Investigation of the adhesion, friction, and wear properties of biphenyl thiol self-assembled monolayers by atomic force microscopy

A comparative study of the tribological properties of self-assembled 1,1′-biphenyl-4-thiol monolayer (BPT) and cross-linked BPT monolayer (BPTC) is presented. BPT monolayers were prepared on a gold substrate by spontaneous self-assembly of biphenylthiol in solution. The cross-linking of BPT was carried out by low energy electron irradiation. Structural characterization by infrared spectra shows that chemical bonds were formed between neighboring phenyl groups after irradiation. The topography, adhesion, friction, and wear properties of BPT and BPTC were studied by atomic force microscopy with silicon nitride and diamond tips. Au(111) and Si(111) substrates were also investigated for reference. Both BPT and BPTC exhibited lower adhesion and friction than Au(111) and Si(111). Cross-linking resulted in a decrease of the adhesive force, whereas the coefficient of friction is increased. The wear resistance of BPT and BPTC slid against a diamond tip were almost identical in the tested range. It appears that cro...