Plasmons, the collective electronic oscillations of metallic nanoparticles and nanostructures, are at the forefront of the development of nanoscale optics. Metallic nanostructures with their geometry-dependent optical resonances are a topic of intense current interest due to their ability to manipulate light in ways not possible with conventional optical materials. As optical frequency nanoantennas, reduced-symmetry plasmonic nanoparticles have light-scattering properties that depend strongly on geometry, orientation, and variations in dielectric environment. Particularly fascinating aspect of these systems is the recently realized possibility of creating optical frequency “magnetic plasmon” responses of comparable magnitude to the “electric plasmon” response. It is of our central interest to understand better the plasmonic system so as to manipulate the energy transport mechanism. With the much more advanced numerical calculations, and based on the Finite Element Method (FEM) and Finite-Difference Time-Domain (FDTD) method, we are now able to study various kinds of nanostructures for different interesting optical properties. With the help of FDTD, we show the geometry dependent dissipation rate in different nanosystems. We brought up a new damped harmonic oscillator model to account for the observed difference. We show that our new model better completes the full map of the energy dissipation mechanism, and the predicted outcome agreed very well with the FDTD calculations. Elliptical nanorings were investigated by applying both FEM and FDTD methods. The mulitiple plasmonic resonaces exhibited by elliptical nanorings and the well tunability of the nanosystem make elliptical nanorings very interesting. Different features can be realized by controlling the aspect ratios of the elliptical nanorings. We show another interesting nanostructures, light bending nanocup as well. Due to its unique light scattering properties, nanocup is a very promising candidate in solar cell applications. We studied more about its light redirection properties with the presence of a dielectric substrate and its sensitivity to the subtle geometry differences. Plasmonic heptamer has been shown to possess an intriguing Fano resonance due to the interference of its hybridized subradiant and super-radiant modes. Neighboring fused heptamers can support magnetic plasmons due to the generation of antiphase ring currents in the metallic nanoclusters. We use such artificial plasmonic molecules as basic elements to construct low-loss plasmonic waveguides and devices. The manipulation of magnetic plasmons in heptamer interconnects can further be expanded to more complex systems, for example, by integrating more optical paths to achieve multiple input and output plasmonic networks. With their compact dimensions, outstanding low-loss propagation characteristics, and range of functionalities, magnetic plasmon-based devices based on these structures should be key to the further development of high- performance energy transport components in informa- tion processing and data storage applications.

Magnetic nanorings are the object of increasing scientific interest because they possess the vortex (stray field free) state which ensures lower magnetostatic interactions between adjacent ring elements in high packing density memory devices. In addition, they have other potential applications such as single magnetic nanoparticle sensors, microwave-frequency oscillators and data processing. The stabilization of magnetization state, types of domains and domain wall structures depends on the competing energies such as magnetostatic, exchange and anisotropy. The nucleation/ pinning of domain walls depends on the local inhomogeneity in shape such as roughness, notches etc, which play an important role in stabilizing domain configurations that can be controlled by magnetic field/spin polarized current etc. The information gained by the study of magnetization reversal in the nanoring devices could help in understanding the possible stable magnetization states, which can be incorporated into the development of magnetic logic and recording devices in a NR-based architecture. The magnetization reversal and the stable states in the symmetric cobalt nanorings (NRs) attached with nanowires (NWs) (at diametrically opposite points), is studied through magnetoresistance (MR) measurements by application of in-plane magnetic field (H). Here, a strong in-plane shape anisotropy is introduced in cobalt thin films by patterning them into NR and NWs. The presence or absence of a DW in the device is detected utilizing the AMR property of the material, where the presence of DW leads to a decrease in the resistance of the probed section of the device. It is demonstrated that the magnetization reversal of the device with smaller width, proceeds through four distinct magnetization states, one of these is the stabilized vortex state that persists over a field range of 0.730 kOe. The effect of width (from 70 nm to 1 µm) and diameter (from 2 µm to 6 µm) on the switching behavior is demonstrated. The magnetization states observed in the MR measurements are well supported by micromagnetic simulations. A statistical analysis of switching fields in these devices was demonstrated by histogram plot (of switching counts) to understand the repeatability and reproducibility of switching characteristics. In addition, the magnetization reversal of permalloy NR is also studied by MR experiment when two NWs are attached to it in two different configurations. It has been demonstrated that a vortex state can be stabilized if the NWs are attached in a way that they are at an obtuse angle with respect to each other (type-II device) which is not the case if the NWs are attached at diametrically opposite points (type-I device). This occurs because the NWs reverse at different fields as they are asymmetric with respect to applied magnetic field at every angle. The angular dependence study of the magnetization states indicates that the vortex state could be always stabilized in the type-II device irrespective of the direction of in-plane applied magnetic field while it is not the case in type-I device. The experimental observations are in good agreement with micromagnetic simulations performed on similar device structures. Further, in the last part of the thesis, the magnetization reversal of geometrically engineered cobalt NR (of width 80 nm) devices are studied by application of H. Two types of cobalt nanoring devices were fabricated. In type-1 devices the NR is attached with two nanowires (NWs) at diametrically opposite positions. In type-2 devices the NR is attached with one NW, whose other end is attached to a 5 µm x 5 µm square pad. In type-2 device, the pad reverses first, thus causing the generation of a DW at the junction of the nucleation pad and the NW. The device type-2 possesses five distinct magnetization states, one of these is the vortex state. Easy nucleation of domain walls (DWs) results in a decrease of switching field corresponding to the reversal of the nanowire. This leads to an increase in the range of fields, where the vortex state exists. In addition, angular dependence of the switching behavior indicates that the vortex state can be stabilized at all in-plane orientations of H. This occurs because of the fact that symmetry was broken due to the presence of single domain wall pinning center which was the junction of the NR and NW. The results of our micromagnetic simulations are in a good agreement with the experimental results. These results are important to understand the role of NWs which allows the formation of vortex state at every angle of the in-plane H. In type-1 device, the simulation shows that when the field is applied at any angle away from the axis of the NW, the vortex state cannot be stabilized. The width dependent study of switching fields indicates, that the switching fields decrease with increasing the width of NR devices due to a reduction of the demagnetization field.

