Abstract
•The structural quality of p-FBT-SAMs was found to depend significantly on the preparation conditions such as the IT.•At short IT, the p-FBT-SAMs were found to mainly include disordered phase (α phase).•With increasing IT, a gradual phase transition from the disordered phase (α) into ordered one (β-phase) was observed.•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 ∼1 min.•The considerable difference in the behavior of these closely related molecules clarifies the delicate balance of the factors which define the energy and the structure of thiol SAMs.
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 ~3 h, the p-FBT-SAMs were found to mainly include disordered phase (α-phase) between the surrounding areas of the gold islands. With increasing IT, a gradual phase transition from the disordered phase (α) into ordered one (β-phase) was observed. Consequently, increasing the IT to ~ 24 h resulted in the formation of well-defined and highly ordered β-phase that is characterized by the commensurate (83×23)R30∘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 ~1 min. The upright-standing p-FBMT molecules were found to form a (3×3)R30∘structure. Despite the absence of a periodic variation in the height of the molecules within the molecular domains, we proposed a larger (23×3)R30∘unit cell containing two inequivalent p-FBMT molecules arranged in a herringbone-type structure instead of the smaller (3×3)R30∘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.