The effect of rippling on the mechanical properties of graphene

TL;DR

This paper investigates how specific ripples at the nanoscale influence the mechanical properties of graphene, highlighting the importance of static ripples over dynamic phonons in its thermomechanics.

Contribution

It presents experimental evidence that static ripples with wavelengths of 5-10nm significantly affect graphene's elastic properties, challenging previous assumptions about flexural phonons.

Findings

Ripples of 5-10nm wavelength influence graphene's mechanics

Vacancies reduce negative thermal expansion coefficient

Applied strain increases Young's modulus

Abstract

Graphene is the stiffest material known so far but, due to its one-atom thickness, it is also very bendable. Consequently, free-standing graphene exhibit ripples that has major effects on its elastic properties. Here we will summarize three experiments where the influence of rippling is essential to address the results. Firstly, we observed that atomic vacancies lessen the negative thermal expansion coefficient of free-standing graphene. We also observed an increase of the Young's modulus with global applied strain and with the introduction of small density defects that we attributed to the decrease of rippling. Here, we will focus on a surprising feature observed in the data: the experiments consistently indicate that only the rippling with wavelengths between 5-10nm influences the mechanics of graphene. The rippling responsible of the negative TEC and anomalous elasticity is thought to be dynamic, i.e. flexural phonons. However, flexural phonons with these wavelengths should have minor effects on the mechanics of graphene, therefore other mechanisms must be considered to address our observations. We propose static ripples as one of the key elements to correctly understand the thermomechanics of graphene and suggest that rippling arises naturally due to a competition of symmetry breaking and anharmonic fluctuations.