Dark-field X-ray imaging makes structural inhomogeneities visible through small-angle scattering, but the directional variants of the method, which also reveal how such features are oriented, have so far been confined to the micrometre scale. In this study, a directional dark-field setup for nanoimaging is presented for the first time, retrieving the orientation of scattering features below the spatial resolution limit of the microscope. The configuration is experimentally simple and can be implemented with existing full-field transmission X-ray microscopes.
The performance was validated at the nanotomography endstation of beamline P05 at the PETRA III synchrotron at DESY in Hamburg. Test structures with line pairs below the resolution limit confirmed that sub-resolution orientations are correctly resolved, and measurements on hierarchical nanoporous silicon showed that orientational changes within the pore network can be cross-correlated across the sample.
Applied to human tooth enamel, the method mapped the directional arrangement of hydroxyapatite nanocrystals with widths of 30 to 70 nanometres, which are bundled into micrometre-sized rods. By making use of shadow regions in the optical configuration, the detectable range of scattering vectors was extended further, demonstrating a pathway towards size-selective dark-field imaging in which different feature sizes can be addressed separately.
The advancement makes the structural anisotropy of nanomaterials quantitatively accessible in a full-field microscope, which is of direct relevance for biomineralisation, advanced materials and nanotechnology applications.
The study is a collaboration between Helmholtz-Zentrum Hereon, the Technical University of Munich, Hamburg University of Technology, DESY, the Paul Scherrer Institute, Fraunhofer IAPT and the Medical University of Vienna, supported by the Collaborative Research Centres CRC 1615 (SMART Reactors) and CRC 986 (M³).
Sami Wirtensohn, Silja Flenner, Dominik John, Peng Qi, Christian David, Manfred May, Patrick Huber, Dirk Herzog, Stefan Tangl, Carina Kampleitner, Kritika Singh, Ingomar Kelbassa, Katrin Bekes, Julia Herzen, Imke Greving (2026). Directional dark field for nanoscale full-field transmission X-ray microscopy. Light: Science & Applications 15, 223.
