Odd–Even Effect in Molecular Packing of Self-Assembled Monolayers of Biphenyl-Substituted Fatty Acid on Ag(111)
2018
Krzykawska, Anna | Szwed, Monika | Ossowski, Jakub | Cyganik, Piotr
Self-assembled monolayers (SAMs) of the homologous series of biphenyl-substituted fatty acids on Ag(111) in the form of (C₆H₄)₂–(CH₂)ₙ–COO/Ag (BPnCOO/Ag, n = 1–4) were studied using infrared reflection absorption spectroscopy (IRRAS), X-ray photoelectron spectroscopy (XPS), and scanning tunneling microscopy (STM). The combination of spectroscopic (IRRAS and XPS) and microscopic analyses (STM) revealed that depending on the parity of the parameter n, which defines the length of the short aliphatic linker, two types of structures are formed by the BPnCOO/Ag. For n = even, highly ordered and stable SAMs are formed in a very short time. For n = odd, the respective monolayers have a disordered liquidlike structure with more canted orientation of the molecular backbone and the anchoring carboxylic group, which results in lower packing density and film thickness compared to the even-numbered SAMs. By comparing obtained results with former odd–even effects reported for analogous SAMs based on thiols and selenols, a common, qualitative model relating the odd–even effect to the monolayer stability and structure is discussed. Our results demonstrate that for BPnCOO/Ag, this odd–even effect is particularly strong and fully controls the ability of molecules to form highly ordered structures. This observation seems to be of key importance for the design of SAMs based on the carboxylic group, which, for correctly designed molecules, forms much better two-dimensionally ordered structures compared to commonly used thiols.
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