ATOMIC-FORCE MICROSCOPY STUDY OF THE PRESSURE-DEPENDENT STRUCTURAL AND FRICTIONAL-PROPERTIES OF N-ALKANETHIOLS ON GOLD

Atomic force microscopy (AFM) has been used to study the effect of pre ssure on the structural and frictional properties of self-assembled mo nolayers of n-octadecanethiol on Au(111). Sharp microfabricated silico n nitride tips (tip radii 100-300 Angstrom) were used. At low load, th e periodicity of the thiol layer is imaged. At higher load, the layer is observed to become disordered. At a critical contact pressure of si milar to 2.3 GPa, a transition from the thiol overlayer to the Au(111) substrate periodicity is observed in the lattice resolution images. T his transition is gradual and reversible. During the transition, frict ional forces first increase and then decrease as the tip-sample separa tion decreases by a distance approximately equivalent to the thickness of the thiol layer.