The secrets of the first few molecular layers near the surface of a material, or adjacent to buried interfaces is an underexplored area of science. Interfaces in rechargeable battery catalysts, semiconductor dielectrics, and two-dimensional materials play important roles in determining energy-conversion efficiency, device performance, and chemical and physical reactivity. X-rays with high photon energies are commonly used to “see” into materials due to their penetrating ability and small wavelength that approximates to the size of molecules and atoms. However, while most soft x-ray spectroscopy techniques can achieve chemical and elemental specificity, they are not able to offer rigorous interfacial specificity. This limitation is usually associated with the optics, laser source, and experimental techniques. Recent developments in nonlinear optics with coherent x-ray free-electron laser sources now make it possible to probe buried interfaces with elemental specificity.
In a recent issue of Physical Review Letters (doi:10.1103/PhysRevLett.120.023901), a research team, led by Craig Schwartz, Richard Saykally, and Royce Lam at Lawrence Berkeley National Laboratory and the University of California, Berkeley, reports the first observation of soft x-ray second-harmonic generation (SHG) in graphite thin films. “This [technique] effectively combines the surface/interface specificity optical second-order spectroscopy with the elemental specificity of soft x-ray spectroscopy,” says Lam, the lead author of this work, “with signal originating primarily from the topmost molecular layer.” The interfacial specificity comes from the enhancement of the second-order nonlinear response to a high-energy, coherent photon beam, which distinguishes itself from surface-specific x-ray spectroscopy that is restricted by the inelastic mean free paths of the photoionized electrons or by the penetration depth of the incident x-rays.
The experimental design of soft x-ray second-harmonic generation (SHG). (a) Schematic showing the energy diagram of the SHG process; (b) experimental setup showing the SHG signal generation from the transmitted free-electron beam on a graphite sample; (c) x-ray absorption SHG spectrum of the graphite sample. FEL is free-electron laser. Credit: Physical Review Letters.
The researchers also developed a first-principles electronic structure framework with a density-function-theory-based supercell approach that matches the experimental spectrum with a calculated linear spectrum for an eight-layer slab of graphite. “This technique should be broadly applicable to a wide variety of materials systems,” Lam says. “Additionally, as the pulses remain coherent in the SHG process, we expect it to be possible to combine soft x-ray SHG with lensless coherent imaging techniques that allow for simultaneous spectroscopic and spatial resolution of materials systems.”
“This work is one of the first to study second-harmonic generation using soft x-ray pulses from a free-electron laser, which could be widely applied to study the interfacial chemical reaction that may be hidden in the materials,” says Liang Zhang, a postdoctoral researcher with expertise in soft x-ray spectroscopy at the Advanced Light Source at Lawrence Berkeley National Laboratory.
