Scanning Probe Microscopy Characterization of Organic Self-Assembled Monolayers

As technology advances, surface coatings become more and more important to assure materials performances. In recent years, molecular coatings have found larger acceptance and uses. Among them, self-assembled monolayers (SAMs) are attractive because they are versatile and their manufacturing approach is easy to scale up. Their mechanical properties, such as elasticity, are generally considered an important quality indicator. The molecular structure and ordering of SAMs is believed to be one of the key causes for their mechanical properties. A direct set of structure-property relationships has not been obtained yet. This is mainly due to the difficulty in achieving mechanical and structural information about SAMs at the same time. Most information currently available on intermolecular interactions in SAMs pertains to highly ordered systems on ideal flat surfaces, i.e. the easiest systems to study. This thesis presents a novel approach to address the question of determining structure and mechanical properties of self-assembled monolayers at the same time. The effective Young’s modulus, E*, of SAMs was measured using the Atomic Force Microscope operated in the bimodal excitation and detection scheme (bimodal AFM) while at the same time imaging at high resolution. Bimodal AFM was first used on alkanethiol molecules self-assembled on Au (111), a model system for SAMs. Surface elasticity has been reliably determined and found to be ligand-length dependent. An interpretation of this behavior is provided in the thesis. A similar investigation has been extended to the characterization of octadecylphosphonic acid SAMs on Al2O3, an industrially relevant system. The monolayer ordering as a function of monolayer formation time was explored, together with the evolution of surface elasticity. The latter allows distinguishing between the consecutive steps of ligand adsorption, monolayer ordering and multilayer formation. The method developed, i.e. simultaneous imaging and mechanical property derivation, was extended to provide localization of the chemical species present in thiolated binary SAMs. Within the systems tested phase separation down to ~10 nm domains could be observed both in the topography and in the elasticity channel. Furthermore, we show in this thesis that this approach can be extended to extract information from more complex biological systems as collagen fibrils. Specifically, in this thesis it is shown that β cyclodextrin addition to collagen fibrils affects the tropocollagen packing density. In conclusion, the results shown in this thesis demonstrate that bimodal AFM allows for accurate characterization of surface nanomechanical properties in organic self-assembled systems, as well as the way those scale with varying molecular ordering.

Stellacci, Francesco
Lausanne, EPFL

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 Record created 2018-01-17, last modified 2020-04-20

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