Structures and Dynamics in Thiolated Diamantane Derivative Monolayers

Self-assembled monolayers (SAMs) of isomeric 1-diamantantyl thiol (1SH) and 4-diamantantyl thiol (4SH) on Au(111) surfaces were characterized using scanning tunneling microscopy (STM) in air. The 4SH molecules form a hexagonal (root 7 X root 7)R19 degrees structure. In monolayers of 1SH, two hexagonal packings, namely, (root 31 x root 31)R8 degrees and a more dense (root 7 X root 7)R19 degrees structure, modulated by rectangular and double row structures were observed. The superstructures correspond to different metastable monolayer states due to intramolecular rotational degrees of freedom. The observed tip-induced structural polymorph changes show that SAMs of different morphologies coexist and may be transformed into each other. Thus, rigid hydrocarbon cages viewed as isotropic dispersion energy donors aggregate effectively within the given unit cell in various relative orientations with only little influence on the packing densities and SAM morphologies.