First-principles Investigations Of Structures And Physical Properties Of Several Two-dimensional Atomic Crystal Materials

The successful exfoliation of graphene and the discovery of its novel physical properties stimulate a large number of researches in two-dimensional atomic crystal materials.Two-dimensional atomic crystal materials include metals,semi-metals,semiconductors,and insulators.The band gap of two-dimensional semiconductor ranges from ultraviolet to infrared.Therefore,two-dimensional atomic crystal materials have potential applications in nanoelectronics,optoelectronics,and new ultra-thin flexible devices.Although a large number of two-dimensional atomic crystal materials have been predicted computationally or successfully prepared in experiments,the two-dimensional atomic crystal materials deserve further exploration to obtain high-performance materials.Based on the first-principles calculation method,this paper predicts several two-dimensional atomic crystal materials,studies their structure,physical properties and their correlations,expands the two-dimensional atomic crystal material library,and provides candidate materials for high-performance electronic devices.The main research results are summarized as follows:1.A new type of hydrogenated graphene(HPHG)was constructed,which is a partially hydrogenated graphene with honeycomb hydrogenated regions formed after periodic hydrogenation of graphene.HPHG is an antiferromagnetic semiconductor whose antiferromagnetic state is very similar to that of zigzag graphene nanoribbons.The projected density of states(PDOS)shows that its magnetism originates from the p_zorbital splitting and partial occupancy caused by the adsorption of H atoms.The graphene nanoflake(GNF)array with antiferromagnetic boundary states embedded in the HPHG can be used as a template for selective adsorption of Au atoms to promote further hydrogenation at the GNF boundary.Then the growth process of HPHG on Ru(0001)substrate was studied.Theoretical calculations found that the process consists of two steps.In the first step,H atoms pass through the graphene from the atop region of the moirépattern formed by the graphene and the Ru substrate,and then occupy all the atop,bridge,and fcc regions to form a honeycomb-shaped H atom intercalation.In the second step,graphene is hydrogenated.H atoms gradually hydrogenate the graphene at the regions with H atom intercalation,and finally hydrogenate all the atop,bridge and fcc regions to form the HPHG on Ru(0001).In the experiment,the partially hydrogenated graphene was obtained by sputtering H atom plasma to Gr/Ru.The scanning tunnel microscope(STM)experimental image is completely consistent with the STM simulation image of HPHG,which proves that the HPHG has been successfully fabricated in experiment.This work provides a theoretical basis for the further application of HPHG.2.A new kind of two-dimensional atomic crystal material,single layer IrSX'(X'=F,Cl,Br,I),is predicted.Theoretical calculations show that this type of material has special physical properties,such as anisotropic high mobility.First,45 semiconductor materials with band gaps larger than 0.1 e V were selected from more than 340monolayer MXX'(X=S,Se,Te;X'=F,Cl,Br,I)compounds belonging to the Pmmn(C_2v)space group.IrSX'(X'=F,Cl,Br,I)is a family of semiconductor materials with smaller effective mass and lower convex hull energy.Phonon spectra and ab initio molecular dynamics(AIMD)simulations at room temperature show that the monolayer IrSX'is dynamically and thermally stable.The single layer IrSF is a direct band gap semiconductor with a band gap of 0.38 e V,while the single layer IrSCl,IrSBr and IrSI are indirect band gap semiconductors with band gaps of 0.30 e V,0.30 e V and 0.20 e V,respectively.By analyzing the energy band structure,it is found that the effective masses of carrieres at the top of the valence band and the bottom of the conduction band are small,which means that such materials have high mobility.Then the carrier mobility of IrSX'is calculated based on the deformation potential theory.It is found that the mobility of the monolayer IrSF is the largest,and the hole mobility along the y axis is as high as 2.1×10⁴ cm²V^-1s^-1,which is higher than the mobility of the monolayer black phosphorus calculated by using the same method and indicates that the monolayer IrSX'have the potential to be applied to high-performance electronic devices.Due to that IrSF is also a ferroelastic material,so we can precisely control the direction of electron migration by combining the ferroelasticity and anisotropic mobility.The hole mobility of IrSF along the y axis calculated by using HSE06 functional is also on the order of10⁴ cm²V^-1s^-1.IrSF also has anisotropic absorbance for visible light.It is calculated that the light absorbance along the y axis is about 34%,and the light absorbance along the x axis is about 2%.In addition to the above properties,the energy ternary phase diagram shows that IrSF and the other three materials have high thermodynamic stability and are expected to be synthesized experimentally.This work provides a theoretical basis for monolayer IrSF and other materials used in semiconductor electronic devices.3.A two-dimensional atomic crystal material,monolayer CrN,and its electrical,magnetic and mechanical properties are studied.In this work,three different phases of CrN,the square phase,the planar hexagonal phase and the sandwich hexagonal phase,are considered.Formation energy calculation indicates that the square-phase CrN has a relatively small formation energy,which is the most stable phase and is most likely to be synthesized by molecular beam epitaxy.Therefore,the physical properties of the square phase are studied.The square phase has D_4h symmetry and its lattice constant is about 4.02(?).The square-phase CrN is a ferromagnetic semi-metallic material.According to the Heisenberg model,its magnetic transition temperature is estimated to exceed 1000 K.By calculating PDOS,it is found that its ferromagnetism and semi-metallic properties are derived from the splitting of the d orbital of Cr atoms.By calculating the different magnetic interactions,it is found that its magnetism is mainly contributed by the super-exchange interaction.In addition,the square-phase CrN has a low Young's modulus and a large critical strain,and its ferromagnetic semi-metallic properties are stable under different strains.Therefore,the square-phase CrN has application prospect in the flexible biocompatible electronic devices.This work provides theoretical support for using monolayer CrN as a flexible electronic device.4.The structure and physical properties of the cleavage surface of CaKFe₄As₄were studied in combination with experiments.The experimental collaborators used STM to characterize the cleavage surface of the low-temperature cleavaged CaKFe₄As₄,and encountered two problems.First,it is impossible to distinguish whether the clusters on the cleavage surface are Ca atoms or K atoms.Second,when scan the cleavage surface of CaKFe₄As₄ with variable bias voltage,the As atomic layer evolves from a?2×?2 structure to a 1×1 structure.The mechanism of the structural transformation caused by variable bias voltage is unclear.To solve the experimental problems,DFT calculations are carried out.The calculations of cleavage energy show that it pays less energy to cleavage CaKFe₄As₄ from the K-As interface than from the Ca-As interface.So the atomic clusters on the surface should be K atoms.Then the structure of the As surface and the PDOS of As atoms on the surface are calculated.It is found that the reason for the structural transformation is as follows.The two As atoms on the surface are not in the same plane and have a tiny difference in height.The difference in height causes the PDOS of the two As atoms to be quite different in certain energy ranges.Therefore,under some bias voltages,the STM image only shows the As atoms of one sublattice with a larger electronic density of states,that is,the?2×?2 structure.At other bias voltages,due to the As atoms of the two sublattices have similar electron density of states,all As atoms can be shown and form a 1×1 structure.This work provides important theoretical support for the following research of the surface of CaKFe₄As₄.