Defect Engineering in Cerium Oxide Nanocrystals: From Catalyst to Quantum Bit

Abstract

Nanocrystalline CeO2 (ceria) is a versatile material that plays a critical role in a wide range of applications from biotechnology and heterogeneous catalysis to quantum information, due to its ability to host a variety of defects and dopants within its crystalline lattice. In this dissertation, we will explore the multifaceted properties of CeO2 and how to manipulate its structure to modulate its function in two different applications: heterogeneous catalysis and quantum information technology. In Chapter 1, we lay out the rich body of studies that elucidate ceria’s structure and function across multiple contexts. We explore what it means to “defect engineer” ceria by introducing dopants to produce an effective heterogeneous catalyst. Then, we discuss the rapidly growing field of quantum information technology, focusing on the emerging interest in ceria as a host lattice for quantum bits, and consider reverse-engineering ceria to generate a defect-free environment. Finally, we provide context for where that positions this dissertation in our effort to contribute knowledge to the field of nanocrystalline cerium oxide. In Chapter 2, we introduce the context for CeO2 nanocrystals and their applications in organophosphate hydrolysis. We unpack the uncertainties in the literature regarding the relationship between specific surface defects and catalytic activity in dephosphorylation. Trivalent dopants serve as a tool for manipulating defects, including the concentration of Ce3+ and oxygen vacancies, thereby influencing the hydrolytic activity of CeO2. By studying the catalytic degradation of dimethyl-p¬-nitrophenyl phosphate (DMNP) as a model organophosphate substrate with doped CeO2 and piecing together a picture of the catalytic surface by powder X-ray diffraction (PXRD), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy, we show that CeO2 exhibits high sensitivity to dopants that generate lattice strain, Ce3+ ions, and oxygen vacancy defects. Consequently, we postulate that achieving high catalytic efficiency in CeO2 requires a balanced active site ensemble, in which defects are maintained at optimal concentrations and distributions on the nanocrystal surface. In Chapter 3, we reframe CeO2 as a chemically tunable wide-bandgap host lattice for its ability to accommodate optically active and spin-active defects. We investigate CeO2 nanocrystals as hosts for near-infrared-compatible Yb3+ ions as optically active spin quantum bits (qubits), by characterizing the as-synthesized Yb3+-doped CeO2 nanocrystals (0.01–10% Yb) by photoluminescence and EPR. Thermal annealing at 700 °C increases crystalline domain sizes (from ~7 nm to ~20 nm) and reduces the concentration of Ce3+ defects, yielding an enhancement in excited-state lifetimes and spin-lattice relaxation time (T1), while phase memory times don’t significantly change, suggesting ulterior dephasing mechanisms not addressed by annealing. Collectively, these results establish defect mitigation, rather than dopant identity alone, as a bottleneck in realizing nanocrystalline CeO2 as a viable quantum host. Through this work, we provide quantitative benchmarks for optical and spin coherence in CeO2:Yb3+ nanocrystals and highlight defect engineering as a critical pathway toward scalable rare earth-based qubits.