Energetics and structural characterization of C₆₀ polymerization in BN and carbon nanopeapods

As in the case of carbon nanotubes, also boron nitride nanotubes may host arrays of C₆₀ molecules and form a nanopeapod (NPP). The observed separation between C₆₀ molecules in BN NPP’s is consistently shorter than in carbon NPP’s, which influences their electronic properties. Here we report on total-energy pseudopotential density functional theory (DFT) calculations for polymerized and nonpolymerized C₆₀ chains, and optimize their atomic structures to provide a description of their energetic landscape. A fully polymerized C₆₀ chain and a C₆₀ dimer are found to be more stable than nonpolymerized C₆₀, respectively, by 0.89 and 0.38 eV∕C₆₀. The geometry and energetics of an encapsulated C₆₀ chain is not significantly different with respect to the isolated molecule. Encapsulation energies in BN and carbon NPP’s are, respectively, 1.56 and 1.67 eV∕C₆₀, which are significantly larger than the calculated activation energy for C₆₀ polymerization, supporting the hypothesis that encapsulated C₆₀’s in NPP’s are partially polymerized. Band structure analysis show that polymerization does not affect the gap width of the C₆₀ chain. BN NPP’s are semiconductors with a gap width determined by the C₆₀. The lowest unoccupied C₆₀ states lie just above the Fermi level in metallic carbon NPP’s and charge transfert could take place, affecting the C₆₀ geometry.