Quynh Nguyen

Plasmonic core@shell nanoparticles offer a versatile platform for controlling light–matter interactions at the nanoscale, because their optical, electronic, and chemical properties can be tuned through the independent design of core composition, shell composition, particle size, shell thickness, and morphology. This thesis explores the synthesis, optical characterisation, and plasmon-driven chemistry of Au based core@shell nanostructures, with the broader aim of connecting nanoscale architecture to emergent optical and photocatalytic behaviour. First, this thesis introduces the fundamental concepts of plasmonics and colloidal nanoparticle synthesis, providing the theoretical and experimental framework required to understand localised surface plasmon resonances, extinction spectroscopy, electron microscopy, and Mie-theory-based optical modelling. Building on this foundation, a scalable colloidal route is developed to fabricate Au@Al-doped ZnO core@shell nanoparticles with independent control over Au core size, ZnO shell thickness, and aluminium dopant concentration. This approach enables systematic tuning of hybrid plasmonic/epsilon-near-zero nanostructures and provides a route towards spectral alignment between the Au core plasmon resonance and the optical response of the doped oxide shell. The second part of this thesis focuses on plasmon-driven chemistry using the light-induced formation of Ag shells on Au nanoparticles as a model reaction. By combining in situ extinction spectroscopy with Mie-theory calculations, nanoparticle concentrations, absorption cross sections, absorbed photon numbers, and internal quantum efficiencies are extracted from steady-state measurements. This analysis shows that smaller spherical Au nanoparticles use absorbed photons more efficiently to drive Ag⁺ reduction, and that the observed size dependence is consistent with a mechanism governed by short-lived, near-surface d-band holes. Finally, the role of crystallographic faceting is investigated, revealing that faceted Au nanoparticles deviate from the behaviour predicted for spherical particles and exhibit selective Ag deposition at edges and corners. Together, these findings demonstrate how controlled synthesis, optical spectroscopy, electron microscopy, and analytical modelling can be combined to link nanoparticle structure to plasmonic function. This thesis therefore provides both synthetic strategies and mechanistic insight for the rational design of next-generation plasmonic core@shell nanomaterials for photonics, sensing, nonlinear optics, and photocatalysis.

Light absorption in plasmonic gold nanoparticles (AuNPs) generates short-lived nonthermalized electrons and holes that can drive redox reactions, making them attractive for photocatalytic applications. However, characterizing these processes and disentangling competing effects due to photothermal heating or plasmonic field enhancement remain challenging. Here, we exploit the light-driven synthesis of silver shells around AuNPs as a model reaction to study the role of nonthermalized carriers in plasmonic gold photochemistry. By coupling in situ extinction spectroscopy with Mie theory calculations, we extract internal quantum efficiencies (IQEs) for AuNPs with different diameters. The size dependence of the IQE scales with the probability of light absorption within 2-3 nm of the Au nanoparticle surface, highlighting the critical role of short-lived d-band holes, which recombine on ultrafast time scales, in driving our chemistry. More broadly, our approach provides a simple yet powerful framework to characterize activation mechanisms and quantify carrier dynamics in plasmonic photocatalysis.

Light absorption in plasmonic gold nanoparticles (AuNPs) generates short-lived non-thermalized electrons and holes that can drive redox reactions, making them attractive for photocatalytic applications. However, characterizing these processes and disentangling competing effects due to photothermal heating or plasmonic field enhancement remains challenging. Here, we exploit the light-driven synthesis of silver shells around AuNPs as a model reaction to study the role of non-thermalized carriers in plasmonic gold photochemistry. By coupling in-situ extinction spectroscopy with Mie theory calculations, we extract internal quantum efficiencies (IQEs) for AuNPs with different diameters. The size dependence of the IQE scales with the probability of light absorption within 2-3 nm from the Au nanoparticle surface, highlighting the critical role of short-lived d-band holes, which recombine on ultrafast timescales, in driving our chemistry. More broadly, our approach provides a simple yet powerful framework to characterize activation mechanisms and quantify carrier dynamics in plasmonic photocatalysis.

A novel colloidal synthesis approach for fabricating Au@ZnO core@shell nanoparticles with tunable core size, shell thickness, and Al dopant concentration.

Plasmonic materials, such as gold nanoparticles (AuNPs), exhibit significant extinction and near-field enhancement across the visible and near-infrared spectrum, attributable to localized surface plasmon resonances (LSPRs). Epsilon-near-zero (ENZ) materials, such as aluminum doped zinc oxide (AZO) are known in non-linear optics for their ability to generate and manipulate light-matter interactions through processes like higher harmonic generation. Combining doped ZnO with plasmonic materials therefore holds promise for enhancing non-linear efficiencies and tuning their operational wavelengths. To date, however, only top-down structures based on plasmonically decorated thin ENZ films have been realized, and no colloidal and scalable route to obtain these hybrid materials has been reported yet. Here, we introduce a novel colloidal synthesis approach for fabricating Au@AZO core@shell nanoparticles with tunable core size, shell thickness, and dopant concentration, allowing for the spectral alignment of the LSPRs of the AuNPs with the non-linear optical properties of the AZO shells. Our method involves the colloidal synthesis of gold cores followed by a controlled sol-gel process to deposit ZnO and AZO shells, resulting in core diameters ranging from 25 to 69 nm, shell thicknesses from 16 to 47 nm, and aluminum doping levels between 0 and 4 at.%. Our procedure widens the range of hybrid plasmonic nanostructures that can be colloidally synthesized, opening new possibilities for the large scale fabrication of high-performance nanomaterials for integration in photonic, photocatalytic, and sensing applications.