Nanotubes for charge storage – towards an atomistic model

Highlights: • we have investigated alkali and halide ions in thin Au and carbon nanotubes • all ions are at the center of the tubes and carry unit charge • they experience image energies of a few electron volts • we explain at an atomic level why narrow tubes store charges better than wider ones - Abstract: We examine the insertion of alkali and halide ions into narrow nanotubes of graphite and gold by density functional theory (DFT). For tubes with diameters less than about 10 Åthe optimum position of the ion is in the center of the tube. Bader analysis and an analysis of the densities of states gave contradictory results for the charge on the halide ions, but we argue on physical grounds that they carry unit negative charge. We have calculated the energies of inserting the atoms into the tubes, where they are ionized. Because of the small system size the work function of the tubes changes during this process, which makes it difficult to interpret these energies. The surrounding tubes screen the ionic charge very effectively; thereby the ion-ion interactions are strongly reduced, which explains, why narrow tubes store charge more effectively than wider one. These effects are stronger for gold than for graphite tubes. We define an effective image radius of a cylindrical tube, and calculate the image energy experienced by the ions