2-DIMENSIONAL STRUCTURE OF DISULFIDES AND THIOLS ON GOLD(111)

In order to find factors which determine the two-dimensional structure of self-assembled monolayers (SAMs), several classes of thiols and di sulfides on gold (111) have been investigated by atomic force microsco py (AFM). SAMs were formed from a series of symmetrical and asymmetric al diethylalkanoate disulfides, omega-hydroxy-and omega-carboxyalkanet hiols, diacetylene disulfides, and different anthracene terminated thi ols and disulfides. In all the cases, two-dimensional crystalline stru ctures could be resolved; even for an asymmetrical diethylalkanoate di sulfide that had a chain length difference of five methylene units. Th e lattices were analyzed quantitatively. Two distinctly different type s of crystalline structures were observed, namely, a hexagonal and a c entered rectangular lattice. For the diethylalkanoate disulfides with short alkyl chains (n less than or equal to 10) both structural phases were observed, domains with a hexagonal lattice existing simultaneous ly with centered rectangular domains. The lenth of the alkyl chain det ermined the probability of finding disulfides in the hexagonal structu re. This dependence on the shape of the molecules as well as the clear contrast of SAMs of asymmetric disulfides suggest that the AFM tip pe netrates into the SAMs and probes, at least partially, the interior of the layers. With the atomic force microscope no difference was observ ed between SAMs formed from thiols and those from disulfides.