Novel Two-dimensional Material Design And Simulation Based On First Principles

With the development in computational methods and computer performance,designing new materials with impressive properties via theoretical calculations has become an important way to promote the advancement of materials science.The nature of material is dominated by its structural characteristics,which specifically consist of the dimensions and the bonding form between atoms.Therefore,the prediction of low-dimensional materials with peculiar bonding may achieve excellent properties and applications.In this thesis,on the basis of density functional theory（DFT）computations,we report the design of new two-dimensional（2D）layered materials with novel geometric configuration,and explore their electronic and mechanical poperties,and potential applications in electrochemical oxygen reduction reaction（ORR）.Firstly,inspired by the rule-breaking planar pentacoordinate carbon（ppC）-containing Be9C24-,we priedicted an intriguing 2D inorganic material,namely Be5C2 monolayer.In Be5C2 monolayer,each carbon atom binds with five beryllium atoms in almost the same plane,forming a quasi-ppC moiety.Be5C2 monolayer appears to have good stability as revealed by its moderate cohesive energy,positive phonon modes and good resistance to high temperature.It is the lowest-energy structure with the Be5C2 stoichiometry in two-dimensional space and therefore holds some promise to be realized experimentally.Be5C2 monolayer is a gapless semiconductor with a Dirac-like point in the band structure and also has an unusual negative Poisson’s ratio.If synthesized,BesC2 monolayer may find applications in electronics and mechanics.The high-level calculations shown that one Si atom could bond with four Ca atoms and one Si atom to form a Ca4Si22-molecule containing planar pentacoordinate silicon（ppSi）.Based on the Ca4Si2 building block,we then extended the ppSi into the periodic system:CaSi monolayer.The analyses of phonon spectrum and molecular dynamic simulations indicated that CaSi monolayer is a thermodynamically and kinetically stable structure.Especially,a global minimum search revealed that the ppSi-containing CaSi monolayer is the lowest-energy structure in 2D space,indicating its great promise for experimental realization.CaSi monolayer is a natural semiconductor with an indirect band gap of 0.5 eV,and it has rather strong optical absorption in the visible region of solar spectrum.More interestingly,the unique atomic configuration endows CaSi monolayer an unusual negative Poisson’s ratio.The rule-breaking geometric structure together with its exceptional properties makes CaSi monolayer a quite promising candidate for electronics,optoelectronics,and mechanics applications.Then we investigated a new 2D transition metal disulfide（TMD）featuring unique structural properties,namely the PdS2 monolayer.Distinguished from other 2D TMDs whichadopt the ordinary 2H or 1T configuration,each Pd atom of PdS2 monolayer binds to four S atoms in the same plane,and two neighboring S atoms can form a covalent S-S bond.The hybrid HSE06 computations demonstrated that the PdS2 monolayer is semiconducting with an indirect band gap of 1.60 eV,which can be effectively reduced by employing a uniaxial or biaxial tensile strain.Especially,PdS2 has rather large hole and electron mobilities,suggesting that the PdS2 monolayer is rather promising for future electronics and optoelectronics.Moreover,our test computations find that PdSe₂ monolayer can take the same configuration as that of PdS2 monolayer.Finally,we systematically explored the potential of utilizing the experimentally available 2D Fe-phthalocyanine（Fe-Pc）monolayer as a catalyst of ORR.In 2D Fe-Pc monolayer,the Fe atoms that only bond with N atoms are distributed separately and regularly,according with the feature of single-atom-catalyst.The computations revealed that O2 molecules can be sufficiently activated on the surface of the Fe-Pc monolayer,and the subsequent ORR steps prefer to proceed on the Fe-Pc monolayer through a more efficient 4e pathway with a considerable limiting potential of 0.68 V.Especially,the Fe-Pc monolayer is more stable than the Fe-Pc molecule in acidic medium,and can present good catalytic performance for ORR on the addition of axial ligands.Therefore,the Fe-Pc monolayer is quite a promising single-atom-catalyst with high efficiency for ORR in fuel cells.