Bonding and orientation in self-assembled monolayers of oligophenyldithiols on Au substrates

The structure and orientation within prototype self-assembled monolayers (SAMs) containing aromatic subunits absorbed on Au substrates has been investigated using infrared (IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and scanning tunneling microscopy (STM). Several oligophenyl(di)thiols, namely biphenylthiol (HS-C6H4-C6H5, BPT), biphenyldithiol (HS-C6H4-C6H4-SH, BPDT), biphenyldimethyldithiol (HS-CH2-C6H4-C6H4-CH2-SH, BPDMT), and terphenyldimethyldithiol (HS-CH2-C6H4-C6H4-C6H4-CH2-SH, TPDMT) have been studied. At least one of these molecules, BPDT, has recently been discussed in connection with the fabrication of a simple electronic device using the aromatic oligophenylthiolate SAM as an active component. Our results reveal that organodithiols with a short oligophenyl backbone, that is, BPDT and BPDMT, do not form well oriented layers but demonstrate that longer backbones, for example, a terphenyl unit, do indeed lead to the formation of SAMs with a high degree of molecular orientation.