Electronic and Optical Properties of Two-Dimensional Transition Metal Dichalcogenide Nanoribbons: Insights from Density Functional Theory and Many-Body Perturbation Methods

Abstract

Two-dimensional (2D) layered materials, such as transition metal dichalcogenides (TMDs), are the subject of intense research interest as platforms for both developing atomically-thin devices and exploring novel physics. Nanoribbons are intriguing, reduced forms of 2D layered materials. They exhibit more spatial confinement effects and rich edge states than their 2D counterparts, which can result in drastic changes in electronic and optical properties. This dissertation presents an investigation of the electronic structure properties of bent TMDs nanoribbons using density functional theory (DFT) and explores their optical absorption and excitonic states through many-body perturbation GW and BSE (Bethe-Salpeter equation) methods.The band structures, band gaps, and projected spin polarization of semiconducting armchair tungsten diselenide (WSe2) nanoribbons are calculated under various bending curvatures and electron/hole doping using density functional approximations (DFAs). Additionally, optical absorption and excitonic states are analyzed using the many-body perturbation GW-BSE approach. The findings reveal that the band gap of the nanoribbon can be modified from direct to indirect or vice versa under appropriate bending curvatures. Furthermore, doping or bending the nanoribbons with proper curvatures leads to spin polarization anisotropy within the bands around the Fermi level. This suggests their potential utilization in compact and controllable magnetic nanodevices and spintronics. The exciton states exhibit mixed or various spin configurations in the electron and hole pairs, which are controlled by the bending, and hold promise for applications in spin-based quantum information processes. Defects are commonly present in 2D TMD materials and significantly alter their properties. The interaction between edge states and defect states in tungsten disulfide (WS2) nanoribbons with line defects under different bending curvatures is investigated using density functional theory (DFT). The results are compared with quasiparticle GW calculations to gain insights into the limitations of DFAs for band gaps and energies of defect states. The investigations uncover interesting semiconducting-to-metallic phase transitions, indicating potential applications in nano-electronics or molecular electronics. Moreover, optical absorption of the bent and defective nanoribbons is calculated using the many-body GW-BSE approach, revealing a tunable optical spectrum and diverse exciton states in the defective WS2 nanoribbons.