Correlating Surface Structures and Nanoscale Friction of CVD Multi-Layered Graphene

Surface structures, ubiquitous in chemical vapor deposition (CVD) grown graphene, determine functional properties of graphene. Since even small changes in the surface structures of 2D nanomaterials may have critical impact on their mechanical and electrical properties due to the large surface area and exceptional ratio of surface area to volume. Therefore, in-depth understanding about the relationship between the structures and the properties is essential to investigate CVD graphene in a variety of scientific and engineering applications. Here, we correlate various surface structures with nanoscale friction of multi-layered graphene islands. We utilized atomic force microscopy (AFM) tip-driven mechanical scratching to obtain super-clean graphene surface, which enabled us to unveil various surface structures such as small-scale and large-scale (folded) wrinkles on multi-layer graphene islands and their effect on interlayer configuration. The network of folded graphene structures formed during the growth was found to induce the top-most graphene layer twisted over the underlying AB-stacked multiple layers of graphene which was revealed by the combinatory analysis of AFM and Raman spectroscopy. This structural change resulted in the increase of interlayer distance, and resultantly influenced the layer-dependent nanoscale friction of multi-layer graphene. Strong layer-dependency of the nanoscale friction was observed only at a high normal load due to the difference in layer-dependent lateral bending stiffness, but no identifiable layer-dependency was observed at the low normal load. Increasing the normal load resulted in the transitional change of the layer-dependency on nanoscale friction, which can be attributed to the decreased interlayer distance between the top-most graphene layer and the underlying graphene layers, resultantly increasing the contribution of different lateral bending stiffness to nanoscale friction. This work provides the in-depth understanding about the correlation between surface structures and layer-dependent nanoscale friction of CVD graphene, highlighting the significance of surface cleanliness of graphene to obtain accurate nanoscale imaging.