The tyranny of the buried interface

Time and again, a major impediment to research progress in condensed matter physics, electrical engineering, materials science, and physical chemistry is the need to understand what is happening in some system at a buried interface. For example, in organic photovoltaic devices, it is of great importance to learn more about what is happening at metal/organic semiconductor interfaces (charge transfer, interfacial dipole formation, Fermi level pinning) and organic/organic interfaces (exciton splitting at the interface between electron- and hole-transporting materials). Another example: in lithium ion batteries, at the interface between either the cathode or the anode and the electrolyte, after the first couple of charge and discharge cycles, there forms the "solid electrolyte interface" (SEI) layer. The SEI is nanoscale in thickness, stabilizes the electrode surface, establishes the energetic lineup between the electrolyte redox chemistry and the actual electrode surface, strongly affects the kinetics of the lithium ion transport, etc.

Unfortunately, probing buried interfaces in situ in functioning systems is extremely hard. There generally is no Star Trek scanner device that can nondestructively reveal atomic-scale details of buried 3d structures. Many of our best characterization approaches are surface-based, or require thinned down samples, and there are always difficult questions about how information gained in such investigations translates to the real situation of interest. This is not a new problem. From the early days of surface science and before, people have been worrying about, e.g., how to connect studies performed in UHV on single crystal surfaces with "real world" situations on polycrystalline surfaces with ambient contaminants. There are some macro-scale interface sensitive approaches (exploiting x-ray standing waves, or interfacial optical effects). Still, the more people working on developing better characterization tools toward this end, the better, even if it doesn't sound terribly exciting to the masses.