X-ray Diffraction Investigation of Disorder in van der Waals Thin Films

Van der Waals (vdW) materials, characterized by their vertically stacked two-dimensional (2D) layers, exhibit unique electronic properties due to the weak vdW interactions between the layers. Using x-ray diffraction with an area detector, we explore vdW thin films of PbI₂ grown by chemical vapor deposition (CVD) as well as resultant 2D perovskite films produced by intercalating PbI₂ films with long chain organic cations. Owing to the weak vdW interaction with the substrate, the films grow flat on any substrate. Furthermore, rotational disorder is observed in the film plane, giving rise to a 2D powder. This 2D powder manifests as well-defined diffraction spots, which we have quantitatively modeled. The area detector is particularly effective for observing stacking disorder that is another consequence of the weak vdW interaction between the layers. The interpretation and modeling of the diffraction images will be discussed and represents a broadly useful approach for investigating layered vdW systems which comprise a ubiquitous class of materials.