Synthesis, Characterization and Surface/Interface Engineering of Confined Functional Materials: Multiferroics and Layered Semiconductors

Functional materials with dimensional and spatial confinements have attracted ever-increasing attention in modern technology (e.g., nanotechnology). Particularly in electronic devices, it is of tremendous importance to control over shape, size and interfaces in fundamental building blocks like semiconductor-based transistors and ferromagnetic/ferroelectric information media. Moreover, such confinements have also aroused extensive intellectual curiosity (e.g., in nanoscience) to investigate unexpected phenomena both experimentally and theoretically. For example, as the size shrinks in at least one dimension, the surface-to-volume ratio increases dramatically; and both inherent properties including band gaps in semiconductors and domain stability in ferroics and external influence through such newly exposed surface/interfaces inevitably change. However, the underlying mechanisms and opportunities are still not fully exploited, especially in novel materials recently discovered, due to lack of appropriate synthesis approach, characterization methodology and functionalization strategy. This dissertation aims at addressing such critical issues in two kinds of functional materials: nanostructured multiferroics (Chapter 2-4) and atomically thin layered semiconductors (Chapter 5-6).;In the first part, to resolve the challenges in synthesis of complex ferroic and multiferroic structures and thus introduce confinements and new functionality, we developed a flexible and convenient sol-gel self assembly approach. Magnetic ZnxMn1-xFe2O 4 thin films were deposited on Si substrates; and their microstructure evolution was studied. Annealing temperatures, crystal lattice, grain size and magnetic properties were successfully correlated. Further, such sol gel approach was extended to prepare multiferroic composite thin films using novel chemical solution with ferroelectric BaTiO3 and magnetic CoFe 2O4 precursors inside. Spin coating and annealing process were adjusted with great care to induce phase separation, heteroepitaxy and complex 3D interfaces. Large magnetoelectric effect in such structures was investigated and explained by strain effect via interfaces between the two nanocrystalline phases.;In order to study ferroic domains and engineer domain walls at the nanoscale, an innovative characterization technique named after angle-resolved piezoresponse force microscopy (AR-PFM) was introduced to characterize the polarization domain configuration particularly the in-plane domain orientation. One typical testbed to demonstrate its capability was BiFeO3 (BFO), the most well-known single phase multiferroic, with up to 8 symmetry-allowed polarization variants. Using AR-PFM we discovered intermediate domains away from these conventional variants and dipole rotation; charges at the domain walls and associated electrostatic energy thus decreased. Such unexpected interface phenomena would facilitate understanding extraordinary properties of BFO and exploring new paradigms in ferroelectrics and multiferroics.;In parallel, synthesis of a new family of materials, i.e., ultrathin layered semiconductors including MoS2, GaS and GaSe was reported. We developed a methodology based on optical contrast with collaboration of atomic force microscopy and Raman spectroscopy to fast and reliably determine the thickness of such ultrathin films. Their multi-functionality in modern electronics and the influence of spatial and dimensional confinement were investigated. Prototypes of field effect transistors based on these layered semiconductors were fabricated to show good mobility and high on/off ratio. Furthermore, we found that the interaction at the surface/interface of such layered materials could either jeopardize the performance of field effect transistors or be used as the platform to detect a variety of hazardous gases including NO2 and NH3.