Study on the synthesis and analysis of distributed metallic nanoparticles and their plasmonic characteristics

Abstract

Light interacts with electrons, atoms, and molecules in materials: light is reflected, refracted, or diffracted when it illuminates certain materials, while molecular structures of materials are sometimes transformed when they are stimulated by light. Since surface plasmons are also an interaction between photons of light and free electrons of metal materials, they can be an attractive research topic in many different fields in science and engineering, such as optical physics, electronics, chemistry, and material science. In this regard, it is necessary to conduct lively interdisciplinary study of surface plasmons to develop plasmonics as one of the next generation technology leaders. As an attempt to conduct the interdisciplinary research on surface plasmons, this dissertation demonstrated both chemical fabrication of various metallic nanoparticle arrays and optical analysis of their surface plasmons. In this thesis, I suggested a new chemical method called site-selective synthesis, which enables to synthesize and align silver (Ag) nanoparticles simultaneously in a designed trench. The mechanism of the site-selective synthesis method is that silver nanoparticles can be nucleated and grown selectively on polyvinylpyrrolidone (PVP) domains by attraction (or repulsion) between silver ions and the hydrophilic PVP island domains in a silica matrix of the trench (or the hydrophobic fluorosilane layer). On the silver nanoparticles in the trench, surface plasmons were excited by obliquely incident light, reradiating the enhanced electromagnetic fields in the far- and near-fields. Even for a large angle of incidence in total internal reflection (TIR), the patterned silver nanoparticle array underwent strong scattering with a high intensity, due to the surface plasmon effect. This research shows the possibility of designing metal nanoparticle arrays for a plasmonic device as well as increasing procedural efficiency in fabrication of metal nanoparticle arrays. When metal nanoparticles become an array structure, plasmonic mixed states occur so that their optical properties change. A combined structure of metal nanoparticles on a metal layer was suggested in order to acquire a surface plasmon resonance (SPR) signal and a fluorescence image simultaneously based on the plasmonic mixed states of propagating surface plasmons (PSPs) and localized surface plasmons (LSPs). Although PSPs and LSPs generally have conflicting properties, i.e. propagation and localization of surface plasmons, they can coexist as a form of the mixed state on the proposed structure at an optimized condition. For the structure of Ag nanocubes with silicon dioxide shells on the Ag layer (AgNC@SiO2), the plasmonic mixed states can appear in either interspaces among AgNC@SiO2 particles, or SiO2 gaps between AgNCs and Ag layers. As a result of scanning structural parameters, including an inter-particle distance, core size, shell thickness, and layer thickness, I found that a slope of the minimum reflectance band on the θ-λR map increased as AgNC@SiO2 particles went close to one another. Decrease of the AgNC diameter and SiO2 shell thickness also influenced on a slope and position of the band. Therefore, the slope of the minimum reflectance band means the proportion of LSPs to PSPs in their plasmonic mixed states. In particular, the real distribution of the inter-particle distances measured in experimental samples was fitted to the Weibull distribution and then the statistical data were fed back to a calculation process. As a result, the recalculation results agreed with the experimental θ-λR maps and fluorescence images. This agreement supports the validity of the simulation approach and the experimental method used for investigating the proposed structure. Those plasmonic mixed properties of PSPs and LSPs will be useful in various practical applications such as bio sensors. As part of an effort to understand a plasmonic coupling between PSPs and LSPs in depth, a densified lattice array structure of AgNC@SiO2 without the Ag layer was proposed. I found the plasmonic intermediate state between PSPs and LSPs on the lattice array of AgNCs with few nanometer gaps through simulation and proved the existence of the intermediate property by using both a self-assembled monolayer (SAM) of AgNC@SiO2 in experiment and a feedback of statistical data for gap distribution to a recalculation. The intermediate state has two distinguishing bands on the θ-λR map on the contrary to a PSP or LSP state which has only one of diagonal and vertical bands (D- and V-bands) on the map. Analysis of Hy-field profiles on the bands demonstrates the reversal transition between PSPs and LSPs in each others band. The cross point between D- and V-bands becomes a metastable state, such as a saddle point, so that a breakage of PSP phase equilibrium, sudden discontinuity of LSPs, and rapid changes of the reflectance occur around that point. As described above, this study suggested a new chemical method for metal nanoparticle arrays and investigated the plasmonic coupling and intermediate state between PSPs and LSPs using two kinds of metallic nanoparticle arrays. In order to conduct this research, chemical and optical approaches are used in the preparation of samples, measurement of plasmonic properties, and simulation analysis of the results. This complex approach is expected to be a model of interdisciplinary research.