Two-dimensional (2D) transition metal dichalcogenides are potential candidates for ultrathin solid-state lubricants in low-dimensional systems owing to their flatness, high in-plane mechanical strength, and low shear interlayer strength. Yet, the effects of surface topography and surface chemistry on the tribological properties of 2D layers are still unclear. In this work, we performed a comparative investigation of nanoscale tribological properties of ultra-thin highly-ordered PtSe2 layers deposited on the sapphire substrates with the in-plane and out-of-plane crystallographic orientation of the PtSe2 c-axis flakes, and also layers consisting of epitaxial PtSe2. PtSe2 c-axis orientation was found to has a moderate impact on the nanotribological and morphological properties of PtSe2, in particular the change in the alignment of the PtSe2 flakes from vertical (VA) to horizontal (HA) led to 1.3 times lower coefficient of friction. This observation was accompanied by an increase in the root-mean-square surface roughness from 1.0 to 1.7 nm for the HA and VA films, respectively. The epitaxial films with larger flakes showed lower friction caused by lowering adhesion when compared to other investigated films, whereas the friction coefficient was similar to films with HA flakes. The observed trends in nanoscale friction is attributed to a different distribution of PtSe2 structure.
Published in: "arXiv Material Science".