RF Sputter Deposition of Nanocrystalline Nd1-xSrxCoO3 Oriented Thin Films

Cobalt-containing perovskite-type oxides have been the subject of intense research mainly due to the possibility of optimizing their structural and physical properties by doping. The range of possible application for these oxides is wide, extending from components in solid oxides fuel cells (SOFCs), oxygen separation membranes and electrochemical reactors, to sensor devices based on their ability to catalytically oxidize CO and CH4 and reduce NO. In addition, perovskite cobaltites have received further attention after the observation of elevated Seebeck coefficients in related layered systems which suggest their possible application as thermoelectric materials (TE). Recent reports showed that doped perovskite cobaltites have relatively high figures of merit (Z) and this directly correlates to the different spin states available for the cobalt ions. As a consequence, the TE materials research is the latest field where cobaltites appeared as promising and new compounds. Their magnetic and magnetoresistive properties are also of recent interest. In particular, a debate has been opened regarding the actual spin-state of Co in lanthanum cobaltites. In fact, it has been shown that in the range from 5 to 1000 K the Co ions pass through three different spin-states (low-spin LS, intermediate-spin IS, and high-spin HS) which are intimately connected to the structural as well as internal parameters such as the metal-oxygen bond lengths. The spin state, in turn, affects the physical properties such as transport, magnetic and optical properties acting as a sort of “Jahn-Teller switch”, where for certain spin configurations the J-T distortion, for the same Co valence state, is suppressed. The internal parameters, such as bond lengths and angles, play a major role in defining the cobaltites physical properties. Much of the work on RECoO3 compounds focussed on bulk or single crystalline materials and less on thin films preparation and characterization which, however, is the useful “physical form” for sensing and catalytic as well as TE micro-devices applications. Moreover, it would be desirable to make available nanocrystalline thin films where the high surface-to-volume ration can enhance the cobaltites physical properties. We report the synthesis and structural and morphological investigation of oriented epitaxial thin films of Nd1-xSrxCoO3 with x = 0, 0.2 and 0.5 on single crystalline substrates (SrTiO3 and LaAlO3) by means of rf-sputtering. The Sr-dopant has been chosen since it has shown to be one of the most soluble and one of the most favourable dopants in terms of structure distortion, with a tolerance factor close to 1. This leads to a less distorted phases where the Co-O-Co hole hopping is favoured. Increasing Sr-concentration gives origin to higher Co valence state and oxygen vacancies concentration, which are thought to be the active sites for gas adsorption. The preparation of oriented Nd1-xSrxCoO3 thin films, such as the object of this comunication, have not been considered in the previous literature