| Abstract
| - We report a study of acid-terminated self-assembled monolayers of alkanethiols of different length, 11-mercaptoundecanoic acid (11-MUA) and 16-mercaptohexadecanoic acid (16-MHDA), on Au(111). Scanning tunnelingmicroscopy (STM), X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV), electrochemical impedancespectroscopy (EIS), and contact angle techniques were used for characterization, and the results were compared withthose obtained from n-alkanethiols of similar chain length, providing a detailed description of the two-dimensionalcrystalline structure. Molecular resolution STM images show that 11-MUA forms a dense-packed monolayer arrangedin a (√3 × √3)R30° structure with a c(2 × 4) superlattice, where the simple hexagonal phase, the c(2 × 4) superlattice,and nonordered areas coexist. 16-MHDA assembles in a uniform monolayer with similar morphology to that of11-MUA, but molecular resolution could not be reached in STM due to both the hydrophilicity of the acid groupsand the poor conductivity of the thick monolayer. Nevertheless, the monolayer thicknesses estimated by XPS andelectrochemistry and the highly blocking character of the film observed by electrochemistry as well as the low watercontact angle are consistent with 16-MHDA molecules forming a compact monolayer on the Au(111) substrate withfully extended alkyl chains and acid groups pointing away from the surface. The results obtained for 16-MHDA werereproducible under different preparation conditions such as the addition or omission of acetic acid to the ethanolicsolution. Contrary to other reports, we demonstrate that ordered acid-terminated self-assembled monolayers are obtainedwith the same preparation conditions as those of the methyl-terminated ones, without any additional treatment.
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