X-Ray Probe Targets Interfaces

I nterfaces are what separate one material from another. We sense the world through interfaces, whether touching the surface of a table or seeing the light reflecting off the edge of a glass. Many other interfaces are less visible but still have a place in our lives. Modern solar cells consist of thin layers where interfaces play an important role for charge separation. Catalytic reactions for chemical energy transformations occur at solid-gas or solid-liquid interfaces. It is extremely challenging to probe these interfaces, as they are often buried under layers or in contact with a liquid or high-pressure gas where the number of interface atoms is extremely small in comparison to the surrounding material [1]. Researchers, therefore, search for techniques that are only sensitive to the two-dimensional interface. Royce Lam from the University of California, Berkeley, and colleagues have developed an interfacial probe utilizing soft-x-ray pulses from a free electron laser [2]. By aiming the pulses at a graphite sample, the scientists have shown that they can detect a nonlinear spectrographic signal that arises from the graphene layers near the surface of the graphite. This demonstration opens up a new field in interface studies, offering the possibility to track surface chemistry reactions with the femtosecond resolution provided by the very short x-ray pulses from free electron lasers.