Functional silicon carbide electrodes for applications in bioelectronics and biosensing

The wide bandgap semiconductor silicon carbide (SiC) is one of the most important materials for the development of electronic devices operating in extreme conditions. However, its exceptional stability and resistance to chemical attack also makes SiC a promising electrode material for bioelectronics and biochemical sensing. The advancement of SiC into such applications requires a detailed understanding of the electrochemical properties of SiC on the one side, and a possibility for the controlled surface modification with functional and biofunctional molecules on the other side. In the framework of this thesis both topics are addressed. The first part of this work is dedicated to a fundamental electrochemical study of n-type 6H-SiC and 4H-SiC electrodes. In particular, we analyze the energetic positions of the SiC band edges and investigate the electron transfer kinetics between SiC and redox molecules in the electrolyte. Importantly, we identify a broad distribution of surface states with energy levels in the bandgap, which significantly affect the charge transfer characteristics for both polytypes. In another set of experiments, we electrochemically investigate the SiC electrodes under UV illumination. We find that the accumulation of photogenerated holes at the interface results in the formation of a porous surface oxide layer; further, we suggest several alternatives to enhance the stability of the SiC substrates under illumination with above-bandgap light. In the second part of this work, we focus on the modification of SiC with self-assembled monolayers (SAMs). Starting from OH-terminated surfaces, different alkyl organosilane and organophosphonate monolayers are immobilized to the SiC electrodes via wet-chemical functionalization methods. We use different surface analytical techniques to examine homogeneity, thickness and grafting density of the SAMs. Furthermore, we successfully demonstrate the covalent nature of the bonds between the SiC substrate and the headgroups of the molecules. As for the non-functionalized samples, a detailed electrochemical study is conducted, and the influence of the SAMs on the resistive and capacitive behavior of the SiC electrodes is investigated. In the final part of this thesis, we utilize the endgroups of the monolayers as anchor sites to graft functional biomolecules to SiC. On the one hand, the electrodes are modified with photosynthetic reaction centers, and a successful electronic coupling between these proteins and the semiconductor is demonstrated. On the other hand, we present two biochemical conjugation protocols to functionalize SiC with DNA and PNA molecules. In addition to examining thickness and surface coverage of these oligonucleotide layers, they are used as receptor molecules to detect biochemical interactions in aqueous electrolytes. In particular, the hybridization of PNA with a complementary DNA strand and the binding of thrombin proteins to aptamer-functionalized SiC electrodes is shown. The results from this work demonstrate that n-type SiC is a promising substrate material for applications in the fields of bioelectronics and biochemical sensing. In particular, the possibility to extract charges from surface-immobilized proteins under photo-stimulation and to convert biomolecular interactions into electrical signals in electrolyte solutions will be of great importance for the development of novel SiC-based biofunctional devices.