Dissertations, Theses, and Capstone Projects

Date of Degree

2-2024

Document Type

Dissertation

Degree Name

Ph.D.

Program

Physics

Advisor

Gabriele Grosso

Committee Members

Andrea Alu

Vinod M. Menon

Matthew Y. Sfeir

Ajit Srivastava

Johannes Flick

Subject Categories

Condensed Matter Physics | Optics | Physics | Quantum Physics

Keywords

Transition Metal Dichalcogenides, Monolayers, Exciton, Dark Exciton, Exciton Transport, Exciton-Exciton Interaction, Exciton Density, Phase Engineering

Abstract

Monolayers Transition metal dichalcogenides (TMDs) have attracted much attention in recent years due to their promising optical and electronic properties for applications in optoelectronic devices. The rich multivalley band structure and sizable spin-orbit coupling in monolayer TMDs result in several optically bright and dark excitonic states with different spin and valley configurations. In the proposed works, we have developed experimental techniques and theoretical models to study the dynamics, interactions, and transport of both dark and bright excitons.

In W-based monolayers of TMDs, the momentum dark exciton cannot typically recombine optically, but they represent the lowest excitonic state of the system and can severely affect the overall optical performances. We performed theoretical and experimental studies that show that compressive strain allows us to visualize intervalley momentum dark exciton in the PL spectrum and that these excitons can find application in strain sensing. We show that in monolayer WS2 the formation of momentum dark exciton is greatly enhanced even with small compressive strain due to intervalley electron-phonon coupling, and their spectral properties strongly correlated with the strain magnitude. Furthermore, we show a similar mechanism for WSe2, however, with tensile due to its qualitatively different band structure than WS2. We exploited this correlation for strain sensing in two-dimensional semiconductors, revealing an optical gauge factor exceeding 104.

We then focused on spin dark excitons that possess an out-of-plane optical transition dipole, strong binding energy, and long lifetime. Therefore, spin forbidden excitons are promising candidates for interaction-driven long-range transportation. Moreover, these excitons are characterized by lower energy and exhibit a significantly higher density as supported by our theoretical model. By employing a high-resolution spatially resolved PL setup in an encapsulated monolayer of WS2, we demonstrated that the strong repulsive interaction arising from their high density and longer lifetime enables these dark excitons to diffuse up to several micrometers. Furthermore, we conduct experiments in the energy landscape and show that the repulsive interaction can provide energy to dark excitons for transportation even in an uphill energy landscape. This repulsion-driven long-range transport of dark states provides a new route for excitonic devices that could be used for both classical and quantum information processing.

Last, we investigated the optical properties of monolayers of TMDs in different structural phases. Monolayers of TMDs occur in the semiconducting 1H phase, whose optical properties are dominated by excitons, and the metallic 1T phase, however less stable than the 1H phase. We developed a method to engineer stable the 1H/1T mixed phase starting with a pristine 1H phase monolayer WS2 by plasma irradiation process. We can control the size of 1T patches by tuning plasma irradiation time. We observe a novel resonance in mixed-phase WS2 monolayers characterized by a lower excitonic energy compared to the bright exciton and exhibits enhanced absorption, extended lifetime, and circular polarization. We attribute the emergence of these unique excitonic states to the interface that forms between two distinct phases. This interpretation gains additional support from our calculations of the dielectric function carried out on the mixed-phase supercell containing both 1H and 1T phases, revealing a novel optical response at lower energies.

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