Molecules on rails: friction anisotropy and preferential sliding directions of organic nanocrystallites on two-dimensional materials

TL;DR

This study reveals that organic nanocrystallites on 2D materials exhibit anisotropic friction and slide preferentially along growth directions, influenced by epitaxial relations, as shown through combined AFM manipulation and molecular dynamics simulations.

Contribution

It introduces a novel approach combining AFM manipulation and simulations to study friction anisotropy and sliding directions of organic nanocrystallites on 2D materials.

Findings

Nanocrystallites slide along growth directions, not pushing directions.

Friction anisotropy is significant and direction-dependent.

Epitaxial relations determine the observed frictional behavior.

Abstract

Two-dimensional (2D) materials are envisaged as ultra-thin solid lubricants for nano-mechanical systems. So far, their frictional properties at the nanoscale have been studied by standard friction force microscopy. However, lateral manipulation of nanoparticles is a more suitable method to study the dependence of friction on the crystallography of two contacting surfaces. Still, such experiments are lacking. In this study, we combine atomic force microscopy (AFM) based lateral manipulation and molecular dynamics simulations in order to investigate the movements of organic needle-like nanocrystallites grown by van der Waals epitaxy on graphene and hexagonal boron nitride. We observe that nanoneedle fragments -- when pushed by an AFM tip -- do not move along the original pushing directions. Instead, they slide on the 2D materials preferentially along the needles' growth directions, which act as invisible rails along commensurate directions. Further, when the nanocrystallites were rotated by applying a torque with the AFM tip across the preferential sliding directions, we find an increase of the torsional signal of the AFM cantilever. We demonstrate in conjunction with simulations that both, the significant friction anisotropy and preferential sliding directions are determined by the complex epitaxial relation and arise from the commensurate and incommensurate states between the organic nanocrystallites and the 2D materials.