Graphene flakes under controlled biaxial deformation.

Charalampos Androulidakis,George Kalosakas,Costas Galiotis,Emmanuel N. Koukaras,John Parthenios,Konstantinos Papagelis

doi:10.1038/srep18219

Abstract

Thin membranes, such as monolayer graphene of monoatomic thickness, are bound to exhibit lateral buckling under uniaxial tensile loading that impairs its mechanical behaviour. In this work, we have developed an experimental device to subject 2D materials to controlled equibiaxial strain on supported beams that can be flexed up or down to subject the material to either compression or tension, respectively. Using strain gauges in tandem with Raman spectroscopy measurements, we monitor the G and 2D phonon properties of graphene under biaxial strain and thus extract important information about the uptake of stress under these conditions. The experimental shift over strain for the G and 2D Raman peaks were found to be in the range of 62.3 ± 5 cm–1/%, and 148.2 ± 6 cm–1/%, respectively, for monolayer but also bilayer graphenes. The corresponding Grüneisen parameters for the G and 2D peaks were found to be between 1.97 ± 0.15 and 2.86 ± 0.12, respectively. These values agree reasonably well with those obtained from small-strain bubble-type experiments. The results presented are also backed up by classical and ab initio molecular dynamics simulations and excellent agreement of Γ-E2g shifts with strains and the Grüneisen parameter was observed.

Highlights

IntroductionBiaxial deformation (stretching) is relevant for thin films or membranes at all scales
As is evident, biaxial deformation is relevant for thin films or membranes at all scales
We have examined several graphene samples within a range of thicknesses from monolayer up to nanographite, and have monitored in detail the response of the Raman phonons. To our knowledge this is the first time that phonon shifts for trilayer graphene are reported under biaxial deformation and this complements the prior work reported under uniaxial strain[19]

Summary

Introduction

Biaxial deformation (stretching) is relevant for thin films or membranes at all scales. In this work a new experimental technique has been developed for subjecting 2-dimensional crystals (such as graphene, MoS2, etc.) to controllable equibiaxial tensile strain gradients The principle of this technique is based on the extension along two dimensions of the three-point bending configuration of plastic bars that have been employed for uniaxial loading as mentioned earlier. A plastic substrate with cruciform shape, as shown, is symmetrically deformed about its centre, inducing an equi-biaxial strain at that position The advantages of this technique compared to those attempted earlier are the following: (a) the biaxial strain is applied at a stepwise and controllable manner, (b) the strain can be directly measured using strain gauges, (c) the setup is capable of loading any 2-dimensional material at moderate strain levels and, (d) the jig is housed under a Raman microscope that allows mapping of stress or strain with submicron resolution. The numerical results are in good agreement with the experimental measurements

Methods

Results

Conclusion