Electron Energy Loss Spectroscopy of Cadmium Oxide Micro-Pillars

Abstract

Recent years have seen an interest in finding new materials for plasmonics in the infrared regime. Highly-doped semiconductors are capable of this, with one of the most popular platforms being doped cadmium oxide (CdO). By varying film thickness and carrier concentration, CdO can sustain both long-range propagating surface plasmon polariton (SPP) and highly confined epsilon-near-zero (ENZ) modes. When layers sustaining SPP and ENZ modes are grown atop one another, a new, hybridized mode is generated showcasing characteristics of both SPP and ENZ modes through strong-coupling interactions. This shows great promise for tunable infrared plasmonic devices.

To date, much of the research on CdO and its plasmonic modes has been approached via traditional light optics, which showcases limited spatial resolution and difficulties in fully characterizing the plasmonic density of states. In this thesis, high spatial and high energy resolution characterization is used to study plasmon modes sustained by CdO structures comprised of SPP/ENZ bilayers via electron energy loss spectroscopy (EELS) in the scanning transmission electron microscope (STEM). Recent advancements in high energy resolution STEM-EELS are leveraged to resolve the multitude of plasmonic resonances within the near- to mid-infrared on individual CdO structures. Spatially resolved spectral maps of individual plasmonic modes are generated through the development of a novel technique allowing the acquisition of STEM-EELS spectrum images across multiple microns while maintaining high energy resolution. These are utilized to study how a variety of environmental and structural factors impact the plasmonic response of the fabricated pillars. A further study performing tilt-resolved EELS is conducted to study how the modes evolve with orientation, and an outlook on EELS tomography beyond the quasi-static approximation is provided. This thesis demonstrates a step forward for the characterization of micron-scale structures using STEM-EELS, as well as a substantial contribution to the understanding of how individual plasmonic components interact with their surroundings.