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  • 學位論文

運用第一原理計算探討二氧化矽基材之表面結構、摻雜與缺陷對石墨烯電子性質之影響

Modulation of the Electronic Properties of Graphene on the Silicon Dioxide Substrates - A First Principles Study

指導教授 : 郭錦龍

摘要


石墨烯具有優異及獨特的電子與光學等性質,因此被認為在各種奈米尺度元件的應用上極具發展潛力。由於石墨烯為二維晶體結構,若要將其應用在奈米元件中則必需依附於基材上,而二氧化矽為奈米元件中最為廣泛使用的基材材料,因此研究二氧化矽基材微結構與石墨烯之間的交互作用便成為極重要的課題。然而,目前對於二氧化矽基材微結構或介面性質如何影響石墨烯電子性質之影響並沒有深入的了解。本研究嘗試運用第一原理計算來探討二氧化矽基材之表面結構、摻雜物與缺陷對石墨烯電子性質之影響。 首先我們探討無缺陷二氧化矽基材中不同的表面結構與石墨烯之交互作用。研究結果顯示當二氧化矽基材表面為緻密結構或含有羥基結構時,石墨烯與基材之間並沒有電荷轉移,僅只在兩者介面間產生電荷重新分佈的現象,使得石墨烯狄拉克點之π能帶與π*能帶間產生微小的能隙值。其中二氧化矽基材表面的羥基造成石墨烯與基材介面間電荷重新分佈的程度較高,使得載子散射的機率增加,並導致石墨烯的能隙值變大。 接著,我們探討二氧化矽基材可能存在的缺陷對石墨烯電子性質之影響。研究結果顯示當石墨烯在含有silanone groups、單牙碳基或價換偶缺陷之二氧化矽基材上時,石墨烯的狄拉克點會高於基材之最低未佔據能態,造成石墨烯有電子轉移到二氧化矽基材,使得石墨烯產生P型摻雜效果。 最後本研究探討二氧化矽基材中存在硼或磷半導體摻雜物時,其摻雜物在基材中可能形成的微結構對石墨烯電子性質之影響。研究結果顯示二氧化矽基材表面結構的差異造成介面間產生不同大小及方向之局域電場,使得硼摻雜物在二氧化矽能隙中所引入的缺陷能階產生變化,因而對石墨烯產生不同的摻雜效果。本研究成果可對石墨烯之相關材料應用在光電元件中的設計與製程改善方面提供更多可行的方向與參考。

並列摘要


Graphene, the two-dimensional network of carbon atoms packed into the hexagonal crystal structure, is one of the most promising materials for fabrication of nanoscale devices due to its excellent electronic and optical properties. Because of its two-dimensional nature, graphene is always exposed to the environment and must be attached to substrates for nanodevice applications. It is of great importance to understand the electronic properties of graphene affected by the environment, such as molecule adsorption or substrate-surface interaction. Of all substrate materials in the devices, silica is the most widely used. Therefore the study of the interaction between graphene and silica substrates has become one of the most significant issues. Although the doping behavior of graphene has been investigated by many experimental and theoretical works, the impacts of the microstructures and the interfacial properties of the underlying silica substrates are not fully elucidated. In this study, we attempted to develop an atomic-scale understanding regarding the modulation of the electronic properties of graphene via underlying silica substrates based on first-principles density-functional theory calculations. First, we studied the energetic and electronic structures of graphene on the defect-free α-quartz (0001) surfaces. We employed three types of dense SiO2 surfaces as well as two different kinds of hydroxylated SiO2 surfaces to investigate the effect of the substrate on the electronic properties of graphene. The interaction between graphene and the OH group on the hydroxylated SiO2 surface with different orientations were investigated in this study as well. These structural models of oxide surfaces were carefully generated using constant-temperature molecular dynamics simulations followed by subsequent atomic relaxation and structural optimization. We considered three inequivalent positions for graphene to be placed on the top of the defect-free SiO2 model surfaces. Our results showed only weak binding energy and that there were no electrons transfer between graphene and the SiO2 substrates for all defect-free surface models considered. Furthermore, our band structure calculations showed that the linear bands of graphene were only slightly changed accompanied with the appearance of a tiny band gap opening. Interestingly, our calculations revealed a much larger band gap opening for graphene to be placed on the top of the hydroxylated surface than that on a dense one, which is mainly attributed to the more significant charge redistribution of graphene by the polar hydroxyl groups on the SiO2 surfaces. Moreover, the OH group on the silica surface resulted in more charge redistribution on the interface between graphene and the substrate, increasing the probability of charge carrier scattering. Next, we explored the electronic properties of graphene deposited on various defective SiO2 substrates. Our results showed that there were no charge transfer between graphene and the silica substrate for the surface containing Frenkel pair or two-membered ring. However, our band structures calculations revealed a route towards the p-type doping effect via the interactions with the silanone groups, monodentate carbonate or valence alternation pairs on the SiO2 surfaces. For the surface containing silanone groups, the doping effect is mainly attributed to the electron transfer from graphene to the silanone groups on the SiO2 surface. For the surface containing monodentate carbonate or valence alternation pairs, the doping effect is mainly induced by the local electrostatic fields that shift the anti-bonding states of the surface atoms to a relatively lower position than the Dirac point. Finally, we investigated the electronic properties of graphene deposited on boron-doped SiO2 substrates. Our results indicated that the boron can be an acceptor or donor that causes the doping effects on a graphene sheet. Manipulation of the surface structures can modulate the work functions of the substrates, which is able to change the positions of the boron-induced defect levels relative to the Dirac point. Interestingly, change of the local dipole moments induced by a layer of adsorbed water may cause the inversion of the doping effect on graphene. On the other hand, it was suggested that doping boron in SiO2 substrates containing surface OH groups can result in a typical n-type doping effect on graphene. Our results may give an aid to material design and process improvement of graphene-related materials in nanoelectronic devices.

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