New Probe for the Secrets of Complex Interfaces

Element-selective method reveals interfacial properties of materials used for water purification, catalysis, energy conversion, and more.

Scientists have devised a spectroscopy method—based on soft X-ray second-harmonic generation—that probes buried graphene layers inside graphite.

The Science

In rechargeable batteries, ultra-thin material sheets are crucial. Reactions occur at the interfaces of these sheets. Scientists want to track these reactions. They need a way to probe buried interfaces with elemental specificity. In principle, soft X-ray spectroscopy techniques could be a powerful tool for probing such complex interfaces. However, these techniques haven’t been available due to limitations in optics and laser sources. For the first time, an international team devised a soft X-ray second harmonic generation technique and used it to characterize individual layers of graphene inside a graphite sample, promising to open new vistas into the nature of complex interfaces.    

The Impact

Knowing how reactions occur at buried interfaces is crucial for energy storage, water purification, and other uses. The new technique offers a way to probe specific elements at buried interfaces. The results from the technique could, one day, let scientists use soft X-rays to track interfacial processes occurring in a quadrillionth of a second.


Understanding the detailed nature of complex interfaces has become a quest of profound significance, as it underlies urgently needed advances in many applications, including water purification, desalination, and reclamation technologies, and is vital to central processes in electrochemistry, atmospheric chemistry, biochemistry, and energy conversion. Scientists developed a new technique to probe interfaces with both surface and element-specific selectivity, demonstrated for the individual graphene layers within bulk graphite. When soft X-ray pulses from a free electron laser enter the material, they excite inner-orbital electrons in the carbon atoms. For carbon atoms making up a graphite interface, these excitations can produce a photon (a packet of light) with twice the energy of the incoming photons. Scientists can use observations of this second harmonic generation to elucidate important interfacial properties, as well as chemical reactions occurring on these interfaces.


Richard Saykally
University of California, Berkeley and Lawrence Berkeley National Laboratory  

CP Schwartz
Lawrence Berkeley National Laboratory  


Soft X-ray second harmonic generation measurements were conducted at the EIS-TIMEX beam line at FERMI facility in Trieste, Italy. R.K.L., A.M.R., J.W.S., and R.J.S. were supported by the Department of Energy (DOE), Office of Science, Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division. C.J.H. and S.L.R. were supported by the U.S. Army Research Laboratory (ARL) and the U.S. Army Research Office (ARO). S.L.R. received a National Science Foundation Graduate Research Fellowship. Simulations were performed as part of a user project with T.A.P., C.D.P., and D.P. at The Molecular Foundry (TMF). Theoretical simulations of nonlinear susceptibility by C.D.P. were carried out within TIMES at SLAC National Accelerator Laboratory supported by the DOE, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division. The calculations were performed using computational resources at the National Energy Research Scientific Computing Center and at TMF, both DOE Office of Science user facilities. N.F., P.M., M.C., and L.P. were supported by the project Single-Shot X-ray Emission-Spectroscopy experiments funded by the Italian Ministry for Education and Research as an in-kind project for the EuroFEL consortium. W.S.D. was supported by the Joint Center for Artificial Photosynthesis, a DOE Energy Innovation Hub supported through the Office of Science. S.T.C. was supported by DOE under contract with the National Renewable Energy Laboratory. D.S., T.C.W., D.N., S.T.C., and C.P.S. were in part supported by the DOE, Office of Energy Efficiency and Renewable Energy, Solar Energy Technology Office BRIDGE program. D.S., T.C.W., D.N., and C.P.S. were also supported by the SLAC National Accelerator Laboratory. Electron yield X-ray absorption measurements were performed at Stanford Synchrotron Radiation Lightsource. Use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory is supported by the DOE, Office of Science, Basic Energy Sciences. Material support was provided by the DOE, Office of Science, Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division. Travel support was provided by ARL and ARO.


R.K. Lam, S.L. Raj, T.A. Pascal, et al., “Soft X-ray second harmonic generation as an interfacial probe.” Physical Review Letters 120, 023901 (2018). [DOI: 10.1103/PhysRevLett.120.023901]

Related Links

APS Physics viewpoint: X-ray probe targets interfaces

Chemical & Engineering News article: Second-harmonic generation with soft X-rays probes buried interfaces

Materials Research Society Bulletin article: Probing the buried interface between graphene layers

Highlight Categories

Program: ASCR , BES , CSGB , MSE , SUF , Hubs

Performer: University , DOE Laboratory , SC User Facilities , ASCR User Facilities , NERSC , BES User Facilities , Foundry , SSRL

Additional: Collaborations , Non-DOE Interagency Collaboration