Theses and Dissertations at Montana State University (MSU)
Permanent URI for this collectionhttps://scholarworks.montana.edu/handle/1/733
Browse
3 results
Search Results
Item Elucidating the impacts of structural heterogeneity on excited state dynamics in solution-processed materials(Montana State University - Bozeman, College of Letters & Science, 2024) Afrin, Sajia; Chairperson, Graduate Committee: Erik Grumstrup; This is a manuscript style paper that includes co-authored chapters.Solution-processed inorganic and organic semiconductors hold enormous promises due to their low manufacturing cost, scalability, and compatibility with flexible substrates. However, solution processing techniques do not require control over crystal growth, which can lead to structural defects within the crystal structure. The defects within solution-processed semiconductors can create significant challenges in optimizing device functionality; therefore, it is crucial to understand the impact of structural defects on photophysical properties. Traditional ensemble measurement techniques can conceal the effects of microscale structural defects on functional properties in the structure-averaged observation of solution-processed materials. The work presented in this dissertation employs time-resolved and spectrally resolved microscopy techniques to investigate the influence of structural heterogeneity on the photophysical properties of microscale solution-processed materials. Measurements collected across multiple discrete and highly crystalline domains of multiple classes of solution-processed materials have helped establish a relationship between the functionality and the local structure of these materials. Initially, the focus was on elucidating anisotropic carrier transport in lead halide perovskites by investigating lattice strain and energetic distribution in microcrystals. Later, the focus shifted towards characterizing and understanding the impact of structural defects on the excited state dynamics in another class of solution-processed material called metal-organic frameworks (MOFs). PCN-222 exhibited rapid exciton transport with time-averaged diffusion coefficients ranging from 0.27 to 1.0 cm2/s and subdiffusive behavior, showing transport slowing on the tens of ps time scale. Subdiffusivity indicated that excited states were rapidly transported through the porphyrin network of PCN-222 before being trapped. Moreover, the first transport measurements and transient absorption microscopic measurements in PCN-222 are reported here. Photoluminescence quenching and heterogeneous relaxation pathways were noted in regions with higher structural heterogeneity. Furthermore, the spectral evolution of porphyrinic PCN-222 MOF was investigated, which revealed excitation-dependent chromophore coupling in the MOF structure. Soret band excitation with enhanced coupling can create more mobile excited states, whereas Q band excitation with reduced coupling will generate fewer mobile excited states. Excitation-dependent chromophore coupling strongly dictates the transport and relaxation properties in MOF microstructures that also illustrate the impact of structural defects on the excited state transport and relaxation dynamics. A significant spectral shift has also been observed in microrods stemming from structural heterogeneity. These findings contribute to a deeper understanding of the impact of structural defects on the photophysical properties of solution-processed materials, facilitating the development of optimized semiconductor devices for various applications. The results reported in this dissertation will not only continue to aid in the characterization of MOFs but will also advance our understanding of excited state dynamics in a variety of solution-processed materials.Item Thermalization and exciton localization in 2D semiconductors(Montana State University - Bozeman, College of Letters & Science, 2023) Strasbourg, Matthew Christopher; Chairperson, Graduate Committee: Nick Borys; This is a manuscript style paper that includes co-authored chapters.2D semiconductors are a promising class of materials to investigate for applications in the next generation of photonic devices. They can be used to generate quantum light and also exhibit correlated many-body phenomena. Many of the novel optoelectronic properties of 2D semiconductors are associated with strongly-bound hydrogen-like states known as excitons. Excitons in 2D semiconductors have binding energies on the order of 100s of meV and are stable at room temperature. At low temperatures, higher-order excitonic states such as charged excitons and biexcitons--multiple-bound excitons that are like hydrogen molecules-- and localized excitons that emit quantum light are also observed. Whether excited optically or electronically, a diversity of high-energy excitons and free carriers are produced directly after excitation. The relaxation and thermalization of these initial states influence the formation of excitons, biexcitons, and localized excitons. Here, I present work that (i) investigates the thermalization of excited states in a prototypical 2D semiconductor, monolayer (1L-) WSe2, and reports the discovery that the generation of charged biexcitons is enhanced with increasing photoexcitation energy, (ii) shows the emergence of quantum emitters (QEs) in a new 2D QE platform: 1L-WSe2 nanowrinkle arrays induced by Au nano stressors, and (iii) uses a novel method to classify the excited-state dynamics of 2D QEs and differentiate emitter populations. A suite of low-temperature energy- and time- resolved optical spectroscopies are used to conduct this work. This work shows how excited state thermalization affects the formation of exciton and biexcitons and investigates the optical properties of an emergent class of 2D quantum light emitters.Item Transient passive mode-locked ND:YAG laser using a semiconductor saturable absorber mirror(Montana State University - Bozeman, College of Letters & Science, 2022) Shaffer, Heather Rose; Chairperson, Graduate Committee: Joseph A. ShawQ-switched-mode-locking in a Nd:YAG bulk resonator was demonstrated using a semiconductor saturable absorber mirror (SESAM). A 10-W-pulsed-diode-pumped Nd:YAG laser system at Quantel USA by Lumibird, Inc. was adapted for mode-locking operation in a breadboard setup. Three SESAM mirrors were tested with initial reflectivities R 0=85%, 90%, and 95% in several cavity configurations to show enhanced sub-nanosecond pulse modulation at the free spectral range of each resonator. Transient Q-switched and long-pulse envelopes are shown with underlying mode-locked pulse modulation.