Unveiling the effect of substrate on graphene via non-destructive multiscale Raman spectroscopy

Graphene has now become a frontier material for various industrial applications due to its remarkable properties such as high electrical conductivity, exceptional mechanical strength and thermal conductivity. Indeed, the quality control of the produced graphene is one of the main issues to be solved for its successful application in various technologies such as flexible electronics, energy storage devices and advanced composites.The samples subject to this study were produced by Graphenea during the work of the H2020 European project CHALLENGES (realtime nano CHAracterization reLatEd techNoloGiES); in particular, these samples consists of graphene grown on copper substrates and then transferred to silicon with different values of resistivity. These samples were then characterised by the Physikalisch-Technische Bundesanstalt (PTB) and the Sapienza University of Rome. The study was mainly focused on the application of advanced optical non-destructive characterisation techniques, in particular correlative microscopy (Atomic Force Microscopy and Raman spectroscopy) and Tip-Enhanced Raman Spectroscopy (TERS), to evaluate their application for in-line quality control. Using monochromatic laser light, Raman spectroscopy can reveal valuable information about strain, defects and compositional variations, while the non-invasive nature of these techniques makes them ideal for industrial applications, allowing real-time monitoring without compromising sample integrity but with relatively low spatial resolution (diffraction-limited). In addition to conventional correlative microscopy using Raman spectroscopy and atomic force microscopy (AFM), tip-enhanced Raman spectroscopy (TERS) is emerging as a powerful tool in the arsenal of graphene characterisation techniques. Combining the high spatial resolution of scanning probe microscopy with the sensitivity of Raman spectroscopy, TERS provides not only high-resolution topographical information, but also rapid chemical mapping of graphene at the nanoscale thanks to the plasmonic effect at the tip. This technique reveals nuances in strain distribution and compositional defects at the nanoscale level, providing unprecedented insight into the local properties of graphene. Understanding the strain and compositional defects in graphene grown on copper and after transfer to silicon is crucial for tailoring the material for specific applications. The choice of substrate can significantly affect the electronic and mechanical properties of graphene, so comprehensive characterisation using advanced optical techniques is essential.