應用第一原理計算石墨烯/氧化石墨烯/石墨烷奈米結構及矽鍺奈米線之電子結構與光學性質

Abstract

本研究探討石墨烯和其相關材料（氧化石墨烯及石墨烷）的能帶結構。石墨烯為單層石墨結構，近年來是非常熱門的新材料。這個材料有非常好的電導性質，很適合做為太陽能光電元件的異質接面。由於石墨烯的氧化及氫化過程是可逆且可控制的，石墨烯和氧化石墨烯和石墨烷的介面對於元件的異質接面將是一個關鍵因素。我們使用第一原理去模擬這類介面的能帶結構，例如半無限大的介面、層和層之間的介面及量子點形式的石墨烯。這些異質介面的電荷轉移也是本研究的重點之一。在特殊情況之下，異質介面會造成石墨烯空間上的侷限效應，這個效應使光學吸收位於量子點。 此外，我們亦探討矽鍺奈米線的光學躍遷現象。經由能帶結構之計算，我們可以知道能隙大小及情況在不同的殼核比例下之變化。從吸收光譜我們發現核的性質在光學躍遷中扮演重要的角色。另外，吸收密度的分析也有助於探討矽鍺奈米線光學躍遷的機制。

In this research, the band profiles of graphene and graphene related materials (graphane and graphene oxide) have been investigated. Graphene, constructed by a single layer of graphite, has been developed in recent years. This two-dimension material is promising in photovoltaic heterojuction since its good conducting properties. Due to the reversible hydrogenation and oxidation process, the interface of graphene, graphene oxide and graphane is a key factor to investigate heterojunctions. We use first principle calculations to evaluate the band alignment properties of interfaces, such interfaces as semi-infinite contact, layer-layer contact and quantum dot of graphene. The charge transfer between two sides of heterostructures is also discussed. In some conditions, the spatial confinement in graphene quantum dot results in optical excitations inside the dot. We also investigate the optical transitions of silicon-germanium core-shell nanowires. With different core diameter and shell thickness, we illustrate the variation of fundamental band gap energy and the direct-indirect transition on band structure calculation. Through the parallel-polarization absorption spectrum, we found that the core property plays an important role on the optical transition. A method called the band-resolved absorption density analysis technique is developed to investigate the mechanism of the optical transition processes.