Quasimetallic behavior of carrier-polarized ${\mathrm{C}}_{60}$ molecular layers: Experiment and theory

Although ${\mathrm{C}}_{60}$ is an insulator with a band gap $E_g$ of $\sim 2.5\mathrm{eV}$, 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 $\sim 0.16\mathrm{eV}$ is found.

Figure 1

A schematic of the device showing LiF protected Al electrodes sandwiching a ${\mathrm{C}}_{60}$ layer. Note that the Al strips are orthogonally positioned.

Figure 2

(a) $I\text{−}V$ curves for three $\mathrm{Al}∕X∕{\mathrm{C}}_{60}\left(250\mathrm{nm}\right)∕X∕\mathrm{Al}$ devices, with $X=\mathrm{Li}\mathrm{F}$ or left unprotected (//) at the top and bottom interlayers. (b) $I\text{−}V$ curves for $\mathrm{Al}∕\mathrm{Li}\mathrm{F}∕{\mathrm{C}}_{60}\left(250\mathrm{nm}\right)∕∕M$, with $M=\mathrm{Al}$, Ag, and Pt. Only the top electrode has LiF.

Figure 3

(Color online) (a) $I\text{−}V$ curves showing increased nonlinearity at lower $T$, for an $\mathrm{Al}∕\mathrm{Li}\mathrm{F}∕{\mathrm{C}}_{60}\left(450\mathrm{nm}\right)∕\mathrm{Li}\mathrm{F}∕\mathrm{Al}$ device. (b) Arrhanius plots of the current as a function of $1∕T$, over a range of $\sim 100°$. The activation energy is $0.155\pm 0.005\mathrm{eV}$.

Figure 4

(a) (Color online) The electron density of states (DOS) for isolated ${\mathrm{C}}_{60}$ near the energy gap. Experimental data (Ref. 10) for ${\mathrm{C}}_{60}$ on Ag are also shown. (b) DOS for the ${\mathrm{C}}_{60}\text{−}\mathrm{C}_{60}{}^{-}$ anion; the gap is further lowered by $\sim 1.3\mathrm{V}$. A level width $\gamma =0.15\mathrm{eV}$ is assumed in the DOS calculations. See Table .