Investigations On The Structures And Properties Of Several Novel Two-dimensional Atomic Crystals By First-Principles Calculations

Two-dimensional atomic crystal materials have developed rapidly in recent years,in which the representative materials of single component are graphene,silicene and germanene,and the typical materials of two components are transition metal disulfide compound（TMDC）,hexagonal boron nitride（h-BN）and so on.Many two-dimensional materials have made great breakthroughs in recent years.However,many aspects in this field are still in the infant stage.At present,there are still lots of two-dimensional materials that have not been systematically studied.In this dissertation,based on first-principles calculations combined with various measurement methods,several two-dimensional atomic crystal materials,B₂H₂,Pt-Te binary phased structures,VSe₂ and1H-TiTe₂ have been systematically investigated.The main research progresses are as follows:1.The atomic structures,electronic band structures,phonon dispersion relationship and strain modification on band of B₂H₂ and its nanoribbons have been systematically explored.The band of intrinsic B₂H₂ exhibits typical metallic properties.When uniaxial strain is applied along Z-s（zigzag-edged direction with mirror symmetry）or A-as（armchair-edged direction without mirror symmetry）direction,a band gap is opened,showing semiconductor characteristics.Its metallic feature maintained when uniaxial strain applied along A-s（armchair-edged direction with mirror symmetry）or Z-as（zigzag-edged direction without mirror symmetry）direction.The metal-semiconductor phase transition can also be realized by applying biaxial strain.According to the boundary and symmetry,B₂H₂ nanoribbons can be classified four items:ANR-s（armchair-edged nanoribbon with mirror symmetry）,ZNR-s（zigzag-edged nanoribbon with mirror symmetry）,ANR-as（armchair-edged nanoribbon without mirror symmetry）and ZNR-as（zigzag-edged nanoribbon without mirror symmetry）.Among them,ANR-s,ANR-as and ZNR-as are semiconductor,ZNR-s is metallic.The ribbon width and strain can regulate the band structure of B₂H₂nanoribbons.Nanoribbons with semiconductor properties（ANR-s,ANR-as and ZNR-as）can be converted to metallic properties by increasing their ribbon widths.Metallic ZNR-s has no response to the change of width.The band of metallic ANR-as and ZNR-s appeared a gap after applying uniaxial strain along the direction of nanoribbon.For example,in ANR-as with a width of 11.0?,a gap of 1.10 eV is opened under 16%strain.Strained ANR-as presents a characteristic of semiconductor.There is no imaginary frequency in the phonon spectrum,indicating the stability of strained ribbon.The systematic investigation on band structure modification of two-dimensional B₂H₂and its nanoribbons by first-principles calculations is helpful to fully understand the electronic properties of B₂H₂,especially its excellent mechanical properties.Meanwhile,the band structure modification of B₂H₂ nanoribbons provides a theoretical basis for the application of B₂H₂ nanoribbon structure in flexible electronic devices.2.Two stable binary new materials containing vacancy Pt-Te were screened out:Pt₄Te₆ and Pt₄Te₇.Both monolayer structures exist in thermodynamic metastable states.Through the calculation of phonon spectrum,it is found that both structures have no imaginary frequency,which proves that their structures are stable.After hydrogen atoms adsorbed on the surface of Pt₄Te₆ and Pt₄Te₇,the calculation of Gibbs free energy showed that both materials presented excellent catalytic activity of hydrogen evolution reaction（HER）.The free energy of Pt₄Te₆ and Pt₄Te₇ were both 0.06 eV when the coverage of hydrogen atoms was low,and the catalytic performance was better than that of commercial Pt surface（ΔG=-0.09 eV）.The free energy of Pt₄Te₇ changed to-0.07 eV when the coverage of hydrogen atoms was high,its catalytic activity remained better than that of Pt surface.Besides,it was found that vacancy concentration had a significant effect on the catalytic performance of complete 1T-PtTe₂.When the vacancy concentration reached 6.25%,the free energy of the system was 0.05,reaching the optimal catalytic activity of HER.The research on the screening of Pt-Te binary structured materials and the calculation of their potential applications on HER catalysis can not only provide useful theoretical reference and guidance for the synthesis of Pt-Te binary layered materials,but also provide a new strategy for the catalytic application of TMDC materials in HER.3.For the one-dimensional stripe phenomenon of VSe₂ discovered by the cooperator,we proposed that the stripe is caused by Se atom vacancies in VSe₂,and its chemical composition is V₄Se₇.And the rationality of this model has been confirmed by AFM,STM and other measurement methods.By increasing the beam flow of Se atoms and adjusting the annealing temperature,the experimental collaborators found that the one-dimensional stripe was transformed back to the complete two-dimensional structure,which further confirmed the rationality of the proposed Se atom vacancy defect.Using natural one-dimensional（1D）stripes,a novel method for the realization of platinum monatomic catalysis was proposed.The calculation shows that the platinum atoms tend to be adsorbed in the line vacancy of the 1D template as a single atom.Subsequently,the statistics of platinum atoms deposition on the 1D template showed that nearly 60%of the platinum atoms were adsorbed as single atom,and about 30%were adsorbed as dimers.A very small amount of platinum atoms was adsorbed as trimers and tetramers,and all forms of platinum atoms were adsorbed in vacancies.These experimental statistics verify the theoretical prediction of platinum dispersion and adsorption on the template.Further free energy calculations predicted the excellent catalytic performance of the system.The 1D template system can greatly improve the catalytic efficiency of the heavy metal platinum as most of the platinum is adsorbed in a form of monoatomic dispersion.Platinum monatomic catalytic design based on VSe₂1D template also provides a new idea for the application of other 2D TMDC materials in HER catalytic field.4.The possibility of 1H phase synthesis of TiTe₂ was explored.Ab initio calculations found that the 1T-TiTe₂ is metallic,while the 1H phase is semiconducting,and the 1T and 1H phases show completely different STM simulation morphologies on the Au substrate.The electron states of several adjacent Te atoms in the surface layer of 1H phase merge together,resulting in a new period and a large bright spot in the STM simulation image.LEED structure characterization and high resolution STM measurement provide experimental evidence for 1H-TiTe₂ model.The theoretical prediction and measurement verification of 1H-TiTe₂ have enriched and expanded the2D material library and are expected to be applied in future devices with new physical properties.