Multi­modal Coherent X-­ray Imaging and Tomography of Functional Materials

X­-ray microscopy is a powerful tool for examining different materials with a combination of field­of­view and resolution that is not easily accessible by electron and optical microscopy, respectively. Due to its ability to penetrate materials, X­-ray microscopy has become an indispensable tool for investigation of the internal properties of volumetric objects.

Absorption contrast­based radiography and computerized tomography have been the most widespread types of X­-ray microscopy techniques. However, with the development of high-­brilliance X­-ray sources, more advanced X­-ray optics, and sophisticated detection schemes, new types of contrast modalities have been realized to provide higher contrast and extend sensitivity to material properties beyond structural and morphological ones.

Dedicated synchrotron X-­ray instruments have been developed for simultaneous sensitivity to different contrast modalities. The main focus of this thesis is on the development and application of coherent X-­ray microscopy techniques for the characterization of functional materials across multiple scales, modalities, and dimensions.

Such flexibility is accessible by phase-­contrast X­-ray imaging techniques, such as scanning coherent X­-ray imaging (ptychography) and grating­-based interferometry. The scanning mode of application for ptychographic imaging allows for combination with correlative studies of electrical and chemical properties of energy materials, such as multilayered thin­film solar cells with close interconnection between electrical performance and the structure of layers at the nanoscale.

Extension of this method into three dimensions via computerized tomography is discussed under the framework that allows for data-­efficient acquisition following the dose fractionation theorem. The framework performs reconstruction of the three-­dimensional volume directly from a multi­dimensional diffraction dataset.

Currently, the instrumental accuracy is limiting the practical realization of such an acquisition, and a fast scheme compensating for the limited accuracy of tomography stages is presented in this thesis. Based on the robustness of ptychographic imaging, a reflection mode configuration of this technique is demonstrated in this thesis.

This configuration allows for bridging the gap between the macroscale field­of­view and high-­resolution imaging, two mutually exclusive properties of imaging techniques. Direct observation of the evolution of materials on a substrate during processing is possible with this new configuration. The resolving capabilities and possibilities of accessing three­-dimensional information using ptychographic imaging in reflection mode are examined.

Part of this thesis, a secondment project with an industrial partner of the MUMMERING ITN, Xnovo Technologies, is devoted to the development of an advanced acquisition scheme for grating­based tensor tomography. Complementary to the phase-­contrast, anisotropic scattering is accessible in grating-­based interferometry as an additional contrast modality.

The investigation of anisotropic structure in three dimensions can be achieved by the flexibility of sample manipulation using a six­axis robotic arm. The robot simulation pipeline and experimental validation of the robot­-assisted data acquisition are presented.