Quantifying Orientational Order of PbI₂ van der Waals Films with X-ray Diffraction using an Area Detector

Van der Waals (vdW) materials, comprised of vertically stacked two-dimensional (2D) molecular layers, exhibit strong in-plane bonds and weak out-of-plane interactions. These systems offer enhanced tunability and the potential for tailoring their electronic and magnetic properties, which are sensitive to their surface interactions. Consequently, it is important to characterize the stacking and orientational properties between the molecularly thin vdW 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 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 modelled. The area detector lends well to the observation of stacking disorder, which is also a result 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 that comprise a ubiquitous class of materials.