Near-field nano holographic tomography with chemical contrast exploiting low-dimensionality constraints

X-ray absorption near-edge structure (XANES) spectroscopy is key to understanding functional materials. We present, to our knowledge, the first use of hard x-ray near-field holography (NFH) for three-dimensional (3D), spatially resolved XANES. While NFH provides lensless imaging with an adjustable field of view, it entails complex reconstruction. Our method jointly reconstructs NFH data acquired at multiple energies and enforces a low-dimensional spectral model based on the physically motivated assumption that each voxel spectrum is a linear combination of a small number of constituents. We demonstrate the approach on a mixed metal oxide catalyst particle, achieving accurate spectral reconstruction and revealing 3D chemical heterogeneity. XANES in NFH mode has the potential to enable nano-scale in-operando XANES microscopy with a large field of view.