Standing-Up Phase of Hexanedithiol Self-Assembled Monolayers on Au(111) Induced by Displacement Reaction

Abstract

The surface structure and adsorption condition of self-assembled monolayers (SAM) samples before and after displacement of pre-adsorbed 1-adamantanethiol (ADT) SAMs on Au(111) by 1,6-hexanedithiol (HDT) molecules were characterized by scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). STM imaging revealed that pre-adsorbed ADT SAMs on Au(111) have fully coverage and a well-ordered phase with a (7x7) packing structure. However, the ordered phase for pre-adsorbed ADT SAMs was changed to the disordered phase via an intermediate phase consisting of two phases after displacement by HDT molecules. In addition, XPS measurements showed that the intensity of the unbound sulfur peak corresponding to a free -SH group at the outer surface of SAMs on Au(111) increases with increasing displacement time. The relative intensities of the unbound sulfur with respect to the bound sulfur were calculated to be 0.076, 0.147, and 1.095 for displaced SAM samples obtained after displacement of 3 min, 2 h, and 24 h, respectively. This provides strong evidence for the formation of a standing-up phase of HDT SAMs via the displacement process. Our results will provide new insight into controlling the adsorption geometry of alkanedithiol SAMs on Au(111).