Reversible loss of Bernal stacking during the deformation of few-layer graphene in nanocomposites

Abstract

The deformation of nanocomposites containing graphene flakes with different numbers of layers has been investigated with the use of Raman spectroscopy. It has been found that there is a shift of the 2D band to lower wavenumber and that the rate of band shift per unit strain tends to decrease as the number of graphene layers increases. It has been demonstrated that band broadening takes place during tensile deformation for mono- and bilayer graphene but that band narrowing occurs when the number of graphene layers is more than two. It is also found that the characteristic asymmetric shape of the 2D Raman band for the graphene with three or more layers changes to a symmetrical shape above about 0.4% strain and that it reverts to an asymmetric shape on unloading. This change in Raman band shape and width has been interpreted as being due to a reversible loss of Bernal stacking in the few-layer graphene during deformation. It has been shown that the elastic strain energy released from the unloading of the inner graphene layers in the few-layer material (~0.2 meV/atom) is similar to the accepted value of the stacking fault energies of graphite and few layer graphene. It is further shown that this loss of Bernal stacking can be accommodated by the formation of arrays of partial dislocations and stacking faults on the basal plane. The effect of the reversible loss of Bernal stacking upon the electronic structure of few-layer graphene and the possibility of using it to modify the electronic structure of few-layer graphene are discussed.

Figure 1

Shifts of the 2D Raman band with strain for graphene flakes in a model nanocomposite. Overall band shift for (a) the monolayer and (b) the bilayer materials.

Figure 2

Shifts of the 2D Raman band with strain for graphene flakes in a model nanocomposite. Overall band shift for (a) the trilayer and (b) the few-layer materials.

Figure 3

High resolution electron micrographs and associated FFTs of the images for (a) chemically exfoliated graphene showing Bernal stacking and (b) CVD-grown graphene showing non-Bernal stacking (the numbers indicate the different number of layers present in the different regions).

Figure 4

Schematic illustration of the loss of Bernal stacking during the deformation of trilayer graphene in a nanocomposite: (a) undeformed structure; (b) deformed structure showing the loss of Bernal stacking through affine deformation; (c) the shear process that take place at the different interfaces along with their values at yield or failure (the A layers are colored black and the B layer is colored red).

Figure 5

Bernal stacked trilayer graphene lattice structure. (a) undeformed material; (b) deformed structure showing an undeformed B layer and the formation of two partial dislocations and a stacking fault between them (the top and bottom A layers are shown identically with the same deformation for clarity). The left-hand side partial dislocation has edge character, and the right-hand side one is mixed edge and screw. The top and bottom (A stacked at edges and C stacked in stacking fault) layers are colored black and the middle layer (B stacked) is colored red.

Figure 6

Transmission electron micrograph showing dislocation arrays (dislocation lines indicated by arrows) in a many-layer graphene flake prepared by mechanical cleavage.