One dimensional nanostructures for nanoelectronics and energy storage: synthesis and advanced electron microscopy characterisation

This research explores the solution based synthesis of functional nanorods and nanowire compositions and the use of electron microscopy techniques to elucidate growth and phase transformations in these nanostructures. The electron microscopy investigations were performed primarily through aberration corrected TEM combined with EDX and in-situ TEM.

Chapter 1 gives an introduction to one- dimensional nanorods and nanowires, including their properties, characterisation and applications. Their growth mechanisms and synthesis techniques are also discussed. The general synthesis methods and characterisation techniques of the nanostructures produced within this work are detailed in Chapter 2.

Chapter 3 reports the synthesis of axial heterostructure Si and Ge (hSG) nanowires (NWs). The NWs were grown from a low-cost Sn powder to achieve a powder form product representing an attractive route from lab-scale to commercial application. Atomic resolution STEM imaging and EDX mappings were used to analyse the interfacial abruptness at the NW heterojunction and the planar defects present in the NWs. The electrochemical performance of these hSG NWs as anodes in a Li-ion battery was examined as a potential anode material to replace standard graphite anodes. The structural evolution and phase changes in the hSG NWs due to Li cycling were also investigated using TEM and EDX.

The use of in situ TEM techniques to track reaction chemistry in nanostructured materials is demonstrated in Chapters 4 and 5.

In Chapter 4, building on the solution based synthesis approach, heterostructure NWs comprising Ni stem and branched Si NWs were grown. In-situ heating TEM experiments were conducted to investigate Ni diffusion and phase transformation in the NWs, where the Ni atoms were supplied from the Ni stem. In situ monitoring of the heating of the NW sample provided unparalleled insights into the role of NW morphology and defect distribution on the rate of Ni diffusion and phase being formed.

Chapter 5 focusses on the in-situ liquid cell TEM investigations of self-assembly processes of nanorods suspended in organic solvents. Cadmium Sulphide nanorods (CdS NRs) were used as a model system. These NRs undergo oriented attachment to form CdS NWs when posttreated with octylamine. Liquid cell TEM was used to dynamically probe the dynamics of CdS NRs in real-time under external stimuli such as reactant flow (octylamine) and electron beam irradiation. The results showed how different environmental stimuli influence the dynamics of CdS NRs, from end-to-end NR attachments to the formation of side-by-side NR chains and vertically aligned self-assembled NRs.

Chapter 6 provides conclusions for each chapter and recommendations for future research on the work completed in this thesis.