This thesis investigates the cation-assisted crystallographic phase transitions and optical property modifications of two-dimensional transition metal dichalcogenides (2D TMDs) using optical techniques. The thesis begins with an introduction to the context of the research (Chapter 1), followed by an overview to the key materials and theoretical concepts in Chapter 2. In Chapter 3 we introduce in the key experimental methods used in the work.

Chapter 4 examines the mechanism of semiconducting hexagonal (1H, 2H) to metallic tetragonal (1T, distorted 1T) phase transition reactions in 2D TMDs during chemical intercalation of lithium cations, employing real-time optical visualization. We directly quantify the dynamics of the phase transition in micrometer-sized TMD flakes with diffraction limited resolution. In addition, we complement the results with ex-situ Raman and photoluminescence measurement. We show this reaction to be a charge-limited surface driven intercalation reaction.

After establishing our ability to probe reaction dynamics in-situ via optical microscopy, in Chapter 5, we explore the effect of optical excitation on this phase transition reaction. We demonstrate that illuminating the material with photons having energy above the band gap accelerates the transition of 2H to 1T phases by more than two orders of magnitudes. This finding also enables a novel and rapid spatial photo-redox phase patterning within mono- and few layered 2D-TMDs. We then demonstrate the improved performance of a phase-engineered photodetector based on mono-layer 1H-MoS2 using this method. We compare chemical lithiation to electrochemical lithiation to develop a detailed mechanistic picture of this process. Based on our findings, we propose a universal route for chemical cation intercalation and phase engineering of TMDs based on redox-potential matching. This is supported by our demonstration of the same phase transition using a newly synthesized solvent of polycyclic aromatic hydrocarbon with lithium and sodium, which significantly shortens the reaction time from several days to just a few minutes, and replaces the highly pyrophoric chemical n-butyllithium, which has been used in this process for the past five decades. This advanced phase engineering method can be applied to a various type of TMDs, such as powder, crystal, and thin flakes, and offers a promising pathway for scalable production of phase-engineered TMDs.

Finally, in Chapter 6 we conduct chemical treatments using the cation-(bis(trifluoromethane) sulfonimide) system on MoS2 grown by metal-organic chemical vapour deposition (MOCVD). By altering the surrounding chemical nature of the cation, we were able to maintain the phase of MoS2 in its semiconducting hexagonal, and only enhance the radiative recombination intensity. This effect is particularly enhanced by adding additional prior treatment using sulfides, which passivate sulfur defects.

In conclusion, through the utilization of various optical characterization methods, we have explored the versatile role of cations in phase engineering and luminescence properties for TMDs.