Understanding and tuning the surface stability of functional thin films
Surface degradation of functional thin-film materials severely limits their long-term performance in a variety of applications, including energy conversion and catalysis as well as information technology. Advanced surface characterization techniques, such as in-situ X-ray photoelectron spectroscopy (XPS), allow probing the surface properties of functional materials close to operating conditions. For example in Sr-doped LaMnO3, a state-of-the-art solid oxide fuel cell cathode material, degradation via the surface segregation of dopant cations can be monitored at elevated temperatures and pressures. A study on electrochemical polarization of three LaMnO3-based materials with markedly different dopant sizes (Ca, Sr, Ba) provides insights into the balance of elastic and electrostatic effects underlying segregation. While segregation is enhanced for high polarization (ΔV ≤ 800mV) of either polarity, the ideal operation point minimizing segregation is found to be shift towards more reducing conditions with increasing dopant size, indicating a stronger contribution of the elastic energy. In a second approach we specifically address the electrostatic attraction of dopants to the surface enriched in oxygen vacancies in (La,Sr)MnO3 by depositing sub-monolayer coverages of metals that modify the surface reducibility. In particular for Hf, the extent of Sr segregation clearly decreases, highlighting a potential pathway for targeted modification of the long-term surface stability of functional doped oxides. Finally, an outlook will be given on research plans at ARCNL based on thin-film growth and in-situ XPS, focusing on the role of structure and composition in the surface stability of thin films relevant for nanolithography applications.