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Investigations of large chiral optical responses in low-dimensional hybrid organic-inorganic semiconductors – methods, mechanisms, and artifacts
Phillips, Alan James
Phillips, Alan James
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2025
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Chiral optical responses have received renewed attention in recent years, due to their potential applications in devices that can interchange between photon, charge, and spin degrees of freedom. Circularly Polarized Light Emission (CPLE) describes the material or device-based emission of circularly polarized photons. Recently, a number of hybrid organic-inorganic semiconductor (HOIS) materials have been shown to exhibit CPLE. However, the seemingly simple measurement of CPLE by HOIS materials has the potential for many critical errors, both experimental and interpretational. This thesis outlines methods to reliably measure and extract the CP components of emission from HOIS materials, and discusses critical considerations for interpreting CPLE responses in HOIS. Additionally, I demonstrate two specific mechanisms that can produce exceptionally large chiral-optical responses in low-dimensional HOIS.
The ionic nature of HOIS materials can lead to significant challenges in accurate determinations of the absorption and emission of CP light, a fact that may contribute to the wide variability and limited reproducibility of these measurements within the literature. Inherent molecular instability under external fields and stresses can lead to artifact-ridden measurements of CPLE due to sample degradation and transformation. Also, in certain experimental setups, linearly polarized effects, such as linear dichroism (LD) and linear birefringence (LB), can produce artifacts that present themselves as large CPLE signals. Further, LD and LB in HOIS systems can also lead to unique Extrinsic effects that can easily be conflated with experimental artifacts, even though they are true chiral-optical responses.
I investigated the CPLE responses of multiple HOIS material systems through techniques such as temperature-dependent, excitation energy-dependent, excitation polarization-dependent, and magnetic field-dependent spectroscopic measurements. My experimental work was highly synergistic with theoretical work by collaborators to determine the mechanistic origins of two specific large chiral-optical response types. First, I show that in an ultra-low symmetry chiral bismuth iodide HOIS system that Extrinsic effects related to LD, LB, and Raman scattering, can produce exceptionally large absorption and emission anisotropies. Second, I demonstrate that in several polar 2D lead iodide HOIS systems with large spin splittings, that optically excited spins can emit CPLE through a Spin Memory effect. The work in this thesis serves to build up the fundamental understanding of CPLE responses from HOIS materials for the future development of technologies that rely on photon/spin/charge-based devices.
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