Theoretical and experimental work on superconductivity has won a number of Nobel prizes in Physics, beginning with the prize to Kamerlingh Onnes in 1913 that included the initial discovery of superconductivity in mercury. Superconductivity has since been at the forefront of research in condensed matter physics. Furthermore, since the first isolation of graphene by Geim and Novoselov in 2004, there has been growing interest in other monolayer and few-layer crystals. Like graphene, other materials can be exfoliated due to the weak van der Waals interactions between layers, primarily the transition metal dichalcogenides (TMDs). Atomically flat and chemically stable thin two dimensional (2D) layers of TMDs have opened up new opportunities for discovering exciting new physics and ultimately developing thin flexible devices. Defect-free exfoliated TMDs are regarded to be ideal materials for use as channels for field effect transistors (FET), which have been shown to possess remarkable electronic properties. Recent advances in field effect-based TMD devices have been achieved using ionic liquid gating and the formation of electrical double layers.Using the techniques previously developed for isolating graphene, few-layer crystals of 1T- and 2H-TaS2 have been obtained in this project to be used as channel materials for FET and ionic field effect transistor (iFET) devices that incorporate DEME-TFSI ionic liquids as a top gate to control the carrier density.In the first experimental chapter (chapter 5) iFETs using a 1 μm thin film of a highly boron-doped diamond (BDD) as the channel material are introduced and the influence of top gating on the transition temperature using a DEME-TFSI ionic liquid is studied. An enhancement in the Tc of the BDD sample under positive top gate potentials is shown as a result of electron doping at the grain boundaries leading to stronger coupling between the grains. The following chapter (Chapter 6) describes low temperature measurements of graphene FET (GFET) devices. These devices were fabricated to enable a reliable and effective calibration for the DEME-TFSI top gate specific capacitance against the known back gate capacitance. This represents a valuable reference for ionic liquid gating studies of TMD materials.The last experimental chapter describes the electrical properties of few-layer 1T-TaS2 (initial section) and 2H-TaS2 (final section) samples used as channels in FET devices. Charge density wave (CDW) transitions in 1T- and 2H-TaS2 are investigated and gating measurements using ionic liquids on these samples are described and summarised. Although no gate influence was seen on the CDW in 2H-TaS2 , a suppression of the CDW transition in cooling cycles of a 1T-TaS2-based FET sample was observed. This suppression demonstrates that accumulation of additional charge carriers in the sample drives it into a metallic state. In a ∼15 nm 2H-TaS2 FET device, strong enhancement of the superconducting critical temperature from 0.8 to 4.7 K is observed with DEME-TFSI top gating. The influence of an additional back gate potential on the device enhances the transition temperature still further up to 5 K. This indicates a co-operative effect between the top and back gates of the sample. It was also demonstrated that 2H-TaS2 crystals are susceptible to intercalation by DEME+ cations in the ionic liquid; a clear enhancement of Tc was observed after simply placing a drop of ionic liquid on a 2H-TaS2 flake without application of a top gate bias. This research project has studied superconductivity in 2D materials and illustrates the capability of ionic liquid gating as a versatile tool to modify the carrier concentration and enhance the critical temperature of a wide range of different materials.
|Date of Award||23 Jul 2018|
|Supervisor||Simon Bending (Supervisor) & Kei Takashina (Supervisor)|