CONTROLLABLE THREE-DIMENSIONAL STRAIN, MICROSTRUCTURE, AND FUNCTIONALITIES IN SELF-ASSEMBLED NANOCOMPOSITE THIN FILMS
Vertically aligned nanocomposite (VAN) configuration has been recognized as the state-of-the-art architecture in the complex oxide epitaxial thin films, which are constructed by two immiscible phases simultaneously and vertically growing on a given substrate and forming various columnar microstructures, such as nanopillars embedded in matrix, nanomaze, and nanocheckboard. Due to its architectural features, VAN structure enables a powerful control on the multifunctionalities via vertical strain engineering, microstructural variations, and interfacial coupling. It provides flexibility in complex oxide designs with various functionalities (e.g., electrical, magnetic, optical, etc.), as well as a platform to explore the correlations between strain, microstructure, and multifunctionalities of the nanocomposite thin films.
In this dissertation, integrated VAN systems with multilayer configuration have been constructed as a new three-dimensional (3D) framework, e.g., inserting 1-3 layers of CeO2 (or LSMO) interlayers into the La0.7Sr0.3MnO3 (LSMO)-CeO2 VAN system and forming 3D interconnected CeO2 (or LSMO) skeleton embedded in LSMO matrix. This new VAN 3D framework enables both lateral and vertical strain engineering simultaneously within the films and obtains highly enhanced magnetotransport properties, such as the record high magnetoresistance (MR) value of ~51-66%, compared with its VAN single layer counterpart. In order to demonstrate the flexibility of this design, other systems such as 3D ZnO framework embedded in LSMO matrix have been constructed to explore the thickness effects of the ZnO interlayers on the magnetotransport properties of the LSMO-ZnO system. The maximum MR value is obtained at the ZnO interlayer thickness of ~2 nm, which enables the optimal magnetoresistance tunneling effect. Meanwhile, the significance of the interlayer selection in the microstructure and magnetoresistance properties of the LSMO-ZnO system has been investigated by varying the interlayer materials yttria-stabilized zirconia (YSZ), CeO2, SrTiO3, BaTiO3, and MgO. The formed 3D heterogeneous framework provides a new dimension to tailor the microstructure, strain and functionalities within the films.
Moreover, a new strain engineering approach with engineered tilted interfaces has been demonstrated by multilayering different VAN layers with various two phase ratio and creating a hybrid nanodumbbell structure within the LSMO-CeO2 VAN thin films. The nanodumbbell structure accomplishes a more efficient strain engineering and exhibits highly enhanced magnetic and magnetoresistance properties, compared with its VAN single layer and interlayer counterparts.
These examples presented in the thesis demonstrate the flexibility and potential of 3D strain engineering in complex VAN systems and a higher level of property control, coupled with unique microstructures and interfaces. Beyond perovskites, these 3D designs can be extended to other material systems for a broader range of applications, such as energy conversion and storage related applications.