Abstract
Abstract Scanning transmission X-ray microscopy is a powerful method for mapping chemical phases in nano-materials. The point spread function (PSF) of a conventional zone-plate-based microscope limits the achievable spatial resolution and also results in spatially resolved spectra that do not accurately reflect the spatial heterogeneity of the samples when the scale of the detail approaches the probe size. X-ray ptychography, a coherent-scattering-based imaging scheme that effectively removes the probe from the image data, returns accurate spectra from regions smaller than the probe size. We show through simulation how the long tails on the PSF of an x-ray optic can cause spectral distortion near a boundary between two spectrally distinct regions. The resulting apparent point spectra can appear mixed, with the species on one side of the boundary seeming to be present on the other even at a distance from the boundary equal to several times the spatial resolution. We further demonstrate the effect experimentally and show that ptychographic microscopy can return the expected spectra from a model system, whereas conventional microscopy does not.
Highlights
Chemical inhomogeneity is key to the functioning of many systems, both biological and technical
We show through simulation on a model system how the point spread function (PSF) of a zone plate, in particular the long tails, leads to spectral distortions
While these data are for an scanning transmission X-ray microscopy (STXM) stack simulated starting with a ptychographic stack, the profiles extracted from the actual STXM stack (Fig. 5, red points) are very similar to those from the simulated STXM, indicating that the mixing does come from the PSF rather than some other source like image registration
Summary
Chemical inhomogeneity is key to the functioning of many systems, both biological and technical. The separation of nanoscale chemical phases is often indicative of the reaction kinetics or material degradation mechanisms (Boesenberg et al, 2013). In both cases, the relevant length scale is often below 50 nm. For instance, there is very little contrast of electron density, so non-spectral techniques produce little information. The important information may be the redox state of a transition metal cathode (Whittingham, 2004). In all these systems, X-ray spectral microscopy has led to important advances. A coherent-scattering based variant of this, ptychography, can improve the resolution to below 10 nm by using a phase retrieval process to reconstruct the probe and
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