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Über dieses Buch

This thesis describes the controlled immobilization of molecules between two cuboidal metal nanoparticles by means of a self-assembly method to control the quantum plasmon resonances. It demonstrates that quantum-plasmonics is possible at length scales that are useful for real applications. Light can interact with certain metals and can be captured in the form of plasmons, which are collective, ultra-fast oscillations of electrons that can be manipulated at the nano-scale. Surface plasmons are considered as a promising phenomenon for potentially bridging the gap between fast-operating-speed optics and nano-scale electronics. Quantum tunneling has been predicted to occur across two closely separated plasmonic resonators at length scales (<0.3 nm) that are not accessible using present-day nanofabrication techniques.

Unlike top-down nanofabrication, the molecules between the closely-spaced metal nanoparticles could control the gap sizes down to sub-nanometer scales and act as the frequency controllers in the terahertz regime, providing a new control parameter in the fabrication of electrical circuits facilitated by quantum plasmon tunneling.

Inhaltsverzeichnis

Frontmatter

Chapter 1. General Introduction

Abstract
Light can interact with certain metals and can be captured in the form of plasmons, which are collective, ultra-fast oscillations of electrons that can be manipulated at the nano-scale. Surface plasmons are seen as a promising phenomenon to potentially bridge the gap between fast operating speed optics and nano-scale electronics. Quantum tunnelling has been predicted to occur across two closely separated plasmonic resonators at length scales (< 0.3 nm) that are not accessible by present-day nanofabrication techniques. In this Chapter, we will give an introduction of this thesis which proves that quantum-plasmonics is possible at length scales that are useful for real applications and demonstrates the fabrication of a molecular electronic circuit using two plasmonic resonators, which are structures that can capture light in the form of plasmons, bridged by monolayer of molecules.
Shu Fen Tan

Chapter 2. Plasmonic Properties, Stability and Chemical Reactivity of Metal Nanoparticles—A Literature Review

Abstract
Theorectically predicted small gaps (< 0.3 nm) is essential for quantum tunneling to occur as mentioned in Chap. 1. Thus, we will give an extensive review on self-assembly of nanoparticles with sub-nanometer separation using top-down and bottom-up approaches in this Chap. 2. We will also review the current state-of-the-art characterization methods such as far-field optics and near-field spectroscopies for investigating the charge transfer plasmon mode. Characterization of such a small gap often requires the need of electron microscopy, therefore an overview of the stability of the metal nanoparticles under the electron beam will be provided. Finally, we further explore the possibility to engineer the chemical composition of nanoparticles in order to allow more flexibility in plasmon frequency-tuning through understanding the reaction kinetics at nanoscale using in situ liquid-cell electron microscopy.
Shu Fen Tan

Chapter 3. Self-Assembly of Silver Nanoparticles with Sub-nanometer Separations

Abstract
A bottom-up method was developed to fabricate structures of the form Ag-SAM-Ag (where SAM = self-assembled monolayer). Silver nanocubes with edge lengths of 30–40 nm were synthesized via established methods, with mixed SAMs of thiolates and dithiolates on their surfaces. The SAMs were used to control the self-assembly process and the width of the sub-nanometer gap sizes. The fabricated structures were characterized by various techniques: transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), Ultraviolet photoelectron spectroscopy (UPS), and UV-visible spectroscopy (UV-Vis). It was demonstrated that the SAMs control the structure’s optical properties and provide molecular-electronic control over the barrier heights of the studied system. The synthesis routines discussed here will be used in Chap. 4 to demonstrate quantum plasmonics.
Shu Fen Tan

Chapter 4. Quantum Plasmon Resonances Controlled by Molecular Tunnel Junction

Abstract
Quantum tunneling between two plasmonic resonators links non-linear quantum optics with terahertz nanoelectronics. Direct observation of and control over quantum plasmon resonances at length scales in the range 0.4–1.3 nm across molecular tunnel junctions made of two plasmonic resonators bridged by self-assembled monolayers (SAMs) were demonstrated. The tunnel barrier width and height are controlled by the properties of the molecules. Using electron energy-loss spectroscopy, a plasmon mode, the tunneling charge transfer plasmon, whose frequency (ranging from 140 to 245 THz) is dependent on the molecules bridging the gap was observed.
Shu Fen Tan

Chapter 5. Stability of Silver and Gold Nanoparticles Under Electron Beam Irradiation

Abstract
The STEM-EELS experiments that have been introduced in Chap.  4 were performed on closely-spaced gold nanocuboids as well. However, the high aspect ratio gaps degraded in many cases as a result of filament formation during electron beam irradiation. In this Chapter, a detailed study on the degradation mechanisms and preventive approaches is given, focusing in particular on the nanoparticle coatings that could act as a protective barrier for minimizing the electron beam induced damage on passivated gold and silver nanoparticles.
Shu Fen Tan

Chapter 6. Real-Time Imaging of Chemical Reactions Between Silver and Gold Nanoparticles

Abstract
In this Chapter, a study on the galvanic replacement reaction of silver nanocubes in dilute, aqueous ethylenediaminetetraacetic acid (EDTA)-capped gold aurate solutions using in situ liquid-cell electron microscopy will be demonstrated. Au/Ag etched nanostructures with concave faces are formed via (1) etching that starts from the faces of the nanocubes followed by (2) the deposition of a gold layer as a result of galvanic replacement, and (3) gold deposition via particle coalescence where small nanoparticles are formed during the reaction as a result of radiolysis. Analysis of the silver removal rate and gold deposition rate provides a quantitative picture of the growth process and shows that the morphology and composition of the final product are dependent on the stoichiometric ratio between gold and silver.
Shu Fen Tan

Chapter 7. Real-Time Imaging of Au–Ag Core-Shell Nanoparticles Formation

Abstract
In this Chapter, we studied the overgrowth process of silver on gold nanocubes in dilute, aqueous silver nitrate solution in the presence of reducing agent, ascorbic acid using in situ liquid-cell electron microscopy. Au–Ag core-shell nanostructures were formed via two mechanistic pathways: (1) nuclei coalescence where the silver nanoparticles absorbed onto the gold nanocubes and (2) monomer attachment where the silver atoms epitaxially deposited onto the gold nanocubes. Both pathways lead to the same Au–Ag core-shell nanostructures. Analysis of the silver deposition rate reveals the growth modes of this process and show that this reaction is chemically-mediated by the reducing agent, ascorbic acid.
Shu Fen Tan

Chapter 8. General Conclusions and Outlook

Abstract
This thesis proves that the quantum plasmon resonances can be controlled by molecular tunnel junctions through bottom-up self-assembly approach. Controlled immobilization of molecules between two cuboidal metal nanoparticles provides two key advances in this plasmonic studies: (1) The plasmonic resonators (nanoparticles) can be brought into close proximity down to sub-nanometer separation in a controllable fashion, (2) The molecules can act as a frequency controllers in terahertz regime, evolving as a new control parameter in the fabrication of electrical circuits facilitated by quantum plasmon tunneling. On the other hand, in situ liquid-cell electron microscopy allows us to elucidate the reaction mechanism and kinetics during chemical reactions such as galvanic replacement reactions and core-shell nanoparticle formation at the nanoscale. Quantitative picture of the growth process is useful for engineering the composition and morphology of plasmonic resonators that could potentially open up more opportunities for application in plasmonics. 
Shu Fen Tan
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