Existence and stability of co-axial and meshed double-walled armchair silicon nanotubes

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A systematic study of armchair double-walled Si nanotubes (DWNT) (n, n)@(m, m) (3≤n≤6; 7≤m≤12) using the finite cluster approximation is presented. The geometries of the tubes have been spin optimized with an all electron 3–21G⁎ basis set and the B3LYP functional. Analysis of the electronic structure properties of these tubes has also been performed with a larger basis set. The study indicates that the stabilities of the Si nanotubes are of the same order as those of single-walled Si nanotubes. It should be possible to experimentally synthesis both single-walled and double-walled Si nanotubes. The binding energy per atom or the cohesive energy of the nanotubes depends not only on the number of atoms but also on the coupling of the constituent single-walled nanotubes. Nanotubes with small interlayer separations, called meshed tubes, do not hold the coaxial cylindrical structure after optimization. The SiNTs (n, n)@(n+3, n+3) are found to have large formation energies and binding energies per atom. For example, (3, 3)@(6, 6), (4, 4)@(7, 7), (5, 5)@(8, 8), and (6, 6)@(9, 9) all have large binding energies per atom, around 3.7eV/atom. All Si nanotubes are found to be semiconductors. However, the band gap, in general, is observed to decrease from single walled nanotubes to double walled nanotubes.