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 alka nethiol self-assembled monolayers (SAMs) containing alkene, cyano, and carb oxylic acid headgroups. The alkene-terminated thiolate SAM displayed a latt ice structure indistinguishable from those of methyl-terminated thiol. The cyano-terminated thiolate SAM showed hexagonal, square, and double-row latt ice structures, indicative of polymorphisin and/or multiple chemical specie s on the surface. The carboxylic acid-terminated thiolate SAM showed a doub le-row lattice structure very similar to that of the cyano-terminated thiol ate SAM suggesting that perhaps the cyano groups had undergone hydrolysis t o form amide and/or carboxylic acid groups. This hypothesis was supported b y the results of a friction-force microscopy experiment on micro-contact pr inted patterns of these molecules.