Abstract
Modern electronic devices utilise logic circuits that are based on modulating the resistance of the device such that it is in a stable and well-defined ‘on’ or ‘off’ state. However, in many materials certain electronic states that span such a modulation of resistance remain inaccessible unless alternative doping methods are used. This thesis presents experimental studies of the significant changes in metallic and insulating behaviour of two two-dimensional (2D) systemsto demonstrate how the electronic properties of these materials can be tuned using chemical composition alteration through alloying and ionic liquid gating.
Part I of this thesis concerns the 2D material 1T-TaS2−xSex (where x = 0 to 2), which is a transition metal dichalcogenide (TMD) compound wherein varying the stoichiometry x sees new electronic phases emerge, including commensurate and incommensurate charge density waves (CDWs), superconductivity, and a Mott insulating state. Here a particular composition, 1T-TaS1.2Se0.8, is grown using chemical vapour transport to produce bulk single 2D crystals which can also be thinned down to flakes using mechanical exfoliation. The bulk crystals are characterised using energy dispersive x-ray spectroscopy to provide an accurate determination of their stoichiometry and assess the success of this growth method. Electronic transport data is then presented to show that this particular x = 0.8 compound exhibits a novel hysteresis effect in resistance during repeated cool-down and warm-up measurement cycles which allows different metallic and insulating states to be accessed. It is then shown that the transitions to these states can be suppressed for faster cooling rates and thinner crystals. It is posited that these transitions relate to the emergence of different CDW phases and arise from changes in the microstructure of the surface during cooling and warming. In order to investigate such microstructure changes, scanning tunnelling microscopy data is then presented to show that a novel region with a greater depletion of states than neighbouring regions emerges that has not been seen before in this family of compounds. The behaviour of these ‘depleted’ regions and other aspects of the microstructure align with the increasingly insulating transport behaviour for different temperature cycles.
Part II of this thesis investigates the oxide perovskite SrTiO3 (STO), which is well-known to form a channel of highly mobile electrons called a two-dimensional electron gas (2DEG) under the influence of large electric fields. However many common methods to dope this 2DEG face limitations in the extent to which low and high carrier concentrations can be accumulated along this channel. This thesis uses the doping method of ionic liquid gating to investigate the effects of
altering the cation structure of the commonly-used gating ionic liquid DEME-TFSI, in order to optimise ion accumulation at this 2DEG interface and induce metal-insulating transitions. The fabrication of STO electric double layer field effect transistors is detailed, and ambient and low temperature transport measurements are performed using these transistors. Using a certain ionic
liquid N111MeOEt-TFSI is shown to completely repress the metal-insulating transition, and may be explained by the occurrence of crystallisation instead of a glass transition on cooling down this ionic liquid.
| Date of Award | 26 Jul 2023 |
|---|---|
| Original language | English |
| Awarding Institution |
|
| Supervisor | Sara Dale (Supervisor) & Simon Bending (Supervisor) |