AVS2010 Session TF-ThA: Multifunctional Thin Films and Characterization

The electrochemical energy storage systems based on Li-based insertion and reconstitution chemistries are a vital aspect of energy technologies. Despite the wealth of device-level and atomistic studies, little is known on the mesoscopic mechanisms of ion diffusion and electronic transport on the level of grain clusters, individual grains, and extended defects.

The development of the capability for probing ion transport on the nanometer scale is a key to deciphering complex interplay between structure, functionality, and performance in these systems. Here we demonstrate how Scanning Probe Microscopy can be utilized to measure Li-ion mobility based on the strong strain-bias coupling in the system when local Li concentrations are changed by electrical fields. The imaging capability, as well as time- and voltage spectroscopies analogous to traditional current based electrochemical characterization methods are developed. The reversible intercalation of Li and mapping electrochemical activity in LiCoO2 is demonstrated, illustrating higher Li diffusivity at non-basal planes and grain boundaries. In Si-anode device structure, the direct mapping of Li diffusion at extended defects and evolution of Li-activity with charge state is explored. The electrical field-dependence of Li mobility is studied to determine the critical bias required for the onset of electrochemical transformation, potentially allowing reaction and diffusion processes in the battery system to be separated at each location. The Scanning Probe Microscopy measurements are compared with classical characterization methods such as cyclic voltammetry and electrochemical impedance spectroscopy. The prospects of Scanning Probe Microscopy for battery characterization are discussed.

This material is based upon work supported as part of the Fluid Interface Reactions, Structures and Transport (FIRST) Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number ERKCC61. Part of worked is performed as a user proposal in the Center for Nanophase Materials Sciences at ORNL.

There is a strong and growing interest in complex oxide interfaces because of the wide range of functional properties exhibited. It is well known that the LaAlO₃/SrTiO₃ (LAO)/(STO) interface exhibits novel electronic conductivity when grown in certain ways. LAO and STO are both band insulators in the bulk, but their interface exhibits n-type electrical conductivity when LAO is grown on TiO₂-terminated STO. Rutherford backscattering spectrometry (RBS) was used to determine interface composition in several pulsed laser deposition (PLD) grown LAO/STO samples prepared at leading laboratories. RBS data collected on these samples clearly show that La diffuses deep into the STO substrate. The clear presence of a shoulder between the low energy side of the La peak and the high energy side of the Sr peak suggests La indiffusion, although this shoulder could also be caused by pulse pile up in the detector, straggling, and/or multiple/dual scattering effects. Therefore, RBS data were taken as a function of beam current, incident beam energy, and film thickness to determine if this shoulder is due to these artifacts, or La indiffusion. It was determined that none of the aforementioned artifacts occur, thereby implicating La indiffusion. The presence of substitutional La at Sr sites in the substrate provides a plausible explanation for the observed n-type conductivity, as La is a shallow donor in STO.

The use of CsI coatings on graphite fiber cathodes has been shown to reduce the field strength required for field emission from approximately 10⁴ V/cm to 250 V/cm. Interestingly, the mechanism for enhanced field emission is poorly understood. We have explored the enhancement mechanism by using simulated cathode structures consisting of CsI films deposited (by thermal evaporation and by pulsed laser deposition) onto graphite and Si surfaces; the films were characterized by x-ray photoelectron, Auger electron, ultraviolet photoelectron, and electron energy loss spectroscopy. Two aspects of the enchantment mechanism have been explored. The first, cathode conditioning, was addressed by characterizing the surfaces of CsI-coated cathodes and (stainless steel) anodes before and after conditioning. The results allow us to assess the change in cathode surface chemistry as well as the extent to which material is transferred between electrodes during conditioning. The second aspect concerns the reported observation that CsI coated cathodes, after use, exhibit the disappearance of I and the appearance of a coating that appears to have wetted the cathode surface. We hypothesize that I depletion leaves behind a film of Cs which, with its low melting point, will flow during cathode operation. To test this hypothesis, we have characterized I depletion from CsI by carrying out electron and photon stimulated desorption from CsI surfaces. The talk will conclude with a discussion of our findings and their relevance to the enhancement mechanism.

One major challenge in the stoichiometric growth of complex oxides, such as YMnO3, is the control of the relative compositions of the constituent materials. Desirable properties of oxide materials, such as ferroelectricity, are highly dependent upon material stoichiometry, making stoichiometry control an important issue. While RHEED (Reflection High Energy Electron Diffraction) analysis is typically used as a qualitative tool, RHEED generated x-rays can be used to give quantitative compositional information. The relative compositions of Y and Mn in MBE grown YMnO3 samples were studied using the grazing exit x-rays generated by RHEED electrons. Comparing the results with RBS characterization suggested that the technique has the potential for real-time compositional analysis.

This work was funded by the Office of Naval Research ( Grant N00014-02-1-0974), the Air Force Office of Scientific Research (MURI grant F49620-03-1-0330), and the National Science Foundation (CIAM-DMR grant 0502825).