On the existence and stability of double-walled armchair silicon carbide nanotubes

A systematic study of type 1 armchair double-walled SiC nanotubes (DWNTs) (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 using the hybrid functional B3LYP (Becke's three-parameter exchange functional and the Lee-Yang-Parr correlation functional) and the all-electron 3-21G* basis set. The study indicates that the stabilities of the double-walled SiC nanotubes are of the same order as those of single-walled SiC nanotubes suggesting the possibilities of experimental synthesis of both single-walled and double-walled SiC nanotubes. The binding energy per atom or the cohesive energy of the double-walled nanotubes depends not only on the number of atoms but also on the coupling of the constituent single-walled nanotubes. The formation energy of the DWNTs is found to be maximum when the interlayer separation is about 3.5 angstrom. The DWNTs (n, n)@(n + 4, n + 4) are found to have large formation energies. In particular, (5.5)@(9,9) DWNT is the most stable tube in our study with a binding energy per atom of 5.07 eV, the largest formation energy of 12.39 eV, an interlayer separation of 3.58 angstrom and a band gap of 1.97 eV. All double-walled SiC nanotubes are found to be semiconductors, with the band gaps decreasing from single-walled nanotubes to double-walled nanotubes. (C) 2011 Elsevier Ltd. All rights reserved.