Research and Projects

Twist angle independent, momentum direct, interlayer exciton in van der Waals heterostructure

In van der Waals heterostructure of semiconductors with type II band alignment, there is a possibility of observation of interlayer excitons (IX) where electrons and holes forming the exciton are located in separate layers. TMDC heterobilayer provides an excellent ground for observing and manipulating IX. However, they are not always observed in these heterostructures, and their observation requires certain conditions, such as appropriate interlayer spacing and momentum mismatch. In this project we demonstrated interlayer excitons at room temperature, which are radiative, independent of the twist angle and lattice mismatch between the materials forming the heterostructure. My approach was to make the heterostructure with the materials (bilayer WS2 and bulk PbI2) having the band extremes at the Brillouin zone center (Γ point) such that the interlayer transition is always momentum-direct and radiative.

Stacking order and layer parity dependent anisotropic excitons in ReS2

ReS2 is known to have two stable stacking order namely ,AA and AB stacking. However, systemic studies of excitonic responses of these two phases are lacking, which led to a lot of discrepancies in the literature. In this project,I investigated the stacking-dependent and layer parity-dependent photoluminescence (PL) properties of rhenium disulfide (ReS2), a two-dimensional layered material with anisotropic optical characteristics. The study investigates two polymorphs of ReS2, namely AA and AB phases, using angle-resolved PL at cryogenic temperatures and second harmonic generation spectroscopy. The findings reveal significant differences between the two phases: AA phase exhibits two excitons and Rydberg excitons, while AB phase shows five emission peaks with strong layer parity dependence. Both phases demonstrate anisotropic excitonic emissions, with the first exciton of AA phas2e and the first three emission peaks of AB phase oriented along the b-axis of the crystal. The research also uncovers that bulk samples of ReS2 cannot be considered as decoupled monolayers, as they show distinct PL responses compared to thinner samples.

This finding have profound implications on design of polarization sensitive optoelectronics using ReS2

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