利用奈米粒子開發新型化學感測器與光電元件

Abstract

本論文之研究目的在於利用實心與中空的金屬奈米粒子開發新型的化學感測器及光電元件。本論文之第一部份簡述奈米粒子的製備、物理及化學性質與相關的應用。在第二部份的研究中，我利用表面電漿共振(SPR)效應及金奈米粒子對烷基硫醇分子（alkanethiol）的高反應性設計了兩種分別可應用在矽及塑膠基板上之化學感測器。首先，我合成了一系列具有不同形貌及不同SPR效應的中空金粒子(HGNs)，並利用光譜橢圓儀(SE)來討論單層中空金粒子陣列之光學常數，如折射率(n)及消光係數(k)。此奈米粒子陣列的消光係數曲線具有一個SPR的特徵峰，其波段則與其在水溶液狀態下的SPR吸收波段相似。另一方面，此奈米粒子陣列的折射率則因為SPR效應出現了異常色散(abnormal dispersion)的特性。接著我比較了中空金粒子跟實心金粒子(SGNs)陣列其光學常數的差異，並利用光譜橢圓儀及中空金粒子所具有的較高靈敏性製備了一種可在不透明基板上偵測環境折射率變化之化學感測器。在本論文第二部分的後段，我利用奈米粒子的SPR散射(scattering)現象製備了一種可撓曲的波導式化學感測器。為此，我改良了傳統的奈米轉印技術來將單層金屬奈米粒子陣列製備在未經任何表面處理的塑膠基板上。透過控制轉印的製程條件及模版（mold）的表面結構，我呈現了一種在三個維度皆可定位奈米粒子的能力：包括微調金屬奈米粒子埋進基板表面的深度及金屬奈米粒子在基板表面排列成二維次微米尺寸圖形的能力，同時其SPR的光學性質亦會隨之改變。接著，我利用三維有限時域差分法（3D FDTD）的光學模擬來討論當奈米粒子埋進表面後，在波導與空氣介面的近場光學行為。與模擬結果相呼應的是，相較於傳統的波導式感測器，在實際的散射信號量測中可獲得將近十倍的提升。這個SPR散射信號的提升主要是來自於，當金屬奈米粒子埋進波導的表面時，這些粒子會同時與波導內所侷限的模態（guiding-mold waves）及穿出表面的消逝波(evanescent waves)發生作用，並將其散射出來。在本論文的第三部份，我則利用金奈米粒子分別設計了三種在光電元件上的應用。首先，我研究透過KrF準分子雷射誘發中空金粒子發生型態轉變的光熱效應，並設計了一種全新光資料儲存系統。中空金粒子通常具有兩個吸收區，包括典型的SPR吸收峰，與來自於金屬材料在深紫外光波段的本質吸收。因此，僅需要單一脈衝的雷射光就能被中空粒子吸收，並且使其溫度上升，導致中空結構的崩解而轉變成實心的粒子，而這個粒子結構的變化則導致其在光學性質上的明顯改變： SPR特徵峰會發生大幅的藍移。利用這個方法，我呈現了此光儲存系統可以具有次微米解析度的的記錄能力，並且利用藍光雷射顯微鏡來作為讀取工具。更進一步來說，若是將此光學儲存系統結合前述奈米金屬粒子轉印技術於可撓曲的基材上，將可開發出一種比傳統光碟高出將近千倍儲存密度的光儲存記錄帶。在本論文第三部分的後段，我利用了奈米金粒子來開發兩種矽太陽能電池表面上最佳化的抗反射層結構(antireflection structure)。首先，利用金奈米粒子在矽的酸蝕刻中所具有的獨特催化能力，透過控制金奈米粒子陣列的密度及蝕刻時間，可以在矽基板上製作具有柱狀、甜筒狀及碗狀的均勻多孔性絨化之形貌。其中由緊密甜筒狀孔洞所形成的類金字塔（pyramidal）結構，則在可見光及近紅外光的範圍內皆呈現了非常低的反射率(小於3%)。而此低反射率則是來自於此結構所具有的漸變折射率以及能將光侷限在孔洞中的表現。除此之外，我進一步討論直接利用奈米粒子陣列作為矽太陽能電池的抗反射層結構之可行性。根據三維 FDTD的模擬結果，我認為金屬奈米粒子在此扮演一種有缺陷的抗反射層結構：當SPR散射發生時，會使得原本應該反射的光線進而穿透到基板內部，但是當SPR的吸收發生時，則會使入射光被粒子本身吸收而無法穿透到基板內部。為了證明這個假設，我設計了數種低反射率的情況，並在模擬及實際的太陽能電池上都看到相似的現象：當反射光原本就極低或不存在的情況下，金屬奈米粒子陣列反而會使得穿透到矽內部的光線減少。而為了兼顧奈米粒子在製程上的優勢及克服粒子本身吸收的缺陷，我選用了介電質奈米粒子(dielectric NPs)來作為抗反射層結構。最後，在模擬及實際的元件中，利用緊密排列的二氧化鈦奈米粒子作為矽太陽能電池的抗反射層結構，將可以在光電流(photocurrents)上獲得超過30%的提昇。

