Mapping plasmon excitations in gold nanostructures using electron energy loss spectroscopy
In this work we present the plasmonic properties of simulated and fabricated Au nanostructures using an electron beam as a source of excitation. The motivation for the this work comes from the applications of these nanostructures and the need to understand the physics behind them. We introduce the concept of plasmonics in chapter 1 where we describe the historical significance and fundamentals underpinning the topic. We provide a description of the conditions under which plasmons can be created and highlight some key concepts which are used throughout this thesis. We then outline the experimental and modelling methodology used to probe the plasmonic properties of our nanostructures in chapter 2. Simulated results are generated using the Metallic NanoParticle Boundary Elemental Method software package, which is capable of calculating the plasmonics of nanostructures using an incident excitation. An overview of the calculations and some initial results regarding substrate effects and size-dependent plasmonics are presented. The experimental procedure is given in this chapter where we describe the fabrication and examination of our Au nanosturctures. In this work we use electron beam lithography as a method of nanofabrication, a description of which is given. We also show some of the optimisation results whilst highlighting some difficulties with this method. Scanning transmission electron microscopy with electron energy loss spectroscopy is the the experimental technique used throughout this thesis and we provide an overview of this method in Chapter 2, detailing some of the optimisation process and data analysis used in this thesis. In Chapter 3 we study the plasmonics of near field transducers, motivated by their use in heat-assisted magnetic recording (HAMR) as a nanoscale heat source. This work begins by examining and comparing the plasmonic properties of what we call ’nanoraindrop‘ and ’nanolollipop‘ geometries, describing the advantages and disadvantages of each. We then review an annular structure that has more tunable plasmon resonances, both energetically and spatially. Using nanodisk and nanoraindrop geometries, we explore this tuning model by controlling the diameter and position of the hole in the annular structures, showing the potential enhancement of the plasmonic properties of nanostructures. Chapter 4 explores more complex and realistic environmental factors of temperature and the surrounding dielectric environment in Chapter 4. Understanding the effect annealing has on nanostructures is important for HAMR as they transducer is exposed to fluctuating high temperatures during the recording process. We show the effect of annealing on the plasmonic and structural properties of our fabricated structures and hypothesise the potential problems prolonged heating could cause in HAMR applications. A realistic dielectric encapsulating layer of Ta2O5 is introduced to the simulations and experiment. To investigate our samples and emulate the experimental system in HAMR we devised and carried out an intricate sample preparation using a focused ion beam. We were able to resolve the plasmonics of the encapsulated nanostructures and compared the results to samples with no encapsulating layer. Finally, in Chapter 5 we examine a different plasmonic system., moving from the purely 2D structures of ealier chapters to considering out of plane coupling of complex dimple systems found in biosensor used in protein detection. The motivation for this work comes from a need to further understand the detection mechanism of these plasmonic sensors, using simulated electron energy loss spectroscopy. Our examination method for this chapter began with breaking down the complex dimple structures into individual solid and aperture components, and simplifying the geometry from a chiral shape to a rod in order to examine the plasmonic coupling in these devices. We then increase the complexity of the model by examining the shuriken structure, building on our understanding gained from the simple rod results, and highlight the coupling mechanism in our dimple models.
plasmonics, STEM EELS, nanofabrication, computational modeling, HAMR, gold nanomaterials
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