Abstract
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.