Spectroscopic study of excitonic and correlated physics in two-dimensional semiconductors and moiré superlattices

Lian, Zhen
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Shi, Sufei, S.S.
Plawsky, Joel, J.P.
Chakrapani, Vidhya, V.C.
Lu, Toh-Ming, T.L.
Shi, Sufei, S.S.
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Chemical engineering
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Two-dimensional (2D) semiconductors and moiré superlattices exhibit unconventional physical properties such as tightly bond excitons, enhanced light-matter interaction, valley degree of freedom and strong electron-electron correlation effects, making them promising for future electronic and optoelectronic applications. To lay the foundation for future device development, it is crucial to understand how the properties of excitons and correlated electronic states can be manipulated.The moiré superlattices of transition metal dichalcogenides (TMDCs) are known to host localized moiré excitons trapped by moiré potential. Correlated insulating states such as Mott insulator and generalized Wigner crystals also emerge in these systems due to their isolated flat bands. Despite recent extensive research into moiré heterobilayers and homobilayers, a fundamental understanding of the moiré coupling in the out-of-plane direction is still lacking. In this study, the moiré excitons and correlated insulating states in multilayer WSe2/ monolayer WS2 moiré superlattices were explored by reflectance contrast spectroscopy combined with microwave impedance measurement. The excitons from the WSe2 layers away from the interface is found to be barely affected by the moiré coupling, suggesting the moiré potential is limited to the interface of the heterojunction. The carrier density to reach each correlated insulating states determined to be the same regardless of the WSe2 layer number, suggesting these moiré systems also possess isolated flat valence band. Modulations of moiré exciton energies and critical temperatures of correlated insulating states were also observed. Under an out-of-plane electric field, the overlap of the flat valence band and the trivial parabolic band was achieved in bilayer WSe2/ monolayer WS2. The correlated insulating state at one hole per unit cell is found to be robust under electric field, suggesting a phase transition from a Mott insulator to an excitonic insulator. TMDC moiré heterobilayers are known to host long-lived, valley-polarized dipolar interlayer excitons, which could be potentially used as single quantum emitters. In the second part of this dissertation, the effects of exciton-carrier interaction and exciton-exciton interaction in WSe2/WS2 moiré heterobilayer were studied using photoluminescence (PL) spectroscopy. At the Mott insulating states, the peak positions of the ground state interlayer excitons exhibit blueshifts, which were attributed to the repulsion between the interlayer excitons and the correlated charge carriers. The magnitudes of the blueshifts are consistent with the electron and hole wavefunctions obtained using first-principles calculations. At high excitation power, additional high-energy interlayer exciton PL peaks were observed, which could be explained by the formation of an excitonic Mott insulator. Additional helicity-resolved PL measurements were performed to confirm this hypothesis. Interestingly, a strong enhancement of the degree of polarization was also observed. Realizing the superpositions of quantum states is an important step for quantum information processing. The third part of this dissertation demonstrates two systems which can be potentially utilized to realize this goal via exciton hybridization. In angle-aligned WSe2/WS2/WSe2 heterotrilayer, a quadrupolar exciton state, which is a hybridized state of two dipolar interlayer excitons with opposite orientations, was observed using PL spectroscopy and was evident by a quadratic peak shift under an out-of-plane electric field. This hybridization effect is due to the coherent hole tunneling between the WSe2 layers, which also leads to an interlayer Mott insulator state, a correlated electronic phase composed of holes shared between two flat bands. In trilayer WSe2/ monolayer WS2 moiré heterostructure, a three-level-hybridized excitonic state composed of two interlayer excitons at different moiré sites and one intralayer exciton was detected using reflectance contrast, which potentially enables information processing based on moiré site degree of freedom. Finally, photocurrent spectroscopy as a sensitive technique to study excitonic effects in 2D semiconductors was demonstrated. The absorption spectra of TiS3 nanoribbons, which are difficult to measure using conventional spectroscopy techniques due to their below-diffraction-limit sizes, were characterized using polarized photocurrent spectroscopy. The energy splitting and shifting of the excitonic states in a lateral electric field were observed in WSe2 photodiodes and are consistent with the excitonic Stark shift effect. This study demonstrates a universal platform to investigate the effects of lateral electric field in 2D semiconductors.
School of Engineering
Dept. of Chemical and Biological Engineering
Rensselaer Polytechnic Institute, Troy, NY
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