Abstract
MoS2, a two-dimensional semiconductor from the transition metal dichalcogenide (TMD) group, has displayed the ability for its electronic properties to be significantly manipulated through ionic liquid (IL) gating. Using an IL as a gate dielectric and through the formation of an electric double layer (EDL) at the MoS2-IL interface, IL gating can induce exceptionally high carrier densities, enabling access to novel electronic phases and phenomena previously unattainable with conventional gating techniques. The synergy between MoS2 and IL gating offers a promising avenue for realizing next-generation electronic devices with enhanced performance and novel functionalities. However, the full potential of this approach can only be realized with a deep understanding of the EDL and its intricate relationship with IL ions and MoS2 under gating. In this thesis, the dependence of IL-gating performance on three core parameters of IL gating were investigated: the physicochemical properties of the cations, temperature dependency, and environmental influences.The prevalence of water in the environment necessitated a study into the effects of water presence in IL gating. Two similar ionic liquids, DEME-TFSI and N1224-TFSI, were studied with a combination of cyclic voltammetry (CV), in-liquid atomic force microscopy and differential EIS measurements performed in air and in an inert atmosphere. A narrowing of the electrochemical window was observed in air and a large increase in redox current was indicative of water-induced redox and hydrogen evolution reaction processes. Differential EIS measured an 88.55 µFcm-2 and 51.75 µFcm-2 increase in the peak cathodic capacitance for DEME-TFSI and N1224-TFSI respectively in air compared to a dry, inert atmosphere. Concurrently, the anodic peak capacitance was 16.59 µFcm-2 lower in air for DEME-TFSI and 13.14 µFcm-2 lower in air for N1224-TFSI. In-situ Raman spectroscopy of IL gated MoS2 in air and in vacuum showed significant degradation of the MoS2 Raman peaks at high gate voltages. Symmetry dependent electron-phonon renormalisation was observed during in-situ gating sweeps, with stronger suppression of the A1g mode when measured in vacuum compared to in air.
The following chapters characterised the relationship between cationic structure and MoS2 gating performance. The electrochemical and gating characteristics were measured on 6 quaternary ammonium based ILs where a clear trend was established between the peak cathodic differential capacitances and the size of the cation; The largest recorded peak cathodic capacitance was 91 µFcm-2 for N1114-TFSI, the smallest cation measured, while the smallest recorded peak cathodic capacitance was 11.7 µFcm-2 for N222MeOEt-TFSI, the largest cation measured. The sheet carrier density n2D of IL gated MoS2 followed a similar trend, where smaller cationic sizes resulted in more than a tenfold increase in induced sheet carrier concentration and reported induced n2D values greater than 1014 cm-2. Resistance-temperature curves of IL-gated MoS2 for temperatures as low as 4 K corroborated the impact of cation size, while revealing anomalies like the potential Kondo-like effect induced by sulfur vacancy band hybridization in certain ILgated configurations. Differential scanning calorimetry measurements revealed the variance in the phase transitions of the ILs, including cold crystallisation.
| Date of Award | 24 Apr 2024 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Sara Dale (Supervisor) & Daniel Wolverson (Supervisor) |