Energetics and structural characterization of C60 polymerization in BN and carbon nanopeapods -: art. no. 205418

As in the case of carbon nanotubes, also boron nitride nanotubes may host arrays of C-60 molecules and form a nanopeapod (NPP). The observed separation between C-60 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 C-60 chains, and optimize their atomic structures to provide a description of their energetic landscape. A fully polymerized C-60 chain and a C-60 dimer are found to be more stable than nonpolymerized C-60, respectively, by 0.89 and 0.38 eV/C-60. The geometry and energetics of an encapsulated C-60 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/C-60, which are significantly larger than the calculated activation energy for C-60 polymerization, supporting the hypothesis that encapsulated C-60's in NPP's are partially polymerized. Band structure analysis show that polymerization does not affect the gap width of the C-60 chain. BN NPP's are semiconductors with a gap width determined by the C-60. The lowest unoccupied C-60 states lie just above the Fermi level in metallic carbon NPP's and charge transfert could take place, affecting the C-60 geometry.