Directional Dark-Field Nanoimaging Achieved in Transmission X-ray Microscopy

Researchers have demonstrated the first directional dark-field setup for nanoimaging, achieving orientation mapping of scattering features below the spatial resolution limit. The method is implemented by adding apertures in front of the condenser in existing transmission X-ray microscopy setups.

com reported that dark-field X-ray imaging visualizes structural inhomogeneities through small-angle scattering, but existing directional dark-field methods have been confined to the micrometer scale.

The directional dark-field nanoimaging method was validated on a golden Siemens star test object with 600 nm structure heights. The Siemens star test object includes nearly horizontally and vertically oriented line patterns with decreasing structural sizes down to 30 nm feature size.

Directional dark-field projections in x- and y-directions are combined to calculate a directional dark-field image showing angular orientation and scattering magnitude.

Line pairs with a pitch of 60 nm, corresponding to 30 nm feature size, were visualized and their orientation mapped using directional dark-field imaging. The total exposure time of the directional dark-field image of the Siemens star was 4 × 300 s = 20 min. The spatial location of sub-resolution features in directional dark-field imaging remains limited to the spatial resolution of the setup.

A hierarchical nanoporous silicon pillar sample was imaged next. The nanoporous silicon pillar was created through additive manufacturing via powder bed fusion by laser beam of an aluminum-silicon alloy followed by a dealloying step. The silicon pillar has a porosity of around 89 percent and features large elongated pores with sizes in the range of 1 μm to 8 μm.

The pores in the silicon pillar consist of individual ligaments ranging from 50 nm to 200 nm in diameter. 28°.

34°. The dark-field signal is a projection that provides the average angular orientation throughout the entire sample thickness at a given pixel position. Aligned sample features scatter strongly perpendicular to their orientation and weakly in parallel directions.

A pillar from the enamel of a human permanent tooth with molar incisor hypomineralization was also imaged. Molar incisor hypomineralization is defined as hypomineralization of the enamel of at least one first permanent molar with or without the involvement of incisors.

Human tooth enamel consists mainly of hydroxyapatite crystals that have a width of 30 nm to 70 nm and a length of 100 nm to 1000 nm.

Hydroxyapatite crystals in enamel bundle into rods, or prisms, that are about 6 μm in diameter. 28°. In the MIH tooth enamel sample, a more marked inter-prismatic space was observed compared to healthy enamel.

The prisms in the imaged MIH tooth enamel appear as circular structures with an opening on one side known as a keyhole structure. The solid support structures on the top and right side of the tooth enamel sample are completely dark and do not contain any scattering structures. Edges create a SAXS signal mainly perpendicular to their longitudinal direction.

The directional dark-field TXM setup becomes sensitive to orientation by covering the condenser with a condenser aperture. Closing the dark-field aperture switches the TXM to normal dark-field modality. The study extends the detectable scattering vector range by utilizing shadow regions in the optical configuration and enables size-selective dark-field imaging.

The method was developed after recent advances extended dark-field capabilities to nanoscale transmission X-ray microscopy, though directional scattering retrieval had remained inaccessible at sub-micrometer resolutions. Directional dark-field imaging has previously been used to investigate microstructure in archeological skeletal remains and orientation within carbon fiber reinforced polymers.

Dark-field imaging has been used to detect lung diseases like emphysema and to improve breast cancer diagnostics by revealing micromorphology of breast calcifications.

Recent developments allow optics-free extraction of the dark-field signal based on the Fokker-Planck equation using multiple distances or multiple energies. The advancement enables the quantitative structural characterization of anisotropic nanomaterials critical to biomineralization, advanced materials and nanotechnology applications.