Effects of chiral indices on the atomic arrangements and electronic properties of Si double-walled nanotubes (6,min)@(9,mout) (min = 0 to 6, mout = 0 to 9) by SCC-DFTB calculations

The geometric structural optimization and electronic properties of double-walled silicon nanotubes (DWSiNTs) (6, min)@(9, mout) (min= 0 to 6, mout= 0 to 9) are studied in terms of the self-consistent charge density functional tight binding (SCC-DFTB) method. In particular, a cross-section configuration transformation of the DWSiNTs 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 sp2 hybridization, 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 DWSiNTs. The DWSiNTs (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. These theoretical studies provide references for the preparation, and future application, of DWSiNTs.