interactions upon Rupture of Au Microcontacts to Self-Assembled Monolayers Terminated with −S(CO)CH3 or −SH

Abstract

Pulloff forces were measured under solvent for Au-coated atomic force microscopy (AFM) tips in contact with −S-acetate-, −O-acetate-, −SH-, or −OH-terminated self-assembled monolayers (SAMs). The SAMs were formed by adsorption of ω-functionalized undecylphosphonic acids on metal oxide substrates. In ethanol and hexadecane, the mean force required to rupture Au/S-acetate microcontacts was 7 times greater than the mean force required to break Au/O-acetate contacts, consistent with the known affinity of S-containing functional groups for Au. Further, rupture force histograms for Au/S-acetate microcontacts under ethanol or hexadecane showed 0.1 nN periodicity. Rupture forces for Au/−SH microcontacts were 4 times greater than for Au/−OH microcontacts under ethanol, and the rupture force histograms showed the same 0.1 nN periodicity. We have assigned this 0.1 nN force quantum to rupture of individual chemical bonds and have estimated the bond energy to be on the order of 10 kJ/mol. The specific interaction corresponding to this energy appears to be abstraction of Au atoms from the tip surface upon pulloff. Our ability to detect these discrete interactions was a function of the solvent in which the measurements were made. For example, in water there was no difference in the mean pulloff force for Au/S-acetate and Au/O-acetate contacts and the histograms did not exhibit periodicity. In general, mean rupture forces for tip−SAM microcontacts are strongly solvent-dependent. To observe single bond rupture forces directly, we argue that the tip−substrate interfacial energy must be negative and larger in absolute value than the substrate−solvent and tip−solvent interfacial energies [i.e., |γsubstrate-tip| > (γtip-solvent + γsubstrate-solvent)]. Otherwise, nonspecific solvent exclusion effects dominate the microcontact adhesion. These measurements show that, whereas rupture forces for tip−SAM microcontacts are solvent-dependent, these forces can be sensitive, under the right conditions, to fluctuations in the number of discrete chemical interactions.