Electron Beam Characterization of Infrared Plasmons
This dissertation focuses on characterizing the distribution and flow of energy in plasmonic nanomaterials with high spatial and spectral resolution. The unique electronic and magnetic excitations of these systems are highly dependent on their local chemical and physical properties, giving rise to novel functionality for next generation nanoelectronic and nanophotonic devices. In particular, our work centers on resolving the detailed interactions between infrared active nanostructures and their surrounding resonant environments with the ultimate goal of directing and concentrating infrared energy. We interrogate these interactions with nanometer-scale precision using scanning transmission electron microscopy (STEM) coupled with electron energy loss spectroscopy (EELS). STEM-EELS has the ability to simultaneously correlate spatial and spectral measurements of individual infrared active nanostructures using an electron probe that operates on the same length and energy scales as the relevant physical processes under investigation.
Our investigations begin with doped semiconductor nanocrystals that host spectrally narrow plasmons in the infrared. Using STEM-EELS we analyze the response of single particle plasmons of individual tin-doped indium oxide nanocrystals, showing how the surface and bulk responses of a single particle can be tuned by controlling the free carrier concentration via the dopant. Doping from 1-10 atomic percent tin will shift the plasmon from lower to higher energies across the near-infrared region, respectively. In combination with theoretical modeling we also demonstrate a unique approach for retrieving the fundamental dielectric parameters of individual nanocrystals. This method, devoid from ensemble averaging, illustrates the potential for electron-beam ellipsometry measurements on materials that cannot be prepared in bulk form or as thin films. We then combine STEM-EELS and theoretical modeling to investigate the capability of colloidal indium tin oxide (ITO) NC pairs to form hybridized plasmon modes, providing an additional route to influence the IR plasmon spectrum. These results demonstrate that ITO NCs may have greater coupling strength than expected, emphasizing their potential for near-field enhancement and resonant energy transfer in the IR.
We follow this work with research into traditional metal plasmonic materials, using specific geometries of gold nanomaterials to develop nanoparticles that host infrared responses. Through a combination of STEM-EELS and theoretical modeling, we delve into identifying the strong-to-weak coupling limits of paired gold nanostructures. These investigations culminate in the characterization of an all plasmonic Fano resonance, a subspace of the weak coupling regime between quasi-discrete and quasi-continuum localized surface plasmon resonances. This work illustrates how STEM instrumentation can experimentally observe nanoscale plasmonic responses that were previously the domain only of higher resolution infrared spectroscopies. We expand our understanding of infrared coupling by precisely nanofabricating, characterizing, and modeling the infrared optical responses of plasmonic nanorhombus assemblies. Ranging from a monomer to a pentamer ensemble, experimental and simulated point spectra, spectrum images, and near-field maps agree well with the results of an analytical coupled normal mode model. The nanorhombus assembly is then expanded into an aperiodic Penrose array, where we identify the effects of long-range lattice structure on near-field particle hybridization. These results show that hybridization across the array is minimally affected by the overall lattice structure, except when probing the center of the array.
History
Date Modified
2022-06-28Defense Date
2022-03-29CIP Code
- 40.0501
Research Director(s)
Jon P. CamdenCommittee Members
Gregory Hartland Paul BohnDegree
- Doctor of Philosophy
Degree Level
- Doctoral Dissertation
Language
- English
Alternate Identifier
1333204251Library Record
6236215OCLC Number
1333204251Program Name
- Chemistry and Biochemistry