High energy-resolution electron energy-loss spectroscopy study of the electronic structure of C60 polymer crystals

Electronic structures of C60 polymer crystals (dimerized C60: dimer-C60, one-dimensionally polymerized C60: 1D-C60 and two-dimensionally polymerized C60: 2D-C60) were studied by using a high energy-resolution electron energy-loss spectroscopy microscope. Spectra were obtained from single crystalline specimen areas of 180 nm in diameter. The dielectric functions of the materials were derived from valence electron excitation spectra by using Kramers-Kronig analysis. The onset energies of the imaginary parts of the dielectric functions ɛ2 (bandgap energies) were 1.6 eV (dimer-C60), 1.4 eV (1D-C60) and 1.5 eV (2D-C60). Those energies are smaller than that of the monomer C60 (mono-C60) crystal of 1.7 eV. Energies of interband transition peaks observed in each ɛ2 of polymerized C60 material were smaller by amounts of 0.5 eV than those of corresponding peak energies of mono-C60. Excitation spectra of 1s electrons to the conduction band showed that the intensity of 1s → π* transitions compared to that of 1s → σ* transitions was smaller than that of mono-C60. It may due to that a part of π-bonds of a C60 cluster changed to σ-bonds to form covalent bonds with neighboring C60 clusters in the polymerization process.