Energetic, geometric, and electronic evolutions of K-doped single-wall carbon nanotube ropes with K intercalation concentration

The energetic, geometric, and electronic evolutions of a K-doped single-wall (10,10) carbon nanotube rope with K intercalation concentration are systematically investigated by using first principles calculations. The existence of a stable intermediate phase $\left({\mathrm{K}}^{\mathrm{exo}}{\mathrm{C}}_{13.3}\right)$ before saturation $\left({\mathrm{K}}^{\mathrm{exo}}{\mathrm{C}}_{6.7}\right)$ for exohedral K doping (outside the tube) is first theoretically confirmed. The optimum K intercalation density in single-wall carbon nanotube ropes with open ends is predicted to be as high as about ${\mathrm{KC}}_{4.2},$ nearly twice the well-known value in graphite. The simple charge transfer model is applicable only in the low-K doping level regime. The nearly free electron states of the nanotube couple with the K $4s$ orbital, and the lower hybridized states do cross the Fermi level as the exohedral and endohedral (inside the tube) K doping densities exceed ${\mathrm{K}}^{\mathrm{exo}}{\mathrm{C}}_{20}$ and ${\mathrm{K}}^{\mathrm{endo}}{\mathrm{C}}_{80},$ respectively.