Study On The Structure And Electronic Properties Of One-dimensional Silicon Nanomaterials

Silicene nanomaterial is a kind of semiconductor material with excellent performance,which has been widely used in the preparation of new devices,and has attracted much attention from researchers.In order to better understand the structure and properties of low-dimensional silicene nanomaterials,self-consistent charge density functional tight-binding（SCC-DFTB）method is proposed in this paper.The geometric arrangement of atoms,bond length and bond angle,system stability,electrical properties and charge distribution of one-dimensional silicene nanoribbons and silicene nanotubes were simulated.The specific work were divided into the following three parts:Firstly,the results show that the degree of warpage of the ZSi NRs is significantly changed by hydrogenation,while the bond length and bond angle are less affected.The edge hydrogenation reduces the Si-dangling bonds in the nanoribbons,which increased the binding energy of nanoribbon and enhanced system stability.In the absence of an electric field,the unhydrogenated nanoribbons with different period widths all exhibit semiconducting properties,and the hydrogenated nanoribbons exhibit metallic or semi-metallic properties.Under the vertical external electric field in the z-direction,the stability and energy gap of the unhydrogenated nanoribbons are sensitive to the change of strength,and the oscillation fluctuation is large.With the increase of strength,the atoms in the outermost four layers of the nanoribbons show obvious charge gain and loss.When the vertical electric field is applied to the hydrogenated nanoribbons,the energy gap changes are related to the period width.Their electrical properties realized the conversion between the semi-metal properties and the direct band gap semiconductor properties.Charge transfer occurs between the adjacent two layers of atoms.Under the same electric field strength,the amount of charge transfer increases from left to right along the direction of the width of the nanoribbon;at the same time,with the increase of the electric field strength,the amount of charge transfer also presents an increasing trend.The parallel external electric field along the x-direction of the nanoribbon has a greater influence on the electrified properties of the nanoribbon than the vertical electric field.The nanoribbon can be regulated by applying weaker electric field intensity in the x-direction.Then,the effects of SW defects and applied vertical electric fields of different strengths on the geometric structure and electrical properties of zigzag silicene nanoribbons with different period widths（widths=2,3,4）were studied.The presence of SW defects leads to the reconstruction of silicon atoms,the interaction between atoms is enhanced,the bond lengths and bond angles at the location of the defect are changed,and the influence of SW-II type defects on the geometric structure of nanoribbons is greater than that of SW-I type defects.The change trend of the binding energy of perfect structure nanoribbon is related to its period width,regardless of the types and number of defects,while the size of the energy gap changes with the number and position of defects,and the transition between semi-metallic and semiconductor properties occurs.Meanwhile,at the same defect concentration,the effect of SW-I defects on the band structure is more obvious than that of SW-II defects.The electric field breaks the mirror symmetry at the SW defect,the electronegativity of some atoms changes with the increase of the electric field strength.The defect types does not affect the charge transfer rule of the nanoribbons,but will change the amount of charge transfer of the nanoribbons.Finally,the geometric structural optimization and electronic properties of double-walled silicon nanotubes（DWSi NTs）(6,m_in)@(9,m_out)(m_in=0 to 6,m_out=0 to 9)are studied.In particular,a cross-section configuration transformation of the DWSi NTs affected by the different inner and outer wall chiral indices is explored.It seems that six different shapes including circular-like,ellipse-like,trilateral-like,quadrilateral-like,pentagon-like,and hexagon-like shapes appear.The interactions between the atoms in the inner and outer walls possess most of the atoms in sp³ hybridization,followed in sp²hybridization,with very few in high-order hypervalent states.The chiral indices of the inner and outer walls as well as the cross-section configuration of the tube have obvious effects on the stability of the DWSi NTs.The DWSi NTs（6,5）@（9,7）,（6,6）@（9,8）,and（6,6）@（9,9）show metal characteristic:others exhibit semiconductor properties with a narrow band gap.Furthermore,the possibility of tuning the band structure by changing the chiral indices is demonstrated to help for the performance needs of different devices,inducing a metal-semiconductor transition and direct-indirect band gap transition.