Graphene and related two-dimensional materials have shown unusual and exceptional mechanical properties1,2,3, with similarities to origami-like paper folding4,5 and kirigami-like cutting6,7 demonstrated. For paper analogues, a critical difference between macroscopic sheets and a two-dimensional solid is the molecular scale of the thin dimension of the latter, allowing the thermal activation of considerable out-of-plane motion. So far thermal activity has been shown to produce local wrinkles in a free graphene sheet that help in theoretically understanding its stability8, for example, and give rise to unexpected long-range bending stiffness6. Here we show that thermal activation can have a more marked effect on the behaviour of two-dimensional solids, leading to spontaneous and self-driven sliding, tearing and peeling from a substrate on scales approaching the macroscopic. We demonstrate that scalable nanoimprint-style contact techniques can nucleate and direct the parallel self-assembly of graphene ribbons of controlled shape in ambient conditions. We interpret our observations through a simple fracture-mechanics model that shows how thermodynamic forces drive the formation of the graphene–graphene interface in lieu of substrate contact with sufficient strength to peel and tear multilayer graphene sheets. Our results show how weak physical surface forces can be harnessed and focused by simple folded configurations of graphene to tear the strongest covalent bond. This effect may hold promise for the patterning and mechanical actuating of devices based on two-dimensional materials.
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We thank H. Nolan, N. McEvoy and G. Duesberg for discussions and assistance with the Raman spectroscopy. We thank A. Fasolino, A. Schirmeisen, J. Coleman and P. McCormack for discussions and J. Sader and J. Pethica for critical reviews of the manuscript. We acknowledge financial support from the Science Foundation of Ireland (SFI) under CRANN CSET 08/CE/I1432 and PI 08/IN/I1932. We also recognize assistance from Horizon 2020 COST Action MP1303.
The authors declare no competing financial interests.
Extended data figures and tables
The growth pattern recorded by AFM imaging shows ribbon pinning and release as contamination defects are pushed aside (insets a and b), followed by rapid growth until next contaminant reached at point c. Final ribbon pinning occurs by encounter with sliver fracture in the flake that exposes the underlying substrate d. In a, a bulge in the fold (yellow arrow) has been disappeared by b as the defect has moved to a lower position along the ribbon head indicated by the green arrow. Ribbon growth direction is to the left. Horizontal error bars indicate uncertainty in exact time of AFM image acquisition used to extract ribbon length values, whereas vertical error bars are omitted as the uncertainty range is too small to see at this scale.
Extended Data Figure 2 Scanning Raman analysis of commensurate stacked layers in a mechanically exfoliated graphene flake with no self-assembled structures present.
a, Optical micrograph of a mechanically exfoliated graphene sheet. The red box indicates the area that was analysed, comprising three- and four-layer-thick graphene sheets. b–d, Maps of the G band intensity (b), 2D band intensity (c) and the FWHM of the 2D band (d). e, Plot of the average Raman spectra within the three- and four-layer areas. f, Comparative plot of the 2D bands within the three- and four-layer areas.
Extended Data Figure 3 Scanning Raman analysis of the self-assembled bilayer graphene ribbons shown in
Fig. 1l. a, Optical micrograph of the mechanically exfoliated bilayer graphene sheet. The red box indicates a single indentation from which the three folded ribbons grew. b–d, Maps of the G band intensity (b), 2D band intensity (c) and the FWHM of the 2D band (d). e, Plot of the average Raman spectra within the folded ribbons (four-layer) and the bilayer sheet. f, Comparative plot of the 2D bands within the folded ribbons four-layer) and the bilayer sheet.
a–c, Ribbon length (a), velocity (b) and width (c) versus time for the self-assembling ribbon system presented in Fig. 4, as extracted from the sequence of AFM images shown below. See Methods for an explanation of the fitting functions. Scale bars, 1 μm.
An animation of successive images of the growth of three ribbons shown in Figure 1. The video consists of images captured by AFM over a 14 day period. The ribbon in the lower left is able to grow over a subsurface defect before stopping, while the ribbon growing towards the right of the image halts upon encountering the edge of the host graphene sheet. (WMV 2167 kb)
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Annett, J., Cross, G. Self-assembly of graphene ribbons by spontaneous self-tearing and peeling from a substrate. Nature 535, 271–275 (2016). https://doi.org/10.1038/nature18304
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