Syntheses and Photophysical Studies of Two-Dimensional Hybrid Organic-Inorganic Semiconductors

Abstract

This thesis focuses on the studies of two families of two-dimensional (2D) hybrid organic-inorganic semiconductors with exciting properties for optoelectronic applications. The first family is 2D lead halide perovskites (LHPs), which have received immense interest due to their solution processability and interesting luminescent properties. However, their uses in energy harvesting and light-emitting applications are limited by the difficulty in their synthesis, the knowledge of their structural-property relationships, and the understanding of their exciton physics. In this thesis, we first present cooling-induced crystallization to produce phase-pure 2D LHPs with controllable chemical compositions. Using single-crystal X-ray diffraction, we refined their crystal structures across temperatures and observed two phase transitions corresponding to structural changes in organic and inorganic sub-lattices. The structural information across these phase transitions was then used to explain temperature-dependent optical properties and address the debate over the origins of their broadband emission. We observed two broadband emission features and distinguished defect-associated from self-trapped exciton (STE) emission by the difference in their temperature-dependent behaviors. Moreover, we found that the temperature dependence of STE emission is strongly correlated with exciton-phonon coupling strength and structural distortion, suggesting a possible tuning of this property by compositional engineering.

The second material is silver phenylselenolate (AgSePh), an emerging 2D metal organic chalcogenolate (MOC) material with blue luminescence, in-plane anisotropy, large exciton binding energy, non-toxic and earth-abundant elemental composition, and a scalable synthetic method. Despite these desirable characteristics for modern electronics, its fundamental studies and device integration are limited by its crystal size and quality produced by current synthetic methods. In this thesis, we report two synthetic advances to produce AgSePh thin films with controllable grain size from <200 nm to >5 µm and microcrystals with increased crystal size from ~2 µm to >1mm. Systematic optical and electrical characterizations through photoluminescence spectroscopy and electrical conductivity suggest higher crystalline qualities with lower defect densities in these samples. Using ⁷⁷Se nuclear magnetic resonance spectroscopy and monitoring reaction kinetics, we provide mechanistic insights that enable the development of generalizable single-crystal growth. Overall, we expect these reported synthetic methods to facilitate the studies on this exciting material and its family.