Structure and Growth of Hexyl Azobenzene Thiol SAMs on Au(111)

Abstract

The structure and growth of functionalized thiol self-assembled monolayers (SAMs) on Au(111) derived from a new compound, 12-(4-((4-hexylphenyl)azo)phenoxy)dodecane-1-thiol (“hexyl azobenzene thiol”), were examined by atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), dynamic contact angles, Fourier transform infrared-reflection absorption spectroscopy (FTIR-RAS), and reflection UV−vis absorption spectroscopy. The hexyl azobenzene thiol SAM showed adsorption kinetics comparable to that of alkanethiols. At the initial stage of SAM growth, formation of nanosize domains resulting from the rearrangement of molecules on the surface was confirmed in a similar manner as for alkanethiol SAMs with long alkyl chains. The hexyl azobenzene thiol SAM exhibited a unique molecular ordering due to the intermolecular interaction between tail groups. Molecular resolution AFM images revealed an expanded lattice (nearest neighbor spacings, a = 5.3 ± 0.1 Å, b = 5.6 ± 0.1 Å; angle between the two axes, φ = 85 ± 1°) relative to that of simple azobenzene-terminated SAMs, due to the influence of the hexyl-terminal groups attached to the para-position of the azobenzene moieties. Average molecular tilt and twist angles (α = 14°, β = 42°) were determined by quantitative analysis of C−H stretching mode for FTIR-RAS data. The reflection UV−vis absorption spectra implied that the long axes of the trans-azobenzene moieties were aligned normal to the plane of surface in fully covered SAMs, while those were more tilted (aligned more parallel to the surface plane) in the submonolayers.