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

摘要

Pull-off force measurements wee carried out between gold-coated atomic force microscope (AFM)cantilever tips modified with self-assembled monolyayers (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 sputteered silicon wafer substarate formed a granular morphology with dimensions of 30-50nm.The radii of curvatures for the two different cantilever tips used in this study were 32 nm adn 53 nm.Pull-off force results wee analyzed using the Derjaguin-Muller-Toporov (DMT)contact mechanics theory to determine both surface free energy (sigma)for oh,ch_3,co_2h AND nh_2-terminated monolayers and the resultant work fo adhesin (W_A)between SAMs.The analysis took into account the coarse morphology of the gold coating.It was found that the GAMMA and W_A values determined wiht the AFM technique approached similar theromodynamic parameters as calculated from the lifshite-van der Waals/Lesis acid-based interaction theory using advancing contact angles;however,the highe ends of the adhesio force distributions significantly exceeded the predicted values.This discrepancy is discussed in therms of multiple contact points experienced by a probe penetrating into the grain structure of the gold coaating.