Let there be (more) light : a study of tungsten based 2D semiconductor-plasmonic hybrid structures

Johnson, Alexander Dawn
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Monolayer transition metal dichalcogenides (TMDs) have emerged as promising materials for optoelectronic applications due to their transition from an indirect to a direct (or nearly direct) band gap when reduced to a single layer thickness; this transition is accompanied by a dramatic increase in light absorption and emission. Though this increase is impressive as compared to their bulk counterparts, the extreme thinness of the material limits its absorption capabilities and non-radiative relaxation channels reduce its quantum yield. In order to take full advantage of the optical properties of TMDs, these issues must be overcome. In this dissertation I will discuss work that focused on enhancing the emission of WS₂ and WSe₂ using Ag structures. Two sets of experiments are presented. The first set studied WS₂-Ag structure hybrids. We varied the thickness of a dielectric spacer that is positioned between monolayer WS₂ and a Ag film to observe how it affects the photoluminescence (PL) of the system. It was found that greater PL intensity was achieved as the spacer thickness decreased; however, PL intensity was reduced if no spacer was present. These results are intriguing because the discovered 1 nm ideal thickness is much smaller than what has been found for other emitters, suggesting that new mechanisms are at play. Additionally, work was done focusing on the effects of metal quality on PL enhancement by comparing the performance of hybrid structures created with thermal verses atomically smooth epitaxial Ag film. Only structures that utilize cyclic re-excitation observed differences in PL enhancement, with better performance from structures created with epitaxial films. The second set studied WSe₂-Ag structure hybrids at low temperatures. Here it is shown that low-lying dark exciton states greatly influences the PL of the hybrids by creating an additional non-radiative relaxation channel for higher energy exciton states. This phenomenon manifests as quenching of the higher energy states when WSe₂ is placed on a substrate that increases the relaxation rate of the dark state. As a result, coupling WSe₂ to plasmonic structures quench the band edge exciton emission, while simultaneously allowing enhancement of lower energy defect bound exciton emission.