Heat-induced phase separation of self-assembled monolayers of a fluorocarbon-hydrocarbon asymmetric disulfide on a Au(111) surface

Heat-induced phase separation of fluorocarbon-hydrocarbon self-assembled monolayers (SAMs) on a Au(111) surface was studied by scanning tunneling microscopy (STM), atomic force microscopy (AFM) and friction force microscopy (FFM). Phase-separated domains were depressed by 0.3 nm with a lateral dimension of 20-200 nm. The friction forces on the higher parts were smaller than those on the lower domains. The higher parts were assigned to the hydrocarbon chains, because the fluorocarbon chains should have a larger friction force acting on them than the hydrocarbon chains against the silicon nitride tip. Phase separation suggests cleavaging of the S-S bonds of disulfides in the SAMs. XPS measurements clearly showed an increase in the isolated Au-S binding species, suggesting the existence of the thiolate (monomer).