Spectroscopic Investigation of Excitons and Interactions in Engineered Transition Metal Dichalcogenides and Perovskites

Abstract

Low-dimensional materials provide a versatile platform for engineering light--matter interactions by enabling precise control over electronic, vibrational, and symmetry-driven effects. Among these systems, transition metal dichalcogenides and lead--halide perovskites are particularly compelling due to their strong excitonic response. In such materials, optical properties are governed by the interplay of exciton--phonon coupling, interlayer interactions, and spin--orbit effects, all of which can be systematically tuned through external perturbations such as strain, dimensionality, structural modification, and applied fields. This thesis explores how controlled engineering of these interactions gives rise to emergent optical phenomena in nanostructured materials.

The first part of this work focuses on the optical properties of \ch{MoS2} nanoscrolls. Rolling two-dimensional monolayers into quasi-one-dimensional nanoscrolls introduces symmetry-broken stacking and a highly inhomogeneous strain landscape. This structural transformation suppresses interlayer coupling, resulting in enhanced photoluminescence emission. Moreover, the combined effects of dimensional reduction and symmetry breaking lead to pronounced broadband optical anisotropy in an otherwise isotropic material, as revealed by polarization-resolved optical spectroscopy.

The second part investigates the manipulation of momentum-forbidden dark excitons in monolayer \ch{WSe2} using periodically modulated strain. By transferring monolayers onto nano-pillar substrates, a periodic potential landscape is created, enabling the localization of strain-brightened dark excitons. Low-temperature magneto-optical measurements reveal the formation of multiple spectrally narrow emission lines with large effective Land\'e \emph{g}-factors, indicative of strongly confined excitonic states analogous to moir\'e-trapped excitons.

The final part of this thesis examines exciton physics in \ch{CsPbBr3} lead--halide perovskite microcrystals, with particular emphasis on the interplay between Rashba spin--orbit coupling and exciton--phonon interactions. Polarization- and temperature-dependent spectroscopy provides direct evidence for exciton-selective phonon coupling, demonstrating that coexisting high-energy and Rashba excitons interact with distinct sets of phonon modes. These findings highlight the role of spin--orbit effects in shaping exciton--phonon dynamics. The thesis concludes with a brief discussion of ongoing efforts to explore light and exciton propagation in this system.