Giant and Tunable Anisotropy of Nanoscale Friction in Graphene

Abstract

The nanoscale friction between an atomic force microscopy tip and graphene is investigated using friction force microscopy (FFM). During the tip movement, friction forces are observed to increase and then saturate in a highly anisotropic manner. As a result, the friction forces in graphene are highly dependent on the scanning direction: under some conditions, the energy dissipated along the armchair direction can be 80% higher than along the zigzag direction. In comparison, for highly-oriented pyrolitic graphite (HOPG), the friction anisotropy between armchair and zigzag directions is only 15%. This giant friction anisotropy in graphene results from anisotropies in the amplitudes of flexural deformations of the graphene sheet driven by the tip movement, not present in HOPG. The effect can be seen as a novel manifestation of the classical phenomenon of Euler buckling at the nanoscale, which provides the non-linear ingredients that amplify friction anisotropy. Simulations based on a novel version of the 2D Tomlinson model (modified to include the effects of flexural deformations), as well as fully atomistic molecular dynamics simulations and first-principles density-functional theory (DFT) calculations, are able to reproduce and explain the experimental observations.

Studying physisorption processes and molecular friction of cycloparaphenylene molecules on graphene nano-sized flakes: role of π⋯π and CH⋯π interactions

“…Finally, we would like to investigate how the molecular friction, depending generally speaking on the roughness and the chemistry of the surface, might impact on the dynamical evolution of the physisorption process. Note that only recently atomic-scale (STM and/or AFM) experiments are able to probe this highly directional effect [65,66] and theoretical studies are still scarce.…”

Anion intercalated graphite: a combined electrochemical and tribological investigation by in situ AFM

“…5). It is known that single graphene layers are prone to give rise to ripple distortion which originates friction anisotropy (Choi et al ., 2011; Almeida et al ., 2016). This friction anisotropy in graphene results from the anisotropic puckering of the graphene layers driven by the tip movement, which is not observable in HOPG.…”

Direct imaging, three-dimensional interaction spectroscopy, and friction anisotropy of atomic-scale ripples on MoS2

“…Considering that the direction dependence, i.e., anisotropy, of friction can be potentially an important design parameter for 2Dmaterial-based solid lubrication in small-scale mechanical systems, we performed FFM measurements to probe friction anisotropy on MoS 2 flakes exfoliated onto SiO 2 . Our work was additionally motivated by previous reports of friction anisotropy on 2D materials including graphene and MoS 2 , where twofold and sixfold symmetries have been reported [16][17][18][19] , as well as irregular anisotropic behavior 20 . While twofold anisotropic behavior was tentatively explained by the presence of ripples 16,17 or stripes formed by molecular adsorbates 18 , sixfold anisotropy was ascribed to the hexagonal symmetry of the atomic structure of the involved materials 19 .…”

“…Despite this seemingly widespread idea-together with a competing theory based on the presence of linearly aligned stripes formed by environmental adsorbates 18 -, the connection to friction anisotropy remains controversial, as the ripples are not directly observed during the experiments. Moreover, the literature also includes friction anisotropy studies that deviate from twofold symmetry 19,20 .…”