Correlations of Electronic and Chemical State on La_(0.7)Sr_(0.3)MnO_3 Dense Thin-Film Cathode Surfaces

摘要

We report on the surface topography, electron tunneling and chemical characteristics of 10nm- and 50nm-thick La_(0.7)Sr_(0.3)MnO_3 (LSM) thin-films at temperatures up to 580°C in 10~(-3) mbar oxygen pressure, using in situ scanning tunneling microscopy and spectroscopy (STM/STS). Coarsening of the grains was found on the 10nm-thick LSM film. High resolution mapping of the tunneling spectra onto the surface topography showed a higher electron exchange rate at select grain boundaries compared to grain surfaces. A threshold-like drop in the tunneling current was observed at positive bias in STS, and is suggested as a unique indicator to the activation polarization in cation-oxygen bonding on LSM. Sr-enrichment and Mn-depletion were found on the surface at high temperature using Auger electron spectroscopy, accompanied by a reduction in tunneling conductance in STS. This suggests that the Mn-terminated surfaces are more active for electron exchange in oxygen reduction compared to the (La,Sr)- terminated surfaces on LSM.