Transmission Electron Microscopy Studies Of Low-dimensional Materials

In this thesis, high-resolution transmission electron microscopy (HRTEM) and electron energy-loss spectroscopy (EELS) have been employed to investigate the microstructures of low-dimensional nanomaterials such as zero-dimensional germanium nanocrystals (Ge-nc), one-dimensional hematite nanowires (Fe2O3NWs) and two-dimensional manganite epitaxial films in details.The thesis consists of four chapters.In chapter1, the basic characteristics, unique properties, and preparation methods of low-dimensional nanomaterials are generally reviewed. In addition, transmission electron microscopy (TEM) specimen preparation for different types of samples is also introduced.In chapter2, Fe2O3NWs were synthesized by thermal oxidation of pure iron foils. Using the HRTEM technique, the microstructure and growth mechanism of Fe2O3NWs are systematically investigated. Three different structures, single-crystalline, bicrystalline, and tricrystalline are observed in the Fe2O3NWs. The formation of bicrystalline and tricrystalline Fe2O3NWs is attributed to the coalescence of two or three single-crystalline nanowires. In addition, a new modulated structure with a periodicity of1.53nm is observed in the single-crystalline nanowires.In chapter3, the surface morphologies, microstructures and formation mechanisms of Bio.4Cao.6Mn03(BCMO)/SrTiO3(STO) and Lao67Ca0.33Mn03(LCMO)/STO manganite epitaxial films are investigated by HRTEM and atomic force microscopy (AFM). It is shown that the BCMO epitaxial films exhibit an island growth mode whereas the LCMO epitaxial films follow a layer by layer growth mode. Combining the critical thickness theory with the atomic diffusion length, an atomic collapse model is proposed to explain the growth mechanism of the perovskite manganite epitaxial films.In chapter4, Ge-nc were synthesized by co-implantation of Si and Ge ions into a SiO2film on (100) Si substrate and fused silica (pure SiO2), respectively. TEM and EELS are employed to investigate the microstructure and chemical composition of the Ge-nc. It is revealed that the absence of nanocavities in the SiO2film/Si substrate is attributed to the presence of Si atoms inside the formed Ge-nc, which can inhibit the outdiffusion of Ge atoms. However, for the fused silica sample, no Si atoms are detected within the Ge-nc, where strong Ge diffusion effects produce a great number of nanocavities.