Higher Coverage Gas Adsorption on the Surface of Carbon Nanotubes: Evidence for a Possible New Phase in the Second Layer

We present adsorption isotherm results for Ne, $\mathrm{C}{\mathrm{H}}_{\mathrm{4}}$, and Xe on bundles of close-ended single-wall carbon nanotubes, for coverages above the completion of the first layer. We find a small, sharp, substep present in the second-layer data for Ne and $\mathrm{C}{\mathrm{H}}_{\mathrm{4}}$, and a weaker feature, that produces an isothermal compressibility peak, for Xe. The size and location of the feature allows its tentative identification as a new, second-layer, one-dimensional phase, in which the atoms sit atop high binding energy sites in the second layer.