Investigation of nanoscale frictional contact by friction force microscopy

Organsilane self-assembled monolayers (SAMs) deposited on mica (APTES/mica and OTS/mica) were investigated by friction force microscopy in air. The frictional force as a function of load was found not to be entirely subject to linear law, especially when the loads were kept low. The JKR model, the DMT model, and the transition regime between these two can be employed to describe the nanoscale contacts. We analyzed and fitted the measured frictional data using the modified Carpick's transition equation. The transition parameter, alpha, extracted from the fitting was used to determine which model should be used to describe the contacts. We found that the frictions both for the tip-APTES/mica system and the tip-OTS system could be described by the transition regime between JKR and DMT models. In addition, using the Carpick's transition equations, we determined the adhesion energies for the tip-sample contacts. This study implies a method to identify the accuracy of a particular continuum model to describe the nanoscale contacts. Considering that many previous studies assumed the contacts were completely subject to either the JKR model or the DMT model, we believe that this work will provide us with more realistic information concerning the nanoscale contacts.