Towards sustainable plasmonics: a study on the shapes and plasmonic properties of magnesium nanoparticles
The growing interest in nanoparticles and their increasingly diverse applications is fuelled by the ability to tune properties via material selection and shape control, promoting intense experimental and theoretical research. For plasmonics, where nanostructures are exploited to focus and channel light in the nanoscale, material and structure effects are studied in relation to optical properties. Understanding such light-matter interactions can inspire the design and synthesis of nanoparticles tailored for various applications, such as in sensing, photocatalysis and biomedicine, to name a few. Modelling shape effects can accelerate this progress and relies on coupling the nanoparticle’s, crystallographically dependent, geometrical representation with electromagnetic simulations. Material selection is also critical. Noble metals, which have been the pillars of plasmonics since the nascence of the field, have certain limitations and do not satisfy the sustainability requirements of modern societies, hence triggering intense research into alternative plasmonic materials. The objective of the current research is thus twofold: to provide a platform for user-friendly and crystallographically correct nanoparticle shape modelling, compatible with available electromagnetic simulation tools, and, helped by this tool, to investigate the shapes and structure-plasmonic property relations for nanoparticles made out of magnesium, a recently introduced plasmonic metal.
In this work, the reader is first introduced to the concepts of localised surface plasmon resonances, Wulff-based approaches to nanocrystal shape modelling as well as the challenges and opportunities of magnesium nanoparticles in plasmonics and beyond. The next three chapters focus on nanoparticle shape modelling commencing by a summary of the already available tools. This is followed by the description of Crystal Creator, a GUI developed to facilitate the generation of shape input for electromagnetic simulations and to model the twinned nanoparticle shapes for various crystallographies. The applicability of Crystal Creator on a variety of nanocrystal structures is demonstrated via the calculation of the optical response of well-known face-centred cubic plasmonic metals as well as the investigation of the single crystal and twinned nanoparticle shapes of metals that adopt body-centered cubic and tetragonal crystallographic configurations. The hexagonal close packed adaptation is then used to understand and predict the nanoparticle shapes of magnesium. The two subsequent chapters focus on the plasmonic properties of the modelled magnesium nanoparticles. Here, electromagnetic simulations in the discrete dipole approximation and single-particle dark-field scattering measurements are employed to unravel size, shape and environment effects in the near-field and far-field optical response of the nanoparticles. Finally, a summary and a concluding discussion on the findings of this work, along with an outlook for future research, are provided in the last chapter.