Quasimetallic behavior of carrier-polarized C60 molecular layers: Experiment and theory

Although ${\mathrm{C}}_{60}$ is an insulator with a band gap ${E}_{g}$ of $\ensuremath{\sim}2.5\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, we show experimentally and theoretically that ${E}_{g}$ is strongly affected by image forces and injected charges. In sharp contrast to the Coulomb blockade typical of quantum dots, ${E}_{g}$ is reduced by Coulomb effects. The conductance of a micron-sized ${\mathrm{C}}_{60}$ film sandwiched between metal (Al, Ag, Au, Mg, and Pt) contacts is investigated. Excellent Ohmic conductance holds for ${\mathrm{C}}_{60}$ layers at room temperature, on using $\mathrm{Al}∕\mathrm{Li}\mathrm{F}$ bilayer contacts. Arrhenius type activated conductivity with an activation gap of $\ensuremath{\sim}0.16\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ is found.