First-principles Prediction Of The Structural And Electronic Properties Of Potential Two-dimensional Silicon Allotropes

Graphene attracted people’s attention to the field of two-dimensional(2D)filed,accelerating the research and discovery of other 2D materials.By replacing carbon(C)atoms in graphene with Group IV elements(Si,Ge,Sn),people have proposed and prepared graphene-like silicene,germylene and stanylene.Other 2D simple substance(for example borene,phosphorene and antimony)and multi-component compounds(such as h-BN and)had also been proposed and synthesized.Element silicon is important to human and it is widely distributed in nature.The crystalline silicon has been selected as the material basis for the semiconductor industry in views of its moderate band gap,good stability,environmental friendliness and low-price.2D silicene has similar band structure as graphene,namely the the Dirac-cone feature formed by band-crossing near Fermi-level.It is believed as the new material candidate for future nano-electronic devices.However,due to its poor stability,silicene needs to be prepared on substrate,which will destroy its electronic properties for real applications.In recent years,other more stable 2D silicon structures have been proposed or synthesized.Here,we report our discoveries of some low-energy 2D silicon allotropes by RG~2code,as well as their stabilities and electronic properties calculated from DFT-based first-principles method.(1)Mirror-symmetry protected node-loop half-metal in 2D Si-Cmma:The RG~2 is employed to systematically search for 2D four-coordinate silicon thin films with a thickness of less than 4 Angstrom and the first-principle calculation software VASP is used to investigate the structures,stabilities and electronic properties of the most stable two candidates Si-Cmma and Si-Pmma.We found that Si-Cmma and Si-Pmma are both dynamically and mechanically stable.Their energies are lower than those of the experimentally synthesized silicene and most of the theoretically predicted 2D silicon allotropes.Si-Pmma is predicted as a common metal and Si-Cmma possess mirror-symmetry protected Dirac nodal-loop.Further research results show that the insulating h-BN can be considered as a growth substrate for Si-Cmma due to the lattice matching.Such a insulating substrate has negligible effect on the electronic properties of the semi-metallic nodal-loop.(2)Low-energy 2D dumbbell silicon allotropes Si-Pbam and Si-P-62m:According to the adsorption-rules of ad-atoms in the previous models of dumbbell silicene,we use RG~2 to systematically search for possible adsorption patterns in 2D 3-coordinated silicon allotropes and we find two new full-adsorption configurations Si-Pbam and Si-P-62m.The structures,stabilities and electronic properties of these two new 2D dumbbell silicon allotropes were further studied by first-principles methods.The calculation results show that they are more energetically stable than most of the previously proposed dumbbell silicon allotropes.The energy band structures shows that both Si-Pbam and Si-P-62m are direct band gap semiconductors,with gaps of 0.96 e V and 0.65 e V,respectively.They are good candidates for absorption materials for solar cell application due to their optical properties.