Energetics and electronic structures of one-dimensional fullerene chains encapsulated in zigzag nanotubes

We report total-energy electronic-structure calculations that provide energetics and electronic structures of fullerenes encapsulated in zigzag carbon nanotubes (so-called peapods). It is found that the space between the nanotube and the encapsulated fullerenes is a decisive factor to determine the energetics for the encapsulation process of the fullerenes in the nanotubes and the stability of resultant structures. The reaction energies for the encapsulation processes exhibit common characteristics depending on the space inside the tube, irrespective of their metallic and semiconducting properties. It is also found that the electronic structures of peapods depend on the space and that they reflect electron states of the encapsulated fullerenes. A relative energy position of the lowest unoccupied states of the encapsulated fullerenes modulates the energy gap of the semiconducting nanotubes. Further, the interwall spacing results in a significant shift of the electron states that have originated from the fullerenes. Our results indicate that the energy gap of the semiconducting peapods exhibits interesting variation depending on both the space and the fullerenes and suggest the possibility of band-gap engineering of the all carbon materials.