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

摘要

We report on the surface topography, electron tunneling andchemical 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 oxygenpressure, using in situ scanning tunneling microscopy andspectroscopy (STM/STS). Coarsening of the grains was found onthe 10nm-thick LSM film. High resolution mapping of thetunneling spectra onto the surface topography showed a higherelectron exchange rate at select grain boundaries compared to grainsurfaces. A threshold-like drop in the tunneling current wasobserved at positive bias in STS, and is suggested as a uniqueindicator to the activation polarization in cation-oxygen bondingon LSM. Sr-enrichment and Mn-depletion were found on thesurface at high temperature using Auger electron spectroscopy,accompanied by a reduction in tunneling conductance in STS. Thissuggests that the Mn-terminated surfaces are more active forelectron exchange in oxygen reduction compared to the (La,Sr)-terminated surfaces on LSM.