Fabrication And Physical Properties Modulation Of Two-Dimensional 3d Electronic Systems

For the specific physical problems,the different dimension leads to different solusions.When the spatial dimension of the material system decreases,that is the sizes of some dimension reach the quantum region,the quantum confiment effects are important,and the physical laws of the macroscopy will be invalid,and the properties of the material can be only describe by the quantum machanics.Due to the quantum effects of the low dimension systems,many novel low dimension quantum states have been discovered,such as quantum well states,interfacial superconductivity,charge density wave,topological superconductivity,interger/fractional quantum Hall effect,etc.In recent years,with the developments of the growth methods,the characterization methods and the maturity of the quantum mechanical theory,it is possible to study the properties of the materials in smaller scales and lower dimensions.For two-dimension systems,the successful preparation of a series of two-dimensinal materials make this field flourish,such as single element two-dimensional materials,two dimensional magnetic materials,transition metal dichalcogenides,metal-organic coordination networks etc.The controllable preparations and the explorations of novel properties of these two-dimension materials are the key steps to promote the developments of the physics and the applications of the states in low dimensions.In this thesis,we succefully prepared three kinds of two-dimensional 3d electronic systems,including two kinds of metal-organic coordination networks and a kind of transition metal dichalcogenides,grown by the metal-organic self-assembly and molecular beam epitaxy.The structures and the electronic properties of these three kinds of two-dimensional 3d electronic systems were systematically characterized by scanning tunneling microscopy and scanning tunneling spectroscopy（STM/S）at atomic level.The three parts are as follows:The two-dimensional Fe-DPBP metal-organic coordination network with honeycomb lattice has been prepared by the self-assembly of a linear molecule with two pyridyl（4,4’-di（4-pyridyl）biphenyl,DPBP）and iron atoms on Au（111）film.The two-,three,and four-fold coordinated Fe atoms has been found in the two-dimensional metal-organic coordination network.As revealed by the scanning tunneling spectroscopy measurements and the density functional theory calculations,the hybrid electronic states of Fe atoms step away from the Fermi level as the number of the coordinated molecules increases.The shifting rate,0.19 e V per pyridyl offers the experimental estimation of the repulsive energy between the iron atom and a pair of ligand electrons.Our work provides insights to understand the coordination geometry and electronic coupling at atomic level.The two kinds of two-dimensional Fe-HAT metal-organic coordination frameworks with kagome lattice have been prepared by the self-assembly of a tripyrazine molecule（1,4,5,8,9,12-hexaazatriphenylene,HAT）and Fe atoms on Ag（111）film.The periodicities of them are 1.3 nm and 2.0 nm,respectively.Furthermore,combined with the difference during growth and the structural models,the Fe centers in the kagome metal organic framework with bigger periodicity are indeed trinuclear Fe centers,which means that we realized the growth of metal-organic framework with multinuclear Fe centers.Our work provides the platform to study the novel quantum states,such as the magnetism,topological properties,and quantum spin liquid,etc.The three phases（kagome,1T,and 1T’）of NiTe₂have been prepared by co-depositing of Nickel atoms and Telluride atoms using the molecular beam epitaxy on the graphene on 6H-Si C（0001）substrate.The kagome phase and the 1T’NiTe₂have never been reported.Besides,the kagome structure has never been realized in transition metal dichalcogenides.The partially reversible phase transition has been realized by tunning the annealing condition,which is,for the first time,the phase transition in group-10 transition metal dichalcogenides.Combined with the band structure calculation of DFT,the kagome and 1T NiTe₂are type-II Dirac fermions candidates.And the number of the Dirac points are 12 and 4,respectively,which indicates the phase transition of these two phases is topological Lifshitz transition.Our work extends the phase transition systems of transition metal dichalcogenides,offering the platforms to study physics of topological materials,the physics of kagome lattice,and the topological Lifshitz transition.