Scanning Tunneling Microscope Study Of Hydrogenated Two-dimensional Material And Two-dimensional Topological Insulators

The emergence of graphene has opened the door towards a new world of two-dimensional(2D)materials.Graphene has multiple excellent properties,such as quasi-particles with zero effective mass,which can faciliate non-dissipative transport.This property can greatly increase its operation speed if it can be applied to integrated circuits.However,due to the existence of nearly zero energy gap,graphene cannot be directly applied in semiconductor devices.Therefore,looking for two-dimensional semiconductor materials with properties similar to graphene has great potential application value.The excellent electronic features of graphene are largely due to its special honeycomb configuration.Therefore,it is an important direction to study the properties of other two-dimensional semiconductors with honeycomb structures.At the same time,in addition to the emergence of graphene triggering the upsurge of 2D materials,people also predicted topological insulators(TIs)from graphene and experimentally discovered a new material system.Among them,the two-dimensional topological insulator or quantum spin Hall insulator has a conductive channel that is topologically protected from backscattering,and two dissipationless electron streams with opposite spins can be formed at the edges.If TIs can be applied to semiconductor devices,it will be possible to solve the overheating problem of the device.And due to the introduction of spin resolution,it is possible to carry more information,which will further improve the performance of the device.Based on the above discussion,in this thesis,we mainly use molecular beam epitaxy(MBE)and scanning tunneling microscope(STM)technology as the key methods to study the hydrogenation of the epitaxially grown honeycomb semiconductor Sn₂Bi on a silicon-based substrate,the structural and electronic characterization of a new type of 2D TIs Ta₂Pd₃Te₅ and the growth and structure characterization of two-dimensional transition metal oxide Mo O₃.The main contents of this thesis are as followings:1.The hydrogenation study of 2D materials is an important means to control the energy band structure and other physical properties.The 2D honeycomb semiconductor Sn₂Bi with nearly free 2D hole dispersion bands grown on the surface of Si(111)may have broad application prospects in device applications.There are a lot of theory papers investigating hydrogenating or functionalizing of Sn₂Bi to verify its model and tune its sturctral and electronic properties,but no relevant progresses has been maded in experiments.Herein,we used Scanning Tunneling Microscope and Scanning Tunneling Spectroscopy(STS)combined with density functional theory(DFT)calculations to study hydrogenation of Sn₂Bi grown by MBE on Si(111)with different hydrogen dose.The STM study shows that a small amount of hydrogen adsorption is random on single sites.Combining the modeling and DFT calculation,it is found that hydrogen adsorption site is on the bridge between the central tin atom and its neighboring tin atoms,which appears in the STM image as it has the characteristic of“butterfly”in three directions.After saturated hydrogen adsorption,the adsorption sites are the same as the point adsorption.The STM image shows groove-like features along the three different directions,indicating that there is a certain interaction between two adjacent hydrogen adsorption sites and through forming a trench-like structure to release stress.After saturated hydrogenation,the energy gap size of the system is significantly increased,which is consistent with the result of DFT calculation.In the energy band structure,the symmetry of the central tin atom is broken after the bonding between this atom and hydrogen atom,resulting in disappearance of the flat band at the bottom of the conduction band with respect to the original Sn₂Bi energy band structure,which is mutually confirmed with the structure model.Moreover,the adsorbed hydrogen atoms can be completely desorbed at a high temperature to restore a clean surface,indicating the reversibility of the hydrogenation process.2.2D TIs have broad application prospects in the field of spintronics devices.Most of materials with quantum spin Hall effect that exist at present are single-layer2D TIs,which have great practical applications limit.The single-layer Ta₂Pd₃Te₅material is predicted to be a 2D TI,and because the interlayer interaction is very weak,the single crystal surface may also exhibit quantum spin Hall effect.We employed XRD and STM to characterize the structure of the cleaved Ta₂Pd₃Te₅ single crystal,which is consistent with the theoretically predicted structure,and the single crystal quality is very high.Applying a voltage pulse between the needle tip and the sample can cause the surface of the sample to break,indicating a very weak interlayer force.Transport,angular resolved photoelectron spectroscopy(ARPES)and STS measurement results all indicated that the bulk Ta₂Pd₃Te₅ single crystal is a narrow band gap semiconductor.DFT results show that Ta₂Pd₃Te₅ has an inverted energy band near the Fermi surface,which are topologically non-trivial.The STS measurements of the step edges found that the local density of states at the step edges has a significant enhancement,and it exists under different step types,cut-off atoms and chemical environments,which provides strong evidence for the existence of topological edge states.3.The MBE method was used to grow 2D molybdenum trioxide on different metal substrates and graphene/Si C.Using STM to characterize the structure of molybdenum trioxide films grown on Cu(111)substrates,we have observed that in the annealing steps performed after in-situ growth,different annealing temperatures can trigger varied structures.X-ray photoelectron spectroscopy analysis of two of the high-quality ordered structures showed that molybdenum trioxide may be reduced after high temperature annealing to form molybdenum oxide compounds of different valences and may oxidize the substrate surface to cuprous oxide.Structural model analysis shows that there may be two possible models,one is an ordered structure composed of a macromolecular model of molybdenum oxide,and the other is a model of cuprous oxide.Both models are reasonable.