Local optical properties of 2D semiconductor/plasmonic heterostructures

The possibility to control the properties of low-dimensional semiconductors via the exploitation of properly engineered architectures shows promising implications for several potential applications in the fields of optoelectronic and quantum technologies. Among the plethora of semiconducting materials, two-dimensional group 6 transition metal dichalcogenides (TMDCs), when thinned down to the three-atoms-thick monolayer (ML), exhibit a transition of the electronic bandgap from indirect to direct, with the bandgap energy falling within the visible spectral range. This, together with other singular properties, makes TMDCs extremely appealing light-sensitive materials for optoelectronics and photonics applications. Among several strategies to enhance light-matter interaction in ultrathin TMDC films, the electromagnetic field confinement and amplification typical of nano-sized metallic objects supporting localized surface plasmon resonances, i.e. light-induced collective electronic oscillations, can significantly strengthen the interaction of atomically-thick TMDCs with light, with the opportunity to exploit hybrid systems to realize plasmon-enhanced devices. In addition, the structural, electronic and optical properties of 2D TMDCs can be properly manipulated via their integration with plasmonic materials. Moreover, strongly-coupled exciton-plasmon systems can be realized by combining few- and single-layer TMDCs with ad-hoc designed plasmonic nanostructures with promising implications both for fundamental research and quantum-based applications. In this context, the research activity reported in this thesis has dealt with the study of the optical properties of spatially-confined systems. Two main classes of nanomaterials were investigated, namely noble metal nanostructures, with specific interest on their plasmonic and thermoplasmonic properties, and 2D TMDCs, with a focus on their excitonic properties. This manuscript mainly deals with the local optical properties which arise when integrating ML-TMDCs with plasmonic nanosystems to form hybrid structures. The experimental investigations on the hybrid systems have in common the exploitation of laterally-resolved optical techniques with micrometric and even nanometric spatial resolution. In detail, I will show how the combination of imaging spectroscopic ellipsometry and imaging photoluminescence spectroscopy can provide a complete picture of the local excitonic properties of TMDC flakes (WS2 in this case) grown by chemical vapour deposition. Deep knowledge on the local excitonic properties of 2D TMDCs proved fundamental for studying how their properties can be tailored by coupling with plasmonic materials. In this thesis, hybrid systems with a double-layer architecture (i.e. ML-TMDC/plasmonic substrate) were realized for two main experimental investigations. The first study dealt with the role played by the morphology of the plasmonic substrate, an ultra-dense array of Au NPs (approximately 10^3 NPs/µm^2), in affecting the plasmon-exciton interaction. In the second experiment, a 2D TMDC/plasmonic heterostructure was implemented as a system to probe the capabilities of tip-enhanced photoluminescence spectroscopy (TEPL) in mapping at the nanoscale the light-emission related properties of ML-TMDCs onto a plasmonic substrate. The last part of the thesis is dedicated to experimental investigations on the ultrafast temperature evolution of impulsively-excited plasmonic systems by means of pump-probe techniques. Two model-free approaches are presented for the direct assessment of the temporal evolution of the electron gas temperature after impulsive photoexcitation of metallic NPs. More in general, the results obtained from these last experimental studies pave the way for the assessment of the relaxation dynamics within physical systems and are inspiring towards further exploration on the phenomena which arise following photoexcitation of low-dimensional semiconductor/plasmonic heterostructures taking place on time scales of the fs-ps, such as the processes of charge and/or energy transfer and those related to hot electrons.