石墨烯與金屬介面的第一原理探討

Abstract

近年來，石墨稀相關研究論文呈現指數性增加，是二維材料研究領域的領頭羊，基於石墨烯在電性上優良的物理特性，可應用於新穎光電元件之可撓式透明電極。因此石墨烯與金屬之間的介面研究(interface between graphene and metal)就是一個重要的課題。本論文透過Material Studio軟體與CASTEP模組，針對無電性改變的石墨烯-鎳介面(G-Ni(111) interface)與有電性改變的石墨烯-金介面(G-Au(111) interface)做理論計算，藉由介面形成能、能帶圖、電子態密度圖、功函數與靜電位能圖的模擬結果，來探討上述兩種金屬與石墨稀介面的物理電性差異。主要發現石墨烯-鎳介面其模擬功函數值偏離鎳金屬的情況，而石墨烯-金介面的模擬功函數值則接近金金屬的狀況，而其與實驗上觀察到的電性改變是否有關仍需討論。此外，模擬上為了找到最穩定結構，須執行收斂性分析，其概念雖然簡單，但是方法尚未有所定論，因此本論文整理提出一種收斂性分析的方法，一定程度上也確保計算的精準度。 在介面形成能的部分，G-Ni(111)介面得到的數值為-0.169 eV，G-Au(111)則為-0.054 eV，前者約為後者的3倍。從能帶圖方面，發現石墨烯的能帶在形成G-Ni(111)介面後狄拉克點消失，出現了新的能帶；G-Au(111)介面的部分，狄拉克點沒有消失，而於費米面向上位移約0.4 eV，為石墨烯受金表面影響而p-type化。在電子態密度中，由於G-Ni(111)在形成介面後，石墨烯的低於費米面部分的曲線有受到鎳的影響位移-1.2 eV，因此猜測有n-type的可能性。靜電位能的角度來看，G-Ni(111)之間並沒有形成介面層，而G-Au(111)之間有形成介面層。綜合模擬結果與實驗結果，可以確定的是，對於電子而言，G-Ni(111)的介面相對於G-Au(111)是容易通過的且不需跨過屏障的。

Recently, the related research paper of graphene has been exponential growth and graphene become the leader of two-dimensional materials field. It based on the excellent electrical properties and suitable, which can be provided in a new optoelectronic components of the flexible transparent electrode. Therefore, the interface between graphene and metal is an important issue. In this thesis, we use Material Studio software and CASTEP module to simulate the condition of interface for graphene-nickel interface (G-Ni (111) interface) with no altered electrical properties and a graphene-gold interface (G-Au (111) interface) with altered electrical properties. The difference of electrical properties between G-Ni (111) interface and G-Au (111) interface can be achieved by analyzes the total energy, work function, electrostatic potential map, band structure and partial density of state. We find that the simulated work function of G-Ni (111) close to the original metal and G-Au (111) is deviates from the original metal. In addition, the appropriate parameters are selected by the convergence analysis, and then perform the geometrical optimization to find the most stable structure. The results of adsorption energy for G-Ni (111) and G-Au (111) are -0.169 eV, and -0.054 eV, respectively. In the band structure result, the Dirac point of graphene disappears after the formation of G-Ni (111), and also form a new energy band. In G-Au (111) result, the Dirac point does not disappear and is shifted upward by about 0.4 eV from Fermi surface. In partial density of state result, G-Ni (111) is deduced that the s-orbital and p-orbital of the Ni (111) surface are mixed with p-orbital of graphene. On the other hand, the density of states near the Dirac cone in G-Au (111) is affected by gold, which leads to asymmetric shape and low left and right height. Based on the simulation results and the experimental results, it can be determined that the interface of G-Ni (111) is easy to pass the with respect to G-Au (111) and does not need to cross the barrier.