part of introduction
Duo to the unusual physical properties, carbon nanotubes have attracted lots of attentions. For the higher mechinical stiffness and greater thermal stability, double-walled nanotubes(DWNTs) may be have more benefit in field emission device comparing to single-walled nanotubes(SWNTs). Their double shell structure offers the possibility of shielding the inner tube for external perturbations. These merits make them very interesting for certain nano-device applications.