Towards tunable topological insulator surfaces: an EELS study of Bi₂Se₃/organic interfaces

McCauley, Mairi (2025) Towards tunable topological insulator surfaces: an EELS study of Bi₂Se₃/organic interfaces. PhD thesis, University of Glasgow.

Abstract

Topological insulators (TIs) such as Bi₂Se₃ can host low-loss plasmons with enhanced transport properties at their surfaces that are promising for plasmonic and optoelectronic applications. However, practical use of TIs in plasmonic devices requires tunable control over these plasmonic properties. This thesis explores the use of organic molecular overlayers as surface dopants to modify the plasmonic behaviour of Bi₂Se₃.

Four different different organic overlayers; C₆₀, graphene, H₂Pc and CuPc, were selected to modify the Bi₂Se₃ surface, whilst enclosing the TI as required for potential device integration. Electron transparent lamellae were prepared using focused ion beam techniques from bulk crystals and analysed using transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS). Plasmon behaviour across insulator/Bi₂Se₃ and Bi₂Se₃/organic interfaces was probed using EELS and momentum-resolved EELS to investigate changes to plasmon energy, confinement and dispersion. Supporting simulations, calculated from bulk dielectric functions using analytical and finite element methods, were used to interpret EELS spectra and highlight localised plasmon modes.

At a Al₂O3/Bi₂Se₃ interface, a surface plasmon mode at 5.3 eV was identified and shown to be confined to within 2 nm of the interface. Its √q plasmon dispersion is characteristic of a two-dimensional π plasmon, similar to π plasmons in free-standing graphene, likely originating from the the first layer of Se atoms within Bi₂Se₃. At a SiO2/Bi₂Se₃ interface, a surface plasmon at 5.6 eV was observed to coexist with the Bi₂Se₃ π plasmon, illustrating changes to Bi₂Se₃ surface behaviour even at insulating substrate interfaces. In contrast, a highly localised plasmon at 6.3 eV was observed at the Bi₂Se₃/C₆₀ interface, confined to the first layer of C₆₀ molecules. It exhibited very little plasmon dispersion and, with the aid of density functional theory calculations, was proposed arise from the formation of a hybridised plasmon mode. This mode also exhibited enhanced signal in the first molecular layer and a significant charge transfer from Bi₃Se₃ to the first layer of C₆₀ molecules, affirming it to be a hybrid plasmon mode.

These findings were expanded upon by performing similar studies on Bi₂Se₃ graphene, Bi₂Se₃/H₂Pc and Bi₂Se₃/CuPc interfaces. The plasmon energy at each interface was observed at 6.0, 6.4 and 6.4 eV respectively, indicating subtle changes with varied organic overlayers. Plasmon modes at these interfaces were observed to be less confined than at a Bi₂Se₃/C₆₀ interface, indicating that hybrid plasmon confinement could be dependent on a well ordered molecular surface. At lower energies, <200 meV, the plasmonic behaviour at Bi₂Se₃/H₂Pc and Bi₂Se₃/CuPc interfaces was observed to differ more substantially, with variations in plasmon energy throughout the Bi₂Se3 layer and trends towards each interface. Bulk Bi₂Se₃ plasmon modes were observed at different energies in each sample and could be a consequence of different carrier densities from different growth methods. Distinct trends in plasmon energy towards each phthalocyanine interface point towards long interaction depths between Bi₂Se₃ and organic layers at lower energies.

This research demonstrates that organic molecular overlayers can modify the interfacial plasmonic properties in TIs and suggests a route towards tunable plasmonic devices. These results are relevant for potential applications in broadband-TI photodetectors, where organic layers could be tuned electrically or optically to modify a TI interface and enable control of absorption properties.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Subjects: Q Science > QC Physics
• Colleges/Schools: College of Science and Engineering > School of Physics and Astronomy
• Supervisor's Name: MacLaren, Professor Donald and Moorsom, Dr. Timothy
• Date of Award: 2025
• Depositing User: Theses Team
• Unique ID: glathesis:2025-85578
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 10 Nov 2025 11:09
• Last Modified: 11 Nov 2025 11:48
• Thesis DOI: 10.5525/gla.thesis.85578
• URI: https://theses.gla.ac.uk/id/eprint/85578

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