Research On Epitaxial Growth And Electronic Structure Properties Of Several Types Of Two-dimensional Single Crystal Thin Films

In the past 30 years,two-dimensional materials have attracted great attentions for their novel electronic,optical and magnetic properties.With the research achievements of theoretical predictions and experimental characterizations,there emerge variant two-dimensional material systems,which have not only uncovered many intriguing physics,but also show promising prospects in applications of next generation of information technology.For example,thin films of MoS₂ can achieve"on"and"off"state regulation for building field effect transistors.The heterostructure of monolayer FeSe/Sr Ti O₃ is superconductor with high transition temperature.Long-range ferromagnetic order is found in monolayer CrI₃ films.These novel phenomena have brought up opportunities for understanding the physics at the two-dimensional limit.Therefore,it is particularly important to study the growth and electronic properties of two-dimensional films.In this thesis,we have prepared Bi,CrTe2 and Te thin films with molecular beam epitaxy and characterized their electronic structures with scanning tunneling microscopy and spectroscopy at cryogenic temperatures.The interpretation of our observations is substantiated with theoretical modeling and density functional calculations.The main findings of our studies are highlighted below.1.Leakage of Cooper pairs through hetero-interfaces leads to superconducting proximity effect,which has been utilized extensively in building functional quantum devices and inducing novel superconductivity.While an atomically sharp interface in real space is known to be crucial for effective Cooper pair proximity transfer,the influence of electronic valleys in the momentum space is yet investigated.Here,we report the observation of valley dependent superconducting proximity effect in a heterostructure with twisted overlapping.The heterostructure is realized by growth of multi-domain Bi(111)films on a single crystal NbSe₂ substrate with molecular beam epitaxy.With spectroscopic imaging scanning tunneling spectroscopy,we identified different types of atomic overlapping in the Bi films,and measured drastic changes of proximity induced superconducting gap sizes on the differently oriented Bi domains.Based on our theoretical model calculation,this phenomenon can be interpreted as valley dependent superconducting proximity coupling between the Bi film and NbSe₂.We also investigated the lateral proximity effect between two adjacent Bi domains,which determines a significant reduction of mean free path of electrons,associated with interfacial scattering.Our study expands the scope of tunable physical properties with the valley degree of freedom.2.According to Hohenberg-Mermin-Wagner theorem,long-range ferromagnetic order and antiferromagnetic order can not exist in two dimensions.The recent discovery of ferromagnetism monolayer CrI₃ films has renewed the traditional view of magnetism in two dimensions.However,intrinsic antiferromagnetic order in monolayer films is still lack.In this regard,we prepared monolayer and bilayer CrTe₂ films on graphene covered SiC(0001)substrate by molecular beam epitaxy.Conventional spin averaged STM measurements reveal that the surface of monolayer CrTe₂film exhibits a 2×1 stripe structure.Such strip structure,according to first-principles calculations,is directly related to its antiferromagnetic order.Further,we investigated the magnetic properties of monolayer CrTe₂ by a spin-resolved scanning tunneling microscopy with a Cr tip.We found that the spin texture of the monolayer CrTe₂ is antiferromagnetic with AABB-like zigzag configuration.Spin contrast of the zigzag structure is reversed with opposite magnetic fields applied perpendicular to the film surface.In addition,there is a characteristic peak related to magnetism at around-500 mV in the spin-resolved spectrum.This peak shifts progressively towards lower energy with increasing perpendicular magnetic fields in both directions until a saturation at 0.5 T.This indicates that the magnetic anisotropy,which is out-of-plane,in the monolayer thin film is small.The magnetic transition is a spin-flop type transition.Our study reports the first direct observation of antiferromagnetic order at atomic scale in two-dimensional limit,which may promote the application of two-dimensional materials in spintronics.3.The two-dimensional Te is a graphene-like layered material with the crystal structure and electronic properties strongly depending on the supporting substrate.We thus systematically study the evolution of geometric and electronic structure of Te atoms onto a NbSe₂ substrate,from sub-monolayer to few layers.In the low coverage,Te atoms are adsorbed on the NbSe₂ surface,forming two types of highly ordered superstructures.By increasing the coverage,the thin film is converted into a semiconducting?-Te phase,accompanied by its energy gap decreasing with the increase in thickness.Combining with the first-principles calculations,we find that charge transfer occurs between the interface of the NbSe₂ substrate and the?-Te film,which enhances the interlayer interaction.Moreover,the interaction between tellurium adatoms and?-Te films leads to?2×?2 surface reconstruction prior to a new single layer,conforming to our theoretical calculations.Therefore,our work clarifies the kinetic growth of tellurium films including the atom adsorption,strain and surface reconstruction.The Te/NbSe₂ heterostructure provides a suitable platform for tuning the exotic electronic properties of 2D Te films via substrate modulation or surface decoration.