Spectroscopic Ellipsometry and Electrochemical and X-ray Photoelectron Spectroscopy Investigation of the Influence of the Crystalline Plane on the Adsorption of α,ω-Alkanedithiols: Mono Versus Bi-Coordinated Configurations

Abstract

The effect of the Au surface crystalline plane on the configuration of self-assembled monolayers (SAMs) of α,ω-alkanedithiols (DTs) is studied via reductive desorption (RD), X-ray photoelectron spectroscopy (XPS), and spectroscopic ellipsometry (SE) experiments. A remarkable difference in the SAM configuration of short DTs, such as 1,4-butanedithiol (C4DT) on both Au(111) and Au(100) is observed; while standing up (SU)-configured SAMs are formed on the (111) plane, lying down molecules form the structure on (100). The higher stability of the bi-coordinated structure on Au(100) is interpreted, indirectly, as higher bonding energies for the RS-Au(100) bond, in agreement with the RD and XPS results. Because hardly no SU phases are detected when adsorbing C4DT on Au(100), a longer DT is needed in order to achieve this configuration, namely, 1,6-hexanedithiol (C6DT). In this case, although the RSC6DT-Au(100) bond is similar to RSC4DT-Au(100) in the bi-coordinated phase, the larger lateral interactions in the SU C6DT become the driving force for the DT lifting (i.e., the mono-coordinated phase formation). SE experiments reveal the thickness of the SU SAMs and detect the differences in the C4DT behavior on both surfaces. A discussion on the effect of aging of the DT immersion solutions on the outcome of the SAM configuration is also presented.