可撓性電極材料石墨烯與氧化金之光學性質分析及其光電元件之應用研究

本論文研究石墨烯及氧化金薄膜的特殊光學特性，並利用石墨烯光學非均向性質開發快速非破壞性之大面積二維奈米材料檢測技術。亦利用反應式濺鍍製備氧化金薄膜，並研究其特殊光學性質及應用。此兩種材料未來皆可用於軟性電子元件之電極，及其相關光電元件之應用。首先，第一部分簡述石墨烯薄膜及氧化金薄膜的製備方法及其相關應用，並對相關文獻整理與探討。第二部分，利用化學氣相沉積製備石墨烯薄膜並得到不同品質的單層石墨烯，藉由光譜儀所量測之穿透反射率來分析在不同入射角不同偏振態下石墨烯的光學吸收係數比值定義為材料的非均向性程度。在傳統的石墨烯品質鑑定上，拉曼光譜具有非破壞性鑑定其結構品質的功能，然而拉曼光譜本身具有訊號強度弱、偵測面積小、設備成本高等缺點。在此我們開發了利用光學非均向性來鑑別大面積石墨烯薄膜品質因子的技術，結果與拉曼光譜具有高度正向的相關性，再者，因石墨烯薄膜不同角度之光學非均向性現象，使其比目前所廣泛使用的氧化銦錫(ITO)電極更適合用於需要大角度穿透性質之元件。本論文的第三部分，開發利用橢圓參數對石墨烯薄膜在不同基板上作為品質檢測之因子，橢圓偏光儀具有大面積快速量測能力，且可針對薄膜成長有即時監控的能力。我們開發利用單一橢圓參數即可偵測石墨烯光學非均向性之快速檢測方法，並利用石墨烯薄膜在紫外光波段具有較強之光學吸收性質分別檢測在不同基板上的品質差異性，其中包括使用二氧化矽薄膜成長於矽晶圓上、銅、玻璃、矽晶圓，除此之外針對大面積的石墨烯薄膜品質偵測，亦可利用橢圓偏光儀空間映像技術(mapping)來分析大面積的石墨烯薄膜品質分佈。亦可利用橢圓參數峰值的移動來判斷石墨烯的層數變化。在第四部份的研究中，我們建立了製備大面積的氧化金薄膜之技術，並利用基本的材料及光學性質分析技術來研究其薄膜的基本特性及光學常數，探討氧化金在其還原反應發生時，會形成鑲嵌在氧化金薄膜的奈米金粒子，具有明顯的侷域表面電漿現象，搭配後續不同的還原條件，可以調控其電漿共振波長從可見光到近紅外光，還原之氧化金基板可作為近紅外光的拉曼增強基材。比較一系列不同濃度之R6G待測分子，我們可偵測到10-10 M之低濃度。在第五部分的研究中，承接先前所開發的氧化金低溫薄膜沉積技術與石墨烯轉移技術，我們結合了氧化金高功函數且高穿透光的特性與石墨烯可撓曲與寬波段吸收的性質，開發大面積可撓式寬波段且具穿透性的高效率光偵測器。石墨烯具有優異的不可透氣性與電性傳導性質，我們利用了石墨烯來大幅減緩氧化金的還原速率，此外亦結合了石墨烯與氧化金的異質結構製作了大面積且低工作電壓（約0.1伏特）的垂直式光偵測器。石墨烯/氧化金光偵測器在寬波段下(從紫外光到紅外光波段)均具有比傳統石墨烯/金光偵測器較佳的光電響應 (photoresponse)，除此之外，我們亦針對不同光波段之照光強度線性區域做探討，相對於紅外光波段，紫外光具有相對較強的光子能量能產生較佳的光電轉換效率(1300 A W-1 at 310 nm )，此現象主要來自於石墨烯薄膜中的熱載子散射與載子間之加乘放大效應而產生較佳的熱電子增益，可適用於皮瓦(picowatt)強度的紫外光弱光偵測。另一方面，石墨烯與氧化金均為具穿透性及撓曲性材料且可適用於塑膠透明基板製程，也因此後續可結合其他電子元件做為可攜式軟性電子元件之應用。

