Engineering Excitonic Properties in van der Waals Solids Using Strain

Passcode: 759432

While electronics sits at the forefront of logic processing and data communication, the ever-increasing trend of on-chip power density has rendered the sustainability of device scaling questionable. 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. To address this challenge, the strain-based approach is being widely explored by the research community. To harness the true potential of excitons as candidates for next-generation sensors, and energy-efficient information processing, it is of paramount importance to understand strain-engineered exciton photophysics in different material systems.

In this work, we first investigate the uniaxial strain modulated exciton photophysics in an organic guest: host blend. We utilize buckled SiO₂ microbeams to tune the excited state photoluminescence from the guest molecules. We verify that the observed spectral shift originates from modulation in molecular density under axial strain. Later, we study piezoelectric modulation of excitonic photoluminescence in a monolayer semiconductor under strong dielectric screening.  As an archetype system, we study the photoluminescence spectral modulation in monolayer WSe₂ transferred on piezoelectric LiNbO₃. Finally, we investigate exciton transport under traveling strain in h-BN/WSe₂/h-BN system. Traveling strain generated by the Rayleigh type SAW wave creates out-of-phase modulation of the monolayer energy bands. 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.

Chair: Professor Parag B. Deotare