Influence of an alkyl spacer on the formation and structure of 4- Fluorobenzenethiol and 4-Fluorobenzenemethanethiol self-assembled monolayers on Au(111)

Self-assembled monolayers (SAMs) prepared by adsorption of 4-Fluorobenzenethiol (p-FBT) and 4-Fluorobenzenemethanethiol (p-FBMT) on Au(111) substrates with different immersion times (ITs) at room temperature have been characterized using scanning tunneling microscopy (STM). The resulting p-FBT and p-FBMT monolayers showed substantial differences concerning their adlayer SAM structure and surface morphology. The structural quality of p-FBT-SAMs was found to depend significantly on the preparation conditions such as the IT. At short IT of similar to 3 h, the p-FBT-SAMs were found to mainly include disordered phase (alpha-phase) between the surrounding areas of the gold islands. With increasing IT, a gradual phase transition from the disordered phase (alpha) into ordered one (beta-phase) was observed. Consequently, increasing the IT to similar to 24 h resulted in the formation of well-defined and highly ordered beta-phase that is characterized by the commensurate (8 root 3 x 2 root 3) R30 degrees superstructure. In contrast to the p-FBT-SAMs, a full-coverage and close packed monolayers were formed in p-FBMT-SAMs only after very short IT of similar to 1 min. The upright-standing p-FBMT molecules were found to form a (root 3 x root 3) R30 degrees structure. Despite the absence of a periodic variation in the height of the molecules within the molecular domains, we proposed a larger (2 root 3 x root 3) R30 degrees unit cell containing two inequivalent p-FBMT molecules arranged in a herringbone-type structure instead of the smaller (root 3 x root 3) R30 degrees unit cell. The considerable difference in the behavior of these closely related molecules which differ only in their structure (an additional methyl group between benzene and S group in p-FBMT), clarifies the delicate balance of the factors which define the energy and the structure of thiol SAMs.