Righetto, Marcello (2018) Optical Nanostructures for Excitonic Devices. [Ph.D. thesis]
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Unrelenting advances in the field of nanoscience are fostering the progress in diverse research fields, ranging from light-emitting to medicine and diagnostics, from energy conversion to communication technologies. Besides representing the most paradigmatic example of nanoscience, semiconductor quantum dots (QDs) avowedly brought revolutions in many of the research fields mentioned above. Nowadays, some QDs-based devices and applications reported efficiencies almost as good as current state-of-the-art technologies. The founding concept of QDs is the application of quantum confinement effects on excitons, i.e., the main players of optical properties in bulk semiconductors. Among the wealth of ensuing properties, the size- and shape- tunability of the electronic excitations and increased coupling with light field aroused much interest. Also, the colloidal approach endows QDs with high processability and low cost, thereby encouraging their implementation in existing technologies and extending their impact to other fields. Howbeit, despite three decades of investigations, the bottom line has not been reached yet, and researchers are still delving deeper into the photophysics of these nanosystems. Though many of the low hanging fruit of QDs have been harvested, higher-lying ones seem to be even more succulent.
This thesis deals with the quest for highly performing nanostructures, as a prerequisite for some high impact optoelectronic applications, e.g., QD-Lasers and QD-Solar Cells. Within this framework, the struggle against fast Auger recombinations and trapping of either hot carriers or cold excitons was addressed mainly by sophisticated core/shell technologies. Thus, the first part of the thesis reports how tuning different shell parameters (e.g., the smoothness of the interface potential, the height of the confining potential, and the interfacial strain) it is possible to exert control on these detrimental recombination processes. Though often disregarded, even the role of organic capping ligand is reconsidered in promoting the outcoupling of QDs excited states and addressing their interaction. Besides the useful and technologically relevant advice gathered within these studies, the primary inheritance of the first part is the comprehensive photophysical scenario, portrayed by a phenomenological model that successfully describes many aspects of the exciton dynamics in QDs. This amount of knowledge was capitalized in the second part of this thesis, dealing with the quest for novel materials, potentially outpacing conventional CdSe-based QDs. Perovskite-based QDs reported promising results, whereas some pitfall in the conventional characterization of carbon-based QDs were discovered. The rationalization of both nature and dynamics of this materials is expected to expedite their development as alternative (and potentially superior) technologies concerning those studied in the first part.
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