Lattice structure of self-assembled monolayers of dialkyl sulfides and calix[4]arene sulfide adsorbates on Au(111) revealed by atomic force microscopy

The lattice structure of a variety of self-assembled monolayers (SAMs) on Au(111), derived from dialkyl sulfides and calix[4]arene-based tetrasulfide adsorbates, was elucidated by atomic force microscopy (AFM). SAMs of n-alkanethiols and a fluorinated alkanethiol were imaged as reference layers. The sulfide-based SAMs were annealed at elevated temperatures in order to improve the monolayer quality. AFM revealed depressions with depths of a single or multiple steps in the Au(111) for SAMs of thiols, as well as sulfides. The tail group lattice of dialkyl sulfides has a (root 3 x root 3)R30 degrees structure identical to that of n-alkanethiols. The results are indicative for the dominating role of intermolecular van der Waals interactions for determining the structure of the dialkyl sulfide-based SAMs. For the calix[4]arene adsorbates the packing of the alkyl chains between the cavity and the substrate could be imaged, which indicates a considerable "information depth" of the AFM measurements. The tilt angle of the alkane segment was found to depend on the rigidity of the cavity.