Non-invasive three-dimensional probing of atomic order in single-crystalline thin films

The emergence of powerful synchrotron sources, fast pixel detectors, and the increasingavailability of high-performance computing has facilitated the acquisition and analysisof high-quality, large-volume diffuse scattering data, providing insights into the realstructures of disordered crystals that were not previously possible. However, its ap-plication to studying local structural order in thin films remains largely unexplored.This limitation arises from the strong substrate contributions, which obscures the weakdiffuse scattering signals from the film, as well as the restricted reciprocal space cov-erage and resolution inherent in grazing incidence X-ray diffraction. This doctoralwork pioneers the extension of single-crystal diffuse X-ray scattering techniques tothin films, enabling atomic-scale characterization of complex domain structures andtheir correlations within and across layers.We introduce and validate a novel methodology for the structural characteriza-tion of single-crystalline thin films, overcoming challenges associated with non-invasiveprobing of nanoscale domain configurations. A key accomplishment is the implemen-tation of high-energy ultra-small grazing-incidence X-ray diffraction, which effectivelymitigates substrate contributions while achieving large reciprocal space coverage andhigh sensitivity to small atomic displacements. The large-volume and high-resolutiondiffuse scattering data is analyzed using three-dimensional difference pair distributionfunction (3D-ΔPDF), which provides invaluable insights into the local atomic structureof single-crystalline thin films.The methodology is first applied to disordered oxide perovskite thin films, wherewe establish a framework for interpreting 3D-ΔPDF features associated with oxygenoctahedral tilts and cation displacements. The approach is successfully validated onthe known structure of SrRuO3 on SrTiO3, revealing a+a−c− tilts and picometer-scalecation shifts with superior spatial resolution compared to other characterization meth-ods. Furthermore, correlation lengths and rotational amplitudes of the oxygen octahe-dral tilts are analyzed as a function of layer thickness and temperature, demonstratingthe potential of the methodology in quantitative analysis. Central to the analysis isthe divide-and-conquer approach that allows for the selective examination of diffusescattering features and ΔPDF signals with distinct structural properties, showcasingthe versatility of the methodology.ivBuilding on this validated framework, we extend our approach to studying com-plex polarization domain correlations in ferroelectric PbTiO3 and dielectric SrTiO3(PTO|STO) superlattices. Our findings reveal the presence of domain correlationswithin and across PTO layers, extending into adjacent STO layers. We also investi-gated temperature-dependent phase transitions of the domain correlations and iden-tified two low-temperature phases with strong anisotropic diffuse scattering and onehigh-temperature pattern with ring-like appearance. In general, thermal stability ofthe low-temperature configurations increases in superlattices with higher bilayer thick-ness, and the isotropic configuration emerges at temperatures significantly above roomtemperature. On the other hand, the only examined superlattice structure, wherethe PTO layers are thicker than the STO layers, exhibits ring-like scattering even atroom-temperature, suggesting that thicker PTO layers experience reduced anisotropicstrain from the STO layers.Finally, complementary 3D-ΔPDF analysis reveals the presence of Néel-type do-main walls in the investigated superlattices. A major breakthrough of this work isthe non-invasive probing of buried layers within the superlattices, which unveils aquadratic-shaped flux-closure polarization configuration induced by tetragonal straineffects — an observation rarely achieved with conventional characterization methods.This demonstrates the method’s capability to identify local structural chirality, a keyfeature in many complex materials hosting nontrivial topologies.