ABSTRACT
Recently, single wall nanotubes (SWNT) have been successfully fabricated inside inert AlP04-5 zeolite channels (Tang, 1998; Wang, 2001 ). The nanotubes are perfectly aligned mono-sized SWNTs with ultra-small diameters of about 4 A, confined inside the zeolite channels with inner diameters of about 7.3 A. Due to their extremely small radius, they show unique and exciting properties, including superconducting fluctuation (Tang, 2001) with a mean field Tc of about 15 K. We consider here the possibility of modifying the properties of these nanotubes by doping. We focus on alkali atom intercalation, in anticipation that the superconductivity temperature can be enhanced if alkali doping is possible. These systems are unique as far as doping is concerned since the confinement of the nanotubes inside the zeolite channels naturally prevents the metal atoms from attaching to the outside of the SWNT. We will see below that alkali atoms inside the nanotube cannot form clusters, but have to line up as a single-atom wire. These ultra-small radius SWNTs thus have some unique structural features that make them ideal platforms for realizing a truly one-dimensional single atom wire, provided that some necessary conditions can be satisfied. These conditions are (i) the alkali atom and nanotube reaction has to be exothermic; (ii) the atoms inside the tube should not form small 3D clusters or 2D patches that are bound to the wall; (iii) the diffusion barrier inside the tube should be small; (iv) the metal atoms must find its way into the tube. We note that previous calculations (Miyamoto, 1995) have shown that intercalating K into nanotubes can be strongly
exothermic. The present calculations focus on ultra-small radius tubes, in particular we consider whether the insertion barrier can be overcome.
