Manipulating the electronic properties of group IV TMDs with ionic liquids

Two dimensional (2D) materials have gained extensive attention and research interest over the past few decades, particularly since the isolation of graphene in 2004 which prompted the development of powerful graphene-based devices. These devices are used for a range of applications including energy generation and storage, chemical sensors, optical devices and high speed electronics. The success of these naturally prompted further investigation into the novel properties and exciting opportunities that other ultrathin devices could hold. More recently, focus has shifted towards a class of 2D materials called transition metal dichalcogenides (TMDs) with the chemical formula MX2, where a plane of transition metal atoms (M) is sandwiched between two layers of chalcogen atoms (X). Depending on the combination of transition metal and chalcogens used, a variety of electronic properties can be exhibited including a tuned transition from the insulating to semiconductive state, and even to the superconductive state. The latter can be achieved with ionic liquids, which are a molten salt at room temperature made of bulky anions and cations, through a technique called ionic liquid gating. Indeed, TMDs can be made into field effect transistors and gated with ionic liquids, which highly dope the material and can induce it into the superconductive state. However, while it is recognised that ionic liquids have powerful properties, they are not yet fully understood, and only a limited number of commercially available ionic liquids are commonly used. Furthermore, certain less explored TMDs such as ZrSe2 and HfSe2 are predicted to have remarkably high charge mobility at room temperature, making their application for ionic liquid gated field effect transistors especially promising in the investigation for exotic electric states, and applications in energy storage/conversion, sensors, and optoelectronics. This PhD will thus aim to manipulate the electronic properties of less explored TMDs, as well as understand the mechanisms involved in ionic liquid gating induced superconductivity in 2D materials by investigating the different interactions between the ionic liquids and the TMDs. Through the exploration of novel fabrication techniques, this PhD also aims to fabricate increasingly high performing ionic liquid FETs, as well as improve methods to manipulate air sensitive TMDs. Observing and controlling gate-induced superconductivity will increase the number of known superconductors as well as help improve understanding of 2D superconductivity. Furthermore, superconductive materials are currently used in quantum computers which promise applications in chemical simulations for individually tuned medical treatments, data encryption, or even computational problems which are currently unsolvable on classical computers. Thus ionic liquid gate-induced superconductivity of TMDs could easily aid the development of quantum computers and their scalability, particularly if it is induced at high temperatures. Finally, compared to the more commonly used back-gating technique, ionic liquid gating allows for the development of devices with flexible substrates which are essential for the development of bioelectronics and wearable probes.