Scanning tunneling microscopic study of layered phosphorus and molybdenum ditelluride

Abstract

Two-dimensional (2D) materials have attracted lots of research attention in the past decade because of their extraordinary electronic and optoelectronic properties as well as potential device applications. Transition-metal dichalcogenides (TMDs) are a genre of 2D materials that cover a wide range of properties. Some TMDs even exhibit non-trivial characters as superconductors, topological insulators, etc. Layered phosphorus is a late but important member in the 2D material family. Single- or few-layer black phosphorus has been widely studied recently. Reports show that it has extraordinary carrier mobility, which makes it competitive in the application in 2D devices. Besides black phosphorus, there are also other layered phosphorus allotropes being proposed and experimentally realized. Blue phosphorus is one of the most attractive allotropes, who is predicted to be as stable as black phosphorus and shows comparable electronic and optical properties. Recently, it has been reported to be successfully synthesized on Au(111) by molecular beam epitaxy (MBE). In this thesis, three new 2D systems are studied by scanning tunneling microscopy (STM). The first is blue phosphorene-like film grown on Au(111). Reports have shown that a blue phosphorene-like superstructure can be stabilized on a (5×5)-cell of Au(111). In this work, the growth path of the blue phosphorene-like epi-film is studied. We find a particular coverage jump of phosphorus in the growth process. It indicates the existence of an interesting mechanism of surface dewetting accompanied by the phase separation. The experimental results are carefully compared and explained by density functional theory (DFT) calculations. The second system is the layered phosphorus grown on Pt(111). As a noble metal, Pt(111) surface can also capture the reactive phosphorus atoms. We find that depending on phosphorus coverage and growth temperature, phosphorus can form five kinds of superstructures on Pt(111) surface. The structural and electronic properties of these superstructures are characterized by STM. The possible links between these structures, depending on coverage and temperature, are proposed. The final system is a new and chalcogen-deficient structural phase, the variational hexagonal (v1H) phase of MoTe1.6, observed in MBE grown MoTe2. This new phase has an unusual structure that belongs to the P-62m (No.189) space group and coexists with the 1H and 1T’ MoTe2. It is found structurally associated with a typical defect – mirror-twin domain boundaries (MTBs) – in the 1H-MoTe2, which can be visualized as evolved from the 1H-MoTe2 when the MTBs have increased to the highest density. The v1H-MoTe1.6 is electronically metallic. By both STM/S measurements and the DFT calculations, some interesting electronic characteristics are also revealed.