Structure and growth of 4-methyl-4′-mercaptobiphenyl monolayers on Au(111): a surface diffraction study

In an attempt to quantify the role played by the rigidity of the molecular backbone on the self-assembly process, monolayers of 4-methyl-4′-mercaptobiphenyl assembled on Au(111) were characterized by grazing incidence X-ray diffraction and low-energy atomic beam diffraction. Two phases of different density were observed. In the low-density 'striped' phase, the diffraction pattern is consistent with a commensurate rectangular (8 × 2√3) surface lattice. Systematic absences and the intensity modulation in the diffraction pattern suggest that the structure can be composed by staggered molecular rows arranged in a head-to-head orientation with their molecular axes close to the surface. The diffraction pattern of the high-density phase can be described by a commensurate (√3 × √3)R30° surface lattice. The measured intensity variation along the (1,1) Bragg rod is consistent with a tilt angle of at most 19° from the surface normal. Therefore, both similarities and differences with the diffraction patterns of the low-density phase and the high-density phase of the monolayers of n-alkanethiol on Au(111) have been detected. The thermal behavior of the monolayers of 4-methyl-4′-mercaptobiphenyl was also examined. Both phases are found to be thermally more stable than the corresponding phases of monolayers of n-alkanethiols. Finally, the growth behavior of the monolayers of 4-methyl-4′-mercaptobiphenyl was investigated and various growth protocols were tried. Compared with the case of n-alkanethiol monolayers, the high-density phase of the monolayers of 4-methyl-4′-mercaptobiphenyl is more difficult to prepare.