關鍵字

光學非均向性；品質因子；石墨烯；大面積檢測；橢圓偏光儀；氧化金；侷域表面電漿共振；表面拉曼增強；金奈米粒子；光偵測器；軟性元件

並列摘要

In this thesis, we investigated that the specific optical properties of graphene and gold oxide (AuOx) films. The optical anisotropy of graphene can be used to develop rapid, non-destructive and large-area characterization technique for two-dimensional nanomaterials. Besides, we used the reactive sputtering process to prepare homogeneous AuOx films, and investigated their specific optical properties and applications. In the first part of thesis, the fabrication methods for graphene and AuOx and their applications on devices are introduced, and the relative literatures are reviewed. In the second part of thesis, we utilized the chemical vapor deposition (CVD) method to prepared single-layer graphene featuring different qualities. By the angle– and polarization–dependent transmittance and reflectance from a spectrophotometer, we were able to analyze and calculate the ratios of optical absorption coefficients between the two kinds of polarized light at different incident angles, and regarded the ratios as the optical anisotropy. Raman spectroscopy is solely demonstrates the nondestructive analysis of graphene quality. But the drawbacks of Raman spectroscopy are weak signals, small detective area, and high-cost equipment. Herein we developed a quality factor based on the optical anisotropy to characterize large-area graphene films, and the results were consistent with the Raman spectroscopy. Moreover, we found that the optically anisotropic graphene was more suitable for the devices demanding high transmittance at large incident angles than the commonly used indium tin oxide (ITO) film. In the third part of thesis, we developed a spectroscopic ellipsometry (SE) based method to identify the qualities of large-area graphene on different substrates. We developed a characterizing method based on only one ellipsometric parameter (Ψ), to rapidly determine the optical anisotropy of graphene. Besides, we exploited the high absorbance of graphene within UV regime to monitor the distinct qualities of graphene on different substrates, including SiO2/Si, copper, fused silica, and Si. Moreover, consider for the large-area graphene, we also utilized high resolution SE mapping to rapidly characterize the local quality in large-area graphene films. Besides, the peaks values and the peak-shifts in the Ψ spectra could provide the information of structural qualities and layer-number of graphene films, respectively. In the fourth part of thesis, we established the fabrication method for large-area and homogeneous AuOx films, and we utilized the material and optical analysis method to study their basic properties and optical constants. We focused on the reduction processes of AuOx films, formation of embedded Au nanoparticles (NPs) and their localized surface plasmon resonance (LSPR) phenomenon. Moreover, the LSPR wavelength of the embedded Au NPs could be tuned from visible to the near-infrared (NIR) by varying the reduction process. Besides, the reduced AuOx films could be applied as surface-enhanced Raman spectroscopy (SERS)–active substrates in the NIR regime. By detecting R6G molecules of different concentrations, the minimum detectable concentration was down to 10–10 M. In the fifth part of thesis, based on the low temperature deposition of AuOx films and the graphene-transferring technique, we combined the graphene and AuOx films to develop a flexible, transparent, high-efficient and broadband-working photodetector under low applied voltage (ca. 0.1V) that took advantages of the high work function and transparency of AuOx films. Graphene possessed superior impermeability to water and gas, and therefore could significantly prevent the AuOx film from reduction. We exploited the heterostructure of graphene/AuOx to develop a vertical-junction photodetector. Comparing to conventional graphene/Au photodetector, the graphene/AuOx photodetector displayed a higher responsivity in a broadband regime from ultraviolet (UV) to infrared (IR). In addition, we investigated the linear-response region of incident power-dependent photoresponse under different wavelengths of incident light. Superior to responsivity in the IR and visible regimes, the graphene/AuOx displays a very high responsivity (1300 A W-1) under a wavelength of 310 nm. Due to carrier-carrier scattering and carrier multiplication, the hot-electron assisted photoresponse would be greater under high energy photon irradiation. This phenomenon can be applied on the low-light detection in the UV regime even in the picowatt scale. On the other hand, graphene and AuOx films all possess transparency and flexibility, and thus are compatible with transparent plastic substrates. And this photodetector can be easily combined with other electronic devices to develop further portable and flexible optoelectronic systems.

並列關鍵字

optical anisotropy ； quality factor ； graphene ； large-area characterization ； spectroscopic ellipsometry ； gold oxide (AuOx) ； localized surface plasmon resonance (LSPR) ； surface enhanced Raman scattering ； Au nanoparticles ； photodetector ； flexible devices