Size Effects on the Electrical Conductivity of Ceria: Achieving Low Space Charge Potentials in Nanocrystalline Thin Films

Thin films of nominally pure and 10 mol % Gd-doped ceria were grown on Al2O3 < 1 (1) over bar 02 > (r-cut) and MgO < 100 > substrates with pulsed laser deposition (PLD). Their electrical conductivity properties were measured using impedance spectroscopy. Oxygen partial pressure and temperature dependence indicate that the nominally pure films are contaminated with acceptor impurities whose concentration is found to vary perceptibly among the samples. Quite remarkably, the nanocrystalline 10 mol % Gd-doped thin films show conductivities that are, as expected, lower than those for epitaxial films but surprisingly much larger than those obtained previously from other comparable nanocrystalline films (e.g., grown on SiO2), indicating that the oxygen vacancies are much less depleted at the grain boundaries. Correspondingly, the space charge potential was found to be unusually small with a value of 0.19 +/- 0.05 V.