Simultaneous Monitoring of Molecular Thin Film Morphology and Crystal Structure by X-ray Scattering

Hybrid materials of pi-conjugated molecular semiconductors combined with two-dimensional (2D) materials exhibit a significant potential for novel (opto)electronics. The overall characteristics of molecular growth, such as molecular packing, crystallographic structure, and morphology, determine the specific application. The growth studies are thus key for understanding and improving these systems. So far, a simultaneous real-time study of the thin film morphology evolution along with its crystalline phase on a 2D substrate has not been performed. Here, we report on an additional feature of surface-sensitive grazing-incidence wide-angle X-ray scattering (GIWAXS). We show that refraction induced multiple scattering effects can be employed to reveal the morphology-related growth modes of a thin organic layer on a 2D underlayer together with the crystallographic structure and molecular orientation. A thorough description of the dynamic scattering effects is provided, allowing a detailed study of the Bragg peak splitting in the out-of-plane direction. We discuss the influence of the material-related parameters, such as stress and growth type, on the mutual position of the split peaks during the growth. Furthermore, we show that the time-resolved GIWAXS enables the observation of the electron density variation in real time. Our findings should be considered as a general feature of GIWAXS that can be applied to various types of thin films, where the temporal evolution of morphology, molecular arrangement, and crystallographic structure are of interest.