Low Temperature Growth And Hydrogenation Of Stanene On Au(111)substrate

In recent years,both theoretical and experimental researches in graphene-like two-dimensional single elemental materials like silicene,germanene and stanene has experienced an explosive increase.Among them,stanene is a two dimensional allotrope of Tin(Sn)withlow-buckled honeycomb structure.It is of special interest owing to its outstanding properties like large-gap two dimensional quantum spin Hall state,topological superconductivity and the near-room-temperature quantum anomalous Hall effect.Unlike graphene,stanene is unable to be achieved by mechanical exfoliation,instead,most of the stanene that reported so far are obtained by epitaxial growth of Sn atoms on various substrates.Au(111)is one of the metal substrates that has been widely studied for the stanene growth.Theoretical calculations predicted that it is possible to achieve honeycomb stanene on Au(111).However,most of the present experimental results show complex Sn-Au alloy when Sn atoms was deposited at room substrate temperature.The experimental growth of pure stanene on Au(111)remains elusive.In this thesis,combining the two technologies of molecular beam epitaxy(MBE)and low-temperature scanning tunneling microscopy(LT-STM)explores the growth mechanism of stanene on Au(111)at low temperature.Successfully obtaining pure honeycomb stanene at substrate temperature of 77K.Then investigating the hydrogenation of stanene on Au(111),reveals the interactions between hydrogen atoms and stanene.The main conclusions obtained in this thesis are as follows:(1)When the substrate temperature is held at 77K,the tendency of Sn-Au alloy is inhibited and a honeycomb stanene is formed on Au(111)surface.When the coverage of Sn atoms is lower than 1ML,Sn atoms are preferentially adsorbed in the face-centered cubic stacking(FCC)area on Au(111)surface,forming irregular stanene islands.With the increasing of Sn coverage,Sn islands become larger and larger,and extend to the hexagonal close-packed(HCP)area,forming a large flat stanene.When Sn coverage is greater than1ML,Sn clusters appear on top of monolayer stanene,which follow the herring-bone structure.(2)The electronic states of honeycomb stanene are characterized by scanning tunneling spectroscopy(STS),which shows a metallic U-shaped STS curve.The shoulder feature that locates at the bias range of(-1.0?-0.8e V)seems to be the edge states of stanene,indicating that the honeycomb stanene that we grow on Au(111)at 77K contains pure Sn atoms,not Sn-Au alloy.When the sample is annealed to temperature higher than 150K,the edge of stanene islands begin to change.With the increasing of the annealing temperature,Sn atoms start to incorporate into Au(111)lattice and finally form(?3×?3)Au₂Sn alloy at room temperature.(3)Using a hot tungsten wire to crack hydrogen molecules into hydrogen atoms and realize the hydrogenation of stanene.STM measurements show that the adsorption of hydrogen atoms changes the overall topography of stanene,producing an effect similar to“etching”stanene.With the increase of H exposure,some small-size“black hole”defects on stanene island gradually evolves into large-size black holes.Additionally,the adsorption of hydrogen atoms leads to the formation of ordered(2×2)structure on Au(111)substrate.The origin of this(2×2)structure is still unknown,which is supposed to relate to the interaction between H atoms and Au(111)surface.(4)I also study the thermol stability of hydrogenated stanene by annealing the hydrogenated sample to different temperatures.When anneal to 150K,the hydrogenated stanene become pure stanene and the(2×2)structure remain unchanged.When the annealing temperature is increased to 200K,Sn-Au alloy appears.Finally,at room temperature(about293K),the whole surface is covered by(?3×?3)Au₂Sn structure.