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Catalysts speed up a chemical reaction or allow for reactions to take place that would not otherwise occur. The chemical nature of a catalyst and its structure are crucial for interactions with reaction intermediates. An electrocatalyst is used in an electrochemical reaction, for example in a fuel cell to produce electricity. In this case, reaction rates are also dependent on the electrode potential and the structure of the electrical double-layer. This work provides a valuable overview of this rapidly developing field by focusing on the aspects that drive the research of today and tomorrow. Key topics are discussed by leading experts, making this book a must-have for many scientists of the field with backgrounds in different disciplines, including chemistry, physics, biochemistry, engineering as well as surface and materials science. This book is volume XIV in the series "Advances in Electrochemical Sciences and Engineering".
This technique has the potential for application in nano-scale systems beyond single-molecule junctions. These results constitute another step toward the development of single-molecule devices with commercial applications. Finally, the methods presented in this thesis offer further insights into the electronic structure of molecular junctions. We show that we can assess energy-level alignment at metal molecule interfaces– this alignment is a crucial parameter controlling the proper- ties of the interface. We also demonstrate that we can probe large regions ( 2eV) of the transmission function which governs charge transport through the junction. By being able to control level alignment, we are also able to offer preliminary studies on single-molecule junctions in the resonant transport regime. Combined, the results presented in this thesis grant new insights into electron transport at the nanoscale and provide new routes for the development of functional single-molecule devices.
Building electronic devices out of single molecules has been the ultimate goal of downscaling electric circuits. Understanding charge transport through single-molecule junctions is central to achieving this goal. To gain deeper insights into charge transport through single molecules, this dissertation centers on detailed experimental modulation and control of charge transport through single-molecule junctions using modified scanning probe microscope break-junction (SPM-BJ) techniques. First, I explored the effect of molecule-electrode contact interfaces. Using force-conductance cross-correlation analysis, I mapped out the correlation between conductance and force of modulated Au-octanedithiol-Au junctions measured with CAFM-BJ. The investigation of the conductance change during junction elongation showed a unique contact tunneling barrier of octanedithiol, which was interpreted by a newly developed contact barrier model. A systematic control of anchoring groups of benzene-based molecular junctions showed that current rectification occurred whenever asymmetric anchoring groups were introduced, which is mainly due to asymmetry in potential drop across the contacts. Second, I studied the impact of DNA's structural change on its conductance. The conductance of poly d(GC)4 DNA duplex was found to decrease by two orders of magnitude during a B- to Z-form structural transition, which is mainly attributed to the breaking of Ï0-Ï0 stacking between adjacent base pairs caused by the transition. Using stretch-hold mode STM-BJ technique, the structural transition was successfully monitored solely based on conductance measurements. Then, I attempted to modify the structure of DNA for functional I-V feature. A DNA-based molecular rectifier was for the first time constructed by site-specific intercalation of coralyne molecules into a custom-designed DNA duplex. Measured I-V curves of the resulting DNA-coralyne complex showed strong rectification with a rectification ratio of 15 at 1.1V. Based on NEGF-DFT calculations, this rectification is mainly caused by asymmetric coupling of the HOMO-1 level to the electrodes when an external bias is applied, an unprecedented rectification mechanism. Finally, Fermi level pinning of charge transfer resonances was investigated in junctions composed of terthiophene containing molecular wires. Taken together, these results not only provide new understanding of charge transport through molecules, they also opened new route for building functional molecular electronic devices.