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The ability to measure, control and understand charge transport in single molecules connected with external electrodes is a basic requirement in molecular electronics. The charge transport depends on the electronic states of the molecules, as well as on other factors, such as the details of the molecule-electrode contact, and the local environment of the molecules. A reliable technique allowing one to measure and control the charge transport in single molecules is highly desired. This dissertation describes charge transport studies in various molecular junctions fabricated by using nanoelectrodes on silicon chip, an adjustable scanning tunneling microscope (STM) break junction, and a combination of mechanical break junction and electrodeposition techniques.
Studying charge transport through single molecules is of great importance for unravelling charge transport mechanisms, investigating fundamentals of chemistry, and developing functional building blocks in molecular electronics. First, a study of the thermoelectric effect in single DNA molecules is reported. By varying the molecular length and sequence, the charge transport in DNA was tuned to either a hopping- or tunneling-dominated regimes. In the hopping regime, the thermoelectric effect is small and insensitive to the molecular length. Meanwhile, in the tunneling regime, the thermoelectric effect is large and sensitive to the length. These findings indicate that by varying its sequence and length, the thermoelectric effect in DNA can be controlled. The experimental results are then described in terms of hopping and tunneling charge transport models. Then, I showed that the electron transfer reaction of a single ferrocene molecule can be controlled with a mechanical force. I monitor the redox state of the molecule from its characteristic conductance, detect the switching events of the molecule from reduced to oxidized states with the force, and determine a negative shift of ~34 mV in the redox potential under force. The theoretical modeling is in good agreement with the observations, and reveals the role of the coupling between the electronic states and structure of the molecule. Finally, conclusions and perspectives were discussed to point out the implications of the above works and future studies that can be performed based on the findings.
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".
In this thesis we studied the conductance of single molecule junctions. We focused on the consequences of nuclear motion on the transport characteristics of the junction and investigated the switching behavior of tautomeric molecules. Two different theoretical methods were used: scattering and density matrix theory. Scattering theory was applied to to two classes of molecules. In the benzene-thiolate systems we demonstrated the importance of respective time/energy scales, comprising the lifetime of the hole on the molecule, the size of the vibronic coupling, and the vibrational frequency, which determine vibrational effects. For the tautomers switching behavior depended significantly on the chemical nature of the hydrogen carrying unit. Density matrix theory was used to compute the current-voltage curve of a 'two electronic states'-'one mode' system, which we compared to similar results, obtained from scattering theory.