Constructions And Structural Characterizations Of Several New-Type Of Two-Dimensional Atomic-Crystal Materials

Graphene,a monolayer of carbon atoms with a two-dimensional(2D)honeycomb structure,has been widely investigated since the exfoliation in 2004.Graphene with a single atomic-layer thickness possesses plenty of unique and extraordinary properties,such as high charge carrier mobility,high optical transparency and room temperature quantum hall effect.Those properties make Graphene show potential applications in electronics,optics,magnetics,biological medicine,catalysis,energy storage and sensor,etc.Thus,Graphene is recognized as the future material and revolutionary material in this century.The great success of graphene has inspired us to explore and discover the other novel 2D atomic-crystal materials beyond graphene.In particular,the 2D groupIV materials(silicene,germanene and stanene)and 2D non group-IV materials(borophene,hafnene,phosphorene,hexagonal boron nitride,and transition metal dichalcogenides)have attracted enormous interest because of their outstanding properties.In this dissertation,combining scanning tunneling microscopy(STM),low-energy electron diffraction(LEED),and density functional theory(DFT)calculation,we mainly investigated the fabrications,atomic structures and electronic properties of several new types of two-dimensional atomic-crystals materials,as follows:Identifying and visualizing the edge terminations of single-layer MoSe2 island.Using the molecular beam epitaxy method,we successfully constructed high-quality,single-layer MoSe2 on Au(111)surface.All of the islands have hexagonal or triangular shapes with two kinds of well-defined edges.STM characterization reveals the atomicresolved images of two edges of MoSe2 island(Mo edge and Se edge).Scanning tunneling spectroscopy(STS)curves show notable differences in positive sample bias for the two types of edges.Density functional theory calculations for several edge configurations of MoSe2 confirm that the STS differences are attributed to the coupling between the pz orbit of Se atoms and the dxz orbit of Mo atoms,and the two types of observed edge terminations are the bare Se edge and selenium-saturated Mo edge.Fabrications,characterizations and functionalizations of new type of 2D atomiccrystal CuSe.After deposition of Se atoms onto Cu(111)surface and post-annealing the sample,we successfully constructed the large-scale,2D monolayer,honeycomb structure CuSe.With different amount of Se atoms,monolayer CuSe can possess two distinct structures: ridge structure and nanopore structure.Combining the STM,LEED and DFT characterizations,we demonstrate the difference between two structures is the variable bond length of Cu-Se.In addition,by deposition of Fe atoms,FePc molecules and C60 molecules onto the intrinsically-patterned 2D materials,we achieve the preferred adsorption,demonstrating that these materials can serve as templates for selective self-assembly of molecules or nanoclusters for selective or dual functionalization.Fabrication,characterization of new type of 2D atomic-crystal AgSe and Ag5Se2.After deposition the Se atoms onto Ag(111)surface and post-annealing the sample,we successfully constructed two kinds of large-scale,2D monolayer silver selenides.Monolayer AgSe demonstrates a honeycomb structure and can be fabricated at roomtemperature.In addition,DFT calculations show the existence of magnetic property in the free-standing AgSe surface.Monolayer Ag5Se2 is fabricated by deposition of more Se atoms and LEED pattern shows that it is a ?7×?7 superstructure with respect to Ag(111)substrate.Construction of Two-Dimensional Chiral Networks through Atomic Bromine on Surfaces.Using atomic bromine and organic molecules(DPA,DcC6 PA,Pentacene),we successfully constructed and characterized the large-area 2D chiral networks on Ag(111)and Cu(111)surfaces by combining molecular beam epitaxy with scanning tunneling microscopy.The Br atoms distribute themselves periodically in the network with the maximum number of-C-H···Br hydrogen bonds.Density functional theory calculations demonstrate that the hydrogen bonds contribute to the stability of the Brorganic networks.In addition,by controlling the ratio of bromine atoms to DPA molecules,different patterns of Br-organic networks were obtained on Ag(111)surfaces.