Nanoscale friction and wear behavior of a CVD-grown aged WS2 monolayer: the role of wrinkles and surface chemistry†
Friction reduction by transition metal dichalcogenide (TMD) monolayers is well documented; however, wrinkle formation on the surface of TMDs takes place due to strain relaxation over time and leads to the deterioration of the tribological properties at a small scale. Herein, we report the role of wrinkles on the wear behavior of a chemical vapor deposition (CVD) grown aged WS2 monolayer and the comparison with wrinkle-free regions. Atomic force microscopy (AFM) was utilized to perform load-dependent experiments, and we noticed that the wear initiated near wrinkles resulted in the disintegration of the monolayer. In contrast, in the wrinkle-free regions, wear occurred at significantly higher loads, similar to that of freshly grown WS2, although the coefficient of friction (COF) was increased due to the changes in surface chemistry as a result of aging, which was confirmed using X-ray photoelectron spectroscopy (XPS). In the presence of wrinkles, a ten-fold reduction in the load-carrying capacity was observed compared to the wrinkle-free regions. Molecular dynamics (MD) simulations were used to corroborate experimental findings, which demonstrate the role of wrinkles in the initiation of wear due to the stress concentration under sliding nanocontacts near the wrinkles. In addition, simulations help establish a relationship between the adsorbed chemical species on the surface and increased COF.