X-Ray Ptychography and Microscopy (Nano-XRM)
X-ray ptychography and microscopy is performed by the Hard X-ray Nanoprobe Beamline operated by the Electron and X-ray Microscopy Group at the Center for Nanoscale Materials (CNM) at Sector 26 of Argonne’s Advanced Photon Source (APS). The Argonne Hard X-ray Nanoprobe provides a platform for nanoscale materials research that uses highly focused coherent X-ray microscopy methods that harness the brilliance of the synchrotron for transformative insight into these materials.
The Hard X-ray Nanoprobe is shared by the CNM and the APS and is a synchrotron-based nanoscale X-ray microscopy beamline that enables two core capabilities: Bragg ptychography and real space structural/chemical correlative mapping. These capabilities leverage several fundamental strengths of hard X-ray microscopy, including: i) large penetration depth; ii) high fundamental sensitivity to strain (10-5 dc/c); iii) chemical sensitivity (parts-per-million trace elemental detection); iv) phase-coherent diffraction methods; and v) the unique per-bunch-brightness of the APS (~100ps), which provides unique insight into a broad range of high-impact questions related to structure-function relationships at the nanoscale.
Bragg ptychography, a scanning coherent diffractive imaging technique that exploits the coherence of the nanofocused X-ray beam, combined with iterative phase retrieval methods, provides nanoscale structure and lattice strain information within crystalline samples at a resolution extending to 5 nm, well beyond that of current hard X-ray focusing optics. The ptychography technique results in a real space complex-valued image of the phase and amplitude locally imparted to the beam by the sample in real space with a flexible field of view—this image is a quantitative map of atomic displacements that allows for flexible analysis of strain, polarization, dislocations and defects within active materials. Scanning nanodiffraction and Bragg ptychography are in high demand at the HXN as tools for probing crystal ordering, defects and phase transitions.
The CNM/APS HXN is also uniquely capable of correlative chemical and structural nanoimaging at a resolution of roughly 25 nm through the use of scanning fluorescence X-ray microscopy (SFXM) in combination with scanning X-ray diffraction microscopy (SXDM).
In SFXM, the spatial distributions of the elements in a sample are mapped by scanning a nanofocused X-ray spot over it as the emitted fluorescence X-rays are measured by an energy-dispersive detector. Characteristic fluorescence X-rays emitted by the sample uniquely identify the elements present in it with 1000X greater sensitivity than electron probes; the incident photon energy can also be tuned over absorption edges to analyze the sample's chemical state.
Nanoscale elemental and chemical mapping with the HXN enables understanding of material properties, such as trace contaminants, second-phase particles, defects and interfacial segregation. Nanoscale structural information, such as crystallographic phase, strain and texture present in atomic lattices are measured at the HXN using SXDM at a spatial resolution down to 25 nm by recording how a crystalline sample diffracts the incident nanofocused X-ray beam while on the Bragg condition as the beam spot is scanned over the sample. This can be done at a sensitivity to lattice strain of 10^-5 dc/c, enabling the measurement of crystalline lattice deformations on the order of less than a picometer, two orders of magnitude beyond the sensitivity of modern electron microscopy techniques. Combining SFXM measurements of the chemical distribution in the sample with SXDM mapping of structure creates a unique path for observation of strain-induced chemical activity in catalysis, structural nucleation of trace elements in energy harvesting materials, and nanoscale strain induced by chemical processes in energy storage materials.