Production and mechanical characterization of graphene micro-ribbons

Patterning of graphene into micro- and nano-ribbons allows for the tunability in emerging fields such as flexible electronic and optoelectronic devices, and is gaining interest for the production of more efficient reinforcement for composite materials. In this work we fabricate micro-ribbons from CVD graphene by combining UV photolithography and dry etching oxygen plasma treatments. Raman spectral imaging confirms the effectiveness of the patterning procedure, which is suitable for large-area patterning of graphene on wafer-scale, and confirms that the quality of graphene remains unaltered. The produced micro-ribbons were finally transferred and embedded into a polymeric matrix and the mechanical response was investigated by in-situ mechanical investigation combining Raman spectroscopy and tensile/compressive tests

Failure Processes in Embedded Monolayer Graphene under Axial Compression

Exfoliated monolayer graphene flakes were embedded in a polymer matrix and loaded under axial compression. By monitoring the shifts of the 2D Raman phonons of rectangular flakes of various sizes under load, the critical strain to failure was determined. Prior to loading care was taken for the examined area of the flake to be free of residual stresses. The critical strain values for first failure were found to be independent of flake size at a mean value of -0.60 % corresponding to a yield stress of -6 GPa. By combining Euler mechanics with a Winkler approach, we show that unlike buckling in air, the presence of the polymer constraint results in graphene buckling at a fixed value of strain with an estimated wrinkle wavelength of the order of 1-2 nm. These results were compared with DFT computations performed on analogue coronene/ PMMA oligomers and a reasonable agreement was obtained.Comment: 28 pages. Manuscript 20 pages, 8 figures. Supporting information 10 pages, 6 figure

Phonon and structural changes in deformed bernal stacked bilayer graphene

We present the first Raman spectroscopic study of Bernal bilayer graphene flakes under uniaxial tension. Apart from a purely mechanical behavior in flake regions where both layers are strained evenly, certain effects stem from inhomogeneous stress distribution across the layers. These phenomena such as the removal of inversion symmetry in bilayer graphene may have important implications in the band-gap engineering providing an alternative route to induce the formation of a band-gap.Comment: 21 pages, 7 figure

Stress Transfer Mechanisms at the Submicron Level for Graphene/Polymer Systems

The stress transfer mechanism from a polymer substrate to a nanoinclusion, such as a graphene flake, is of extreme interest for the production of effective nanocomposites. Previous work conducted mainly at the micron scale has shown that the intrinsic mechanism of stress transfer is shear at the interface. However, since the interfacial shear takes its maximum value at the very edge of the nanoinclusion it is of extreme interest to assess the effect of edge integrity upon axial stress transfer at the submicron scale. Here, we conduct a detailed Raman line mapping near the edges of a monolayer graphene flake that is simply supported onto an epoxy-based photoresist (SU8)/poly­(methyl methacrylate) matrix at steps as small as 100 nm. We show for the first time that the distribution of axial strain (stress) along the flake deviates somewhat from the classical shear-lag prediction for a region of ∼2 μm from the edge. This behavior is mainly attributed to the presence of residual stresses, unintentional doping, and/or edge effects (deviation from the equilibrium values of bond lengths and angles, as well as different edge chiralities). By considering a simple balance of shear-to-normal stresses at the interface we are able to directly convert the strain (stress) gradient to values of interfacial shear stress for all the applied tensile levels without assuming classical shear-lag behavior. For large flakes a maximum value of interfacial shear stress of 0.4 MPa is obtained prior to flake slipping

Phonon and Structural Changes in Deformed Bernal Stacked Bilayer Graphene

We present the first Raman spectroscopic study of Bernal bilayer graphene flakes under uniaxial tension. Apart from a purely mechanical behavior in flake regions where both layers are strained evenly, certain effects stem from inhomogeneous stress distribution across the layers. These phenomena such as the removal of inversion symmetry in bilayer graphene may have important implications in the band gap engineering, providing an alternative route to induce the formation of a band gap

Raman 2D-Band Splitting in Graphene: Theory and Experiment

We present a systematic experimental and theoretical study of the two-phonon (2D) Raman scattering in graphene under uniaxial tension. The external perturbation unveils that the 2D mode excited with 785nm has a complex line-shape mainly due to the contribution of two distinct double resonance scattering processes (inner and outer) in the Raman signal. The splitting depends on the direction of the applied strain and the polarization of the incident light. The results give new insight into the nature of the 2D band and have significant implications for the use of graphene as reinforcement in composites since the 2D mode is crucial to assess how effectively graphene uptakes an applied stress or strain.Comment: 30 pages, 5 figues, published in ACS Nan

Phonon and Structural Changes in Deformed Bernal Stacked Bilayer Graphene

We present the first Raman spectroscopic study of Bernal bilayer graphene flakes under uniaxial tension. Apart from a purely mechanical behavior in flake regions where both layers are strained evenly, certain effects stem from inhomogeneous stress distribution across the layers. These phenomena such as the removal of inversion symmetry in bilayer graphene may have important implications in the band-gap engineering providing an alternative route to induce the formation of a band-gap.Comment: 21 pages, 7 figure