以軟 X-ray 、可見光研究與調變石墨烯材料的能帶結構

Abstract

石墨烯的層數、氧化程度與摻雜程度皆會影響本身能帶結構與電子特性，本研究是以X-ray 與可見光在這三面(層數、氧化程度與摻雜程度)上來研究與調變石墨烯的能帶結構。首先在層數方面，我們以掃描光電子顯微鏡研究不同層數的石墨烯X-ray光電子能譜(XPS)，搭配靜電力顯微鏡（EFM）研究石墨烯功函數(work function )隨層數的變化，發現石墨烯的碳原子1s深層能階(C1s core-level)能量(E1s, core-level to vacuum level)隨著層數增加而降低，單層與多層石墨烯的E1s能量分別為289.42 eV 與289.13 eV。從石墨烯的E1s與層數的關係中，根據鍵極長強度相關性理論(bond-order-length-strength correlation theory, BOLS)，我們描繪出E1s與碳原子有效配位數(effective coordination number)關係，並推測出單顆自由的碳原子與鑽石的能量分別為288.17 eV 和 288.91 eV。透過不同層數的石墨烯XPS解析及有效配位數解析的E1s研究，我們也間接證了石墨(graphite)的光電子能譜存在著表面深層能階位移現象(surface core-level shift, SCLS)，在多層石墨烯上的SCLS約0.09 eV。 藉由氧化石墨烯(graphene oxide, GO)還原後的還原氧化石墨烯(reduced graphene oxide, RGO)是一種常用於大量製作石墨烯的方法。GO與RGO之間有顯著的電子特性差異，因此我們想了解氧化程度對石墨烯能帶結構影響，並且也想了解是否能透過soft X-ray照射來達到GO的還原。在此部分，我們藉由時間解析的XPS量測，證明了在soft X-ray的照射下GO可以被還原成RGO。GO上的碳氧鍵(C-O bond)數量隨著X-ray曝照時間成指數衰減。其還原常數與X-ray 所引致的低能量二次電子的強度成正向關係，因此氧化石墨烯的還原我們主要歸咎於X-ray引致的二次電子所造成C-O鍵的裂解，而非X-ray本身。透過價帶頂（Valence Band Maximum）與EFM功函數量測，我們描繪出了GO到RGO的能帶結構改變，期間C1s core-level到Valence Band Maximum的能帶寬度是固定，E1s則隨GO的還原而降低。 石墨烯特殊的Dirac點能帶結構，使其電阻值明顯受摻雜程度也就是功函數大小影響。為有效地調控石墨烯摻雜程度，我們結合偶氮聚合物DR1-PMMA製作成石墨烯/DR1-PMMA場效電晶體，藉由DR1分子極化方向與強度改變來調控石墨烯的摻雜，其電阻值變化可用於石墨烯記憶體應用上。三位元的石墨烯記憶體操作可以被實現，在室溫下我們藉由光助極化法(photo-assisted poling)寫入 “1” 狀態，藉由光去極化法(photo-depoling)可抹去“1” 態回到 “0” 狀態，其中“1”與“0”狀態的電阻變化差可達 60%。由於在室溫下DR1分子是被固定的，因此石墨烯/DR1-PMMA 場效電晶體元件於外加電場0.2 MV/cm下仍可保有原其寫入狀態。

The band structure and electronic properties of graphene are influenced by number of layer (NL), oxidized level and doping level itself. In this thesis, we would like to use soft X-ray and visible light to study and modulate the band structure of graphene under the three aspects of NL, oxidized level and doping level. In the aspect of NL, we began characterizing the dependence of photoelectron spectra of mechanical exfoliation method (MEM) produced graphene on NL by the scanning photoelectron microscopy (SPEM). The work function of graphene with various NL was measured by electrostatic force microscopy (EFM). According the results of SPEM and EFM, we found that the C1s core-level energy (E1s, core-level to vacuum level) of graphene decreases with the increase of NL. The E1s s of single layer and multilayer graphene (MLG) are ~289.42 eV and ~289.13 eV, respectively. Based on the bond-order-length-strength correlation theory (BOLS), we derived the relationship between E1s and effect coordination number of carbon atom, and obtained the energies of a free carbon atom (288.17 eV) and diamond (288.91 eV). According the evolution of XPS with various NL of graphene and the coordination-resolved E1s, we supported the existence of surface core-level shift (SCLS) on graphite. The magnitude of SCLS of MLG is 0.09 eV. Reduced graphene oxide (RGO) from the reduction of graphene oxide (GO) is one of ways to produce graphene hugely. We are interesting in the influence of oxidized level to the electronic band structure of graphene, and also would like to know if the soft X-ray exposure can induce the reduction of GO. Therefore, we demonstrate that X-ray irradiation is able to induce the reduction of graphene oxide (GO) first by the characterization of time-resolved X-ray photoelectron spectroscopy (XPS). The number of CO bonds of GO exhibits an exponential decay with exposure time. The X-ray reduction rate constant of GO is positively correlated with the intensity of low-energy secondary electrons excited from substrates by soft X-ray exposure. The reduction of GO is due to the secondary electrons induce the dissociation of the CO bonds, not X-ray itself. According the results of XPS, valence band maximum and EFM measurements, we obtained the variation in band structure from GO to RGO. The energy band of C1s core level to valence band maximum from GO to RGO is rigid, and E1s decreases with reduction of GO. Because of the unique band structure of graphene at Dirac point, the resistance of graphene is obviously influenced by the doping level, i.e. the magnitude of work function. To efficaciously modulate the doping level of graphene, we fabricated a graphene/DR1-PMMA field-effect transistor (FET), where the DR1-PMMA is an azobenzene molecule copolymer. The polarization of DR1 molecules can be built and erased by photo-assisted poling (PAP) and photo-deploying (PD) operations with visible light, respectively. The ternary–logic memory of graphene/DR1-PMMA FET can be performed in the room temperature by PAP and PD operations. The PAP operation writes “1” resistance states into the graphene/DR1-PMMA FET. The PD operation erases “1” resistance states to “0” resistance state. The resistance change ratio between“1” state and “0” state is 60%. Under the room temperature, the fixed DR1 molecules result the graphene/DR1-PMMA FET device maintaining in the written states under an external electric field as high as 0.2 MV/cm.