Refractory oxides based multilayered core@shell plasmonic nanostructures

Abstract

Plasmonic devices have received a great deal of attention in the past decades due to their ability to concentrate electromagnetic radiations to sub-diffraction limited spots of the order of a few nanometers while enhancing the field intensity by a few orders of magnitudes. These remarkable properties have facilitated the development of a wide range of technologies, including near-field optical microscope, photodynamic therapy, plasmonic biosensors, subwavelength waveguides, and heat-assisted magnetic recording in data storage, to name just a few.

However, to this date the applications of plasmonics have mainly been restricted to mild temperature ranges not so far above room temperature, as most plasmonic materials, and in particular the best ones made of noble metals, are soft and have relatively low melting points, which are easily deformed at elevated temperatures. This has become an urgent bottleneck issue holding back the commercialisation of many plasmonic technologies and set significant barriers restricting highly important nanoscale research in high-temperature environment, such as solar thermophotovoltaics and the burgeoning studies of nanoscale thermodynamics that are fundamental to nanochemistry.

Despite the much research efforts of expanding plasmonic materials beyond the noble metals (gold, silver, copper, and aluminium) to include refractory materials such as metal nitrides, silides, germanides and many other compounds and 2D materials such as graphene, alternative plasmonic materials with optical properties comparable to noble metals but can operate effectively in high-temperature environment are still elusive. To this date, noble metals still stand as the best plasmonic materials with unrivalled optical properties that few other materials can compete with.

In this research, experimental, numerical, and analytical investigations of core@shell plasmonic nanostructures consist of noble metals and refractory oxides are presented. Such structures show significantly improved thermal stability due to the protective oxide layers, while retaining the excellent plasmonic properties of noble metals.

A theoretical model which can analytically evaluate the optical properties of multilayered spheroidal core@shell nanostructures of arbitrary numbers of layers is developed. A variety of multilayered nanostructures of different geometries and materials are investigated, including nanospheres, nanospheroids and nanorings, and a range of oxides, including SiO2, Al2O3, HfO2, V2O5, and Mo2O3.

The theoretical model is a powerful tool for designing novel nanostructures with custom-designed optical properties tailored to suit specific applications. Samples of a wide variety of geometries were fabricated using electron beam lithography. The optical properties and thermal stability of the samples were characterised, which demonstrate that effective control of the optical and thermal properties of the nanostructures can be achieved by varying the constituents and structural parameters. Specific discussions were made with regard to the heat-assisted magnetic recording technology. A novel design of the optical delivery system consists of an elliptical plasmonic antenna and a V-shaped waveguide is proposed, which exhibit many advantages over the traditional lollipop design of the near-field transducer.

The research in this thesis provides an important step to facilitate plasmonics research and applications in high-temperature environment.

Date of Award	Jul 2021
Original language	English
Awarding Institution	Queen's University Belfast
Sponsors	Engineering & Physical Sciences Research Council
Supervisor	Fumin Huang (Supervisor) & Marc Sorel (Supervisor)