Cluster modeling of three types of double-walled armchair silicon carbide nanotubes

A systematic study of three types of armchair double-walled SiC nanotubes (DWNT) (n,n)@(m,m) (3 a parts per thousand currency sign n a parts per thousand currency sign 6; 7 a parts per thousand currency sign m a parts per thousand currency sign 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 possibility 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 singlewalled nanotubes and their types. A study of the binding energies, Mulliken charges, density of states and HOMO-LUMO gaps has been performed for all nanotubes from (n,n)@(n + 3,n + 3) to (n,n)@(n + 6,n + 6) (n = 3 - 6). Type 2 DWNTs do not preserve the coaxial geometry when the difference in the chirality of the outer and inner tubes is 5 or less. For type 3, this occurs when the chirality difference is 4 or less. The gaps of types 2 and 3 DWNTs are lower than those of the corresponding single-walled nanotubes and are significantly less than those of type 1 DWNTs.