MBE Growth And Scanning Tunneling Microscope Study Of Borophene

Since the discovery of graphene in 2004,the study of two-dimensional materials has been developed greatly.Two-dimensional materials for elements in the same main group with carbon,such as silicene,germanene,and stanene,are successively prepared.Similar to graphene,these new two-dimensional materials have honeycomb lattice structures and Dirac cone electronic structures in the energy band,and therefore hold rich physical properties.But unlike graphene,these two-dimensional materials are predicted to have certain size of energy gaps.The existence of energy gap on the one hand makes some of the new physical phenomena such as quantum spin Hall effects may be come true in these materials,on the other hand also helps to expand the application of two-dimensional materials.Therefore,the constant discovery of new two-dimensional materials and enriching the properties of two-dimensional materials are important topics in the field of two-dimensional materials research.In this paper,we successfully prepared a new type of two-dimensional material,borophene,by molecular beam epitaxy(MBE).Combined with scanning tunneling microscopy(STM)and first-principles calculations,we studied the structure and properties of borophene.The main contents of this paper are as follows:1.Borophene was prepared by epitaxial growth of boron on Ag(111)substrate.By the study of scanning tunneling microscopy,it is found that there are two kinds of borophene on the surface of Ag(111),S1 and S2,where S1 is the low temperature phase and S2 is the high temperature phase.Combined with the first-principles calculation,we have given reasonable models of these two structures.The electronic properties of these two structures are characterized by scanning tunneling spectroscopy and density functional theory calculation,which show that they all have metal properties.The oxidation resistance of borophene was tested by oxidation experiment.In addition,two metastable structures of borophene on the Ag(111)surface were observed,namely S3 and S4.Scanning tunneling spectroscopy study shows that they also have metal properties.A special kind of domain boundary is observed in S3,with a boundary state near the domain boundary.2.We have attempted to grow borophene on Au(111)surface by molecular beam epitaxy.We mainly used two ways,heating growth and room temperature deposition and re-annealing.During the heating growth process,boron is difficult to spread on Au(111)surface,mainly existing in the forms of clusters,and Au(111)surface is obviously characterized by B destruction.During the room temperature deposition re-annealing process,some flat films can form on the surface,but these membrane structures have a lattice structure similar to that of the substrate Au(111).Meanwhile,Au(111)always exhibits a disruptive feature.It is difficult to determine whether these films are true borophene or alloy of B and Au.3.The borophene nanoribbons were prepared on the surface of Ag(110)by molecular beam epitaxy.By STM characterization,B has four structures on Ag(110)surface.Combined with the first-principles calculation,we have given reasonable models of these four structures.Unlike the borophene structure on Ag(111)surface,these four structures are superstructures of borophene and Ag(110)substrates.We also calculated the width distribution of these borophene nanoribbons and found that they had good uniformity.