Synthesis and Characterization of Low-Dimensional Materials for Dynamic Reconfigurability in  Mixed-Dimensional Heterostructures

Olding, Jack Nicklaus

doi:https://doi.org/10.21985/n2-61qm-y185

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Synthesis and Characterization of Low-Dimensional Materials for Dynamic Reconfigurability in Mixed-Dimensional Heterostructures

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This thesis describes the synthesis and photophysical characterization of low-dimensionalmaterials—including thin-film semiconductors, colloidal quantum dots, and molecules—with the broader motivation of integrating them into mixed-dimensional heterostructures with novel responses to external stimuli. Due to their high surface area to volume ratio and incomplete dielectric screening, mixed-dimensional heterostructures have high sensitivity to their environment and external stimuli, providing new response functions and reconfigurability. A primary goal of this thesis is the synthesis and characterization of materials that would enable dynamic reconfigurability through external stimuli, including emerging low-dimensional ferroelectrics and molecules with large excited-state dipole moment. The spontaneous and reversible electrical polarization in ferroelectrics is a potential source of bistable reconfigurability in mixed-dimensional heterostructures, and there are fundamental questions to explore regarding ferroelectricity in these atomically thin materials and devices. In contrast to the bistability of ferroelectrics, molecules with large excited-state dipole moments can introduce transient responses with the duration and magnitude dependent on the molecular photophysics. The epitaxy of 2D ferroelectric SnS on layered MoS2 by a scalable, low-temperature growth technique is demonstrated. Growth conditions that favor layer-by-layer growth are found and the interlayer interactions driving epitaxy in the non-isostructural heterostructure are determined, guiding future growth of 2D/2D heterostructures. Claims of ferroelectricity in 4 colloidal cadmium chalcogenide quantum dots are examined. While results from the literature are reproducible, the evidence for ferroelectricity is shown to be a misinterpretation of the polarization measurement caused by significant conductivity in the test sample. Finally, a molecule with a large excited state dipole moment, CPPZ, is synthesized and integrated with other low-dimensional materials to optically modulate the local electric potential. The effect of the excited-state dipole on the excitonic states of colloidal nanocrystals and work function of 2D semiconductors are investigated by ultrafast optical spectroscopy and Kelvin probe force microscopy, respectively. While no Stark effect is observed in the former case, a dramatic change in the work function is observed for the 2D semiconductor on CPPZ compared to the isolated 2D semiconductor case. Understanding the synthesis and properties of materials with exceptional responses to light or electric fields will enable their integration into mixed-dimensional heterostructures with novel responses.

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