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  • 學位論文

石墨烯薄膜應用於新穎光電元件之特性研究

Investigation of novel optoelectronic devices based on CVD grown graphene films

指導教授 : 陳永芳

摘要


由於二維材料石墨烯的優異電性及光學特性,如極高的載子遷移率、高透明度、以及可調變的費米能階,石墨烯在近年來引起重大的關注,認為是一種有潛力的導電材料並可廣泛運用在光電元件及奈米材料上。本論文中,我們利用化學氣相沉積法生長之大面積石墨烯為基礎,應用並製作新穎的元件,並探討其在光電物理領域的潛在應用。研究分為四個主題,簡要介紹如下: 1. 石墨烯之表面電漿感測器 此研究中,透過石墨烯的電子傳導對金屬奈米粒子產生的表面電漿有高靈敏的反應,因此可利用此特性將石墨烯作為一高靈敏的表面電漿感測器。在實驗中,由於金屬奈米粒子在相對應的光頻率照射下有強烈的局部性表面電漿震盪,當此震盪作用在石墨烯材料,我們觀測到表面電漿的震盪使石墨烯的光電流反應有七倍以上的增幅。藉由整合此二維材料石墨烯及金屬奈米粒子,我們可以更簡易的探究表面電漿。 2. 利用壓電材料增強石墨烯與有機半導體異質接面的光反應 由於石墨烯與有機半導體聚(3–己基噻吩) (P3HT)的異質接面因為費米能階的平衡下存在著蕭特基位障,當受到頻率可激發P3HT之入射光照射產生的電子電洞對因此被此位障促使的電場分離而流向並參雜石墨烯,此過程可得到石墨烯電流對光的反應。在此實驗中,我們利用壓電材料的極化特性作為元件的基板改變了石墨烯本身的費米能階,進而成功增強石墨稀與有機半導體之位障電場,相比一般二氧化矽基板,壓電材料基板的光反應有十倍的提升。這種獨特的現象更可運用在提升光感測器及太陽能電池元件。 3. 利用場效調變之高效能垂直穿隧式光電元件 二維材料的整合對於新穎元件的發展十分重要,因應縮小元件尺寸的需求,垂直式堆疊元件的研究更是受到注目。我們的研究是利用石墨烯/介電質/半導體的垂直堆疊元件成功製作出多功能的光電元件,藉由電場閘極調變石墨烯的費米能階,間接調控了載子垂直穿隧介電質並傳導至發光半導體的能力。經由這樣的設計,我們製作一個可穩定且有效率的發光二極體,此光電元件亦整合了電晶體及可調變之光感測器,此一多用途的光電元件更有益於光通信與面板顯示器的發展。 4. 可變色的金氧半結構發光電晶體 在傳統的彩色平面顯示器,需要結合多個元件及複雜的工序才能達成多色的發光輸出。在此研究,我們發展一新型的發光二極體並結合電晶體的場效調變石墨烯穿隧至發光材料氮化鎵的能階選擇,因而得到發光頻譜的電致調變。藉由添加量子點的光轉換降頻特性,為此元件調變發光頻譜的範圍增加了另一個方,向進而得到更廣泛的調色範圍。此外,在特定的輸入電壓及場效下,元件亦可放出疊加為白色的光源。此實驗提升發光二極體的發光範圍且簡化了複雜的多層次製程,希望能在面板及新穎顯示器的發展上有其應用價值。

並列摘要


In this thesis, we have reported the design, fabrication and characterization of nanoscale optoelectronic devices made with CVD grown graphene films. Due to the advantages of this 2-D material such as high electron and hole mobility, high transparency and tunable Fermi level, graphene has been considered as a promising conducting material. It is believed our research shown here can serve as a key step for the further development of novel functional optoelectronic devices. 1. A Graphene-Based Surface Plasmon Sensor We present the application of graphene as a plasmon sensor. It was found that the electronic transport of chemical vapor deposition CVD-synthesized graphene is sensitive to surface plasmons generated by the illumination of metal nanoparticles. The observed change in electronic conduction can be up to seven times larger than the intrinsic photoresponse of graphene. A study of the mechanism revealed local field-assisted oxygen desorption induced by surface plasmons to be the cause of this intriguing behavior. A detailed investigation of the wavelength and spacing dependence of the plasmon–graphene coupling proves that graphene can be used as a sensitive, high resolution electronic plasmon detector. This finding shows the potential of devices exploiting the novel properties of graphene and surface plasmons. 2. A Highly Sensitive Graphene-Organic Hybrid Photodetector with a Piezoelectric Substrate To create a sensitive photodetector, the transparent and conductive properties of graphene and the optical and photovoltaic properties of poly(3-hexylthiophene) (P3HT) are combined as a hybrid composite. Based on the inherent nature of the band alignment between graphene and P3HT, the photogenerated holes are able to transfer to the graphene layer and improve the photoresponse to be much better than the traditional layer by layer organic system. Additionally, the graphene is deposited on a piezoelectric Pb(Zr0.2Ti0.8)O3 (PZT) substrate, and the photoresponse of such composite photodetectors is found to be ten times larger than on SiO2 base. It is demonstrated that the electric field of the polarization of piezoelectric substrate helps the spatial separation of photogenerated electrons and holes and promotes the hole doping of graphene to enhance the photoconduction. A detailed investigation of graphene layers, thickness of P3HT and time evolution shows that the composite of graphene and P3HT on PZT can be used as a sensitive photodetector and has potential as an effective solar cell. Moreover, with the replacement of P3HT by a thin layer of bulk heterojunction of polymer and fullerene, the photosensitivity can be further increased by more than one order of magnitude. 3. Tunneling-injection in vertical quasi-2D heterojunctions enabled efficient and adjustable optoelectronic conversion The advent of 2D materials integration has enabled novel heterojunctions where carrier transport proceeds through different ultrathin layers. We here demonstrate the potential of such heterojunctions on a graphene/dielectric/semiconductor vertical stack that combines several enabling features for optoelectronic devices. Efficient and stable light emission was achieved through carrier tunneling from the graphene injector into prominent states of a luminescent material. Graphene’s unique properties enable fine control of the band alignment in the heterojunction. This advantage was used to produce vertical tunneling-injection light-emitting transistors (VtiLET) where gating allows adjustment of the light emission intensity independent of applied bias. This device was shown to simultaneously act as a light detecting transistor with a linear and gate tunable sensitivity. The presented development of an electronically controllable multifunctional light emitter, light detector and transistor open up a new route for future optoelectronics. 4. Covering the whole color space by a color-changing light emitting transistor We here report a novel light emitting transistor based on graphene/insulator/semiconductor whose color can be changed over the whole chromaticity space. This behavior is enabled by the independent adjustment of the emission intensity of two peaks in the semiconductor emission spectrum. Device bias was found to control the accumulation of charge carriers at the interface and could selectively enhance or suppress emission from the band edge and deep acceptor levels. Electrostatic gating of the graphene resulted in the selective blocking of tunneling injection into states at the band edge. The independent intensity control of two features in the emission spectrum enables the simultaneous adjustment of saturation and hue. By adding a layer of down-converting quantum dot emitters, the whole chromaticity space can be accessed as demonstrated by the selective emission of white light, which opens new application in optoelectronic devices ranging from displays to solid state lighting.

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