Main Group Monoelemental Two-dimensional Materials:MBE Growth And STM Study

Two dimensional materials have nowadays been the research focus in condensed matter physics and materials science.Monoelemental two dimensional materials(ME2DMs)comprising group IVA and VIA elements are potential candidates for succeeding and/or complementing silicon-based semiconductors owing to their moderated bandgap,carrier mobility,bandgap tunability,outstanding thin-body electrostatic control and one dimensional topologically protected edge states.However,the production of high-quality 2D materials with outstanding electronic properties are still major challenges.It is of great significance to study the growth mechanism and electronic properties from atomic scale.In this thesis,by using molecular beam epitaxy(MBE)and scanning tunneling microscopy and spectroscopy(STM/STS),we have fabricated stanene,phosphorene,antimonene,and bismuthene via introduction of buffer layers.The roles of two types of buffer layers,alloy and oxide,in modulating the interfacial interaction are explored from the perspective of experiment and theory.These studies have far-reaching theoretical significance for expanding applications of 2D materials in the field of electronic devices.The contents are described as follows:1.Two-dimensional structures evolution of group IVA and VA elements on Au(111)The growth behavior of Sn,P,Sb and Bi on Au(111)surface are investigated through tuning coverage and temperature.Sn forms((?))Au2Sn surface alloy.The existence of herringbone reconstruction of Au(111)substrate indicates the weak interfacial interaction between Sn and substrate.P forms the typical(4×4)Au-BlueP honeycomb network on Au(111)regardless of the coverage,which is the evidence of self-limiting growth mode.Sb assembles into multiple intermediate phases comprising dimer pairs and coin-shaped aggregates depending critically on the coverage.Increasing the coverage and annealing temperature converts these dimer pairs to(100)face of AuSb2 surface alloy.Bi form(5×3)periodic stripes on Au(111)at monolayer thickness,without alloying process.In the process of growth,electronic configuration affects the bonding energy between elements and substrate.The differences of relative atomic mass and atomic radius lead to strain and surface fluctuation at interfaces.As a result,these factors induce the diversity of structures.These studies reveal that interfacial interaction is the key of affecting surface 2D growth and structural characteristics.2.Effect of metal surface alloy buffer layer on structures/electronic properties of 2D materials.Noble metal substrates tend to form surface alloy with many elements.We fabricated stable((?))Au2Sn alloy and found that following Sn forms different growth intermediates by changing substrate temperatures.These intermediates undergo a series of phase transitions and finally transform to β phase stanene with 0.51 nm lattice constant that matches well with that of surface alloy at 530 K.In another work,we obtained stable,metallic(3×(?))Cu2Te2 alloy on room temperature Cu2Sb/Cu(111)substrate.It is different from previous studies that Cu2Te2 alloy only exists at low substrate temperature.We fabricated high-quality β phase antimonene on Sb alloyed Cu(111)and Cu(110)substrates.Owing to the threefold and twofold symmetry with different lattice constants of two substrate,the antimonene both exhibits hexagonal lattice but with difference in lattice constants(0.45 nm/0.39 nm).Cu2SbCu(111)and CuSb-Cu(110)introduce 7.5%tensile strain and 6.8%compressive strain into antimonene lattice,respectively.Combining STS and theoretical calculations,both the bandgap and gap type of antimonene are modulated.Surface alloy buffer layers in three works all prevent the alloying between deposited elements and substrates.Au2Sn induces the change of band structures and Rashba splitting at interfaces due to the orbital hybridization and charge transfer.Cu2Sb increases the energy barrier of Cu3Te2 formation,which is favorable to form Cu2Te2 phase.Cu-Sb alloy realizes the strain engineering towards electronic properties of antimonene.In this part we successfully modulate the interfacial interaction between elements and substrates through introduction of alloy buffer layers,and realize the control of structures and semiconducting properties of 2D materials.3.Effect of copper oxide buffer layer on the structures/electronic properties of 2D materials.The diverse structures and fruitful electronic properties,like high dielectric constant,wide band gap and so on,in oxide thin film endow them with feasibility in tunning the interfacial interaction.We annealed Cu(111)in oxygen and obtained honeycomb lattice copper oxide film,on which we successfully synthesized high-quality phosphorene,antimonene and bismuthene.P forms metallic β phase phosphorene with 0.42 nm lattice constant on Cu3O2/Cu(111)after annealing.Sb undergoes a series of phase transitions and forms highquality,large-scale β phase antimonene with 0.435 nm lattice constant.Tensile strain in antimonene transforms it to a direct bandgap semiconductor with 0.37 eV gap size.This transformation is attributed to the different deformation energy of conduction bands.After deposition of Bi on Cu3O2/Cu(111),a phase transition from black-phosphorus-like bismuthene to blue-phosphorus-like bismuthene is characterized by high-resolution STM.Employing the phase transition,we also demonstrate the construction of an atomically sharp in-plane homojunction along the phase boundary.DFT calculations reveal a one-dimensional edge state along the homojunction as a result of strong spin-orbit coupling.Copper oxide buffer layer prevents the alloying between elements and substrate in the fabrication of antimonene and bismuthene.While in the growth of phosphorene,buffer layer weakens the interfacial interaction between P and Cu(111),changing the cluster-like growth mode.Copper oxide also introduces strain into the lattice of phosphorene,antimonene and bismuthene,resulting in metallic property,direct bandgap semiconductor and one dimensional topologically protected edge state in homojunction,respectively.Change of defect density in copper oxide proves that the interface can affect growth mode.The wide bandgap(～1.2 eV)in copper oxide thin film reduces the charge transfer at interface,which is helpful to characterize intrinsic electronic properties.These works demonstrate that dielectric oxide materials are promising substrates for the controllable growth and modulation of 2D nanostructures,which can extend to technologically important dielectric layer surfaces,such as HfO2,Al2O3.