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In this thesis the potential of silicon carbide as electrode material for applications in bioelectronics and biosensing is studied. The electrochemical characteristics of n-type silicon carbide electrodes are analyzed, and methods for organic and bioorganic surface functionalization are presented. The formation of an electronic junction between photoactive proteins and the semiconductor electrode is demonstrated as well as the detection of biomolecular interactions in aqueous electrolytes., In dieser Arbeit wird das Potential von Siliziumkarbid als Elektrodenmaterial für Anwendungen in der Bioelektronik und Biosensorik untersucht. Das elektrochemische Verhalten von N-Typ Siliziumkarbid-Elektroden wird analysiert, und Methoden zur organischen und bioorganischen Oberflächenfunktionalisierung werden vorgestellt. Die Herstellung einer elektronischen Verbindung zwischen photoaktiven Proteinen und der Halbleiter-Elektrode wird ebenso gezeigt wie die Detektion von biomolekularen Wechselwirkungen in wässrigen Elektrolyten.

Understanding the mechanisms of charge transfer across the semiconductor/electrolyte interface is a basic prerequisite for a variety of practical applications. In particular, electrically active surface states located in the semiconductor band gap are expected to play an important role, but direct experimental evidence of surface states has proven to be challenging, and further experimental studies are required to verify their influence on the exchange of charge carriers between semiconductor and electrolyte. Due to its wide band gap, chemical stability, and controllable surface termination, silicon carbide (SiC) provides an excellent model system for this purpose. In this report, we provide a fundamental electrochemical study of n-type 6H-SiC and 4H-SiC electrodes in aqueous electrolytes containing the ferricyanide/ferrocyanide redox couple. Cyclic voltammetry and impedance spectroscopy measurements are performed over a wide range of potentials to determine the energetic positions of the SiC band edges and to investigate the electron-transfer kinetics between SiC and the ferricyanide molecules. For both polytypes, a broad distribution of surface states with energy levels close to the conduction band is found to mediate electron transfer, resulting in deviations of the observed charge transport characteristics from the predictions of well-established models. Moreover, a detailed evaluation of the impedance data allows for explicit correlation of the charge-transfer resistance associated with the ferricyanide reduction reaction with the potential-dependent distribution of surface states. In addition to the relevance of our studies for advancing the implementation of SiC in biosensing, electrocatalytic, and photocatalytic applications, the presented methodology can also be adopted for fundamental electrochemical investigations of other semiconductor electrodes.

The photoelectrochemical characterization of silicon carbide (SiC) electrodes is important for enabling a wide range of potential applications for this semiconductor. However, photocorrosion of the SiC surface remains a key challenge, because this process considerably hinders the deployment of this material into functional devices. In this report, we use cyclic voltammetry to investigate the stability of n-type 6H-SiC photoelectrodes in buffered aqueous electrolytes. For measurements in pure Tris buffer, photogenerated holes accumulate at the interface under anodic polarization, resulting in the formation of a porous surface oxide layer. Two possibilities are presented to significantly enhance the stability of the SiC photoelectrodes. In the first approach, redox molecules are added to the buffer solution to kinetically facilitate hole transfer to these molecules, and in the second approach, water oxidation in the electrolyte is induced by depositing a cobalt phosphate catalyst onto the semiconductor surface. Both methods are found to effectively suppress photocorrosion of the SiC electrodes, as confirmed by atomic force microscopy and X-ray photoelectron spectroscopy measurements. The presented study provides straightforward routes to stabilize n-type SiC photoelectrodes in aqueous electrolytes, which is essential for a possible utilization of this material in the fields of photocatalysis and multimodal biosensing.