Growth and Structural Transition of Aromatic thiol Self-Assembled Monolayers on Au(111)

Abstract

The formation, structure, thermal stability and electrochemical behaviour of aromatic thiol self-assembled monolayers (SAMs) on Au(111) were characterized by means of scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), contact angle (CA), cyclic voltammetry (CV), and thermal desorption spectroscopy (TDS) to understand the effect of the alkyl spacer between the phenyl group and sulfur atom and the effect of the number of phenyl group at the molecular backbone. Benzenethiol (BT) and benzenemethanethiol (BMT) were used for understanding alkyl spacer effect and BMT and biphenyl methanethiol (BPMT) were used for understanding the effect of the number of phenyl group. Although XPS spectra for BM and BMT SAMs are nearly the same, the surface structures and thermal desorption behaviors significantly differ from each other. BT SAMs on Au(111) were composed of disordered phases, whereas BMT SAMs have well-ordered phases containing vacancy islands. It was found that the strong desorption peak for parent mass species (m/z = 110, C6H5SH+) was observed from BT SAMs at about 227 °C, whereas no desorption peak for them (m/z = 124, C6H5CH2SH+) was observed from BMT SAMs. Interestingly, the dominant TD peak for the benzyl fragments (m/z = 91, C6H5CH2+) formed via the cleavage of C-S bond was observed from BMT SAMs at around 127 °C. From this study, we clearly revealed that the small modification of chemical structure of molecules by inserting of a methylene spacer between the phenyl group and sulfur atom affects 2D SAM structures, adsorption conditions, and thermal desorption behaviors and stability. In addition, we examined the formation of the BMT and BPMT SAMs by changing the temperature condition. ndition at immersion by 24 hours, STM imaging revealed that BMT SAMs at room temperature for 24 h exhibit (√3 ? √3)R30° packing structure, while the SAMs at 50 °C have paired row phase containing larger vacancy islands. However, more defects and disorder phase were formed at 75 °C. On the other hand, the surface morphology of BPMT SAMs hardly changed regardless of temperature due to strong lateral interactions. It was also found that BMT SAMs show a significant structural change from (√3 ? √3)R30° packing structure to (√3 ? 3√3) structure after annealing of pre-covered SAMs at 90 °C for 1 h, whereas BPMT SAMs maintained the surface structure. TDS measurement showed that structural transition of BMT SAMs is closely related to dimerization of sulfur headgroups driven by thermal annealing.