Plasmonic gold nanostructures: optical properties and application in mercury detection

This thesis investigates the application of plasmonic gold nanostructures for mercury detection. Various gold and silver single nanostructures and gold nanostructure assemblies were characterised in detail by correlated single nanostructure spectroscopy and electron microscopy. Several routes for mercury detection were explored: plasmon resonance energy transfer (PRET) upon Hg2+ binding to immobilised gold nanoparticle-organic ligand hybrid structures and amalgamation of single immobilised gold nanorods upon chemical and upon electrochemical reduction of Hg2+ ions. The amalgamation approach showed large potential with extraordinary shifts of the nanorods’ scattering spectra upon exposure to reduced mercury; a result of compositional and morphological change induced in the nanorod by amalgamation with mercury. A shift of 5 nm could be recorded for a concentration as low 10 nM Hg2+. Through detailed time-dependent experiments insights into the amalgamation mechanism were gained and a model comprising 5 steps was developed. Finally, spectroelectrochemistry proved to be an excellent way to study in real time in-situ the amalgamation of mercury with gold nanorods paving the way for future work in this field.

Ultrahigh Purcell factor in low-threshold nanolaser based on asymmetric hybrid plasmonic cavity

A low-threshold nanolaser with all three dimensions at the subwavelength scale is proposed and investigated. The nanolaser is constructed based on an asymmetric hybrid plasmonic F-P cavity with Ag-coated end facets. Lasing characteristics are calculated using finite element method at the wavelength of 1550 nm. The results show that owing to the low modal loss, large modal confinement factor of the asymmetric plasmonic cavity structure, in conjunction with the high reflectivity of the Ag reflectors, a minimum threshold gain of 240 cm−1 is predicted. Furthermore, the Purcell factor as large as 2518 is obtained with optimized structure parameters to enhance rates of spontaneous and stimulated emission.

Self-assembly of gold nanocrystals into discrete coupled plasmonic structures

Development of methodologies for the controlled chemical assembly of nanoparticles into plasmonic molecules of predictable spatial geometry is vital in order to harness novel properties arising from the combination of the individual components constituting the resulting superstructures. This paper presents a route for fabrication of gold plasmonic structures of controlled stoichiometry obtained by the use of a di-rhenium thio-isocyanide complex as linker molecule for gold nanocrystals. Correlated scanning electron microscopy (SEM)—dark-field spectroscopy was used to characterize obtained discrete monomer, dimer and trimer plasmonic molecules. Polarization-dependent scattering spectra of dimer structures showed highly polarized scattering response, due to their highly asymmetric D∞h geometry. In contrast, some trimer structures displayed symmetric geometry (D3h), which showed small polarization dependent response. Theoretical calculations were used to further understand and attribute the origin of plasmonic bands arising during linker-induced formation of plasmonic molecules. Theoretical data matched well with experimentally calculated data. These results confirm that obtained gold superstructures possess properties which are a combination of the properties arising from single components and can, therefore, be classified as plasmonic molecules

Au nanorods-semiconductor nanowire hybrid nanostructures : nanofabrication techniques and optoelectronic properties

The objective of this thesis is the exploration and characterization of novel Au nanorod-semiconductor nanowire hybrid nanostructures. I provide a comprehensive bottom-up approach in which, starting from the synthesis and theoretical investigation of the optical properties of Au nanorods, I design, nanofabricate and characterize Au nanorods-semiconductor nanowire hybrid nanodevices with novel optoelectronic capabilities compared to the non-hybrid counterpart. In this regards, I first discuss the seed-mediated protocols to synthesize Au nanorods with different sizes and the influence of nanorod geometries and non-homogeneous surrounding medium on the optical properties investigated by theoretical simulation. Novel methodologies for assembling Au nanorods on (i) a Si/SiO2 substrate with highly-ordered architecture and (ii) on semiconductor nanowires with spatial precision are developed and optimized. By exploiting these approaches, I demonstrate that Raman active modes of an individual ZnO nanowire can be detected in non-resonant conditions by exploring the longitudinal plasmonic resonance mediation of chemical-synthesized Au nanorods deposited on the nanowire surface otherwise not observable on bare ZnO nanowire. Finally, nanofabrication and detailed electrical characterization of ZnO nanowire field-effect transistor (FET) and optoelectronic properties of Au nanorods - ZnO nanowire FET tunable near-infrared photodetector are investigated. In particular we demonstrated orders of magnitude enhancement in the photocurrent intensity in the explored range of wavelengths and 40 times faster time response compared to the bare ZnO FET detector. The improved performance, attributed to the plasmonicmediated hot-electron generation and injection mechanism underlying the photoresponse is investigated both experimentally and theoretically. The miniaturized, tunable and integrated capabilities offered by metal nanorodssemicondictor nanowire device architectures presented in this thesis work could have an important impact in many application fields such as opto-electronic sensors, photodetectors and photovoltaic devices and open new avenues for designing of novel nanoscale optoelectronic devices.

Engineering metallic nanoparticles for enhancing and probing catalytic reactions

Recent developments in tailoring the structural and chemical properties of colloidal metal nanoparticles (NPs) have led to significant enhancements in catalyst performance. Controllable colloidal synthesis has also allowed tailor-made NPs to serve as mechanistic probes for catalytic processes. The innovative use of colloidal NPs to gain fundamental insights into catalytic function will be highlighted across a variety of catalytic and electrocatalytic applications. The engineering of future heterogenous catalysts is also moving beyond size, shape and composition considerations. Advancements in understanding structure-property relationships have enabled incorporation of complex features such as tuning surface strain to influence the behavior of catalytic NPs. Exploiting plasmonic properties and altering colloidal surface chemistry through functionalization are also emerging as important areas for rational design of catalytic NPs. This news article will highlight the key developments and challenges to the future design of catalytic NPs.