In this thesis two issues in nanotechnology have been addressed. The first is the comprehensive characterization of engineered nanomaterials prior to their examination in toxicology and environmental studies. The second is the development of a method to produce nanostructure arrays over large areas and for low cost. A major challenge when assessing nanomaterial’s risks is the robust characterization of their physicochemical properties, particularly in commercial products. Such data allows the critical features for biological outcomes to be determined. This work focused on the inorganic oxides that were studied in powdered and dispersed forms as well as directly in consumer sunscreen products. The most important finding was that the commercial sunscreens that listed titania or zinc oxide as ingredients contained nanoscale materials. Cell free photochemical tests revealed that ZnO particles without any surface coating were more active at generating ROS than surface coated TiO2 nanoparticles. These studies make clear the importance of exposure studies that examine the native form of nanomaterials directly in commercial products. The second part of this thesis presents the development of a new method to fabricate gold nanoring and nanocrescent arrays over large areas; such materials have unique optical properties consonant with those described as metamaterials. A new shaped nanosphere lithography approach was used to manipulate the form of silica nanospheres packed onto a surface; the resulting array of mushroom structures provided a mask that after gold evaporation and etching left either golden rings or crescents over the surface. The structures had tunable features, with outer diameters ranging from 200 to 350 nm for rings and crescent gap angles of ten to more than a hundred degrees. The use of a double mask method ensured the uniform coverage of these structured over 1 cm2 areas. Experimental and theoretical investigations of the optical properties of the arrays revealed the optical resonances in the infrared region. Finally, in the course of developing the nanorings, etch conditions were developed to deposit large area arrays of polystyrene nanodoughnuts with diameters from 128 to 242 nm. These non-conductive structures provide an ideal template for further attachment of magnetic of optically emissive nanoparticles.

This thesis describes supramolecular approaches to porphyrin nanorings. Cyclic porphyrin arrays resemble natural light harvesting systems, and it is of interest to probe the photophysical effects of bending the porphyrin aromatic π-system. A general overview of the synthesis and photophysical properties of porphyrins and their arrays is carried out in Chapter 1. The electronic structure of porphyrins is examined, and how conformational effects in oligomers, such as inter-porphyrin torsional angle and backbone bending influence the π-conjugation pathway. The structures of light harvesting complexes are discussed. Chapter 2 describes the design and synthesis of a complementary 12-armed template designed to coordinate linear porphyrin oligomers in the correct conformation for cyclisation to give a cyclic porphyrin dodecamer. Chapter 3 demonstrates two approaches to a cyclic porphyrin dodecamer ring. Firstly, a classical templating approach using the 12-armed template is described. The limitations of this approach in the quest for larger nanorings are discussed. Vernier templating, which utilises a mismatch in the number of binding sites between a ligand and its receptor is introduced as a general strategy to the synthesis of large nanorings. This is demonstrated by the synthesis of cyclic dodecamer from a linear porphyrin tetramer and a hexadentate template via a figure-of-eight intermediate. The general utility of the Vernier method to large nanorings is explored in Chapter 4 with steps towards the synthesis of a cyclic tetracosamer, consisting of 24 porphyrin subunits. In preliminary experiments, an improved route to the cyclic porphyrin octamer is described. Finally, the photophysical properties of the nanoring series are explored in Chapter 5 as a function of size and conformation. Femtosecond photoluminescence spectroscopy shows that even in cyclic dodecamer, exciton delocalisation over the entire porphyrin backbone occurs on a sub-picosecond timescale, and parallels are drawn with the dynamics of natural light harvesting complexes.