In this thesis, we utilized unique physical (optical, photothermal) and chemical properties (chemical affinity, catalytic) of metal nanoparticles (NPs) to develop specific chemical sensors and optoelectronic devices. In chapters 1-3, we briefly introduce the research background, literature reviews and the experimental details. In chapter 4, we suggest two kinds of metal-NP based chemical sensors on Si or flexible substrates. (i) We describe the optical constants of self-assembled hollow gold nanoparticle (HGN) monolayers determined through spectroscopic ellipsometry (SE). The extinction coefficient (k) curves of the HGN monolayers exhibited strong surface plasmon resonance (SPR) peaks located at wavelengths that followed similar trends to those of the SPR positions of the HGNs in solution. The refractive index (n) curves exhibited an abnormal dispersion that was due to the strong SPR extinction. The values of Δn and kmax both correlated linearly with the particle number densities. From a comparison of the optical constant values of HGNs with those of solid Au nanoparticles (SGNs), we used SE measurements to demonstrate a highly sensitive Si-based chemical sensor. HGNs display a slightly lower value of k at the SPR peak but a much higher sensitivity to changes in the surrounding medium than do SGNs. (ii) We fabricated a flexible SPR-based scattering waveguide-sensor by directly imprinting HGNs and SGNs onto flexible polycarbonate (PC) plates—without any surface modification—using a modified reversal nanoimprint lithography (rNIL) technology. Controlling the imprinting conditions, including temperature and pressure, allowed us to finely adjust the depth of embedded metal NPs and their SPR properties. Consistent with the three dimensional FDTD simulations, experimentally We obtained an almost one order of magnitude enhancement in the scattering signal after transferring the metal NPs from a glass mold to a PC substrate. We attribute this enhanced signal to the particles strong scattering of the guiding-mode waves and the evanescent wave simultaneously. In chapter 5, we suggest three kinds of NPs enhanced optoelectronic devices. (i) We demonstrate a new optical data storage method: photomodification of HGN monolayers induced by one-shot of deep-ultraviolet (DUV) KrF laser recording. A single pulse from a KrF laser heated the HGNs and transformed them from hollow structures to smaller solid spheres. This change in morphology for the HGNs was accompanied by a significant blue-shift of the SPR peak. If this spectral recording technique could be applied onto thin flexible tapes, the recorded data density would increase significantly relative to that of current rigid discs. (ii) We describe the preparation of optimal textured structures on Si surfaces through metal-assisted etching using SGNs as catalysts in HF/H2O2 solution. We obtained uniformly textured Si layers containing cylindrical, conical, and bowl-shaped features. A textured surface possessing close-packed pyramidal features with dimensions on the subwavelength scale exhibited the lowest reflectance (< 3%) over the entire visible and NIR spectrum. This low reflectance arose from the refractive index gradient of the Si surface and light trapping phenomena. (iii) We systematically investigated the phenomenon of light trapping in Si solar cells coated with metal and dielectric nanoparticles. Based on the FDTD simulations, we suspected that SGN arrays could be considered as deficient single-layer antireflection coatings: they could reduce the amount of reflected light, scattering it into the Si substrates, while strongly absorbing incident light in the plasmonic resonance wavelength regime. We obtained strong evidence supporting this hypothesis from our observation that the degree of light trapping in Si solar cells was dramatically suppressed when using the SGN arrays under a variety of low reflection conditions. Therefore, we replaced the SGNs with dielectric NPs, which possess lower extinction coefficients and better antireflection ability. Finally, we used a simple, rapid, and cheap solution-based method to prepare close-packed TiO2 NP films on Si solar cells; these devices exhibited a uniform and remarkable increase (ca. 30%) in their photocurrents.