Isotope tells how the layers mechanically couple in graphene

The interfacial shear coupling (ISC) is significant for both the fundamental theory of solid mechanics and stability design of 2D devices. This work studies the ISC of bilayer graphene as a representative of 2D vdW stacks, by substituting one of its layers with the isotope and Raman spectroscopy.
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The assembly of two-dimensional (2D) materials into van der Waals (vdW) stacks has opened new avenues for fundamental scientific studies in the past decade, and the interfacial vdW interaction has been extensively recognized as playing the most critical role in determining the innovative properties of these stacks. Besides most previous work that has focused on the electronic properties, the mechanical coupling between the layers of 2D vdW stacks is also important in governing their structural integrity (durability) and mechano-electronic performances. Especially, considering that an atomic-thick vdW stack often has the same type of deformation under a macroscopic load, it is of great significance to study the relative in-plane deformations of all its layers, that is, the shear deformation and the corresponding interfacial shear strength (ISS) of the stack, governed by the interfacial shear coupling (ISC) between its layers.

The key to studying the ISC of a 2D stack is to obtain the strain/stress information of each layer. Unlike measuring the adhesion energy between 2D layers by nanoindentation using the 2D material-wrapped atomic force microscope tip, there are two bottlenecks in observing the in-plane deformation of each layer experimentally. One is to build 2D stacks with ultraclean vdW interfaces with as few impurities and defects as possible, and the other is to develop a reliable strain measurement technique with high accuracy, which can distinguish the signal from each layer even in a homogenous vdW stack.

Isotope-labeled Raman spectroscopy for AB-BLG and the evolution of the corresponding Raman peaks with increased strain. a Overlapped and differentiated Raman peaks from BLG composed of identical and isotope carbon atoms, respectively. b Schematic of the experimental setup for the shear property measurement of BLG by Raman spectroscopy. c OM image of a BLG island transferred on formvar/PDMS substrate. d A typical Raman spectrum of AB-stacked 12C/13C BLG, compared with that of AB-stacked 12C/12C BLG. e Raman G peaks evolution of AB-stacked 12C/13C BLG with increased PDMS tensile strain. Scale bar in (c): 5 μm.

In recent years, Raman spectroscopy has been widely used for strain measurement in 2D materials by the shift of their spectral peaks, owing that it can relate the lattice elongation of a crystal with its spectral feedback. However, when this technique is used for a homogeneous 2D stack, even consisting of only two layers, their Raman signals will totally overlap. Chemical vapor deposition (CVD) -synthesized BLG has a natural and better-defined interface for quantitative characterization. To avoid using the non-deterministic and subjective parameters to fit the Raman spectrum, the 13C isotope is used to label one layer of the BLG (Figure 1a) during the CVD process. And to monitor the strain variation as large as possible, the pyramid stack (Figure 1b) which has been proven as the most efficient structure to transfer stress between graphene layers, has been adopted. In this work, we focus on the analysis of AB-stacked BLG, in which the projection of half of the atoms in the top layer is at the carbon-ring center of the bottom layer, as illustrated in the OM image (Figure 1c). AB-stacked 12C/13C BLG islands are selected via their broad Raman 2D peaks, which are at ~2640 cm–1 and merged by eight subpeaks (Figure 1d). In-situ Raman measurements on these isotope-labeled BLG were conducted during the stretching of flexible substrates, and a typical G peak evolution of the 12C bottom layer and the 13C top layer is shown in Fig. 1e.

After converting the peak shift into strain by the formula, the strain distribution and its evolution of the two layers can be given, then the interfacial shear stress between the two layers can be calculated.

For more details of this work, please see our recent publication in npj 2D Materials and Applications:

Interlayer shear coupling in bilayer graphene, https://www.nature.com/articles/s41699-022-00314-8.

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