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.