Electronic Properties of Carbon Nanotubes Intercalated with Li+ and Mg2+: Effects of Ion Charge and Ion Solvation
The influence of bare and solvated cations imbedded inside single-walled carbon nanotubes (SWCNTs) on the SWCNT electronic properties is studied by ab initio electronic structure calculations. The roles of ion charge and ion solvation are investigated by comparing Li+ vs Mg2+ and Li+ vs its solvatocomplex with two ethylene carbonate (EC) molecules, [Li(EC)2]+. Two achiral nanotubes with similar radii but different electronic structure are considered, namely, the metallic, (15,15) armchair, and semiconducting, (26,0) zigzag, SWCNTs. The intercalation process is energetically favorable for both CNT topologies, with all bare cations and the solvatocomplex under investigation, with the doubly charged Mg2+ ion exhibiting the highest energy gain. Insertion of the bare ions into the SWCNTs increases the electronic entropy. The electronic entropy changes because the ions introduce new energy levels near the Fermi level. Those initially empty levels of the cations accept electron density and generate electronic holes in the valence band of both SWCNT topologies. As a consequence, the semiconducting (26,0) zigzag SWCNT becomes metallic, exhibiting hole conductivity. Solvation of the bare Li+ ion by EC molecules completely screens the influence of the ion charge on the SWCNT electronic properties, independent of the topology. The last fact validates the common practice of employing standard, nonpolarizable force field models in classical molecular dynamics simulations of electrolyte solutions interacting with CNTs. The strong dependence of the nanotube electronic properties on the presence of bare ions can be used for development of novel cation sensors for mass spectroscopy applications.