Spatiotemporal dynamics of excitons and electron-hole plasma in atomically thin semiconductors with a moiré potential

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2022-05

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Transition metal dichalcogenides (TMDs) are layered semiconductor materials that can be exfoliated into atomically thin monolayers and deliberately stacked into vertical heterostructures to engineer artificial 2D materials with unique optical and electronic properties. When two such monolayers are stacked together with a small twist angle, the resulting heterostructure contains a moiré potential that can localize or impede the transport of optically generated excitons. In a similar way, thin layers of hexagonal boron nitride (hBN) can be exfoliated, twisted, stacked, and employed as a substrate to provide an externally sourced moiré potential to a TMD monolayer and thereby modify its exciton diffusion. In this dissertation, I present novel experimental results to demonstrate the above phenomena and explore the spatiotemporal dynamics of both excitons and electron-hole plasma that can form through optical excitation in TMD semiconductor structures with a moiré potential. Specifically, we stacked two TMD monolayers, MoSe₂ and WSe₂, into a vertical heterostructure with a small relative twist angle (near 60° or H-stacking) and hBN encapsulation. By performing spatiotemporally resolved pump probe measurements with different pump powers and repetition rates, we probed diffusion in this heterostructure across three orders of magnitude of excitation densities. While the moiré potential impedes the diffusion of interlayer excitons at low excitation densities, high excitation densities above the Mott density allow for the formation of an electron-hole plasma that undergoes a sub-picosecond rapid expansion driven primarily by coulomb repulsion and Fermi pressure. In a separate sample, we explore a WSe₂ monolayer that is placed on top of an hBN substrate with two distinct regions. One region of this substrate contains only a single thin layer of hBN, while the other substrate region has an additional thin hBN layer that is stacked above the first layer with a small relative twist angle. The WSe₂ monolayer that resides above the hBN layers only feels a moiré potential on top of the twisted substrate. Spatiotemporally resolved pump probe measurements show that the WSe₂ monolayer has reduced exciton diffusion on top of the twisted hBN substrate.

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