P07 focuses on micro-beam applications of high-energy x-ray diffraction and scattering. All offered techniques involve complex sample environments, large samples, and/ or data collection over a wide range in reciprocal space up to high momentum transfer Q. More details on the available capabilities are given in the list below.
Grazing incidence diffraction and scattering
The study of surfaces, interfaces, and thin films requires to probe the structure of a single monolayer up to several tens of nanometers of a material on or within a bulk sample. For this purpose, grazing incidence (GI) geometry is the most effective way to limit the penetration depth to the surface-near/ interface-near region. GI refers to illuminating the sample under an incident angle smaller or around the critical angle of total external reflection, which amounts to values < 0.1° at high photon energies. Therefore, highly precise positional alignment of the surface/ interface with respect to the x-ray beam is a necessary requirement to successfully perform a GI experiment. In addition, a tightly focused x-ray beam is essential to restrict the x-ray footprint resulting from the very shallow incidence angle not to exceed the sample dimension.
EH2 hosts a dedicated surface diffractometer designed for high-energy GI measurements. The x-ray beam is routinely focused down to a vertical size of ~2 μm (FWHM), confining the footprint to the millimeter range which matches common sample sizes from 1 cm2 up to 2 inch diameter. Dedicated reactors and vacuum chambers can be installed to follow growth and phase change processes such as thin film deposition and crystallization, catalytic reactions at the gas-solid interface or the electrode-electrolyte interface. Depending on the nature of the sample, different diffraction and scattering techniques are available incl. reciprocal space mapping and crystal truncation rod (CTR) measurements for single-crystal surfaces and epitaxial films, and total scattering in GI and pair distribution function (PDF) analysis for disordered polycrystalline and amorphous layers. Complementary x-ray reflectivity measurements can be performed to derive information on the film thickness, roughness and density.
X-ray diffraction computed tomography (XRD-CT)
Complementary to full-field tomographic imaging methods such as absorption contrast CT, XRD-CT is a small-beam scanning technique that yields spatially resolved structural information in the form of diffraction patterns throughout the volume of a sample. High-energy x-rays are the probe of choice to non-destructively visualize the interior of large objects on the range of millimeters to centimeters. Neither soft and hard X-rays or electrons, nor neutrons provide sufficient penetration and spatial resolution, respectively, to provide an adequate insight into the core of enclosed systems on this length scale.
The spatial resolution is determined by the size of the focal spot of the beam. In EH2, the standard microfocus beamsize is 2 × 30 μm2 (FWHM), with smaller beamsize (about half size, higher divergence) available on request. When recording total scattering patterns up to high momentum transfer Q, a pair distribution function (PDF) for each volume element (voxel) can be calculated that describes the local structure in real space. The combined capabilities for XRD-CT and PDF-CT form the basis to study objects and devices with crystalline and amorphous or nanocrystalline components, which applies to e.g. batteries, catalysts, and bones. Due to the tomographic scanning procedure of translational and rotational movements, these techniques are limited in time-resolution but allow to follow slow changes that take place on the scale of hours, such as aging of catalysts, charge/discharge of batteries, and chemical reactions in sealed containers with slow kinetics.
Microbeam diffraction and scattering
In general, EH2 offers capabilities for high-energy diffraction and scattering on bulk samples from single crystals to polycrystalline, disordered and amorphous solids as well as liquids. Similar experimental conditions are available at beamline P21.1, which is jointly operated together with P07-EH2 by the same staff members in the P07211 team. Successful proposals for these two stations are distributed to the best benefit for the users taking into account differences in beam parameters and available beamtime. Considering the EH2 share of 1/3 of total beamtime at P07, those experiments that necessarily require a small focal size, maximized flux, or tunable energy are prioritized at EH2. In addition to the above surface and XRD-CT methods, micro-XRD measurements e. g. on very small or twinned single crystals or in scanning mode on heterogeneous bulk samples fall into this category.
Multimodal approaches
Besides recording the atomic structure, some experiments benefit from obtaining chemical information. An energy-resolving point detector for x-ray fluorescence measurements is part of the beamline equipment. In the frame of the Virtual Laboratory for Instrumental Analysis (VIA), more complementary techniques to the x-ray scattering and diffraction experiments are available. These include e.g. spectroscopic methods (UV-vis-NIR, FTIR, Raman) and mass spectrometry, partly for online measurements at the beamline and partly for offline sample characterization in the lab. More information on the instrumentation pool and how to get access is found on https://photon-science.desy.de/facilities/petra_iii/sample_environment__laboratories/analytical_methods_via/index_eng.html
