Synthesis and nanoscale characterization of hierarchically assembled molecular nanosheets

Chemical functionalization of molecular two-dimensional (2D) materials towards the assembly of hierarchical functional nanostructures is of great importance for nanotechnology including areas like artificial photocatalytic systems, nanobiosensors or ultrafiltration. To achieve the desired functionality of 2D materials, these need to be characterized down to the nanoscale. However, obtaining the respective chemical information is challenging and generally requires the application of complementary experimental techniques. Here, we demonstrate the synthesis and chemical characterization of hierarchically assembled molecular nanosheets based on about 1 nm thin, molecular carbon nanomembrane (CNM) and covalently grafted, single-molecule layer cobalt(III) catalysts for the light-driven hydrogen evolution reaction (HER). We employ X-ray photoelectron spectroscopy (XPS) and tip-enhanced Raman spectroscopy (TERS) to access both the transversal and lateral chemical information of the synthesized nanosheets with nanometer resolution. TERS and XPS data provide detailed information on the average and local surface distribution of the catalyst as well as mechanistic details of the grafting reaction. The proposed approach represents a general route towards a nanoscale structural analysis for a variety of molecular 2D materials - a rapidly growing materials class with broad prospects for fundamental science and applications.