Spectroscopy of Electronic States in Organic and Nano-Crystalline Materials Under High Pressure

Author
Abstract
Semiconductor nanocrystals and organic semiconductors have the potential to revolutionize optoelectronic devices. Both classes of materials have been used in photovoltaic and display technologies and are promising candidates to improve both of these sectors. From a physical chemistry point of view, they are also intriguing systems to study in order to gain insight into the fundamental physics of how light interacts with semiconducting materials. Knowledge of how their crystalline structure and electronic properties are coupled is important from both fundamental and application perspectives.

This thesis presents the work I have completed on the pressure-response of both inorganic semiconductor nanocrystals and organic semiconductors using multiple high-pressure generating apparatus. After a description of the experimental procedures, two chapters focus on the pressure-induced photoluminescence (PL) spectral shifts in perovskite nanocrystals. The PL spectra are used to probe changes in the optical band gap of the perovskites as a function of pressure. The differences between the behavior of the perovskites CsPbBr3 and CsPbI3 can be traced to differences in the crystal structure due to the ionic radii of iodide versus bromide. Studies on the size-dependent pressure-response of CsPbBr3 nanocrystals show that the pressure-induced shift of spectral features may depend on the degree of quantum confinement in various sizes of nanocrystals.

The final chapter focuses on an organic semiconductor, rubrene. Rubrene is a material that possesses the ability to convert low energy photons into higher energy photons through a process called triplet-triplet annihilation (TTA) upconversion. Organic upconverting materials generally require a partner molecule which absorbs the lower energy light (sensitizer). The work in this thesis on rubrene shows that rubrene ions trapped in the crystal can act as the sensitizing species, therefore allowing upconversion without an added sensitizer. The TTA process happens through intermolecular coupling of neighboring molecules, so it should be sensitive to the crystalline structure. Our results show that the upconversion PL and intrinsic PL in rubrene are quenched by applied pressure, due to changes in the intermolecular coupling.
Year of Publication
2020
Academic Department
Department of Physics
Degree
Ph.D.
Number of Pages
145
Date Published
2020-11
University
University of Colorado Boulder
City
Boulder
JILA PI Advisors
Advisors - Other
Niels Damrauer
Gordana Dukovic
Joel Eaves
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