Research master thesis

Abstract

The ongoing miniaturization of electronic devices has shifted the attention from tradition silicon-based technologies to two-dimensional (2D) materials. Among these, the family of transition metal dichalcogenides (TMDs) play an important role as potential candidates to replace silicon due to their unique electronic properties. However, synthesizing high-quality monolayers of TMDs remains a significant challenge. First-principles approaches offer a powerful framework to investigate the electronic and transport properties of these materials with high accuracy, providing insights into phenomena such as carrier mobility. Important for the evaluation of the material in potential applications. Tungsten disulfide (WS2), in monolayer (ML) and bilayer (BL) form, has attracted interest due to its relative high carrier mobility (compared to other TMDs such as MoS2). While most studies focus on monolayers, bilayers offer additional tunability. Among the different techniques used to enhance mobility, such as doping or strain, the application of an external electrical transverse to the atomic plane presents an interesting approach. This thesis explores the transport properties of ML and BL WS2 using first-principles simulations. The electronic and vibrational properties, along with the mechanisms responsible for limiting carrier (electron and hole) mobility, are investigated using Quantum Espresso and Perturbo. These tools are capable of approximate the solution the Schr¨odinger and Boltzmann transport equations under different approximations. The presented results confirm that an external electric field has minimal impact on the electronic and vibrational properties in the single layer. In contrast, the double layer exhibits significant changes: the bandgap narrows, and the energy separation between valleys closer to the Fermi level increases with field strength. These modifications alter the electron-phonon interaction, leading to enhanced carrier mobility, reaching up to 95% of the ML value. Ultrafast dynamics simulations further confirm reduced scattering rates and improved drift velocities. These findings exhibit the tunability of TMDs via external fields and highlight BL WS2 as a viable alternative to monolayers for scalable electronic applications.