Analysis of atomic force microscope pull-off forces for gold surfaces portraying nanoscale roughness and specific chemical functionality

Abstract

Pull-off force measurements were carried out between gold-coated atomic force microscope (AFM) cantilever tips modified with self-assembled monolayers (SAMs) of thiols and similar SAMs prepared on gold-coated silicon wafer surfaces in air with a relative humidity level less than 15%. The gold sputtered silicon wafer substrate formed a granular morphology with dimensions of 30-50 nm. The radii of curvatures for the two different cantilever tips used in this study were 32 nm and 53 nm. Pull-off force results were analyzed using the Derjaguin-Muller-Toporov (DMT) contact mechanics theory to determine both surface free energy (γ) for OH, CH₃, CO₂H and NH₂-terminated monolayers and the resultant work of adhesion (W _A) between SAMs. The analysis took into account the coarse morphology of the gold coating. It was found that the γ and W _A values determined with the AFM technique approached similar thermodynamic parameters as calculated from the Lifshitz-van der Waals/Lewis acid-base interaction theory using advancing contact angles; however, the high ends of the adhesion force distributions significantly exceeded the predicted values. This discrepancy is discussed in terms of multiple contact points experienced by a probe penetrating into the grain structure of the gold coating.

Keywords:

• Adhesion
• atomic force microscopy
• chemical force microscopy
• contact angles
• pull-off forces
• surface free energy
• surface tension.