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 unsymmetrical dialkyl sulfides with varying chain length was carried out using a scanning force microscope. Adhesion forces measured between dialkyl sulfide-coated probes and dialkyl sulfide-coated substrates show a direct correlation with variations in chain length as a result of chain interdigitation. The dependence of the friction force upon the sliding velocity and the applied load was investigated. The friction-velocity relationship shows a distinct behavior, i.e., the friction force initially increases with velocity, reaches a maximum, and then decreases. The absolute friction forces increase with increasing chain length and external load. The maxima of the friction forces shift to lower velocities (critical velocity) with increasing applied load, but shift to higher velocities with increasing chain length. This behavior is explained using the concept of friction anisotropy. The friction forces increase linearly with the applied load at a fixed sliding velocity and the calculated friction coefficients show an increase with increasing chain length of the molecules. The chain length dependence of adhesion and friction forces is used to differentiate between chemically similar dialkyl sulfides in compositional imaging. Chain length specific adhesion force mapping and friction force imaging is demonstrated on patterned surfaces bearing up to three different dialkyl sulfides created by the micro-contact printing technique. The influence of the medium on the adhesion and friction is addressed by performing measurements in different solvents.