金奈米結構其基板調節表面電漿共振之分析

Analysis of Substrate-Mediated Surface Plasmon Resonance of Gold Nanostructures

王俐雲(Li-Yun Wang)

指導教授 ： 林鶴南

國立清華大學/工學院/材料科學工程學系/碩士(2014年)

https://doi.org/10.6843/NTHU.2014.00356

摘要

金奈米結構其區域表面電漿共振(localized surface plasmon resonance, LSPR)特性伴隨顯著的吸收和散射現象以及區域電場能量增強、且化學穩定性佳，被廣泛運用於生物和化學感測。對於較複雜的區域電漿共振情況，結構的對稱性改變或是基板的存在是至關重要的影響因素，為了討論該因素研究文獻多以複雜的製程方法製作具對稱性衰退的奈米結構。
本實驗以不同的製程方法製備三種於介電質基板表面的金奈米結構：金奈米粒子、單根金奈米線和金奈米薄膜邊界結構，藉由暗視野顯微術觀察散射光譜，並以有限差分時域法(finite-difference time-domain, FDTD)模擬分析受基板調節的電漿模態。
高對稱性的金奈米粒子於基板表面可觀察到其電漿模態具不同程度的紅位移和多模態之間耦合，並於光譜中觀察到四極共振模態。由於金奈米粒子其電漿共振模態較為單純，可作為各種模態耦合的基礎原理。
對於以原子力顯微微影法(AFM lithography)製備的一維單根金奈米線其電漿共振模態主要的是由於基板調節的影響：其紅光散射波峰(621 nm)是由不同電場偏振激發的偶極模態耦合、並受基板誘發耦合後的結果，而位於藍綠光的共振波峰(485 nm)則與奈米線高度有關。此外，靠近基板之結構的些微變化即可導致不可忽略的模態位移，證明討論基板調節對其電漿共振特性影響的重要性。
於靠近金奈米薄膜邊界結構的基板表面產生誘發電荷，使得其電漿共振特性徹底改變。具有相似基板誘發電場的兩個模態疊加後形成主要的紅光散射波峰(645 nm)，而其他較明顯的散射波峰(588和507 nm)則是分別由結構的垂直邊緣和底部邊緣受基板調節所產生的共振波峰。當奈米結構與介電質基板的接觸面積越大時，基板調節現象對於結構表面電漿特性的影響越顯著。

關鍵字

區域性表面電漿共振 ； 多模態耦合 ； 基板誘發耦合 ； 電漿結構 ； 原子力顯微微影法

並列摘要

Gold nanostructures with optical properties of localized surface plasmon resonance (LSPR), which is related to strong light absorption and scattering and local electromagnetic field enhancement, have been applied in chemical and biological sensing owing to their high stability. To analyze the hybridization of LSPR modes, which are modified by symmetry breaking and a substrate, complicated fabrication processes have been reported in the literature.
In this thesis, substrate-mediated surface plasmon resonance modes of three types of gold nanostructures including nanoparticles, single nanowires and thin film patterns on dielectric substrates are investigated by dark-field optical microscopy and finite-difference time-domain (FDTD) simulation. These three nanostructures are created by relatively simple fabrication techniques.
In a spectrum of gold nanoparticles with spherical symmetry on a dielectric substrate, distinct resonance peak shifts and quadrupolar resonance peaks resulting from the degeneracy and hybridization of LSPR modes can be observed. Moreover, the system can be served as a model for LSPR hybridizations.
For single gold nanowires, which are fabricated by atomic force microscopic lithography, the substrate-mediated hybridization is a significant factor in LSPR modes. In a scattering spectrum, the peak at 621 nm in wavelength is resulting from the superposition and substrate-mediated hybridization of two LSPR modes excited upon the polarized electric field. On the other hand, the peak at 485 nm is related to the thickness of the nanowire. Furthermore, a slightly geometric change of the structure close to the substrate surface might provide observable shifts of LSPR modes, that explains the importance of the substrate-mediated hybridization.
The plasmon modes observed at the edges of gold thin film patterns are thoroughly adjusted by the existence of the induced image charge on the dielectric surface. In a scattering spectrum, the main peak at 645 nm is due to the superposition of plasmon modes with similar substrate-induced electric field. In addition, minor peaks at 588 nm and 507 nm are attributed to the substrate-mediated plasmon modes from the vertical and the bottom parts of edge structures, respectively. The above results reveal that the substrate-mediated hybridization is enhanced with a larger contact area between nanostructures and the dielectric substrate.

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