Employing Surface Curvature for Spatially Resolved X-Ray Reflectivity: Graphene Domains on Liquid Copper

Here we demonstrate the possibility of utilizing X-ray reflectivity for visualization with ~μm spatial resolution of a surface with a heterogeneous electron density due to a partial coverage by another nanometrically thin material. It requires the sample to be convexly bent, thus reflecting the collimated incident beam onto a magnified image recorded by a position-sensitive detector. By the use of a small, about ten microns, intense, and parallel beam such as provided by the most recent synchrotron sources, one can record such spatially resolved X-ray reflectivity with 0.1‒1 kHz frame rate. We demonstrate the use of the method for in situ, time-resolved characterization of single-layer graphene domains during their chemical vapor deposition on a naturally curved surface of a liquid copper drop. This method can follow the growth kinetics, including the coverage ratio, two-dimensional crystal (flake) sizes, and distances between flakes. By taking a single snapshot, we can reconstruct the individual X-ray reflectivity curves, of both covered and non-covered parts of the liquid surface, and thus deduce the corresponding electron density profiles perpendicular to the surface. The technique has a promising perspective for in situ study of two-dimensional materials, ultra-thin films, and self-assemblies on liquid as well as solid surfaces.