UC Davis

The rapidly increasing interest in Ge-based nanostructures is motivated by important advantages of this material compared to other semiconductors commonly investigated in the broad field of nanotechnology. Ge nanoparticles are promising materials for many applications such as photovoltaics, transistors, light emitting devices, bioimaging, and energy storage devices.Due to the strong covalent bonding nature, synthesis of high-quality Ge nanoparticles with uniform size, well-defined morphology, composition, and surface chemistry is a main scientific challenge in the colloidal chemistry community and finding a reliable, scalable, cost-effective, and environmentally friendly synthetic route is of key importance for their further applications. Besides technical interest, access to defined nanoscale structures and compositions is also essential for uncovering their intrinsic properties and expanding the chemistry of group 14 nanomaterials. In this dissertation, both direct synthetic methods and post transformation approaches are explored for rational synthesis of germanium-based nanomaterials with great control of the size, shape, and composition. The detailed reaction mechanisms are revealed through a systematic study on the reaction conditions such as temperature, precursor concentrations, reaction time, etc. Amorphous solid Ge nanoparticles and hollow Ge nanoparticles are synthesized via galvanic replacement reaction with Ag NPs serving as a sacrificial template. Ag@Ge core-shell nanoparticles with controlled shell thickness are also successfully synthesized as an intermediate. Their structural, optical, and electrical properties have been investigated in detail by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and transmission electron microscopy (TEM), and supported by UV-Vis, Powder X-ray Diffraction (XRD), Energy-dispersive X-ray spectroscopy (EDS), etc. In addition, we provide an in situ method to observe the phase transition process of amorphous nano-germanium through a combination of both joule heating and high-energy electron beams at atomic resolution. The dynamic process of thermally induced structural evolution of the Ag@Ge binary system is also studied by TEM in real-time. The insights developed here can be extended to other metal semiconductor systems of interest and are crucial to developing the materials necessary for rigorous mechanistic studies of the effect of ligands and precursors on reactivity and selectivity. The two different synthetic approaches represent a way for achieving nanoparticles with strong covalent bonds as well as complex morphology, which would in turns increase the reproducibility of colloidal synthesis and prepare for scale-up and future practical applications.