Two-dimensional (2D) materials, layered structures having atomic- to nano-scale thickness, have shown unique physical properties such as high in-plane stiffness combined with extremely low out of plane bending rigidity. 2D materials have been used in variety of applications including micro- and nanoelectronics devices, sensors and energy storages, as well as in coatings and lubricant additives to minimize friction and wear. Moreover, dissimilar 2D materials can be stacked on top of each other – like in a LEGO game – to build heterostructures which will leverage the unique features of all constituent 2D materials. Since 2D materials have the highest surface to volume ratio among all classes of materials, understanding the physics of the interfacial properties, i.e. friction and adhesion, of these materials can lead to the discovery of new approaches for tuning and improving their characteristics, thereby opening new avenues for their production, assembly and application.
The nanotribological properties of graphene and MoS2, the two most frequently researched 2D materials, have been extensively investigated using atomic force microcopy (AFM) experiments. However, the tribological properties of graphene and MoS2 cannot be quantitively compared since all the previous nanoscale friction measurements on both materials were performed by alternating from one sample to the other (i.e. one sample at a time). This sequential approach precludes direct comparison because the results are strongly affected by experimental conditions, including probe shape and size, surface chemistry and environmental conditions, which can vary from test to test.
A group of researchers from USA, Canada and Spain, for the first time, characterized nanoscale friction of graphene, MoS2, and a graphene/MoS2 heterostructure at different normal loads in a single measurement, using AFM experiments and molecular dynamics (MD) simulations with complementary density functional theory (DFT) calculations. Graphene exhibited lower friction than MoS2 and the heterostructure across a wide range of normal loads in both AFM experiments and MD simulations. Also, the friction of the heterostructure was occasionally lower than that of the monolayer MoS2 friction. Through careful examination of the possible mechanisms of energy dissipation at the atomic scale – including thermal activation, tip change, roughness, adhesion and elastic deformation – the origin of the friction contrast between sliding on graphene versus MoS2 was identified as the energy barrier that the tip must overcome to slide over each surface. The energy barrier for graphene obtained from quasi-static MD simulations was much smaller than that of monolayer MoS2 and the heterostructure. Complementary DFT calculations confirmed this finding and the energy barrier breakdown revealed that the difference between MoS2 and graphene was mainly due to the higher dispersion contribution to the sliding barrier for MoS2 which stems from the higher polarizability of sulfur atoms in MoS2 compared to carbon atoms in graphene.
The direct comparison of the tribological properties of 2D materials and heterostructures from a single measurement advances the state of the art for studies of friction on these materials. Further, the approach can be extended going forward to assess other mechanical, electronic and thermal properties of 2D materials more generally.
The details of this study can be found in the original article: “Vazirisereshk, M. R.; Ye, H.; Ye, Z.; Otero-De-La-Roza, A.; Zhao, M.-Q.; Gao, Z.; Johnson, A. T. C.; Johnson, E. R.; Carpick, R. W.; Martini, A. Origin of Nanoscale Friction Contrast between Supported Graphene, MoS2, and a Graphene/MoS2 Heterostructure. Nano Letters 2019. DOI: 10.1021/acs.nanolett.9b02035”.
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