Reversible Mechanical and Electrical Properties of Ripped Graphene

We examine the mechanical properties of graphene devices stretched on flexible elastomer substrates. Using atomic force microscopy, electrical transport measurements, and mechanics simulations, we show that microrips form in the graphene during the initial application of tensile strain; however, subsequent applications of the same tensile strain elastically open and close the existing rips. Correspondingly, while the initial tensile strain degrades the devices’ transport properties, subsequent strain-relaxation cycles affect transport only moderately, and in a largely reversible fashion. Graphene’s electrical and mechanical robustness even after partial mechanical failure is unique among conducting thin films. This understanding of the creation and dynamics of rips in graphene is relevant to the design of flexible graphene-based devices which are required to function under strain.

Figure 1

(a) False-color optical image of a graphene bridge device (outlined by dashed line) patterned on a polydimethylsiloxane (PDMS) substrate with gold contact pads (light yellow). The length $L$ and width $W$ of the bridge are described in the text. The scale bar is $25\mu \mathrm{m}$. Inset: A schematic illustration of the device geometry. (b) The mechanical stretching stage with PDMS inserted between the clamps. The devices are stretched along the axis of the microbridge. (c) Offset Raman spectra for a bare PDMS region and a graphene device region. The G and 2D peaks, at $1599{\mathrm{cm}}^{-1}$ and $2659{\mathrm{cm}}^{-1}$ respectively, in the spectra from the device region confirm the presence of graphene.

Figure 2

(a)–(f) AFM phase measurements of graphene on a polymer substrate at approximately 0%, 5%, 0%, 5%, 10%, and 0% strain (applied along the horizontal axis), as labeled. Rips are evident as light gray, elongated vertical features. An example of a rip that opens and closes with applied strain is indicated by the dashed line. Dark spots present in each image are debris on the substrate surface; white halos surrounding some of the debris are indicative of graphene slightly delaminating from the substrate. Elongated horizontal features are strain-dependent wrinkles. (g) AFM phase and (h) height data. Variations in the height data distinguish between wrinkles and rips in the graphene, which have similar signatures in the phase data. The scanned area in each image is $25\mu {\mathrm{m}}^2$.

Figure 3

Coarse-grained simulations show the elastic opening and closing of rips during initial and subsequent tensile loading cycles, in good agreement with AFM measurements in Fig. 2. The graphene region was simulated at 0%, 5%, 0%, 5%, 10%, and 0% strain applied along the horizontal axis, as labeled. Values given in nanometers refer to the rip lengths. The vertical contraction of the graphene region at higher strain values is due to the Poisson effect.

Figure 4

(a) Electrical resistance of a graphene device vs applied tensile strain. The initial application of strain significantly increases the resistance while subsequent strain-relaxation cycles over the same strain range yield smaller, mostly reversible changes in the resistance. (b) Three consecutive strain-relaxation cycles (cycles 3, 4, 5), showing largely reversible transport characteristics.