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Title: Studies of surface plasmons in nanostructures
Other Titles: Biao mian deng li zi bo yu na mi jie gou zhong de yan jiu
Authors: Lin, Shangxin ( 林賞心)
Department: Department of Electronic Engineering
Degree: Doctor of Philosophy
Issue Date: 2011
Publisher: City University of Hong Kong
Subjects: Plasmons (Physics)
Surface plasmon resonance.
Nanostructures -- Optical properties.
Notes: CityU Call Number: QC176.8.P55 L56 2011
xiv, 136 leaves : ill. 30 cm.
Thesis (Ph.D.)--City University of Hong Kong, 2011.
Includes bibliographical references.
Type: thesis
Abstract: Research on surface plasmons (SPs), one of the major field in nanophotonics, explores the properties of a confined electromagnetic field over dimensions in the order of or smaller than the working wavelength. SPs resonance (SPR) is an optical resonance phenomenon where conduction electrons are coupled in metallic nanostructures with an external electromagnetic field. The distinct resonance condition is characteristic of an SPR, and an associated enhanced optical near-field is formed in the SPR. This partially confined optical near-field provides an effective route to design SPs-based materials and devices in several fields such as molecular sensors, organic solar cells, light emitting diodes and etc. Many innovative studies have been conducted over the years. However, there is still a need for a more in-depth comprehension of SPs for additional practical applications. The present work focused on the impact of SPs on the photophysical properties of chromophore, especially in photoluminescence (PL) enhancement. An optical illumination on metal/dielectric interfaces agitates surface waves (SPs), resulting in an enhanced optical near-field in the sub-wavelength dimension. This enhanced field can simultaneously quench or enhance PL of nearby chromophore (competing process), depending on the spacing between metal and chromophore as well as the spectral overlap between the emission band of the chromophore and the SPR band. PL enhancement can be achieved using a delicate design, in which either propagating SPs in planar waveguide or localized SPs (LSPs) are in the metallic nanostructure. Given these conditions, the present thesis emphasizes the adoption of known numerical modeling methods, including the quasi-static method for very small nanoparticles, the Mie theory for nanoparticles with regular spherical shapes, and discrete dipole approximation for nanoparticles with arbitrary shapes. Considering the working wavelength in the visible and near-infrared (NIR) region, silver (Ag) and gold (Au) were chosen as research materials. With instructions for numerical calculation, a variety of different contexts for fabrication were introduced. For instance, an Ag film sustaining the propagating SPs was deposited on a dielectric substrate via thermal evaporation, whereas Ag/Au nanoparticles with various shapes, such as spherical Ag nanoparticles, Ag nanorods, and Ag nanoprism, among others, were synthesized using the wet chemical method. The synthetic mechanism of Ag/Au nanoparticles was in accordance with the classical thermodynamic theory. To enhance the PL of chromophore PL at concerning working frequency, samarium (Sm) and cadmium chalocogenides were chosen as active chromophores. An Sm3+-SU8/Ag/SiO2 configuration was fabricated to investigate the SPs-assisted PL enhancement. The sharp and distinct spectra of Sm3+ (562, 598, and 644 nm corresponding to 4G5/4 → 6H5/2, 6H7/2, and 6H9/2, respectively) facilitated the investigation on the role of SPs during the PL enhancement process. The PL intensities between the sample with and without an Ag film were compared. In addition, the different PL enhancement factors were also determined by varying the thickness of the Ag film. For instance, the PL enhancement factor at room temperature was 8.3 for the sample with a 30 nm Ag film, whereas this factor dropped to 6.6 for the sample with a 70 nm Ag film. Moreover, to eliminate unwanted influence from lattice vibration, the PL enhancement factor was measured at a very low temperature (10 K). At 10 K, the PL enhancement factor was 2.3 for the sample with a 30 nm Ag film, whereas this factor increased to 3.7 for the sample with a 70 nm Ag film. Numerical analysis revealed for the first time that the electron density in metallic nanostructures is the key component in tunneling the PL enhancement factor. This discovery provided a simple and versatile way to tunnel the emission spectral performance of the Sm3+-complex in an Sm3+-SU8/Ag/Au/SiO2 multilayer because of the different work functions of Au (5.10 eV) and Ag (4.52 eV). The dispersion relation of SPs in such a multilayer configuration was deduced, and the relation validity was proved with the experimental results, in which the PL peak intensities of the Sm3+-complex centering at 562 and 598 nm changed without any modification in the molecular structure of the Sm3+-complex. To verify the SPs that were active in the PL enhancement process in the tri-layer system, the Ag film was printed with a periodic pattern, which can convert the propagating SPs into detectable scattering light. A self-assembly monolayer (SAM) pattern was formed by employing polystyrene nanosphere (NS), a cheap and conventional method. However, the hydrophobicity of the Ag film and the hydrophilicity of the surfactant of the polystyrene NS hindered the formation of the SAM pattern. Triton X-100 was used for the pre-decoration of the Ag film due to its capacity to change the hydrophobic characteristic of the Ag film to hydrophilic. A large scale SAM pattern was then fabricated, and the image of the SPs from the pattern was observed during the PL enhancement process. The present study would open up new opportunities for highly efficient and wavelength-selective electroluminescent devices. Moreover, the thesis investigated the emission performance of europium (Eu3+) ions in an Ag nanoparticles system. The Ag nanoparticles were synthesized either via the Ag film annealing method or the polyol reduction method. These methods showed potential as good LSPs hosts for PL enhancement of Eu3+ ions. The PL enhancement factor and lifetime of Eu3+ ions influenced by LSPs were measured to study the influences of LSPs on radiative and nonradiative decay process. Furthermore, considering the applications of SPs in bio-labeling or solar cells, the modulation of active nanoparticles with noble metallic nanoparticles was a prerequisite and had provided fundamental research interest. The modulation of cadmium (Cd) chalocogenides nanoparticles with Ag nanoparticles emerged against a number of technical issues, such as forward cationic ion exchange in mutual nanoparticles, conjugation efficiency, and occurrence of PL quenching in very close mutual distance (smaller than 5 nm), among others. To overcome these challenges, the present study is the first to propose the synthesis of CdS and Ag nanoplate (CdS-Ag NP) hybrid by a forward-reverse cation exchange. The morphology, crystallinity, and atomic composition of the CdS-Ag NPs were investigated using energy dispersive X-Ray spectroscopy and high resolution transmission electron spectroscopy. The CdS-Ag NPs were Cd2+-rich CdS nanoparticles covalently bonded to the surfactant of the Ag NPs. PL enhancement of measured CdS was attributed to the matching of the emission bands of CdS and the tailor-made LSPR bands provided by the Ag NP. In addition, the application of the CdS-Ag NP in HeLa cell imaging was also demonstrated. The present results provided a simple and flexible methodology for conjugating complex nanoparticles, thus offering promising practical applications in nanotechnology. In conclusion, the present thesis presented a systematic investigation on the properties of SPs and their applications. Following the essential elements on SP research, the experimental designs of SPs applications were proposed using a top-down methodology.
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