At the nanoscale, materials’ properties differ vastly from the bulk. For example, nanoparticles (NPs) of plasmonic materials can interact strongly with light and sustain resonant oscillations of their free electron density, giving rise to absorption, scattering and local electric field enhancement. These phenomena enable applications in a variety of fields such as sensing, surface-enhanced spectroscopies, photothermal cancer therapy and photocatalysis. Many properties of NPs, including the optical properties of plasmonic NPs, are strongly dependent on NP size and shape. Understanding the shapes of NPs, in particular those arising from twins in the crystal structure, is vital to understanding their properties.

Much NP technology is dominated by face centred cubic (FCC) metals, including Au, Ag and Pd. In this thesis, the synthesis and NP shapes of non-FCC nanocrystals are studied. First, the synthesis of hexagonal close packed (HCP) Mg NPs, an earth-abundant plasmonic material, is investigated. The NP shapes are explained and the effects of reaction parameters on the products of a colloidal synthesis are systematically probed. NP sizes are selected between 80 nm and over a micrometre by varying the reaction time, overall reaction concentration, temperature, electron carrier, and metal salt additives.

Next, seed mediated growth syntheses of Mg NPs are developed. These methods are a common technique to manipulate NP size and shape for other metallic NPs. The reaction starts with a rapid nucleation step using Li biphenyl as a strong reducing agent. Then the NPs are slowly grown by converting the reducing agent to Li phenanthrene, which is less strongly reducing than Li biphenyl, and using an ice bath to suppress further nucleation. However, control over the amount of growth remains a challenge. The reaction is then further adapted by employing a new reducing agent. By investigating the synthesis route, small Mg NPs with new morphologies are formed. These small seeds are then again grown by altering the reducing agent during the synthesis. Finally, based on these batch syntheses, Mg NPs are synthesised for the first time in a continuous flow system. These systems present opportunities for enhanced control over the reaction products by facilitating more efficient initial mixing. A stable system for reproducible synthesis of Mg NPs is reported, with the capability to form the first coloured colloids of Mg NPs, indicating improved size and shape dispersity.

Beyond HCP crystallinity, other crystal systems are also gaining interest in several fields. For instance, body centred cubic (BCC) NPs such as Fe and W have applications in catalysis, and body centred tetragonal (BCT) NPs such as In have plasmonic applications. Both the catalytic and plasmonic properties of NPs are strongly dependent on the NP morphology, including twinning. The shapes of BCC and BCT NPs are investigated computationally, showing that distinguishing twinned NPs from their single crystal counterparts is challenging for many BCC NPs, but often more straightforward for BCT NPs. Modelling the electron diffraction signal shows that distinguishing twinned BCC and BCT NPs from single crystal signals is impossible if NPs are measured along many of the most likely viewing directions, as the diffraction spots from each twin overlap. This insight into NP shapes with crystallographies beyond the more readily understood FCC system may reveal why twinned BCC NPs have not yet been reported. Based on these results, reliable methods are suggested to diagnose twinning in BCC and BCT NPs.

This thesis paves the way for the use of alternative, non-FCC materials in nanotechnology, including for the large-scale use of Mg as a low-cost and sustainable plasmonic material and for the characterisation of BCC and BCT NPs, opening the door to improved control and understanding of novel NP shapes.