Engineering Excitonic Properties in van der Waals Solids Using Strain
Creators: name: Datta, Kanak
Publisher: name: My University
Year: 2022
Subjects: subject: Strain Engineering
subject: Transition Metal Dichalcogenides
subject: Solid State Solvation
subject: Surface Acoustic Wave
subject: Exciton Transport
subject: Electrical Engineering
subject: Physics
subject: Engineering
subject: Science

description: While electronics sits at the forefront of logic processing and data communication in microprocessors, the ever-increasing trend of on-chip power density has rendered the sustainability of device scaling questionable. This has also predominantly inspired the exploration of alternative pathways for energy-efficient data communication and information processing. Excitonics, not only offers a viable alternative for low-power logic processors but also, low-footprint communication. At room temperature, excitons only exist in a handful of semiconducting media - organic materials and recently discovered transition metal dichalcogenides (TMD) monolayers. While the room temperature existence makes it technologically relevant, controlling the exciton dynamics remains a challenge. The thesis addresses the challenge by exploring strain-based approaches. We first investigate the uniaxial strain modulated photophysics in an organic excitonic guest: host blend Alq3: DCM, where the excited state properties are governed by a phenomenon called solid-state solvation. We utilize buckled SiO2 microbeams to apply axial strain on an overlying host: guest thin film and tune the excited state photoluminescence from the guest molecules. We further verify that the observed spectral shift indeed originates from modulation in local dielectric polarization triggered by the molecular density under axial strain. We also demonstrate dynamic tuning of the molecular fluorescence by electrostatic actuation of a Si3N4 microbeam fabricated on a Si3N4/Si substrate. Later, we study piezoelectric modulation of excitonic photoluminescence in a monolayer semiconductor at room temperature under strong dielectric screening. As an archetype system, we study the photoluminescence spectral modulation in monolayer WSe2 transferred on piezoelectric LiNbO3 substrate. Strong screening from the substrate leads to enhanced exciton dissociation and electric field-dependent shift of exciton emission energy – both can be further tuned by optically generated free carrier density. Finally, we investigate exciton transport under traveling strain in monolayer WSe2 encapsulated by bulk h-BN (h-BN/WSe2/h-BN). We generate high-frequency Rayleigh type SAW on piezoelectric Y-cut LiNbO3 substrate by patterning interdigitated electrodes (IDTs). Traveling strain generated by the SAW wave creates out-of-phase modulation of the monolayer energy bands - also known as the Type – I modulation. We also demonstrate acoustic steering of the photogenerated exciton density by precision control over the instantaneous SAW phase using phase synchronized time-correlated single photon counting scheme (TCSPC). We find that the acoustic transport of excitons in monolayer TMDs at room temperature is limited by the intrinsic exciton mobility and the spatial extent of the monolayer. Lower intrinsic mobility results in the ‘weak coupling’ of the photo-generated excitons to the traveling strain wave.