Scanning probe microscopy tip-sample interactions in primary alcohols of varying chain length

Interactions between chemically functionalized tips and substrates in scanning probe microscopy are usually investigated by utilizing an intervening liquid medium, however the results may be influenced in various ways by the medium itself. In this study the chain length of a series of primary alcohols is shown to affect adhesion measurements. The measurements are performed between a layer of phosphonic acid adsorbed on an aluminum substrate and an aluminum-coated silicon nitride tip. The phosphonic acid/oxidized aluminum system has been characterized previously by vibrational spectroscopy and forms a stable, well-defined system for studying the effects of the medium on adhesion. Adhesion forces between the tip and sample are observed to decrease as the alcohol chain length is increased, and to increase as the dielectric constant of the medium increases. The experimental results of this study cannot be explained by the use of a single model for the role of solvents in force-curve measurements, indicating that a combination of mechanisms may be responsible for adhesion in this system. (C) 2000 American Vacuum Society. [S0734-2101(00)17304-X].