Nanotribological properties of unsymmetrical n-dialkyl sulfide monolayers on gold: Effect of chain length on adhesion, friction, and imaging

A nanotribological study of self-assembled monolayers on gold of unsymmetri cal dialkyl sulfides with varying chain length was carried out using a scan ning force microscope. Adhesion forces measured between dialkyl sulfide-coa ted probes and dialkyl sulfide-coated substrates show a direct correlation with variations in chain length as a result of chain interdigitation. The d ependence of the friction force upon the sliding velocity and the applied l oad was investigated. The friction-velocity relationship shows a distinct b ehavior, i.e., the friction force initially increases with velocity, reache s a maximum, and then decreases. The absolute friction forces increase with increasing chain length and external load. The maxima of the friction forc es shift to lower velocities (critical velocity) with increasing applied lo ad, but shift to higher velocities with increasing chain length. This behav ior is explained using the concept of friction anisotropy. The friction for ces increase linearly with the applied load at a fixed sliding velocity and the calculated friction coefficients show an increase with increasing chai n length of the molecules. The chain length dependence of adhesion and fric tion forces is used to differentiate between chemically similar dialkyl sul fides in compositional imaging. Chain length specific adhesion force mappin g and friction force imaging is demonstrated on patterned surfaces bearing up to three different dialkyl sulfides created by the micro-contact printin g technique. The influence of the medium on the adhesion and friction is ad dressed by performing measurements in different solvents.