利用奈米壓印微影技術開發金屬微奈米複合結構之表面電漿元件

Using Nanoimprint Lithography to Develop Metallic Micro/Nano Hybrid Structure-Based Plasmonic Devices

游振傑(Chen-Chieh Yu)

指導教授 ： 陳學禮

國立臺灣大學/工學院/材料科學與工程學研究所/博士(2014年)

https://doi.org/10.6342/NTU.2014.00626

摘要

金屬微奈米複合結構為近年來在發展光學元件上的新趨勢，因複合結構同時具有微米結構與奈米結構的優點。在過去，微奈米複合結構多以電子束或光學微影術製作，然而這些製程都具有低產率或高成本的缺點，使得這些複合結構的發展受到限制。奈米壓印微影技術普遍被認為是一種快速，高產率，以及穩定的製程，且適合用來製作微米及奈米結構。在本論文利用一種改良的奈米壓印微影技術—奈米直壓金屬技術—來製作以奈米粒子為主的微奈米複合結構。
金屬奈米粒子與薄膜複合結構因其特殊的非等向電漿共振模態而吸引廣大科學家的興趣。金屬奈米粒子上的侷域性表面電漿若與金屬膜上之表面電漿產生耦合，會大幅增加其之間的電磁場強度。本論文發展了一種高解析的“奈米粒子-薄膜游標尺“，其對於奈米粒子與薄膜中間隔離層的厚度與折射率相當地靈敏。在此種游標尺中非等向的電漿共振模態可以直接利用橢圓參數來分析，不需要後續的光學模擬，相當的方便。在可以得到的資訊中，本論文為第一個提出利用橢圓參數來分析奈米粒子-薄膜游標尺的電漿共振模態，並呈現出橢圓參數量測是一個非常方便與穩定的方式。此種複合結構會具有高靈敏度的原因來自於其奈米粒子與薄膜之間的電漿耦合，其對於折射率的靈敏度可以達到每單位折射率的變化產生314奈米的波長移動，相對於其他以奈米粒子為主的感測器而言，是非常優異的。此外，奈米粒子-薄膜游標尺對於隔離層厚度的解析度也很高，可以解析埃長度等級上的差異。在本研究中，隔離層每一埃厚度的變化可以造成9奈米的波長移動。利用此種奈米粒子-薄膜游標尺相當靈敏的特性，發展了一種可以辨識不同胺基酸的感測平台。
另一方面，低成本且快速的奈米直壓金屬技術被用來優化奈米粒子與薄膜複合結構。本論文利用了奈米直壓金屬技術來製作高靈敏電漿共振天線結構。此結構建立在奈米粒子與壓印金屬鏡面上，可藉由入射角改變共振波段，其波段從可見光至近紅外光的寬波段都可產生極大的電場增益。因此其非常適合作為表面增強拉曼散射基材。不像其他表面增強拉曼散射基材需要不同形狀或週期才能達到寬波段增益，此奈米粒子-壓印金屬鏡天線結構只需單一結構即可達成寬波段的增益效果。此天線結構的電場增益來自於奈米粒子上的侷域性表面電漿共振與壓印金屬鏡上周期性結構所激發的表面電漿共振之間的共振耦合。此外，由於表面電漿共振的波段可以藉由入射角調整，因此共振耦合波段也會隨之改變。在論文中提出了此奈米複合結構來做為可見光與近紅外光激發的表面增強拉曼散射基材，並可適用於不同激發波長(532、633、及785 奈米)。此外，此複合結構具有相當高的靈敏度的表面增強拉曼散射基材，其可以偵測待測物(例如羅丹明)至10^-15莫耳濃度以下。
更進一步地，本論文提出一種由高折射率介電質奈米粒子與金屬奈米結構的複合系統來作為短程表面電漿波的奈米聚焦結構。藉由金屬奈米結構激發表面電漿波，並將其聚焦在介電質奈米粒子與金屬奈米結構的界面，局部的電磁場可以被大幅的增益。另外，利用介電質奈米粒子特殊的表面性質，此種奈米聚焦系統可以有許多的應用。例如此種系統可以被用在增益吸附在介電質奈米粒子上的分子之拉曼訊號。除此之外，介電質奈米粒子表面的反應速率亦會被局部的強電磁場所加速，因此本論文所提出的奈米聚焦結構可以同時達到促進與觀察介電質奈米粒子表面反應的雙重功能。在本論文中示範了同時促進與觀察乙醇氣體分子脫附過程以及光催化反應，並更進一步地在鋁金屬表面於紫外光波段進行表面電漿波的奈米聚焦，這是在過去的奈米聚焦結構無法達到的。因此，此種以介電質奈米粒子與金屬結構的奈米聚焦複合結構並不受限於一定要使用貴重金屬，證實了其在未來光控制處理與環保的晶片元件及感測器應用上的潛力。
最後本論文提出了利用波浪狀金屬膜結構作為一高靈敏度的感測器。藉由奈米直壓技術，波浪狀金屬膜結構可以在單一步驟即製作完備。此種波浪狀金屬膜結構同時可藉由表面電漿共振以及折射率匹配現象做為感測的依據。波浪狀金屬結構展現了超高靈敏度(每單位折射率變化可產生800奈米的波長移動)。並由於特殊的折射率匹配現象，可以量測金膜/基材的表面電漿共振模態來感測折射率，此種方法非常的方便且不需要昂貴的光譜儀設備。最後，本論文利用奈米粒子與波浪狀金屬膜複合結構來感測生物分子，證實此種結構確實可以發展成為高靈敏度的生化感測器。

