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

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.