Alternative plasmonic materials and techniques for high temperature application and beyond

  • Hugh Littlehailes

Student thesis: Doctoral ThesisDoctor of Philosophy

Abstract

The development of plasmonic near field transducers (NFTs) capable of operating in high temperature environments is essential for implementation in a growing number of applications, including heat assisted magnetic recording (HAMR), nanoscale heat transfer, and solar energy harvesting. At present, the best plasmonic materials are the noble metals, such as Au, Ag, and Cu. However, despite excellent plasmonic properties, these materials are prone to deformation and melting on the nanoscale, in harsh conditions and at high temperatures, rendering them unviable. In this thesis we investigate a range of materials and antenna geometries for their plasmonic efficiency and thermal stability to assess their suitability for application in harsh environments.

We investigate the structural and optical properties of 100 nm thick films of the intermetallic alloys Au3Zr, and Au3Hf, fabricated by DC magnetron sputtering at a range of deposition temperatures, from room temperature to 427 ◦C, and annealed at various vacuum levels, to determine their suitability as refractory plasmonic materials. We find both materials can be fabricated by this method and show plasmonic capability in the visible and near infrared range, although are characterised by large ε′′ values, in excess of their ε′ values, such that they do not meet the criteria for HAMR: |ε′|>|ε′′| and a high quality factor, Q, (discussed in Chapter 1). Both materials are stable when annealed at 10−8 Torr, but are partially oxidised when annealed again at 10−6 Torr, suggesting encapsulation would be necessary for any applications at elevated temperature. Films of Au3Zr were fabricated to 50 nm with a 5 nm Ti adhesion layer to more closely represent application in HAMR. However, whilst improvement is seen in the optical properties compared to the 100 nm films, the thinner films are more susceptible to oxidation. Both Au3Zr and Au3Hf are compared against equivalent films of the 4:1 stoichiometry. We determine that Au4Zr fabricated at RT outperforms Au3Zr, with the reverse seen when fabricated at elevated temperature. Whereas, Au4Hf appears better than the 3:1 stoichiometry, in ε′ until 1020 nm, although with higher ε′′ values from 500 nm onwards. Both Au4X (X = Zr, Hf) films fabricated at elevated temperature indicate better oxidation resistance than equivalent Au3X films when annealed at a vacuum level of 10−6 Torr. Alkali metals have also displayed promising plasmonic properties, although their reactivity limits their ready application. However, recent research indicates that with appropriate encapsulation, elemental sodium can be used to make plasmonic devices that are stable for extended periods of time. We perform a numerical investigation of the application of elemental Na as a plasmonic NFT for use in HAMR, by simulating a variety of NFT geometries, and different environmental conditions, using the Metallic NanoParticle Boundary Element Method (MNPBEM) software package. We find that compared to Au devices the plasmonic resonances generated in the Na NFTs have a greater intensity, higher quality, and form more discrete modes. We extend the model to include a variety of encapsulating materials to more accurately represent the application of Na to a HAMR setup; determining the extent of the redshift and Q-factor enhancement for each material to best inform the appropriate encapsulation material for the hypothetical application to a Na NFT.

Finally, we optimised the method for the fabrication of near field transducers on dedicated heating chips, to allow for investigation of the plasmonic response of devices by STEM-EELS at elevated temperature, with a placement accuracy of 400 nm, and a repeatable 20 nm fabrication resolution. Whilst heating studies were not carried out, they are now feasible. We explore the possibilities for improved near-field enhancement in droplet geometry NFTs with an aperture, and then also a split in the tip. Simulated and experimental STEM electron energy loss spectroscopy results agree that improved dipole enhancement is achieved for latter, likely caused by intracoupling within the device geometry.

Date of AwardJul 2023
Original languageEnglish
Awarding Institution
  • Queen's University Belfast
SponsorsEngineering & Physical Sciences Research Council
SupervisorFumin Huang (Supervisor) & Donald MacLaren (Supervisor)

Keywords

  • Plasmonic
  • intermetallic
  • STEM EELS
  • gold alloys
  • nanofabrication
  • HAMR
  • sodium
  • computational modelling
  • MNPBEM

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