Effect of the crystallographic c-axis orientation on the tribological properties of the few-layer PtSe2

TL;DR

This study investigates how the crystallographic c-axis orientation of ultra-thin PtSe2 layers affects their tribological properties, revealing that orientation influences friction, surface roughness, and electrical characteristics.

Contribution

It provides a comparative analysis of nanoscale tribological behavior of PtSe2 with different c-axis orientations, highlighting the impact of surface orientation on friction and morphology.

Findings

Horizontal c-axis orientation reduces the coefficient of friction.

Vertical orientation increases surface roughness.

Epitaxial films exhibit lower friction due to reduced adhesion.

Abstract

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 epitaxial PtSe2 layers. PtSe2 c-axis orientation was found to has an impact on the nanotribological, morphological and electrical properties of PtSe2, in particular the change in the alignment of the PtSe2 flakes from vertical (VA) to horizontal (HA) led to the lowering of the coefficient of friction from 0.21 to 0.16. 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 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.