Exploration of Structure-Property Relationship and Growth Mechanism of 1D Nanowires Using Transmission Electron Microscopy
Abstract
One-dimensional (1D) nanostructured materials, e.g., nanowires (NWs) have fascinating properties and applications in various fields including electronic and energy conversion devices. Despite tremendous progress in the NW research field, a thorough understanding of NW structure-properties correlation and their rational synthesis with desired properties are two main roadblocks to the wider application of different types of NWs, which are the main focus of this dissertation.The first part of this dissertation is a collaborative work, and the goal was 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) interac- tion on 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 investigate their interrelation with the observed proper- ties. Individual NW was examined using a transmission electron microscope (TEM) to obtain high-resolution TEM images, diffraction patterns (DP), and morphology at lower magnifications. But while examining NbSe3 NW, it was discovered that the NW needs to be tilted 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 considering other factors (change in heat capacity and Debye temperature) to explain the phenomenon. For the second part of this work, the growth mechanism of boron carbide NWs was explored using TEM-based cross-sectional examination. Because of their unusual structural complexity and exceptional bonding, boron carbide materials are known for their excellent chemical and physical properties and have potential applications in high-temperature thermoelectric devices. To obtain improved thermoelectric performance, our previous group members synthesized boron carbide NWs and performed extensive characterization of their structures and thermal properties. However, rational synthesis of boron carbide NWs with desired properties could not be obtained yet. For this purpose, a thorough understanding of the growth mechanism is crucial. To investigate the growth mechanism and understand the effect of each reaction parameter, cross-sectional TEM examinations were conducted on multiple growth substrates. Those substrates were prepared in different experimental conditions with varying reaction parameters (Ni film as a catalyst, diborane and methane as precursor gases, and annealing temperature and time). During the 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, voids were formed in the SiO2, which could be nucleated due to the coincidence of the microchannels in the SiO2 and stress in the Ni film/substrate interface during annealing. Higher temperatures showed a contradictory trend in the agglomeration and void nucleation behavior. Diborane was seen to etch the SiO2 layer and facilitate the diffusion of the particles whereas methane exhibited the opposite effect. Some nanostructures with a catalyst on top grew in the latter case. Diborane and methane together generated thin films with nanostructures in the absence of Ni film. It can be hypothesized that Ni-catalyzed diborane species cause the etching of the SiO2 which is suppressed when both precursors are present. With all the reaction parameters present in the chamber, the diborane and methane react predominantly to form BxCy film, and NWs grow from this film with the help of the catalyst. Although etching is prevented, the diffusion of particles into SiO2 could still be observed. Compositional analysis of the particles and nanostructures and cross- sectional examination of more growth substrates are required to verify the hypothesis. Despite the lack of compositional information, cross-sectional TEM images presented in this dissertation provide useful data such as the interaction of the catalyst with the substrate, and precursor gases individually and in combination that is not explored extensively before.