Georgia Tsoukleri

Subjecting a Graphene Monolayer to Tension and Compression

The mechanical behaviour of graphene flakes under both tension and compression is examined using a cantilever-beam arrangement. Two different sets of samples were employed involving flakes just supported on a plastic bar but also embedded within the plastic substrate. By monitoring the shift of the 2D Raman line with strain, information on the stress transfer efficiency as a function of stress sign and monolayer support were obtained. In tension, the embedded flake seems to sustain strains up to 1.3%, whereas in compression there is an indication of flake buckling at about 0.7% strain. The retainment of such a high critical buckling strain confirms the relative high flexural rigidity of the embedded monolayer.

Compression Behavior of Single-Layer Graphenes

Central to most applications involving monolayer graphenes is its mechanical response under various stress states. To date most of the work reported is of theoretical nature and refers to tension and compression loading of model graphenes. Most of the experimental work is indeed limited to the bending of single flakes in air and the stretching of flakes up to typically approximately 1% using plastic substrates. Recently we have shown that by employing a cantilever beam we can subject single graphenes to various degrees of axial compression. Here we extend this work much further by measuring in detail both stress uptake and compression buckling strain in single flakes of different geometries. In all cases the mechanical response is monitored by simultaneous Raman measurements through the shift of either the G or 2D phonons of graphene. Despite the infinitely small thickness of the monolayers, the results show that graphenes embedded in plastic beams exhibit remarkable compression buckling strains. For large length (l)-to-width (w) ratios (> or =0.2) the buckling strain is of the order of -0.5% to -0.6%. However, for l/w < 0.2 no failure is observed for strains even higher than -1%. Calculations based on classical Euler analysis show that the buckling strain enhancement provided by the polymer lateral support is more than 6 orders of magnitude compared to that of suspended graphene in air.

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 785 nm 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.

Development of a universal stress sensor for graphene and carbon fibres

Carbon fibres are a significant volume fraction of modern structural airframes. Embedded into polymer matrices, they provide significant strength and stiffness gains by unit weight compared with competing structural materials. Here we use the Raman G peak to assess the response of carbon fibres to the application of strain, with reference to the response of graphene itself. Our data highlight the predominance of the in-plane graphene properties in all graphitic structures examined. A universal master plot relating the G peak strain sensitivity to tensile modulus of all types of carbon fibres, as well as graphene, is presented. We derive a universal value of—average—phonon shift rate with axial stress of around −5ω0−1 (cm−1 MPa−1), where ω0 is the G peak position at zero stress for both graphene and carbon fibre with annular morphology. The use of this for stress measurements in a variety of applications is discussed.Carbon fibres (CF) represent a significant volume fraction of modern structural airframes. Embedded into polymer matrices, they provide significant strength and stiffness gains over unit weight as compared to other competing structural materials. Nevertheless, no conclusive structural model yet exists to account for their extraordinary properties. In particular, polyacrynonitrile (PAN) derived CF are known to be fully turbostratic: the graphene layers are slipped sideways relative to each other, which leads to an inter-graphene distance much greater than graphite. Here, we demonstrate that CF derive their mechanical properties from those of graphene itself. By monitoring the Raman G peak shift with strain for both CF and graphene, we develop a universal master plot relating the G peak strain sensitivity of all types of CF to graphene over a wide range of tensile moduli. A universal value ofaverageshift rate with axial stress of 1 1 1 0 ~ 5 (cm MPa ) ω − − − − is calculated for both graphene and all CF exhibiting annular (“onion-skin”) morphology.

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 up to -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.

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.

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.

Embedded trilayer graphene flakes under tensile and compressive loading

The mechanical response of embedded ABA trilayer graphene flakes loaded in tension and compression on polymer beams is monitored by simultaneous Raman measurements through the strain sensitivity of the G or 2D peaks. A characteristic peculiarity of the investigated flake is that it contains a trilayer and bilayer part. The Bernal stacked bilayer was used as a strain sensor aiming to assess the efficiency of the load transfer from the polymer matrix through shear to the individual graphene layers. For the trilayer graphene in tension, both peaks are redshifted and splitting of the G peak is reported for the first time. In compression, the studied sample was an almost isolated trilayer, in which both peaks are blue-shifted up to a critical compressive strain. This critical strain is found to be one fourth of the value found in the case of single layer graphene despite the higher bending rigidity that trilayer exhibits over the much thinner monolayer.

Axial Deformation of Monolayer Graphene under Tension and Compression

The mechanical response of single layer graphene is monitored by simultaneous Raman measurements through the shift of either the G or 2D optical phonons, for low levels of tensile and compressive strain. In tension, important physical phenomena such as the G and 2D band splitting are discussed. The results can be used to quantify the amount of uniaxial strain, providing a fundamental tool for graphene-based nanoelectronics. In compression, graphenes of atomic thickness embedded in plastic beams are found to exhibit remarkable high compression failure strains. The critical buckling strain for graphene appears to be dependent on the flake size and geometry with respect to the strain axis. It is shown that the embedded flakes can be treated as ideal plates and their behavior can be described by Euler mechanics.