石墨烯與新穎奈米材料複合元件之特性研究

Photoelectric properties of composite devices consisting of graphene and novel nanostructured materials

鄭世豪(Shih-Hao Cheng)

指導教授 ： 陳永芳

國立臺灣大學/理學院/物理研究所/博士(2013年)

https://doi.org/10.6342/NTU.2013.11134

摘要

在這篇論文中，我們主要研究的方向為設計、研製、分析石墨烯與新穎半導體奈米材料複合元件。由於石墨烯的狄拉克費米能量色散特性，導致二維石墨烯具有許多獨特的性能，如高電子遷移率，高的光學透明性，低的薄層電阻，極大的可撓性和表面電漿子效應。我們的研究發現許多有趣與高效能之特性。本論文包含了三大主題，其摘要如下:

1. 垂直排列的無機奈米柱與有機材料混合的太陽能電池與可撓式增強的性能效應：先進新穎的可撓式太陽能電池
一種新的可撓式有機/無機混和光伏太陽能電池元件與彎曲更高的性能已得到證實。利用聚對苯二甲酸乙二醇酯/石墨烯/垂直對齊的氧化鋅奈米柱/聚（3 - 己基噻吩）：苯基-C61-丁酸甲酯（P3HT：PCBM）/銀電極的混合的層狀結構，組成為可撓式太陽能電池。這個結果跟其它之前可撓式太陽能電池有顯著的不同，其功率轉換效率和短路電流密度，可以在增加彎曲角度下有顯著的提高。提高電源的轉換效率最高可達44.8％。而該設備返回到原來的狀態，功率轉換效率恢復到它的初始值。這種獨特增強的現象可以歸因於光捕集效應。成功的利用氧化鋅奈米柱的幾何結構，以及優秀的光學透明性，導電性，和機械可撓的特性的二維石墨烯晶體，製作出優良的可撓式太陽能電池元件。

2. 利用石墨烯的表面電漿效應增強氧化鋅奈米柱的雷射激發
我們的研究中提出將石墨烯(還原式石墨烯氧化物)與氧化鋅組成奈米複合材料，會使得氧化鋅的激發光強度增強與降低激發雷射的門檻。根據理論計算，我們可以發現增強受激輻射的機制可歸因於氧化鋅表面的粗糙程度可以有效的誘發石墨烯的表面電漿進而增強該波段的躍遷激發。我們描述的方法對於未來的高效率光電元件是非常有用的，也提供了石墨烯另一個利用的可能。

3. 超高響應的全碳材料組成光探測器: 一個可行且傑出的整合零維與 二維石墨烯。
我們利用全碳材料製造出超高靈敏度的光探測器，由零維的石墨烯量子點與二維的石墨烯晶體以及碳膠所組成。值得注意的是，在光照下，光電流響應高達4×107 AW-1。這樣高表現的效應其基本的機制出現的幾個重要因素如下。首先，其中之一可以歸因於由於適當的能帶結構可導致石墨烯量子點和石墨烯之間的界面引起良好的電荷轉移，與光電效應產生的電子和電洞的空間分離。此外，匹配良好的石墨烯量子點與石墨烯之間的一致性原子的界面，與石墨烯量子點的良好的對光吸收率，以及石墨烯良好的連續性電子結構和石墨烯下產生的等電位連接網絡傳輸的石墨烯薄片上也扮演重要的角色。因此，我們的研究結果表明一種高效的全碳材料製成光電探測器，並結合不同維度的材料混成元件。這樣的元件是一種全碳材料組成，為價格便宜，無毒，高效的光電器件的進一步發展走出一條新的途徑。

關鍵字

石墨烯 ； 氧化鋅 ； 石墨烯量子點 ； 表面電漿 ； 混合型太陽能電池 ； 光捕捉效應 ； 光電流增益 ； 光檢測器

並列摘要

In this thesis, we report the design, fabrication, and characterization of graphene-based nanoscale semiconductors and optoelectronic devices. Because of its Dirac-fermionic energy dispersion, two-dimensional graphene possesses many unique properties, like high electron mobility, high optical transparency, low sheet resistance, great flexibility and surface plasmon effect. Accroding to our research, several intriguing phenomena and highly efficient devices have been discovered. The highlight of our research is briefly described as follows.
1. Vertically Aligned Inorganic Nanorods/Organic Hybrid Solar Cells with Bending Enhanced Performance: An Advanced Alternative for Flexible Solar Cells
A new flexible organic/inorganic hybrid photovoltaic (HPV) device with bending enhanced performance has been demonstrated. The layered structure of the HPV consists of polyethylene terephthalate (PET)/graphene/vertically aligned ZnO nanorods/poly (3-hexylthiophene): phenyl-C61-butyric acid methyl ester (P3HT: PCBM)/Ag. Unlike all previous reports for flexible organic photovoltaic devices, it is found that the power conversion efficiency and the short-circuit current density of the device can be enhanced with increasing bending angles. The highest enhancement of power conversion efficiency can reach up to 44.8 % compared with its flat counterpart. While the device returns to the original condition, the power conversion efficiency recovers to its initial value. This unique property can be attributed to the enhanced light trapping effect due to the geometric structure of ZnO nanorods as well as the outstanding optical transparency, conductivity, and mechanical flexibility characteristics of two–dimensional graphene crystal.

2. Enhancement of laser action in ZnO nanorods assisted by surface plasmon resonance of reduced graphene oxide nanoflakes
We report the discovery of an enhancement of the random laser action in a nanocomposite comprising reduced graphene oxide nanoflakes and ZnO nanorods. We show that both emission intensity and lasing threshold exhibit an obvious improvement. Based on our theoretical calculations, the mechanism underlying the enhanced stimulated emission can be attributed to coupling between the optical transition and the surface plasmon resonance of the reduced graphene oxide nanoflakes, induced by the ZnO nanorod surface roughness. The approach we describe here will be very useful for the future development of high-efficiency optoelectronic devices and offers an alternative route for application of reduced graphene oxide.

3. All Carbon-Based Photodetector with Ultrahigh Responsivity: A feasible eminent integration of zero and two dimensional graphene
A photodetector with ultrahigh sensitivity based on the composite made with all carbon-based materials consisting of graphene quantum dots (QDs), two dimensional graphene crystal, and conducting carbon paste electrodes has been demonstrated. Under light illumination, remarkably, photocurrent responsivity up to 4 × 107 AW -1 can be obtained. The underlying mechanisms are dominated by several important factor, simultaneously. Firstly, the ultrahigh sensitivity can be attributed to the spatial separation of photogenerated electrons and holes due to the charge transfer caused by the appropriate band alignment across the interface between graphene QDs and graphene. Besides, the well-matched bonding network arising between graphene QDs and graphene under coherent atomic interface, the large absorptivity of graphene QDs, and the excellent continuity of electronic structure for the carriers transport on the graphene sheet also play significant roles. Our result therefore demonstrates a highly efficient all carbon-based photodetector, which serves as an excellent example for the integration of unique physical properities of nano-carbons with different dimensionalities. Together with the associated mechanism, it can pave a new route for the further development of all carbon, cheap, non-toxic and highly efficient optoelectronic devices.

並列關鍵字

graphene ； ZnO ； graphene quantum dots ； surface plasmon ； hybrid solar cell ； light trapping effect ； photocurrent gain ； photodetector

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