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. 2020 Mar 27;11(1):1595.
doi: 10.1038/s41467-020-15446-y.

Tunable Macroscale Structural Superlubricity in Two-Layer Graphene via Strain Engineering

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Free PMC article

Tunable Macroscale Structural Superlubricity in Two-Layer Graphene via Strain Engineering

Charalampos Androulidakis et al. Nat Commun. .
Free PMC article

Abstract

Achieving structural superlubricity in graphitic samples of macroscale size is particularly challenging due to difficulties in sliding large contact areas of commensurate stacking domains. Here, we show the presence of macroscale structural superlubricity between two randomly stacked graphene layers produced by both mechanical exfoliation and chemical vapour deposition. By measuring the shifts of Raman peaks under strain we estimate the values of frictional interlayer shear stress (ILSS) in the superlubricity regime (mm scale) under ambient conditions. The random incommensurate stacking, the presence of wrinkles and the mismatch in the lattice constant between two graphene layers induced by the tensile strain differential are considered responsible for the facile shearing at the macroscale. Furthermore, molecular dynamic simulations show that the stick-slip behaviour does not hold for incommensurate chiral shearing directions for which the ILSS decreases substantially, supporting the experimental observations. Our results pave the way for overcoming several limitations in achieving macroscale superlubricity using graphene.

Conflict of interest statement

The authors declare no competing interests.

Figures

Fig. 1
Fig. 1. Sample characterization and experimental setup.
a Optical image of the single layer with a folded part at the left. The dashed yellow line indicates the scan line, and the red line marks the presence of a wrinkle (see the AFM image in Supplementary Fig. 1). The scale bar is 20 microns. b Spectra of the 2D peak of the single and folded areas. c Schematic representation of the stress transfer mechanism from the polymer to the bottom layer and from the bottom layer to the top. d Schematic of the experimental setup is shown with two single-layer graphenes stacked in an incommensurate state.
Fig. 2
Fig. 2. The shift of the 2D peak under tension.
a The evolution of the 2D peak for the folded bilayer graphene under various levels of tensile strain. b The shift of the 2D peak of the bottom and top single-layer graphenes of the folded bilayer. c The shift of the single-layer graphene solely. The error bars represent standard deviation.
Fig. 3
Fig. 3. Strain-transfer mechanism.
Maps of the frequency of the 2D peak for various levels of strain showing the distribution profile of the frequency of the 2D Raman peak across the length (a) of the bottom and (b) top single-layer graphene of the folded bilayer. In (a), (b), the data are plotted with the same scale on the y axis for comparison, and in (c), a zoom version of the results of (b) is presented for clarity.
Fig. 4
Fig. 4. Interface and interlayer shear stress.
The interfacial shear stress (IFSS) for the cases of exfoliated single-layer graphene/polymer, exfoliated graphene/graphene and CVD graphene/graphene (ILSS) for various levels of tensile strain. The error bars represent standard deviation.
Fig. 5
Fig. 5. Shearing of CVD graphene–graphene sample.
a The position of the 2D peak versus distance for the top CVD single-layer graphene under various levels of tension. In-plot values correspond to slopes shown with dashed lines. Similar values of the slopes indicate similar ILSS. b The average shift of the 2D peak per increment of strain for the bottom and top CVD single layers for a distance of 30 microns. c Raman mapping over a 3-mm distance of a CVD/CVD bilayer specimen with a scan step of 10 μm. The average position is presented by the dashed lines and the standard deviation with the shaded colours. The corresponding average values of strain are also mentioned in the graph. Note: The experiment in (c) performed with a laserline excitation of 785 nm resulting in different frequencies compared to (b),  for which a laserline of 514 nm was used. In SI, the response for the whole strain regime is presented. d AFM image (left) of the as-deposited CVD on the polymer bar, and (right) its x2 magnification. The scale bar is 2 microns.
Fig. 6
Fig. 6. Interlayer shear stress from MD simulations.
ILSS values for various chiral directions at a temperature of 300 K, specifically, a armchair, b zigzag, c 7.5°, d 15.0° and e 22.5°, sliding with respect to the armchair sublayer. It is observed that when shearing a monolayer graphene in achiral directions relative to the underline graphene, the stick-slip motion breaks down and the ILSS is significantly decreased. This is performed by employing the LCBOP potential.

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