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Scanning tunneling microscopy is used to monitor the influence of a controlled nanometer scale surface roughness on structure and stability of physisorbed, crystallized monolayers at the interface between graphite and molecular solutions. The basal plane of graphite is modified by creating atomic scale defects, which are subsequently thermally oxidized to yield monolayer deep circular pits of controlled two-dimensional density and diameters. Alkanes and alkylated oligothiophenes are adsorbed as flat-lying lamellae from solution. Monolayer crystallization is inhibited when the mean distance between the pits becomes comparable to the size of a single molecule. On a time scale of 100 ms, the critical pit diameter for the formation of ordered domains within a pit is two to three lamella widths (6−8 nm). These domains are stabilized and decoupled from those on the terrace. When the pit diameter is increased by a factor of 8−10, the stability inside the pit can be increased by 4 orders of magnitude. This means ...

In this paper we report on the effect that a single fluorine substitution in a hydrocarbon chain of stearic acid has on the molecular ordering of the material and, in addition, the effect that the fluorines have on the scanning tunneling microscopy (STM) contrast of an alkyl chain physisorbed on graphite. The area demand due to a single fluorine substitution is increased by only approximately 1%, demonstrating a very similar steric impact of hydrogen and fluorine. The required accuracy in the measurement is achieved by evaluating the moire pattern due to the adsorbate and substrate lattices. The predicted change in theSTM contrast caused by the fluorine atom was observed for the first time

Monolayers of α,ω-dihexylsexithiophene (α,ω-T6D2) and a non-regioselectively as well as a regioselectively substituted 2,5-didodecylsexithiophene (2,5-T6D2) were physisorbed from solution onto the basal plane of highly oriented pyrolytic graphite (HOPG). Scanning tunnelling microscopy (STM) in situ at the solid/fluid interface revealed that 2,5-T6D2 forms highly ordered monolayers with both the sexithiophene backbones and the alkyl chains oriented parallel to the graphite surface, which is in contrast to vacuum-deposited films on silicon, for which no order has previously been found. On HOPG a difference in the regularity and the average value of intermolecular spacings within monolayers of non-regioselectively and regioselectively substituted 2,5-T6D2 was observed, which is the first time the influence of regioselectivity in a substitution has been detected by STM. Common to the two- and three-dimensional crystal structures of 2,5-T6D2 is a one-dimensional structural unit. α,ω-T6H2 can also form highly ordered monolayers at the interface between an organic solution and the basal plane of graphite with the sexithiophene aromatic backbone and the hexyl side chains oriented parallel to the substrate in a lamella structure. However, in this case it cannot be excluded that other, even more stable adsorbate structures may exist, which could not be imaged by STM.