銀(111)晶面上矽烯的結構與成長模式之探討

Structure and Growth Evolution of Silicene on Ag(111) surface

李國威(Guo-Wei Lee)

指導教授 ： 林登松

國立清華大學/理學院/物理學系/博士(2014年)

https://doi.org/10.6843/NTHU.2014.00619

摘要

因為獨特的蜂巢狀對稱性排列，使得二維結構材料例如石墨烯有許多奇特及引人注目的特性。若由矽原子所組成的二維結構一般被稱為矽烯。但是自從在銀(111)基板上成功地沉積出矽烯以來，在之前的研究中不是使用矽烯的單位向量就是使用銀基板的單位晶格來描述矽烯的成長結構。
我們使用低溫掃描穿隧式顯微鏡(LT-STM)、X射線核心層光電子能譜(XPS)以及第一原理的密度泛函計算(DFT)來研究矽烯成長在銀(111)晶面上的成長模式與結構變化的一連串過程。我們的結果首先證實了在成長單層矽烯時，根據成長環境不同矽烯的確存在許多不同的樣貌結構，而且矽烯與銀(111)基板晶格之間存在有一定程度上的關聯。
在初期矽烯成長在銀基板上，矽原子都集中吸附在銀(111)晶面的上層平台而形成暫穩態的條狀結構，這些條狀結構顯然是由於銀原子被拋射出來所形成的。而且經由仔細分析STM影像之後，我們發現矽烯成長晶面邊界上有特殊的排列結構以及莫利雲狀圖紋有著不同於之前所發現特殊的角度，這些證據都隱約暗示著矽烯附蓋層與銀基板間不必然具備重合的關係。為了使我們的假設得到驗證，第一原理基於密度泛函的計算中表明，針對矽烯對銀(111)晶面的平移位移結合能的能障在每個矽原子約為0.02-0.06電子伏特。而熱能的影響在溫度230 ℃時極有可能可以克服這個位能障。此外，X射線核心層光電子能譜實驗中，單層與多層的矽烯成長下的束縛能變化，也提供另一項證據來驗證矽烯的成長模式與變化過程。
本論文共分為六章，大致編排如下：在第一章簡介研究背景與動機，包含最近在矽烯這個領域所得到的重大進展、發現以及挑戰等。第二章則介紹了實驗技術的方法與操作原理，例如超高真空技術、低溫掃描穿隧式顯微術和X射線光電子能譜。針對基板尺寸及蒸鍍源製作也有詳細地描述。與此同時，理論計算中關於密度泛函的原理、計算推導與物理上的應用會在第三章中詳細地描述。接著在第四及第五章中將分別針對自立式矽烯與矽烯成長在銀(111)晶面上的實驗結果與理論計算等作詳細分析與討論。在第五章中，為了釐清矽烯在銀(111)晶面上的結構變化與成長模式，我們不只提出了STM 與XPS的實驗結果，也運用了第一原理的密度泛函計算來更深入的研究成長相變。最後在六章中總結了這本論文的主要貢獻，首先闡明了隨著矽原子覆蓋率的增加使得矽烯在銀(111)晶面上的結構相變過程，再者我們認為矽烯附蓋層與銀基板之間不用符合一致相對應的關係。

關鍵字

矽烯 ； 掃描穿隧電子顯微鏡 ； 光電子能譜 ； 第一原理 ； 密度泛函

並列摘要

Two-dimensional (2D) materials such as graphene exhibit peculiar and attractive properties because of the unique symmetry of honeycomb π-orbital network. A graphene-like 2D material consists of Si atom is referred to as silicene. However, since a silicene has been experimentally synthesized on Ag(111) surface, the overlayers superstructure are described as either the unit vectors of silicene based or the primitive cell vectors of Ag(111) lattice in previous studies.
We conducted the low-temperature scanning tunneling microscopy (LT-STM), X-ray core-level photoemission spectroscopy (XPS), and performed ab-initio calculations based on density functional theory (DFT) to investigate the growth processes and structural evolutions of silicene on Ag(111) surface. The results confirm the existence of various overlayer structures reported previously and that the lattice parameters of several overlayer structures at sub-monolayer coverage are in some degree related to the corresponding substrate supercells. In the early stage of silicene growth on silver terrace at 230 °C, the adsorbed silicon atoms are incorporated into the upper steps and form at each edge a stripe that consist of precursor structures. The stripe areas expand into domains apparently by further ejections of Ag atoms at the upper terraces. Moreover, the detailed analysis of silicene domain boundaries and the Moiré-like superstructures in the STM images strongly suggest that the overlayer silicene sheets and silver substrate lattices have no coincident relationships. In order to clarify our supposition, first-principles calculations based on DFT show that the barrier of binding energy for the translational displacement of silicene on Ag(111) is about 0.02–0.06 eV per Si atom. The thermal energy has big possibility to overcome the energy barrier at 230 °C. In addition, the core-level photoemission spectra show apparent binding energy shifts for the sub-monolayer and multilayer silicene sheets, providing another evidences to confirm the growth evolution.
This dissertation is organized into six chapters. In chapter 1, we described the background and motivations of this research firstly, and then followed by the view of major progress, findings, and challenges in the reports of silicene field recently. Chapter 2 describes the technique methods and operations of experimental apparatus, such as UHV, LT-STM, and XPS. The information of substrate and evaporation source is also presented in this chapter. Furthermore, the theoretical calculation based on DFT, including the principle, process, and applications are introduced in chapter 3 briefly. Then, the detailed discussions and results of the free-standing silicene and silicene growth upon Ag(111) substrate are described in chapter 4 and 5, respectively. In order to clarify the structure and growth evolution of silicene growth on Ag(111), we not only conducted STM and XPS experiments but also performed first-principles calculations based on DFT by VASP to investigate the various configurations. Finally, the two main results in chapter 6, the structure and phase transition depend on different coverages and the non-coincidence relationship between overlayer silicene and silver substrate are summarized clearly in the end.

並列關鍵字

silicene ； STM ； XPS ； First-principle ； DFT

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