N. Papageorgiou, M. Graetzel, O. Enger, D. Bonifazi, F. Diederich,
J. Phys. Chem. B 2002, 106, 3813-3822
Carboxylated fullerene derivatives adsorbed on nanocrystalline ZrO2 films deposited on conducting glass, display reversible electrochemical behavior with currents being 200 times higher than measured on a monolayer of the redox active fullerene molecule adsorbed on the conducting support. Despite the insulating oxide layer on which the fullerene is adsorbed, cross-surface charge transfer results in the participation of the internal surface of the oxide in the redox process. A mechanism of charge transport involving electron injection from the conducting support, followed by lateral electron hopping within the fullerene monolayer on the oxide, is proposed. Apparent diffusion coefficients as high as 1.5 × 10-8 cm2 s-1 were measured for the electron hopping process. A percolation threshold for electronic conductivity was found at a surface coverage between 40 and 60% of a full monolayer. Co-adsorbed spacer molecules, however, were seen to modify the percolation limit, as no threshold was observed up to 60% fullerene coverage. Significant implications are envisaged with regard to prospective applications in e.g. nano-optoelectronics, electrochemical devices, sensors, solar cells, and redox targeting of adsorbed biomolecules.