Valence electronic structure of cross-linkedC60polymers:In situhigh-resolution photoelectron spectroscopic and density-functional studies

Abstract

When a ${\mathrm{C}}_{60}$ film is irradiated with a $3\phantom{\rule{0.3em}{0ex}}\mathrm{keV}$ electron beam, a cross-linked ${\mathrm{C}}_{60}$ polymer is formed and exhibits metallic electron-transport $(I\text{\ensuremath{-}}V)$ properties in air at room temperature [J. Onoe et al., Appl. Phys. Lett. 82, 595 (2003)]. To elucidate the origin of the metallic $I\text{\ensuremath{-}}V$ characteristics of the cross-linked polymer, we examined the valence photoelectron spectra of the polymer using in situ high-resolution ultraviolet photoelectron spectroscopy (UPS) and found that the spectrum for the cross-linked ${\mathrm{C}}_{60}$ polymer came across the Fermi level $({E}_{F})$. To understand the UPS results for the ${\mathrm{C}}_{60}$ polymer, we performed first-principles calculations of the band structure for three kinds of optimized three-dimensional unit cells of one-dimensional (1D) cross-linked ${\mathrm{C}}_{60}$ polymers with a cross-linkage consisting of both six- and seven-membered rings and of five- and eight-membered rings (P58). It was found that one quasi-1D P58 cross-linked ${\mathrm{C}}_{60}$ polymer shows semimetallic properties, which provides one possible explanation of both previous (metallic $I\text{\ensuremath{-}}V$ characteristics) and present (valence photoelectron spectra) experimental results.