Tunable Multifunctionalities in Oxide-based Phase Change Nanocomposite Thin Films

Phase change materials (PCMs) has emerged as advanced functional materials for efficient thermal energy storage and release. Compared to other organic and inorganic PCMs, oxide-based PCMs have attracted growing interest because of small volume expansion, minor leakage issue, and moderate latent heat. In this dissertation, two special cases of oxide-based PCMs is discussed, i.e., vanadium dioxide (VO₂), and Bi-based perovskite/supercell structures. Specifically, VO₂ emerges as a focus of research because of its well-known semiconductor-to-metal transition (SMT) upon heating close to 68 °C. The intrinsic coupling of SMT and R-M1 structural change makes VO₂ a favorite material both scientifically interesting and technologically important for potential sensor and memory device applications. On the other hand, BFMO supercell structure originates from the double-perovskites, while the substrate-induced epitaxial strain induces the stacking and commensurate modulations of Fe/Mn double layers. The significantly enhanced multiferroic response is attributed to its non-centrosymmetric structure.

In this dissertation, a comprehensive study on the FM integration and novel approaches to achieve broad range transition temperature (T_c) tuning is explored in VO₂ thin films. Specifically, three novel metal/VO₂ nanocomposite designs are discussed, i.e., Pt/VO₂, Ni/VO₂ and Li/VO₂, with different morphology and Tc tuning mechanisms. First, by reconstructing the energy band structure at the metal/VO₂ interface, bidirectional T_c tuning in Pt/VO₂ nanocomposites can be achieved owing to the size dependent work function of Pt NPs. Next, by engineering the morphology by lattice matching, diffusion kinetics, and interfacial mixing, the exploration on Ni/VO₂ nanocomposites achieve the combined goals of Tc tuning and magnetic incorporation/magneto-optical (MO) coupling. Finally, by varying Li concentration during the metal-ion intercalation, T_c of both VO₂(B) and VO₂(M1) thin films can be systematically tailored because of structural deformation and the change in charge carrier density. The demonstration of metal/VO₂ nanocomposite thin films reveals a promising approach to fulfill various working environments for VO₂-based novel electronics, photonics, and spintronics. Furthermore, the microstructure evolution of the ultrathin BFMO supercell structure as well as its physical properties is first studied. The robust ferromagnetic and ferroelectric response is preserved in the ultrathin structure less than 10 nm, making it an attractive candidate for next-generation spintronics based on 2D materials.