Surface Science of Low-Dimensional Materials: Controlling Chemical and Optoelectronic Structure, Connecting Dissimilar Materials, and Improving Atmospheric Stability

Martin, Julia

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Surface Science of Low-Dimensional Materials: Controlling Chemical and Optoelectronic Structure, Connecting Dissimilar Materials, and Improving Atmospheric Stability

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Materials engineering for improving the quality of life and tackling global issues is largely driven by surface chemistry approaches. Such strategies aim to solve not only seemingly inconsequential concerns like protection from the elements and non-stick cookware but also more critically pervasive challenges associated with atmospheric pollution, climate change, chemical warfare threats, cost-scalability factors of industrial processes, among others. For many traditional, widely-applied materials such as silicon or graphene, a diverse breadth of knowledge exists surrounding covalent derivatization strategies to impart chemical or optoelectronic tunability, achieve surface passivation, improve dispersibility in organic solvents or incorporation into polymers, among others. This thesis work aims to borrow from such established covalent derivatization techniques and exploit them for new and emerging materials for applications in solar energy and photocatalysis, sensing, electromagnetic shielding, protective barrier materials, and others. The studies herein primarily surround two classes of low-dimensional materials, bismuth-based oxyhalides and titanium-based carbides and oxides. In the case of BiOI(001), to bridge the gap between the cleanliness of a freshly cleaved surface and that available from purely chemical etching means, we explore a combination of wet chemical treatments and quantify the resulting interfacial chemical states and electronic structure. Ultimately, in combination with overlayer models of idealized oxide-terminated or iodide-terminated BiOI 2D surfaces, angle-resolved X-ray photoelectron spectroscopy, ARXPS, demonstrate an oxide-dominated surface for nascent BiOI and an iodide-dominated surface for BiOI following tape exfoliation or following sequential HF etching and sonication in acetone which each demonstrate distinct electronic structure. Beyond BiOI, we present the first report of the optical properties and electronic band structure for a recently discovered one-dimensional lepidocrocite (1DL) titania that we believe to be highly quantum confined based on a comparably high band-gap energy compared to other lepidocrocite titania structures. Transient absorption, TA, spectroscopy further demonstrate broadband, long-lived sub-gap photoexcitations that we postulate to originate from Ti3+ defects and oxygen vacancies based on reports surrounding optically "black" TiO2. Nonetheless, significant work remains to better understand the nature of these transient state dynamics and the implications for realizing highly efficient applications in photocatalysis and Li-based battery systems. For studies surrounding Ti3C2Tx MXene, we derivatize the material with covalent organosilanes and ultraviolet photoelectron spectroscopy, UPS, demonstrate ~500 meV shifts in work-function values resulting from relative surface dipoles. The magnitude and direction of the interfacial dipoles are tunable based on the chemical functionality of the derivatizing molecules, and we discuss these results in the context of shielding, sensing, and battery applications. Beyond the studies herein, our derivatization strategies should broadly inform present approaches for covalently connecting dissimilar materials to enhance carrier transport and conductivities, improve atmospheric stability, design highly sensitive and selective sensors, and more.

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