First Principles Approach for Optical Excitations in Transition Metal Dichalcogenides

Abstract

This cumulative thesis presents a comprehensive \textit{ab initio} framework for investigating many-body excited states in two-dimensional transition metal dichalcogenides and their heterostructures, combining density functional theory and advanced configuration interaction methods. The developed methodology captures subtle fermion-fermion interactions manifesting as excitonic complexes, enabling highly accurate predictions of optical properties.

The first study focuses on monolayer WSe₂, where many-body screened configuration interaction calculations reveal the existence of negatively charged trions involving the Q-valley of the conduction band. These Q-valley trions are energetically more favorable than their conventional K-valley counterparts and exhibit increased singlet-triplet splittings, providing new insights into valley-dependent optical features consistent with experimental observations.

Building on this, the second investigation examines bilayer Janus transition metal dichalcogenide heterostructures, such as MoSSe–WSSe and WSSe–WSSe. The results show that intrinsic structural asymmetry and interface-induced polarization enable spin-allowed interlayer exciton and trion ground states, contrasting with the spin-forbidden states typical of conventional bilayers. Furthermore, the application of external strain offers additional means to tune the optical brightness and properties of these interlayer states, opening pathways for precise control in optoelectronic applications.

Lastly, more complex heterostructures are investigated, which are constructed by introducing a twist in the loosely van der Waals bound bilayers. To this extent, a generalized force-field relaxation scheme tailored for large moiré heterostructures is developed. By optimized parameterization against first principles calculations, this approach accurately reproduces atomic reconstructions for both commensurate and incommensurate systems, especially at small twist angles. These relaxed structures reveal significant modifications of the interlayer potential landscape, emphasizing the importance of in-plane and out-of-plane atomic displacements on excitonic confinement and localization.

Together, these studies establish a versatile and predictive computational platform for exploring and engineering many-body phenomena in 2D materials. The insights gained into valley physics, interlayer excitonics, and structural relaxation underpin future strategies for designing tailored optoelectronic, valleytronic, and quantum devices based on layered two-dimensional heterostructures.