Adsorption of alkylthiol self-assembled monolayers on gold and the effect of substrate roughness: a comparative study using scanning tunneling microscopy, cyclic voltammetry, second-harmonic generation, and sum-frequency generation

Abstract

Self-assembled monolayers (SAMs) of alkylthiols on gold have called considerable attention due to several applications in the modification and control of surface properties, such as wettability, tribology, and biocompatibility. Therefore, understanding the adsorption and molecular structure of these SAMs is crucial to optimize their quality for a given application. Although many studies in this area have been performed, the effects of substrate morphology on the quality of long chain alkylthiols on gold have not been satisfactorily clarified. This study focuses on the adsorption and conformation of SAMs of 1-hexadecanethiol (C16SH) as a function of adsorption time, substrate roughness, and morphology. The adsorption of C16SH on atomically flat Au surfaces was analyzed via cyclic voltammetry (CV), scanning tunneling microscopy (STM), and second-harmonic generation (SHG), while the molecular conformation and orientation were characterized via sum-frequency generation (SFG) vibrational spectroscopy. We show that CV and STM are much more sensitive to defects in the monolayer than SFG spectroscopy, while SHG is useful to monitor the final stages of adsorption (defect healing). However, SFG spectroscopy suggests that the disordered regions observed in STM are not due to flat-lying molecules but to conformationally disordered upright alkylthiols. Finally, the formation of a nearly perfect monolayer is only obtained at long adsorption times for very flat substrates, with the packing and organization of the alkyl chains depending strongly on the roughness of the gold surface. These findings may have important implications to the preparation of high-quality SAMs for sensitive applications and also highlight the advantages and drawbacks of each technique for assessing the quality of SAMs.