Stable nanotube construction conditions and electronic properties of possible Si double-walled nanotubes (n_in,m_in)@(6,m_out) (n_in=3, 4) by SCC-DFTB calculations

The geometric structural optimization and electronic properties of double-walled silicon nanotubes (DWSiNTs) (n(in),m(in))@(6,mout) (n(in) = 3, 4, m(in) = 0 to nin, mout = 0 to 6) are studied in terms of the self-consistent charge density functional tight binding (SCC-DFTB) method. A suitable spacing range of initial model that can form a stable DWSiNT is explored. Most of the nanotubes present circular-like shapes of their cross-sectional configu-rations. When a DWSiNT has the same chiral angle, 0 or 30 in degree, in its inner and outer walls, a regular periodic structure develops. The bond length, inner and outer wall diameters, and wall spacing are also signif-icantly affected by changes in the chiral index. The stability of the nanotube is affected by three factors: the configuration, the diameter, and the regularity of the atomic arrangements. The electrons in the inner wall are transferred to the outer wall, and the electron distribution of the atoms in the outer wall tube has the regional distribution of the interval between the electron-obtaining and the electron-losing regions. Among the seven tubes with fully commensurate inner and outer walls, DWSiNTs (3,2)@(6,4) exhibit semi-metallic characteristics; the others exhibit semiconductor-like properties with a narrow band gaps. The band gap of DWSiNTs is changed by tuning the chirality index, resulting in the transition between direct and indirect band gaps of nanotubes, which can be adjusted to match the performance needs of different devices, inducing a metal-semiconductor transition and direct-indirect band gap transition. These theoretical studies provide references for the prepara-tion, and future application, of DWSiNTs.