Date of Award
Doctor of Philosophy
Emily A. Smith
Environmental concerns over use of fossil fuels to generate power and the finite supply of these resources have driven major efforts for alternative energies. At the same time, the development of nanotechnology has blossomed to propose strategies and materials for renewable and less energy-intensive end-user devices, such as solar cells and LED lighting. Two examples of promising candidates for energy applications are germanium-based nanocrystals and lead halide perovskite nanocrystals.
Germanium-based materials have limited absorption efficiency due to their indirect band gap. To address this, germanium-tin alloy nanocrystals were synthesized to promote direct band gap character. A full characterization demonstrated tin incorporation, but a direct band gap was not observed. Addition of a cadmium sulfide shell typically results in improved photoluminescence, and the incorporation of tin into germanium-tin/cadmium sulfide core/shell nanocrystals resulted in up to 15× improvement over pure germanium/cadmium sulfide nanocrystals. This is likely due to improved epitaxy (smaller lattice mismatch) between the core and shell material.
Lead halide perovskite nanocrystals have demonstrated amazing potential for solar energy capture but are hampered by stability concerns. All-inorganic cesium lead halide perovskite nanocrystals have been prepared to impede the typical degradation pathways (ambient moisture and oxygen). To assess nanocrystal stability the photophysics of cesium lead halide nanocrystals were measured as a function of halide content under ambient conditions, solar simulated light, and heating. We observed several phenomena including crystal growth (liberation of ligands), photoannealing, crystalline phase changes, and shifting time constants for single crystal photoluminescence data. All of these observations lead to a more realistic picture of the stability of these nanocrystals, which will still likely require encapsulation or surface protection to be effective in long-term device use.
In addition to materials synthesis and characterization, new instrumental techniques are critical for the next generation of energy capture and storage solutions. To this end we constructed a saturated excitation microscope in order to measure the photoluminescence of inorganic semiconductor quantum dots, capable of subdiffraction imaging through demodulation at harmonics (nf, n = 2, 3, etc) of the excitation modulation frequency (f). By demodulating at 3f, a 36% increase in resolution was observed compared to the fundamental image, which will be useful characterizing thin films, nancrystals, and other devices where small defects can have large impacts on performance.
Boote, Brett, "Investigation of the photophysical properties of energy-relevant inorganic nanocrystals" (2019). Graduate Theses and Dissertations. 17408.