New Techniques for Interface Characterization by Combining Scanning Probe Microscopy with Electrochemistry and Nanofluidics

Abstract

Upcoming challenges in energy storage systems and the growing demand for automatized high-throughput screening assays initiated the development of new approaches in atomic force microscopy (AFM). A better understanding of the structure-activity relationship in such systems is a prerequisite for further rational design. However, it requires an in-depth understanding of the interfacial properties on a fundamental level. In this respect, AFM provides the possibility to locally probe and manipulate the interface. Novel concepts in probe microscopy incorporate nanofluidics or analytical techniques such as electrochemistry, which allow to simultaneously study multiple material properties. However, a major prerequisite for novel AFM concepts are suitable cantilever probes. The objective of this thesis covers the development and evaluation of combinatoric AFM methods and the understanding of physico-chemical interactions and processes taking place at the probe tip. In the first part of the thesis, AFM cantilevers bearing a nanoelectrode tip have been used to sense locally resolved faradaic processes on electrodes with heterogeneous reactivity. Additionally, this approach provides access to the topography of the sample and surface properties such as adhesion. As size and surface properties of the nanoelectrode tip dramatically influence its resolution, test samples are a major prerequisite for monitoring the quality of the tip. Here, three different and simple to prepare micro-structured electrode samples have been evaluated, which can be fabricated without the need of expensive or complicated equipment. It was demonstrated that these type of electrochemical probes are able to resolve heterogeneities in redox-reactivity with a lateral resolution < 100 nm. The AFM additionally allows to study macroscopic aggregation and adsorption processes of colloids on chemically heterogeneous surfaces by directly measuring interaction forces on the single particle level. In this project the colloidal probe technique has been used to study direct interaction forces of latex particles on orthogonally functionalized gold nano-mesh electrodes. By laterally scanning the chemically heterogeneous surface with the latex probe, electrostatic interactions and adhesion forces have been measured as a function of the pH. The measured electrostatic focusing effect towards the nano-holes properly explains the macroscopic self-assembly of these particles on the nano-mesh electrode. Since the preparation of 'classical' colloidal probes is restricted to the µm-level, industrial relevant nanoparticles were so far hardly accessible for direct force measurements. Due to recent progresses in micro-fabrication techniques, it became possible to construct micro-channeled AFM cantilevers with a defined aperture. These cantilevers allow for combining the force-sensing capabilities of an AFM with nanofluidic techniques, for example to aspirate or eject fluids at the aperture by an externally applied pressure. Furthermore, the so-called FluidFM technology allows the…