Structural Investigation of 1,1 '-Biphenyl-4-thiol Self-Assembled Monolayers on Au(111) by Scanning Tunneling Microscopy and Low-Energy Electron Diffraction

Self-assembled monolayers (SAMs) of 1,1'-biphenyl-4-thiol (H-(C6H4)(2)-SH) on Au(111) were prepared from solution or via vapor deposition in ultrahigh vacuum and characterized by scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and X-ray photoelectron spectroscopy (XPS). In contrast to the typically observed for densely packed alkane-thiol SAMs on Au(111) (root 3 x root 3)R30 degrees structure, the densely packed aromatic biphenylthiol SAMs prepared by both methods exhibit an unusual hexagonal (2 x 2) structure. Upon annealing at 100 degrees C, this structure evolves into the (2 x 7 root 3) structure resulting in the formation of highly ordered pinstripes oriented along the < 1 -1 0 > directions. Lower density SAMs, prepared by vapor deposition in vacuum, show mixed structures comprising the hexagonal (2 x 2) structure and two rectangular arrangements with the unit cells of (3 root 3 x 9) and (2 root 3 x 8). An extinction of the (3 root 3 x 9) structure in the favor of the (2 root 3 x 8) structure is observed upon annealing at temperatures of similar to 100 degrees C.