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'-mercaptobi phenyl assembled on Au(111) were characterized by grazing incidence X-ray d iffraction and low-energy atomic beam diffraction. Two phases of different density were observed. In the low-density striped' phase, the diffraction p attern is consistent with a commensurate rectangular (8 x 2 root 3) surface lattice. Systematic absences and the intensity modulation in the diffracti on pattern suggest that the structure can be composed by staggered molecula r rows arranged in a head-to-head orientation with their molecular axes clo se to the surface. The diffraction pattern of the high-density phase can be described by a commensurate (root 3 x root 3)R30 degrees surface lattice. The measured intensity variation along the (1,1) Bragg rod is consistent wi th a tilt angle of at most 19 degrees from the surface normal. Therefore, b oth similarities and differences with the diffraction patterns of the low-d ensity 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 t hermally more stable than the corresponding phases of monolayers of n-alkan ethiols. Finally, the growth behavior of the monolayers of 4-methyl-4'-merc aptobiphenyl was investigated and various growth protocols were tried. Comp ared with the case of n-alkanethiol monolayers, the high-density phase of t he monolayers of 4-methyl-4'-mercaptobiphenyl is more difficult to prepare. (C) 2000 Elsevier Science B.V. All rights reserved.