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Title: Development of strontium molybdate and strontium niobate for plasmonic devices
Author: Wells, Matthew Philip
ISNI:       0000 0004 7658 5295
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2018
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Plasmonic phenomena have been proposed as a mechanism for the advancement of a wide range of technologies, ranging from solar energy harvesting, optical and chemical sensing, to high-speed computing. A review of literature on the subject however reveals that, despite the strong motivation for the development of plasmonic technologies, there exist relatively few practical plasmonic devices. This is substantially due to a lack of appropriate materials suitable for use in the relevant frequency regime of each application. Consequently, investigations are carried out into two potential alternative plasmonic materials, namely strontium molybdate (SrMoO3) and strontium niobate (SrNbO3). Thin films of both materials, deposited by pulsed laser deposition (PLD), are comprehensively characterised by spectroscopic ellipsometry, x-ray diffraction, atomic force microscopy, scanning electron microscopy, and secondary ion mass spectrometry, along with DC resistivity and AC Hall effect measurements. Both strontium molybdate and strontium niobate are shown to exhibit optical properties well suited to plasmonic applications operating in the visible to near infrared spectral regime, particularly those utilising the local heat generation capabilities of plasmonic nanoparticles. However further experiments identify a propensity to oxidation present in both materials, thus restricting their use in a number of high temperature applications. Moreover, strontium molybdate is shown to degrade rapidly when exposed to water, further limiting its range of practical applicability. Additional experiments seek to address these prohibitive instabilities; specifically, the degradation mechanisms of strontium molybdate under exposure to high temperatures and in water are both found to be significantly suppressed by the introduction of an in situ nitrogen annealing step to the PLD routine. This is found to induce an additional crystalline phase at the surface of the film which acts as a passivation layer, enabling the material's desirable optical properties to be retained to temperatures in excess of 600° C. Similarly, investigations are conducted into the fabrication and properties of multilayer strontium molybdate/strontium niobate structures, from which it is found that the degradation of strontium molybdate in water can be entirely mitigated by the addition of a 10 nm strontium niobate capping layer. This development facilitates the use of thin-film patterning processes such as colloidal lithography, thus enabling the fabrication of plasmonic nanoantennas from each material. In summary, this thesis describes the introduction of two new materials to the field of plasmonics. The specific applications to which they may be best suited are identified, along with their principal degradation mechanisms, and potential mitigation techniques for these degradations.
Supervisor: Petrov, Peter ; Ryan, Mary Sponsor: Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral