Engineering the electronic structure of two dimensional chalcogenide materials

As two dimensional materials technology matures and the focus turns to optimization, fine tuning the optoelectronic properties of materials is becoming necessary for progress. The focus of this thesis is on developing methods to achieve such modulation in terms of variation of the band gap and introduction of dopants within the two dimensional chalcogenide family of semiconductors. Changing the structure of materials can change the band structure within the optically relevant band gap region of materials by either changing the energies of existing bands to modify the band gap, or by introducing new `defect' states within this region.

Three methods of such optoelectronic property modulation are investigated within this thesis - uniaxial strain, point defects and edge defects. The investigations into uniaxial strain focus on band gap variation in monolayer MoS2, TiS3 using first principle calculations and in monolayer WS2 experimentally. The first principle calculations reveal a high directional dependence of uniaxial strain on the band gap response in TiS3 compared to MoS2, while the experimental investigation focused more on optimization of strain introduction - achieved through deposition of WS2 on pillars.

The point defects study focuses on the potential for p and n type doping within four layer MoS2 through the introduction of oxygen and chlorine dopants - finding oxygen an unsuitable candidate for p type doping (without inclusion of substrate effects) and chlorine a candidate for electron donation. Edge defects are also investigated using first principles calculations in a WS2 system, focusing on the impact of different edge terminations on the introduction of optically relevant defect states. Armchair terminations are found to reduce the band gap significantly, with terminations between armchair and zigzag directions able to modulate this reduction to some degree, and zigzag terminations resulting in a metallic structure.