1863 small 2010, 6, No. 17, 1863–1867 © 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Heterostructures DOI: 10.1002/smll.200902348 Recent developments in the field of thin-film growth tech- nologies have allowed control at an atomic level of deposited layers, thus opening new perspectives in the field of engi- neering of multilayers and heterostructures based on complex oxides. [1] In particular, it is expected that oxide heterostruc- tures, with almost ideal interfaces, may lead to interesting artificial materials with novel properties. Artificial thin-film oxide structures make the already complex individual bulk properties even more interesting through their interaction at the interface. Following such an approach, a number of het- erostructures have been tailored which show extraordinary properties that do not belong to the individual layers. These range from superconductivity at the interface between non- superconducting layers to high-mobility 2D conductivity at the interface between insulating oxides. [2,3] The number of possible combinations of these oxides is enormous, and the potential for novel behavior having practical applications represents a strong motivation for this research. The same approach can be applied to heterostructures based on oxide ionic conductors provided that the issues concerning structural match at the interface are solved. The interest in heterostructures based on oxide ionic conductors is driven by the space-charge-zone effects at the interface, which can increase the charge-carrier concentration locally, and by interface mobility effects, the latter being of particular relevance in the case of materials with high defect density and relatively low mobility. The potential impact of oxide ionic conductor superlattices has been shown for superlattices based on CaF 2 and BaF 2 layers, grown by molecular beam epitaxy, which exhibited an increase in ionic conductivity Enhancement of Ionic Conductivity in Sm-Doped Ceria/ Yttria-Stabilized Zirconia Heteroepitaxial Structures Simone Sanna, Vincenzo Esposito, Antonello Tebano, Silvia Licoccia, Enrico Traversa,* and Giuseppe Balestrino* with increasing number of interfaces. [4] For the thinner indi- vidual layers, the conductivity increase of the superlattices was up to two orders of magnitude larger than the conduc- tivity of BaF 2 films over the whole measured temperature range. These results were found to be in excellent agreement with a model based on fluoride atom redistribution at the interface. More recently, superlattices made of SrTiO 3 (STO) and ZrO 2 :8 mol% Y 2 O 3 (yttria-stabilized zirconia, YSZ) layers were investigated, and showed a colossal enhancement of ionic conductivity, which was several orders of magnitude larger than the YSZ conductivity, even at room tempera- ture. [5] However, these findings were successively questioned because the researchers omitted to consider the ionic and electronic conductivity contribution from the much thicker STO substrates they used. [6] To gain further insight into these findings, herein we investigate the electrochemical properties of superlattices made of alternate layers of 20 mol% samarium-doped ceria (Ce 0.8 Sm 0.2 O 2- δ , SDC) and 8 mol% YSZ, fabricated by pulsed laser deposition (PLD). The choice of coupling SDC and YSZ aimed to have both layers in the superlattices made of oxygen-ion conductors with compatible crystallographic features. These materials are highly defective, so that even- tual interface effects are expected to be related to mobility enhancement phenomena rather than to space-charge-region effects. [7] SDC has a larger ionic conductivity than YSZ, the conventional solid oxide fuel cell (SOFC) electrolyte mate- rial, and is thus intensively studied to reduce the SOFC operating temperature in the 600–800 °C range. [8–10] Several superlattice films were prepared by varying the number of SDC/YSZ bilayers N, keeping the same total thickness, and the superlattice modulation length Λ (the thickness of an individual SDC/YSZ bilayer), keeping the number of inter- faces constant. Electrochemical measurements showed a size- able increase in conductivity with increasing number of SDC/ YSZ interfaces, which does not have an electronic origin as demonstrated by measurements at low oxygen partial pres- sure ( pO 2 ). The conductivity enhancement is attributed to interfacial tensile strain effects rather than to interface space- charge regions, due to the presence of a large defect concen- tration in SDC and YSZ compounds. Doped ceria films having a fluorite structure can be deposited on several non-fluorite substrates by PLD, starting from a polycrystalline target and maintaining the correct stoi- chiometry. [11] In particular, doped ceria has been shown to grow on (001) MgO substrates as well as on (001) LaAlO 3 Dr. S. Sanna, Dr. V. Esposito, Prof. S. Licoccia, Prof. E. Traversa NAST Center & Department of Chemical Science and Technology University of Rome Tor Vergata Via della Ricerca Scientifica, 00133 Rome, Italy E-mail: traversa@uniroma2.it Dr. A. Tebano, Prof. G. Balestrino CNR-SPIN & Department of Mechanical Engineering University of Rome Tor Vergata Via del Politecnico, 00133 Rome, Italy E-mail: giuseppe.balestrino@uniroma2.it Prof. E. Traversa International Research Center for Materials Nanoarchitectonics (MANA) National Institute for Materials Science (NIMS), 1–1 Namiki Tsukuba, Ibaraki 305-0044, Japan