Real-Time X-Ray Studies of Materials Processes

Karl Ludwig

Our research investigates how materials evolve on atomic and nano- length scales during growth, patterning or electrochemical function using real-time x-ray techniques. Many of the experiments use the high brightness of synchrotron x-ray sources – the National Synchrotron Light Source (NSLS-II) at Brookhaven National Laboratory on Long Island, the Advanced Photon Source (APS) at Argonne National Laboratory outside of Chicago, the Cornell High-Energy Synchrotron Source (CHESS) at Cornell, and the Linac Coherent Light Source (LCLS) at SLAC.  Where possible, our research makes contact with fundamental theory and simulation.

In the last few years, our detailed interest has been in two directions – understanding surface and thin film processes and studying the relationship between atomic structure and function in solid oxide fuel cell cathodes. Many of our in-situ studies utilize a unique ultra-high vacuum growth and surface modification facility that we have helped develop at the NSLS-II. We have been using it to examine surface morphology evolution during ion bombardment (which can cause the spontaneous growth of surface nanostructures) and issues related to the growth of wide-bandgap group III-V semiconductor films using atomic layer epitaxy (in collaboration with the Eddy group at the Naval Research Laboratory).

Increasingly our experiments utilize coherent x-ray scattering, which provides the ability to probe nanoscale dynamics during growth and patterening. Partially coherent x-ray beams are created using small (few microns) slits in conjunction with a high-brilliance 3rd generation synchrotron source. The disorder on the surface produces speckle patterns in the scattered x-ray intensity. The evolution of the speckle pattern can then be related to the underlying dynamics of structural changes.  The LCLS is the world's first hard x-ray laser and offers unique new opportunies for coherent scattering on femtosecond time scales that we are now exploring.

A second major direction of our work is in solid oxide fuel cells, which offer the potential for highly efficient energy conversion. However improvements in cathode function are needed before their potential can be fully realized. In collaboration with Profs. Pal, Basu and Gopalan in Engineering and Prof. Smith in Physics, we are examining in-situ the near-surface atomic structure of cathode materials in order to better understand the relationship between function and structure.