Ionic tuning of the electrical and optical properties of two-dimensional materials
Keywords: Materials Science and Engineering.

Two-dimensional transition metal dichalcogenides (TMDCs), which consist of atomically thin layers stacked together by van der Waals interaction, exhibit unique physical and chemical properties due to the quantum confinement effect and their respective d orbital filling. In particular, the semiconducting TMDCs are of both technological and fundamental interests, and tremendous efforts have been devoted to the in-depth understanding of the material properties and the development of next-generation electronic and optoelectronics devices. An important aspect involved in these studies is the modulation of the material properties. Conventional approaches such as alloying, chemical modification, heterostructure assembly, and electrical gating have been successfully used to bring out new properties in TMDCs, yet a versatile tuning method that can achieve an even wider range of tuning in a reversible and continuous manner remains elusive. Hereby, the concept of ionic tuning is introduced where the interaction between the mobile ions and TMDCs is exploited to tune the materials both at the interface and within the crystal. In this thesis, I will present my work on two different kinds of ionic tuning techniques, ionic gating and intercalation. In Chapter 2, the concept and working principle of ionic gating is first introduced, followed by the discussion of my work on developing a novel solid-state ionic gating platform based on a fluoride ion conductor LaF3. Solid-state LaF3 encompasses the benefits of both oxide dielectrics and liquid electrolyte dielectrics. A maximum carrier density of 4 x 10^13 cm-2 can be induced at 220 K in n-type MoS2 (thickness about 10 ~ 20 nm), inducing an insulator-metal transition, by applying a much lower gate voltage than that needed with Si-based transistors. In the two chapters that follow, two kinds of applications with LaF3 gating are discussed. In Chapter 3, LaF3 gating is incorporated into optical absorption and photoluminescence measurements to study the exciton physics in monolayer WSe2. LaF3 serves as a nice platform for the experiment because of the mechanical robustness and the ionic nature of the operation. With LaF3 gating, a maximum hole density of 3.5 x 10^13 cm-2 can be induced at 180 K, which enables the discovery of an excitonic Mott transition from the exciton-dominant regime to the electron-hole plasma regime. In Chapter 4, the development of gate-voltage-dependent ARPES with LaF3 is presented. The goal of this work is to develop a powerful tool to study how the band structure evolves with carrier doping in a dynamic and reversible manner. In the process, multiple technical issues are identified, and their effects and potential workarounds are discussed. In Chapter 5, the second ionic tuning approach, intercalation, is introduced. The concept and techniques for intercalation are first discussed, followed by the discussion of my work developing a facile low-temperature solvent-based chemical intercalation technique that induces dramatic changes in a two-dimensional semiconductor SnS2. In combination with lithography, seamless lateral heterostructures can be fabricated. Motivated by this work, I further developed an electrochemical intercalation approach that can insert various kinds of ions into layered SnS2. The technical development and issues are elaborated. Finally, the outlook for future research based on ionic tuning is presented. In terms of ionic gating, the combination of solid-state LaF3 gating with diverse material characterization techniques including spectroscopy techniques, synchrotron-based equipments, and surface probe measurements. Moreover, the solid nature of the LaF3 crystals enable doping-dependent studies on liquid phase systems such as catalytic reactions. As for intercalation, further development and understanding of the intercalated compounds and the intercalation process, both chemical and electrochemical, are crucial. Moreover, the exploration of sequential multi-ion intercalation, where more than one kind of ion is intercalated into the same flake, could be an interesting direction, involving the study of ion-host interaction and the interaction between ions with different valence states, sizes and chemical properties