Structure and Dynamics at Interfaces

Welcome to this issue of SRN. We present to you an overview of new developments in X-ray methods for investigating the structure and dynamics at interfaces. Once a very specialized topic, understanding the structure, kinetics and dynamics at surfaces and interfaces has become increasingly popular in the last years. For one, fascinating aspects of fundamental science, such as understanding the effects of altered symmetry at interfaces, become more accessible with the improved performance of synchrotrons and free electron lasers delivering ever more intense beams in smaller volumes. This progress has enabled grazing incidence measurements across both scattering and spectroscopic methods. The increasing range of time scales accessible also opens new avenues for exploring dynamics at surfaces. Second, these new capabilities have opened up the way for in-situ and operando investigations of technologically relevant systems such as green catalysts or smart sensors. The challenges in investigating weakly scattering liquid and biological interfaces are also reduced so that there is impressive progress in these fields.

The first article by Oliver Seeck presents an overview of surface and interface methods, outlining the history and method developments over the years. The experimental conditions required for surface sensitive X-ray techniques using the reduced penetration length of X-rays for investigating surfaces and interfaces are outlined and illustrated with exemplary beamline setups ranging from classical Grazing incidence geometry to recent applications such as surface Pair-Distribution Function of surface atoms requiring high energies and the resulting challengingly low angles of incidence. Other methods discussed include the Crystal Truncation Rods, which allow highly surface sensitive investigations and the usefulness of both grazing incidence small and wide angle scattering for coherent imaging and surface X-ray Photon Correlation Spectroscopy. For the future, we can look forward to the development of fast and reliable data real time analysis methods using artificial intelligence routines to provide valuable results in time to feedback to the experiment workflow.

Coherent Bragg diffraction methods are valuable not only for imaging internal structure and strain otherwise invisible but also for imaging domains. Ian Robinson reviews Bragg Coherent Imaging of Fluctuating Domains, illustrating two examples of domain dynamics in crystalline solid state samples using Bragg Coherent Diffraction. Domains are associated with sub-atomic phase shifts in the structure, which are revealed by the strong phase contrast associated with Bragg diffraction. Bragg Ptychography can visualize slow magnetic fluctuations, for example, in La_2−xSr_xNiO₄ where antiferromagnetic spin-ordered phases occur. Domains in ferroelectric BaTiO₃ have been imaged using Bragg Coherent Diffraction Imaging to follow electric field induced movement of internal domain walls, providing a possible explanation for the nanomaterial’s anomalously large dielectric constant, which is used in the manufacture of multilayer ceramic capacitors.

Joseph Strzalka and co-authors describe methods for X-ray photon correlation spectroscopy at interfaces using grazing incidence at the newly upgraded APS. They provide a brief review of the principles of GI-XPCS and discuss its recent applications in soft matter systems such as Brownian motion in complex fluids within confined nanoscale volumes and the stress relaxation of 2D Langmuir monolayers of organic-soluble iron oxide nanoparticles at the air/water interface. Additionally, the contribution presents experimental considerations related to GI-XPCS including state of the art instrumentation and discusses diffuse and multiple scattering effects. Approaches for monitoring and mitigation of radiation damage, a particular problem for dynamics, are also outlined.

Wei Bu and ChemMatCARS colleagues describe some of the beamline past successes along with new possibilities for Liquid Interface Science at the APS, ranging from chemical to life sciences, and bioengineering. The beamline’s instrumentation and software development, outreach activities and diversity programs are also outlined. Liquid interfaces provide rich and exciting model systems for investigating the assembly and growth of molecules, ions, or nanoparticles and their response to perturbations, such as changes in temperature, pH or electric potential. Specific examples including electrostatics at aqueous/organic interfaces, transport of ions from aqueous to organic solvents, anionic phosphatidylserine enzyme-lipid interactions are presented.

We do not forget that lower energy X-ray methods, Ultrafast Photoemission Spectroscopy, and Photon Engineering Techniques, which were recently developed at NSRRC Taiwan, are described by Ping-Hui Lin and co-authors. They have developed a pump-probe scheme for Time-resolved Photoemission Spectroscopy to investigate the dynamics of chemical bonding and electron-hole recombination post-photon stimulation, gaining information on the optical and transport characteristics of emerging materials. They also present Time-resolved Angle-resolved Photoemission Spectroscopy of the femtosecond timescale to investigate non-equilibrium dynamics of many-body effects and collective excitations, such as electron-electron interactions, electron-phonon interactions, and excitons across a range of timescales.

We hope you enjoy the articles, which provide only a small peek into the wealth of exciting science now possible at surfaces and interfaces.

Yours sincerely,
Hyunjung Kim and Bridget Murphy

Disclosure statement

No potential conflict of interest was reported by the author(s).n