Year

2016

Degree Name

Doctor of Philosophy

Department

Institute for Superconducting and Electronic Materials

Abstract

Silicene, the silicon-based counterpart of graphene, has attracted intensive interest due to its unique characteristics and a wide range of promising applications. In this thesis, we investigate its growth mechanism, EPC strength, and oxygen adsorption functionality. The details are as following:

1. Epitaxial growth mechanism of silicene layers fabricated on a Ag(111) surface by MBE deposition were reported in this chapter. The coverage effect and the structural defects have been identified by using STM imaging. It is found that substrate temperature plays a critical role in determination of the silicene superstructures. Several types of defects are observed in different silicene superstructures, which are most likely induced by the low coverage effect or the interface lattice mismatch between silicene and Ag(111). Furthermore, the silicene sheet prefers to initially arise at the terrace edge of the substrate. Our results imply that the growth mechanism of all silicene superstructures follows the Stranski-Krastanov growth mode.

2. The special coupling between Dirac fermion and lattice vibrations, in other words, electron phonon coupling, in silicene layers on an Ag(111) surface was probed by in-situ Raman spectroscopy. The tensile strain, induced by the lattice mismatch between silicene and the substrate, and the charge doping from the substrate, modulate EPC strength. The Raman spectrum clearly reveals evolution of defect peaks with coverage of silicene layers. The peaks at low frequency correspond to the different electron scattering modes occurring at the zigzag and armchair edges. This work implies that Raman spectroscopy allows unambiguous, fast, and nondestructive identification of silicene layers, which is critically lacking in this emerging research field so far.

3. Monolayer silicene grown on Ag(111) surfaces prove a band gap that is tunable by oxygen adatoms from semimetallic to semiconducting type. We find that the adsorption configurations and amounts of oxygen adatoms on the silicene surface perform as the critical factors for band gap engineering, which is determined by different buckling degrees in √13×√13, 4×4, and 2√3×2√3 superstructures. The Si-O-Si bonds are the most energy-favored species formed on √13×√13, 4×4, and 2√3×2√3 structures under oxidation, which is verified by in-situ Raman spectroscopy as well as first-principles calculations. The silicene monolayers retain their structures when fully covered by oxygen adatoms. Our work demonstrates the feasibility of tuning the band gap of silicene with oxygen adatoms, which, in turn, expands the base of available two-dimensional electronic materials for devices with properties that is hardly realized with graphene oxide.

4. Epitaxial silicene shows a strong interaction with the substrate that dramatically affects its electronic structure. The role of electronic coupling in the chemical reactivity between the silicene and the substrate is still unclear so far. The hybridization between Si and Ag induces a metallic surface state, which can gradually decay by oxygen adsorption. XPS results manifest the decoupling of Si-Ag bonds as well as the relatively oxygen resistance of Ag(111) surface after oxygen treatment. First-principles calculations have also illustrated the evolution of the electronic structure of silicene during oxidation. It has been demonstrated experimentally and theoretically that the high chemical activity of 4×4 silicene is attributable to the Si pz state, while the Ag(111) substrate exhibits relatively inert chemical behaviour.

5. Silicene is a monolayer allotrope of silicon atoms arranged in a honeycomb structure with massless Dirac fermion characteristics, similar to graphene. It ensures development of silicon-based multifunctional nanoelectronic and spintronic devices operated at room temperature due to strong spin-orbit coupling. Nevertheless, until now, silicene could only be epitaxially grown on conductive substrates. The strong silicene-substrate interaction may depress its superior electronic properties. A quasi-free-standing silicene layer that has been successfully obtained through oxidization of bilayer silicene on the Ag(111) surface. The oxygen atoms intercalate into the underlayer of silicene, which can isolate the top layer of silicene from the substrate. In consequence, the top layer of silicene exhibits the signature of a 1×1 honeycomb lattice and hosts massless Dirac fermions due to much less interaction with the substrate. Furthermore, the oxidized silicon buffer layer is expected to serve as an ideal dielectric layer for electric gating in electronic devices. These findings are relevant for the future design and application of silicene-based nanoelectronic and spintronic devices.

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