X-Ray Microscopy

Group Leader: Jörg Maser

The Center for Nanoscale Material’s Hard X-Ray Nanoprobe at sector 26 of the Advanced Photon Source is a next-generation hard X-ray microscopy and X-ray imaging beamline with highest spatial resolution. The system provides X-ray fluorescence, X-ray diffraction, coherent diffraction, and transmission imaging with hard X-rays at a spatial resolution of 30 nm or better. This unique instrument will not only be key to the specific research areas of the CNM; it will also be of general utility to the broader nanoscience community in studying nanomaterials and nanostructures, particularly for embedded structures.

The combination of diffraction, fluorescence, and transmission contrast in a single tool will provide unique characterization capabilities for nanoscience. Current hard X-ray microprobes based on Fresnel zone plate optics have demonstrated a spatial resolution of 50 nm at a photon energy of 8-10 keV. With advances in the fabrication of zone plate optics, coupled with an optimized beamline design, an initial spatial resolution of 30 nm has been obtained. Advanced X-ray optics are expected to improve the spatial resolution further. The nanoprobe will cover the spectral range of 3-30 keV, and the working distance between the focusing optics and the sample will typically be in the range of 10-20 mm.

Jörg Maser (left) and Robert Winarski, CNM X-Ray Microscopy Group, at the hard X-ray nanoprobe beamline on Advanced Photon Source (APS) Sector 26. The nanoprobe uses brilliant X-rays with photon energies from 3 to 30 keV to probe the properties of nanoscale materials with a spatial resolution of 30 nm. The system provides a combination of scanning-probe and full-field transmission imaging.

Modes of Operation

Transmission. In this mode, either attenuation or phase shift of the X-ray beam by the sample can be measured. Absorption contrast can be used to map the sample’s density. Particular elemental constituents can be located using measurements on each side of an absorption edge to give an element-specific difference image with moderate sensitivity. Phase-contrast imaging can be sensitive to internal structure even when absorption is low and can be enhanced by tuning the X-ray energy.

Diffraction. By measuring X-rays diffracted from the sample, one can obtain local structural information, such as crystallographic phase, strain, and texture, with an accuracy 100 times higher than with standard electron diffraction. Coherent diffraction methods in Bragg and SAXS geometry provide information on subregions of the sample inside the focal spot.

Fluorescence. Induced X-ray fluorescence reveals the spatial distribution of individual elements in a sample. Because an X-ray probe offers 1,000 times higher sensitivity than electron probes, the fluorescence technique is a powerful tool for quantitative trace element analysis, important for understanding material properties such as second-phase particles, defects, and interfacial segregation.

Spectroscopy. In spectroscopy mode, the primary X-ray beam’s energy is scanned across the absorption edge of an element, providing information on its chemical state (XANES) or its local environment (EXAFS), which allows the study of disordered samples.

Polarization. Both linearly and circularly polarized X-rays will be available. Contrast due to polarization is invaluable in distinguishing fluorescence and diffraction signals and imaging magnetic domain structure by using techniques such as linear and circular dichroism and magnetic diffraction.

Tomography. In X-ray tomography, one of these modes is combined with sample rotation to produce a series of two-dimensional projection images, to be used for reconstructing the sample’s internal three-dimensional structure. This is particularly important for observing the morphology of complex nanostructures.

In summary, a hard X-ray nanoprobe provides advantages, such as being noninvasive and quantitative, requiring minimal sample preparation, giving suboptical spatial resolution, having the ability to penetrate inside a sample and study its internal structure, and having enhanced ability to study processes in situ. Another important distinction from charged-particle probes is that X-rays do not interact with applied electric or magnetic fields, which is an advantage for in-field studies. The design of the nanoprobe beamline aims to preserve these potential advantages.

Research Activities

• Hard X-ray nanoprobe
• Highest-resolution optics for hard X-rays
• Time-resolved stroboscopic measurements
• Full-field imaging
• Scanning probe fluorescence, diffraction, and transmission phase contrast imaging
• Polarization-dependent scattering
• General nanomaterials characterization with X-rays, including coherent diffraction