| In2Se3 is a two-dimensional ferroelectric material with interrelated in-plane and out-of-plane ferroelectric polarization,and the polarization direction can be switched under external electric field,which has great potentials in the applications of field-effect transistors,non-volatile memories,phase-change random access memory,and photodetectors,etc.Defects are often inevitable during material growth,the presence of atomic vacancies and defects can affect the electronic structure and charge distribution of ferroelectric materials,and then have a complex impact on their ferroelectric polarization.Further understanding of the defects and their effects on the electronic structure has become a key prerequisite for the applications of functional ferroelectric materials.Furthermore,ferroelectric materials can be stacked with different materials to form van der Waals heterojunctions,and the ability to switch the polarization under electric field provides a new method for tuning their physical properties.In this thesis,we use scanning tunneling microscopy(STM)as the main experimental technique to characterize the atomic structure of In2Se3 and the effect of defects on its electronic structure.By using molecular beam epitaxy(MBE),we have successfully grown Pb ultra-thin films on the In2Se3 substrate and created an ideal superconductor/ferroelectric heterostructure Pb/In2Se3 to further explore the effect of the ferroelectricity on the electronic structure and superconducting properties.In Chapter 1,we first introduce the properties of ferroelectric materials,the underlying mechanism of ferroelectric polarization,and recent progress on the twodimensional ferroelectric materials.Then,we summarize the structure and physical properties of the In2Se3.In Chapter 2,we introduce the experimental techniques and equipments used in the research,including ultra-high vacuum technology,MBE technique,Raman spectroscopy,and the working principle and main structure of STM.In Chapter 3,we show the effect of defects on the surface morphology and electronic structure of the In2Se3 samples.The coexistence of two phases of α-In2Se3 andβ’-In2Se3 is observed for the first time in real space by STM,and the high-resolution atomic STM topography shows that α-In2Se3 has an uneven surface with clear defects,in the meantime,a complete uppermost Se hexagonal lattice was revealed,which indicates that the defects observed on the α-In2Se3 here are atomic defects below the surface layer.By taking scanning tunneling spectroscopy(STS)measurements at different positions of a-In2Se3,the underlying atomic defects have a hole doping effect with shift the Fermi energy for about 90 meV.Another interesting observation is that the β’-In2Se3 has a defect-free periodic nanostripe structure.In Chapter 4,we study the effect of a-In2Se3 on the Pb quantum well state(QWS)and superconducting transition temperature(Tc).The STM image shows that the Pb films have a 1 × 1 lattice structure and exhibit a moire superlattice structure with a period of 2.5 nm,which is caused by the lattices of Pb and α-In2Se3 with an angle of 11.3°.The corresponding STS spectra show that the QWS of Pb islands on the a-In2Se3 substrate disappear,and no superconducting gap was observed on the Pb film at 4.5 K.The underlying mechanism of the change in the electronic structure and superconductivity of Pb films needs to be further explored.In the fifth chapter,the main results were summarized and future prospective was proposed. |
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