Preparation And Characterization Of Ultrathin Quantum 2-D Materials Grown On Single Crystal

Ultra-thin quantum two-dimensional(2-D)materials have many novel properties which cannot be observed in macroscopic bulk materials.Dirac fermions were found in graphene,and similar properties were found in graphene-like silicene material.Ultra-high superconducting transition temperature(Tc)was found in 2-D superconductors.A lot of physical properties were found in these 2-D materials,such as ultra-high electrical conductivity without energy dissipation and ultra-high thermal conductivity.In this thesis,we fabricate high-quality silicene films on Ag(111)substrate and 2-D single-layer FeSe films on SrTiO3(001)substrate using molecular beam epitaxy(MBE).By using scanning tunneling spectroscopy(STM),we study the atomic and electronic structure of silicene and superconductivity of FeSe films.The main results are listed as follows:(1)The complex superstructures of silicene on Ag(111)remain controversy for many years.By using ultra-low temperature STM,we study the silicene superstructures meticulously.We find there are five atomic-arranged structures of(4x4)and((?)×(?))superstructures.And we named them as(4×4)-?,(4×4)-?,((?)×(?))-?,((?)×(?))-? and((?)×(?))-?.(4×4)-?,((?)×(?))-? and((?)×(?))-?superstructures were not observed before.And by high-resolution images of((?)×(?))-a and((?)×(?))-? superstructures,we identify the real atomic structure of these two superstructures and solve the different understanding of these superstructures.We find that the(2(?)×2(?))silicene superstructure is out-of equilibrium,inherently highly defective and inhomogeneous:it is a patchwork of fragmented silicene pieces.It could not form a long range order.And we find a dewetting process in the growth process of(2(?)×2(?)×)silicene superstructures.The silicon atoms will get into the silver substrate(2)By means of MBE,we grow high-quality single-layer FeSe films on Nb-doped SrTiO3(001)substrates.There are many stripe-liked domain boundaries(DBs)on the single-layer FeSe films due to the lattice mismatch between FeSe and STO.During the post-annealing procedure,we find that superconductivity firstly emerges at stripe-like DBs And the superconductivity gets better with increasing annealing time and decreasing density of dumbbells,which are resulted by the excess Se atoms.With large energy scale d?/d?measurements,we find an electron doping effect at boundaries which should be a reason for superconductivity enhancing on DBs.With atomic STM images of dumbbell defects and DBs,we make a detailed research for the structure of DBs and dumbbells.A balls structural model of dumbbell and a new zigzag structural model of DBs are presented to help understand the structures of dumbbell and stripe-like DBs.The lattice constant compression of 1.5%also may be a reason for superconductivity enhancing at boundaries.Thses results strongly indicate that DBs play an important role in the origin of enhanced superconductivity in FeSe films grown on SrTiO3(001).It provides an important experimental complement to understand the mechanism of superconductivity in single-layer FeSe films.