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Abstract

Transition Metal Dichalcogenides (TMDs) represent a new frontier for optoelectronic device applications. While these materials have found success in prototype devices, their development into consumer-grade devices requires a deeper understanding of their fundamental properties is needed before development into consumer-grade devices. Ultrafast spectroscopy provides a route to ascertain the charge transfer, many-body processes and relaxation dynamics of these systems. In this thesis, I use Two-Dimensional Electronic Spectroscopy to probe fundamental properties of TMD materials. I present results on the TMD MoS2, a prototypical system that has been extensively studied in regards to its fundamental properties and potential device applications. 2DES provides new insight into the sub-50 fs dynamics of TMDs, revealing a rich class of energy transfer and valley crossing dynamics which occur below the time resolution of other techniques such as pump-probe spectroscopy. Harnessing the power of 2DES, I first conduct a comparison of the 2DES spectra in monolayer and overgrown MoS2, revealing enhanced dark state activity which manifests on the 150 fs timescale in overgrown samples, as well as transfer from the B to the A exciton which proceeds on the 20 fs timescale. I next undertake cryogenic studies on monolayer MoS2, which uncover the dominance of Bandgap Renormalization, which is an effect of collective excitation on single excitons, over the explicitly many-body optically-induced biexciton formation. We find that the bandgap renomalization process is phonon-assisted in MoS2, and is distinct from the spectral diffusion process. Using helicity-resolved 2DES, we reveal spin and valley crossing on the 10 fs timescale independent of temperature or fluence in wafer-scale TMDs, far faster and different in mechanism than what is observed in exfoliated samples. We ascribe this fast crossing to a Dexter-like intervalley coupling mechanism, as has been suggested in the literature. Future planned experiments include additional investigation into the kinetics of the bandgap renormalization process, experiments on TMD heterostructures, examining of the linewidth broadening mechanisms of MoS2 at cryogenic temperature, dynamics of electron-hole liquids and higher-lying excitonic complexes in TMDs, and valley and spin crossing studies on other TMDs.

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