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Yang, Dongyang

University of British Columbia

Atomically thin Transition metal dichalcogenide (TMD) with different stacking orders can exhibit distinct quantum phenomena when interacting strongly with external fields. Rhombohedral (R)-stacked TMDs, where neighboring layers are oriented in the same direction, can be created via chemical synthesis or artificial stacking with a small twist. Understanding how atomic registry determines the properties of TMD homo-bilayer is crucial for revealing exotic physics in 2D semiconductors. This thesis uses advanced optical spectroscopy to explore excitonic and correlated phenomena in both homogeneous and twisted R-stacked TMD homo-bilayers. In R-stacked MoS₂ bilayers, we observe spontaneous electrical polarization induced by asymmetric interlayer coupling and Berry phase effects. Excitonic effects help reveal the electronic band structure, while a novel photovoltaic effect, driven by the depolarization field (DEP) in Gr/R-MoS₂/Gr heterostructures, shows potential for scalable optoelectronic applications. Using non-degenerate pump-probe photocurrent spectroscopy, we extract an intrinsic photocurrent speed of ∼2 ps, contributing to the understanding of ultrafast carrier dynamics. The out-of-plane electrical polarization in R-stacked MoS₂ can be switched under an external electric field through in-plane sliding motion, known as sliding ferroelectricity. We develop an optical method to probe domain wall motion in R-stacked MoS₂ homo-bilayers and trilayers, demonstrating that the pinning and de-pinning of domain walls drive the polarization switching. This leads to unconventional two-dimensional ferroelectric behavior, which could open new avenues for device engineering. Finally, we investigate the strongly correlated physics in R-stacked MoSe₂ bilayers with a small twist, revealing a series of correlated insulating states at both integer and fractional fillings in Γ-valley flat bands. Contrary to continuum-band calculation, we observe a Mott-insulator state instead of a semi-metal on a half-filled honeycomb lattice, offering insights into the semi-metal-to-insulator transition. This thesis provides the latest understanding of optical and electronic properties in rhombohedral-stacked TMDs, making them a promising platform for exploring both fundamental condensed matter physics and future applications.

Text

2024-10-01

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/89310

Physics

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/