Exploration of structure-property relationship and Growth mechanism of 1D nanowires using transmission electron microscopy

One dimensional (1D) nanostructured materials are the materials having one external dimension outside the nanoscale range and the other two dimensions in the nanoscale range such as nanowires (NWs). Due to their unique one dimensionality, 1D NWs are considered as the ideal systems to investigate many foundational concepts in physical science and engineering. Along the same line, NWs can be used to establish correlation between their structural parameters, dimensionality, size, and functional properties. Inspired by the vapor-liquid-solid (VLS) growth mechanism provided by R.S. Wagner, NW synthesis started in the early 1990s. In the following three decades, NW research field expanded significantly in terms of synthesis, characterization of structures and properties. However, correlation of the properties with well-understood NW structures and rational synthesis of different NWs with desired properties are two main roadblocks for the wide application of different types of NWs, which are the main focus of this dissertation.
This dissertation consists of two parts. The first part is a collaborative work with researchers from Vanderbilt University and Pennsylvania State University and the goal is to find if any correlation exists between structural parameters, morphology (such as lattice constants and dimensions) and thermal properties of niobium selenide (NbSe3) NWs. NbSe3 is a chain-like structure with molecular chains joined by van der Waals (VdW) force and suitable for the exploration of the effect of electron-phonon (e-ph) interaction on the thermal conductivity. However, superdiffusive thermal transport was observed for ultra-thin NbSe3 NWs (hydraulic diameter <26 nm), which led us to measure the structural parameters and find interrelation with the observed properties. Individual NW was examined using transmission electron microscope (TEM) to obtain high resolution TEM image, diffraction pattern (DP) and morphology at lower magnifications. But measuring the lattice constants from NbSe3 NW, which is a monoclinic structure, was extremely challenging. It was discovered that the NW needs to be tilted to out-of-plane to obtain all the lattice parameters. The process is constrained due to the tilt limitation in the TEM. A DP roadmap was built from the measured and simulated DPs to facilitate the TEM examination and analysis. The obtained results eliminated the structural effect from the unusual thermal properties, and led to consider other factors (change in heat capacity and Debye temperature) to explain the phenomenon.
For the second part of this work, growth mechanism of boron carbide NWs was explored using TEM-based cross-sectional examination. Boron carbide materials are known for their fascinating chemical and physical properties which arise due to their unusual structural complexity and exceptional bonding. Bulk boron carbides are widely used for refractory, ballistic armor, and nuclear applications. They also have potential applications in the high temperature electronic and thermoelectric devices. In order to obtain improved thermoelectric performance, boron carbide NWs were synthesized and extensive characterization of structures and thermal properties were performed by our previous group members. However, rational synthesis of boron carbide NWs with desired properties could not be obtained yet. For this purpose, a through understanding of growth mechanism is crucial.
In order to explore growth mechanism, cross-sectional TEM examinations were conducted on multiple samples prepared in different experimental conditions. The approach was to identify the effect of different reaction parameters on the NW growth. Cross-sectional sample preparation required strenuous effort and the results largely depended on the quality of the prepared samples. Reaction parameters that were considered include Ni film as catalyst, diborane and methane as precursor gases, and annealing temperature and time. During annealing of Ni film on the SiO2/Si substrate, catalyst-substrate interaction could be observed. The contrast in the TEM images demonstrated that Ni film agglomerated into particles and diffused into the SiO2 layer. Also, void formation in the SiO2 was found which could be nucleated due to the coincidence of microchannels in the SiO2 and stress in the Ni film/substrate interface during annealing. Higher temperature showed a contradictory trend in the agglomeration and void nucleation behavior. In the other two experimental conditions, diborane and methane were added to the reaction chamber separately. Diborane was seen to etch the SiO2 layer and facilitate the particles diffusion whereas methane exhibited the opposite effect. Some nanostructures with a catalyst on top grew in the later case. With diborane and methane together generated thin films with nanostructures in the absence of Ni film. The condition with all reaction parameters generated a thin film on the SiO2 layer. NWs could be growing from this film. However, the NW/thin film/SiO2 interfaces are not clear due to the overlapping layers.