MONOLAYERS OF ASYMMETRICAL DIETHYLALKANOAT DISULFIDES ON GOLD(111) - THE INFLUENCE OF CHAIN-LENGTH DIFFERENCE ON ATOMIC-FORCE MICROSCOPE IMAGES

Several symmetrical and asymmetrical long-chain diethylalkanoat disulf ides were synthesized and characterized. The differences in the chain lengths of the disulfides ranged between zero and five methylene units . Self-assembled monolayers of these disulfides formed on Au(111) surf aces were studied by atomic force microscopy (AFM) in order to obtain information about the origin of the image contrast. Domains with hexag onal lattices (lattice constants 5.2-5.3 Angstrom) were detected for a ll derivatives, independent of the chain length differences. This indi cates that the contrast arises roughly at a depth of 4-8 Angstrom in t he monolayers at imaging forces between 0.2 and 3 nN. For SAMs of diet hylalkanoat disulfides, the image contrast is therefore not entirely d ue to the terminal functional groups of the molecules, but is due to t he lattice formed by the closely packed alkane chains in the bulk part of the monolayer. These results are of general importance with respec t to the interpretation of AFM images of organic monolayers.