Giant and Tunable Anisotropy of Nanoscale Friction in Graphene.

Carlos A Achete,Rodrigo Prioli,Rodrigo B Capaz,Marcelo De Cicco,Clara M Almeida,Ricardo Paupitz,Douglas S Galvão,Benjamin Fragneaud,Marcos G Menezes,Luiz Gustavo Cançado

doi:10.1038/srep31569

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.

Highlights

The mechanical behavior of materials at the nanoscale may present completely novel and unexpected features as compared to their bulk counterparts
We demonstrate that friction anisotropy in graphene is tunable by the normal force and can reach values as high as 80%, which represent a giant enhancement with respect to graphite
Using a combination of simulations based on the Prandtl-Tomlinson model[16,17], fully atomistic molecular dynamics and density-functional theory (DFT), we explain this behavior as arising from the anisotropic amplification of tip-induced out-of-plane deformations of the graphene sheet, not present in graphite

Summary

Introduction

The mechanical behavior of materials at the nanoscale may present completely novel and unexpected features as compared to their bulk counterparts. According to Lee et al.[8], this effect originates from out-of-plane (flexural) elastic deformations, which become more prominent as the number of layers decreases, leading to puckering of the graphene sheets at the mechanical contact This increases the surface-tip interaction, which in turn leads to higher friction forces. 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 These results represent a novel mechanism of energy dissipation in 2D systems, opening new possibilities for the design and control of nano-mechanical systems involving single layer materials

Methods

Findings

Conclusion