Influence of tip materials on the friction force microscopy of epitaxial graphene on SiC(0001): comparison of diamond and silicon tips in experiments and atomistic simulations
Friction force microscopy (FFM) with silicon tips on epitaxial graphene supported by SiC(0001) previously revealed a sharp increase in friction at a threshold normal force, linked to the intermittent rehybridization of graphene and the formation of single-layer diamond above 12 GPa. In this study, the FFM behavior of a diamond tip is compared with that of the silicon tip. The diamond tip exhibited a similar abrupt increase in friction, but the threshold normal force was approximately twice as high for comparable tip radii. Simulations of graphene on SiC(0001) sliding against hydroxylated amorphous carbon (a-C) and silicon oxide (SiO2) slabs show that both systems exhibit low shear stress at low pressures, which increases at higher pressures due to bond formation between the graphene and counter slabs. The pressure dependence differs slightly: SiO2 reaches a shear stress plateau around 6 GPa, while a-C continues to increase gradually. For a-C, the transition threshold shifts to higher pressures, consistent with FFM results. Temperature lowers the transition threshold significantly, while sliding velocity has minimal impact on shear stress. These findings provide insights into the stability of low-friction interfaces between epitaxial graphene and the key materials which come into contact with graphene in current micro-electro-mechanical systems.