關鍵字

微奈米複合結構 ； 奈米壓印微影技術 ； 表面電漿共振 ； 電漿耦合 ； 表面增強拉曼散射 ； 奈米聚焦 ； 表面電漿元件

並列摘要

Plasmonic micro/nano hybrid structures are an emerging idea in recent optical device. The hybrid structures possess advantages of both the individual micro- and nano-structures. Typically, electron beam lithography and photolithography are used to fabricate the designed structures. However, the low throughput or high cost of these lithography methods limit the development of such hybrid structures. Nanoimprint lithography is well-known as a rapid, high-throughput, and robust method for fabricating micro- and nano-structures. Therefore, it will be much suitable to construct micro- and nano-structures by using nanoimprint lithography. In this thesis, an improved lithography—the direct nanoimprint-in-metal method—is used to develop nanoparticle (NP) -based micro/nano hybrid structures.
Metallic NP-film hybrid structure has attracted great interests due to the unique, anisotropic plasmonic gap modes. The coupling between the localized surface plasmon resonance (LSPR) and the surface plasmon resonance (SPR) supported by the metal film largely enhances the electromagnetic field. In this thesis, I develop an ultrasensitive nanoparticle (NP)-film caliper that functions with high resolution (angstrom scale) in response to both the dimensions and refractive index of the spacer sandwiched between the NPs and the film. The anisotropy of the plasmonic gap mode in the NP-film caliper can be characterized readily using spectroscopic ellipsometry (SE) without the need for further optical modeling. To the best of our knowledge, this paper is the first to report the use of SE to study the plasmonic gap modes in NP-film calipers and to demonstrate that SE is a robust and convenient method for analyzing NP-film calipers. The high sensitivity of this system originates from the plasmonic gap mode in the NP-film caliper, induced by electromagnetic coupling between the NPs and the film. The refractometric sensitivity of this NP-film caliper reaches up to 314 nm/RIU, superior to those of other NP-based sensors. The NP-film caliper also provides high dimensional resolution, down to the angstrom scale. In this study, the shift in wavelength in response to the change in gap spacing is approximately 9 nm/A. Taking advantage of the ultrasensitivity of this NP-film caliper, a platform for discriminating among thiol-containing amino acids was developed.
On the other hand, the low-cost, rapid, and direct nanoimprint-in-metal method is used to improve the origin NP-film hybrid structures. The direct nanoimprint-in-metal is used to prepare an incident angle–tuned, broadband, ultrahigh-sensitivity plasmonic antennas from nanoparticles (NPs) and imprinted metal mirrors. By changing the angle of incidence, the nanoparticle-imprinted mirror antennas (NIMAs) exhibit broadband electromagnetic enhancement from the visible to the near-infrared (NIR) regime, making them suitable for use as surface-enhanced Raman scattering (SERS)–active substrates. Unlike other SERS-active substrates that feature various structures with different periods or morphologies, the NIMAs achieve broadband electromagnetic enhancement from single configurations. The enhancement of the electric field intensity in the NIMAs originate from coupling between the LSPR of the NPs and the periodic structure–excited surface plasmon resonance (SPR) of the imprinted mirror. Moreover, the coupling wavelengths can be modulated because the SPR wavelength is readily tuned by changing the angle of the incident light. Herein, this thesis demonstrate that such NIMAs are robust substrates for visible and NIR surface-enhanced resonance Raman scattering under multiple laser lines (532, 633, and 785 nm) of excitation. In addition, the NIMAs are ultrasensitive SERS-active substrates that can detect analytes (e.g., rhodamine 6G) at concentrations as low as 10^–15 M.
Moreover, a simple hybrid configuration based on high-index dielectric nanoparticles (NPs) and plasmonic nanostructures is proposed for nanofocusing of submicron-short-range surface plasmon polaritons (SPPs). The excited SPPs are locally bound and focused at the interface between the dielectric NPs and the underlying metallic nanostructures, and greatly enhance the local electromagnetic field. Besides, taking advantages of the surface property of dielectric NPs, versatile functionality for this system can be achieved. For example, the nanofocusing of submicron-short-range SPPs can be applied on enhancing the Raman signals of molecules adsorbed on the dielectric NPs. Besides, the interfacial reaction rate on the dielectric NPs surface can be improved by the local strong electromagnetic field. Therefore, the proposed nanofocusing configuration can promote and probe the interfacial reactions simultaneously. Promoting and probing the desorption of ethanol gas molecules as well as the photo-degradation of MB molecules are demonstrated in this study. Moreover, the nanofocusing of SPPs is demonstrated on relatively cheap metal surface (Al) in the ultraviolet (UV) regime, which is not achieved by conventional tapered waveguide nanofocusing structures. Therefore, the nanofocusing of submicron-short-range SPPs by dielectric NPs on plasmonic nanostructures is not limited to lossless, noble metals, and reveals the potential on future light management and on-chip green devices and sensors.
In the last, the metallic corrugated structures was prepared for use as highly sensitive plasmonic sensors. Relying on the direct nanoimprint-in-metal method, fabrication of this metallic corrugated structure is readily achieved in a single step. The metallic corrugated structures are capable of sensing both surface plasmon resonance (SPR) wavelengths and index-matching effects. The corrugated Au films exhibit high sensitivity (ca. 800 nm/RIU), comparable with or even higher than those of other reported SPR-based sensors. Because of the unique index-matching effect, refractometric sensing can also be performed by measuring the transmission intensity of the Au/substrate SPR mode—conveniently, without a spectrometer. In the last, this thesis demonstrate that the NP-corrugated Au film hybrid structure is capable of sensing biomolecules, revealing the ability of the structure to be a highly sensitive biosensor.

並列關鍵字

micro/nano hybrid structure ； nanoimprint lithography ； surface plasmon resonance ； plasmonic coupling ； surface-enhanced Raman scattering ； nanofocusing ； plasmonic device

參考文獻

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