LATTICE IMAGING OF SELF-ASSEMBLED MONOLAYERS OF PARTIALLY FLUORINATEDDISULFIDES AND THIOLS ON SPUTTERED GOLD BY ATOMIC-FORCE MICROSCOPY

The structure of self-assembled monolayers (SAMs) of various fluorinat ed disulfides, perfluoroalkylamide thiols, and a mixed alkyl perfluoro alkylamide disulfide on sputtered gold was studied by atomic force mic roscopy (AFM). AFM, performed both in air and in ethanol, revealed the monolayer structure with molecular resolution on the polycrystalline gold substrates. For all partially fluorinated disulfides containing e ster groups, a hexagonal lattice with a lattice constant of 5.8-5.9 An gstrom was found. A mixed alkyl perfluoroalkylamide disulfide formed a hexagonal lattice of a slightly larger lattice constant (6.1 Angstrom ), whereas the lattice observed for fluorinated thiols containing an a mide group was either hexagonal (5.7-5.8 Angstrom) or distorted hexati c (5.6, 6.2, 5.6 Angstrom), depending on the length of the perfluoroal kane segment and the imaging force. The observed deviation from hexago nal symmetry is attributed to the distorting effect of hydrogen bondin g between neighboring amide groups within the monolayer. For short per fluoroalkane segments the distortion is observed at low imaging forces , whereas for long perfluoroalkane segments significantly higher imagi ng forces are necessary in order to observe the distortion. The force dependence of the measured lattice symmetries for different chain leng ths suggests that the AFM tip penetrates into the SAM and probes at le ast partially the interior of the SAM.