Novel Layered and 2D Materials for Functionality Enhancement of Contacts and Gas Sensors

II The cover image shows partial intercalation of gold (in yellow) into Ti3SiC2 observed by scanning transmission electron microscopy; a result from my research. During the course of research underlying this thesis, I was enrolled in Agora Materiae, a multidisciplinary doctoral program at Linköping University, Sweden. Abstract Chemical gas sensors are widely-used electronic devices for detecting or measuring the density levels of desired gas species. In this study, materials with established or potential applications for gas sensors are treated. For the case of high-temperature applications (≈600 °C), semiconductor-based gas sensors suffer from rapid oxidation of the metallic ohmic contacts, the same cause-of-failure as for the general case of high-temperature semiconductor electronics. 4H-SiC is an ideal semiconductor for high-temperature applications. Ti3SiC2 is a known ohmic contact to 4H-SiC with the known two-step synthesis process of post-annealing of pre-deposited Ti/Al multilayers or sputter-deposition of Ti3SiC2 films at > 900 °C. Here, sputter-deposition of Ti on 4H-SiC at > 900 °C is presented as a novel single-step method for the synthesis of Ti3SiC2 ohmic contacts, based on a concurrent reaction between sputter-deposited Ti and 4H-SiC. Ti3SiC2, similar to any other known ohmic contact, degrades rapidly in high-temperature oxidizing ambient. To try to overcome this obstacle, noble-metal diffusion into Ti3SiC2 has been studied with the goal to retain ohmic properties of Ti3SiC2 while taking advantage from the oxidation resistivity of noble metals. A novel exchange intercalation between Ti3SiC2 and Au is discovered which results in almost complete exchange of Si with Au giving rise to novel Ti3AuC2 and Ti3Au2C2. Ti3IrC2 is also synthesized through exchange intercalation of Ir into Ti3Au2C2. All the aforementioned phases showed ohmic properties to 4H-SiC. This technique is also studied based on Ti2AlC and Ti3AlC2 resulting in the synthesis of novel Ti2Au2C and Ti3Au2C2, respectively. Using Ti3AuC2 and an IrOx/Au capping layer, an ohmic contact was manufactured which maintained ohmic properties and showed no structural defects after 1000 h of aging at 600 °C air. Ti3SiC2 is a member of a large family of materials known as Mn+1AXn phases. While exchange reactions of Si (or Al) planes inTi3SiC2 (Ti2AlC and Ti3AlC2) is presented here, a worldwide research already exists on chemical removal of the same atomic planes from different Mn+1AXn phases and the synthesis of Mn+1Xn sheets known as MXenes. I performed a theoretical study regarding simulation of electronic and structural properties of more than120 different possible MXene phases. The …