Plasmonic resonances in 2D and 3D nanostructures investigated by monochromated electron energy loss spectroscopy (mono-EELS)
Open Access
- Author:
- Moradifar, Parivash
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- October 06, 2020
- Committee Members:
- Nasim Alem, Dissertation Advisor/Co-Advisor
Nasim Alem, Committee Chair/Co-Chair
Venkatraman Gopalan, Committee Member
Joan Marie Redwing, Committee Member
Christopher Noel Giebink, Outside Member
Thomas E Mallouk, Special Member
John C Mauro, Program Head/Chair - Keywords:
- Plasmonic
Topological Insulator
Metamaterials
Metalattice
Si-Ge Core-Shell
Silver Plasmonic
Electron Energy loss Spectroscopy
Scanning Transmission Electron Microscopy
In-situ TEM
Mono-EELS
S/TEM
Defects
Polygonal Defects
Confinement
Interconnectivity
In-Plane Heterostructure
Bi2Te3
Sb2Te3
2D Chalcongenide
Hollow Structure
Cavity Arrays
Long-Range Ordering
High Pressure Confined Chemical Vapor Deposition
HPcCVD
Low-Loss EELS - Abstract:
- Surface plasmons enable routing and manipulating of light on length scales below the diffraction limit. Surface plasmons are commonly excited by coupling to an electromagnetic field leading to a confined local field enhancements effect that can be used to strengthening the sub-diffraction limit light-matter interaction and open a path for novel applications. Various noble metal based (gold and silver) nanostructures such as nanoparticles and nanorods have been extensively studied for their plasmonic responses in the visible range. Different surface plasmon resonances as well as localized electric field enhancements were observed in these structures. New materials such as metamaterials and low-band gap semiconductors are considered to be two novel paths to produce new plasmonic materials with exotic physical properties and lower resistive losses in comparison to conventional noble metal-based nanostructures. This thesis explores the plasmonic behavior of 2D and 3D nanostructures using monochromated electron energy loss spectroscopy (Mono-EELS) in conjunction with scanning transmission electron microscopy (STEM) to identify and spatially resolve the electronic excitations and surface plasmon modes. The 3D nanostructures are HPcCVD synthesized metalattice nanostructures (subgroup of metamaterials) and are novel periodic and long-range interconnected plasmonic nanostructures. The structure is comprised of a SiO2 close-packed template infiltrated by either Ag or Si-Ge. The effect of confinement, interconnectivity and substrate is explored. The 2D nanostructures consist of Bi2Te3-Sb2Te3 in-plane heterostructures. This thesis explores the effect of heterointerface, intrinsic and extrinsic defects in the 2D nanostructures and their ability to manipulate and modulate the surface plasmon response through a systematic investigation. It is shown that state-of-the-art characterization techniques can be applied to uniquely understand the fundamentals behind the plasmonic behavior of such complex systems. The first chapter – the introduction gives a brief overview of plasmonics, excitation of surface plasmons and conventional noble metal based plasmonic nanostructures. The current state of plasmonic technology and the potential plasmonic platforms are introduced, as well as the need for alternative plasmonic building blocks extending beyond noble metals. In addition, a broad overview of available characterization tools for studying surface plasmon resonances is given. Finally, recent advances in the understanding and characterizing plasmonic properties using aberration corrected electron microscopes in conjunction with monochromated electron energy loss spectroscopy (EELS) is covered. EELS spectrum imaging gives the ability to map the spatial distribution of surface plasmon resonances with nanoscale resolution, giving unique insight into these nanoscale and highly localized excitations. The second chapter introduces the effects of long-range interconnectivity, confinement and substrate as possible tools for manipulating the surface plasmons in Ag metalattice. The effects are investigated using STEM-EELS in conjunction with CST calculations. Various plasmon modes originating from confined interstitial sites are identified and their spatial distribution mapped (throughout the IR-Vis regime). Furthermore, it touches upon the effect of using a substrate with large optical contrast. The effects of substrate and confinement are identified as possible tools for manipulating the surface plasmon resonances in this nanostructure. To qualitatively analyze the STEM-EELS datasets and attribute the physical origin of the observed effects, a python-based data analysis process was developed. Tools and scripts based on the Hyperspy python package were developed as a part of the author’s doctoral research to handle the complex datasets, apply the necessary corrections and to extract and visualize the spatial distribution of surface plasmon resonances. The third chapter focusses on the Si-Ge core-shell metalattice nanostructure, a semiconductor-based platform with long range interconnectivity. The combination of long-range interconnectivity and an extended array of cavities (open geometry) offers an enhanced plasmonic response in these nanostructured model systems. We report on nanoscale probing of the various electronic excitations and visualizing their spatial distributions including interface/surface, bulk and interband transitions using STEM-EELS. The experimental findings are qualitatively compared with CST calculation. Furthermore, the effect of the degree of etching and probe position (electron beam acts as local excitation source) on the electronic excitation variations is explored. The fourth chapter focuses on a the novel hexagonal 2D Bi2Te3-Sb2Te3 in-plane heterostructures (contain a heterointerface between Bi2Te3 and Sb2Te3) and 2D Sb2-xBixTe3 alloy. The systems are studied under static conditions at room temperature in the first section followed by exploring the structural dynamics, compositional stabilities and real time monitoring of defects in heterostructures and alloy samples at elevated temperatures using in-situ S/TEM. The sublimation and preferential sites for defects formation, as well as growth in both samples are explored and correlated to the strain and in-homogeneity of the structure. The experimental observations are further combined with DFT simulation to gain additional insight into the underlying mechanisms responsible for defect initiation and expansion in the systems. The fifth chapter investigates the plasmonic response of 2D Bi2Te3-Sb2Te3 in-plane heterostructures as novel plasmonic crystals beyond the typical noble metal materials. The effect of defects, heterointerface and edges is explored as a path to alter the plasmonic response of the sample. Similar to chapter 2 and chapter 3, STEM-EELS spectrum imaging has been used to identify and spatially resolve the plasmon resonances. The role of defects on the plasmonic response of the flake is further explored by intentionally introducing polygonal extrinsic defects into the flakes through thermal sublimation processes during an in-situ TEM annealing experiment. To help assess various plasmonic resonances and resolve their spectral contributions, singular value decomposition (SVD) is utilized and the experimental plasmonic modes are further combined with the e-DDA simulations. The sixth chapter contains the conclusions and puts the work in this thesis in context – explaining how advanced electron microscopy and spectroscopy techniques in combination with the right data analysis is a promising tool and can be applied to gain a fundamental understanding of complex plasmonic nanostructures and advancing the field of plasmonics. The seventh chapter introduces possible future directions in correlating EELS with cathodoluminescence (CL) as a complementary technique to gain additional insight into the nature of the observed modes and how the new generation of spectroscopy techniques can help to understand the quasi elastic and vibrational features (phonon excitations) further. This dissertation contains a considerable amount of the work undertaken during my PhD and explores the application of advanced microscopy techniques: STEM, In-situ TEM and EELS to study the plasmonic response of several novel 2D and 3D plasmonic building blocks. Chapters 2 to 5 contain a specific sample/approach and have each resulted in a draft article for submission in the coming months. In combination, the chapters give a detailed picture of the plasmonic response of such samples and show how advanced techniques can uniquely shed light on the complex plasmonic behaviors.