The influence of headgroup on the structure of self-assembled monolayers as viewed by scanning tunneling microscopy

Molecular resolution scanning tunnel ng microscopy images are shown of alkanethiol self-assembled monolayers (SAMs) containing alkene, cyano, and carboxylic acid headgroups. The alkene-terminated thiolate SAM displayed a lattice structure indistinguishable from those of methyl-terminated thiol. The cyano-terminated thiolate SAM showed hexagonal, square, and double-row lattice structures, indicative of polymorphisin and/or multiple chemical species on the surface. The carboxylic acid-terminated thiolate SAM showed a double-row lattice structure very similar to that of the cyano-terminated thiolate SAM suggesting that perhaps the cyano groups had undergone hydrolysis to form amide and/or carboxylic acid groups. This hypothesis was supported by the results of a friction-force microscopy experiment on micro-contact printed patterns of these molecules.