The study of ionic materials on nanometer scale is of great relevance for efficient miniaturized devices for energy applications. The epitaxial growth of thin films can be a valid route to tune the properties of the materials and thus obtain new degrees of freedom in materials design. High crystal quality SmxCe1-xO2-δ films are here reported at high doping level up to x=0.4, thanks to the good lat-tice matching with the (110) oriented NdGaO3 substrate. X-ray diffraction and transmission electron microscopy demonstrate the ordered structural quality and absence of Sm segregation at macroscopic and atomic level, respectively. Therefore, in epitaxial thin films the homogeneous doping can be obtained even with high dopant content not always approachable in bulk form, getting even an improvement of the structural properties. In situ spectroscopic measurements by x-ray photoemission and x-ray absorption show the O 2p band shift towards the Fermi level which can favor the oxygen exchange and vacancy formation on the surface when the Sm doping is increased to x=0.4. X-ray absorption spectroscopy also confirms the absence of ordered oxygen vacancy clusters and further reveals that the 5d eg and t2g states are well separated by the crystal field in the undistorted local structure even in the case of high doping level x=0.4.
The design and characterization of a HHG source conceived for Time and Angle Resolved PhotoElectron Spectroscopy (TR-ARPES) experiments are presented. The harmonics are selected through a grating monochromator with an innovative design able to provide XUV radiation for two distinct TR-ARPES setups.
The knowledge of the picosecond dynamics of the energy level alignment between donor and acceptor materials in organic photovoltaic devices under working conditions is a challenge for fundamental material research. We measured by means of time-resolved Resonant X-ray Photoemission Spectroscopy (RPES) the energy level alignment in ZnPc/C60 films. We employed 800 nm femtosecond laser pulses to pump the system simulating sunlight excitation and X-rays from the synchrotron as a probe. We measured changes in the valence bands due to pump induced modifications of the interface dipole. Our measurements prove the feasibility of time-resolved RPES with high repetition rate sources.
The role of trivalent rare-earth dopants on the cerium oxidation state has been systematically studied by in situ photoemission spectroscopy with synchrotron radiation for 10 mol % rare-earth doped epitaxial ceria films. It was found that dopant rare-earths with smaller ionic radius foster the formation of Ce3+ by releasing the stress strength induced by the cation substitution. With a decrease of the dopant ionic radius from La3+ to Yb3+, the out-of-plane axis parameter of the crystal lattice decreases without introducing macroscopic defects. The high crystal quality of our films allowed us to comparatively study both the ionic conductivity and surface reactivity ruling out the influence of structural defects. The measured increase in the activation energy of films and their enhanced surface reactivity can be explained in terms of the dopant ionic radius effects on the Ce4+ → Ce3+ reduction as a result of lattice relaxation. Such findings open new perspectives in designing ceria-based materials with tailored properties by choosing suitable cation substitution.
Samaria-doped ceria (SDC) thin films are particularly important for energy and electronic applications such as microsolid oxide fuel cells, electrolyzers, sensors, and memristors. In this paper, we report a comparative study investigating ionic conductivity and surface reactions for well-grown epitaxial SDC films varying the samaria doping concentration. With increasing doping above 20 mol % of samaria, an enhancement in the defect association is observed by Raman spectroscopy. The role of such associated defects on the films̀ oxygen ion transport and exchange is investigated by electrochemical impedance spectroscopy and electrochemical strain microscopy (ESM). The measurements reveal that the ionic transport has a sharp maximum in ionic conductivity and drops in its activation energy down to 0.6 eV for 20 mol % doping. Increasing the doping concentration further up to 40 mol %, it raises the activation energy substantially by a factor of 2. We ascribe the sluggish transport kinetics to the “bulk” ionic-near ordering in case of the heavily doped epitaxial films. Analysis of the ESM first-order reversal curve measurements indicates that these associated defects may have a beneficial role by lowering the activation of the oxygen exchange “surface” reaction for heavily doped 40 mol % of samaria. In a model experiment, through a solid solution series of samaria doped ceria epitaxial films, we reveal that the occurrence of associated defects in the bulk affects the surface charging state of the SDC films to increase the exchange rates. The implication of these findings is the design of coatings with tuned oxygen surface exchange by controlling the bulk associated clusters for future electrocatalytic applications.