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

Here the possibility of utilizing X-ray reflectivity for visualization with approximate to mu m spatial resolution of a surface with a heterogeneous electron density due to a partial coverage by another nanometrically thin material is demonstrated. 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, 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. 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 is demonstrated. This method can follow the growth kinetics, including the coverage ratio, 2D crystal (flake) sizes, and distances between flakes. By taking a single scan, the individual X-ray reflectivity curves can be reconstructed, of both covered and noncovered parts of the surface, allowing to deduce the corresponding electron density profiles perpendicular to the surface. The technique has a promising perspective for in situ study of 2D materials, ultrathin films, and self-assemblies on liquid as well as solid surfaces.

Automated summary

This study demonstrates the possibility of using X-ray reflectivity to visualize the surface with a heterogeneous electron density due to a partial coverage by another nanometrically thin material with μm spatial resolution. The sample needs to be convexly bent to reflect the collimated incident beam onto a magnified image recorded by a position-sensitive detector. By utilizing small, intense, and parallel beams, spatially resolved X-ray reflectivity can be recorded with a frame rate of 0.1-1 kHz. The application of this method for in situ characterization of single-layer graphene domains during their chemical vapor deposition on a naturally curved surface of a liquid copper drop is also demonstrated.