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

Abstract

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.

Tunable macroscale structural superlubricity in two-layer graphene via strain engineering

“…As mentioned above, the top single layer is stretched only by the strain transferred through shearing from the bottom graphene. By balancing the shear to axial forces at the graphene/ graphene interface, the interlayer shear stress can be estimated -as also in the case of graphene/polymer stress transferfrom the following equation 25 :…”

Stress-transfer from polymer substrates to monolayer and few-layer graphenes

“…In this regard Raman spectroscopy has proven to be the most efficient method; the Raman peaks shift with the application of mechanical load 26 and by monitoring the shift rates of the peaks (2D 27 and G 28 ), the level of stain in the graphene can be back-tracked in a straightforward manner 20,22 . Raman maps can be taken for the 2D and G phonons across graphene flakes at small steps, even at the sub-micron level, providing high resolution for the strain distribution in graphene 20 . The Raman maps are converted to values of strain based on the average shift rate which has been examined in numerous works to be in the range of ~50-64 cm −1 /% for the 2D peak 20,27,[29][30] .…”

Optical detection of strain and doping inhomogeneities in single layer MoS2

“…A simply-supported MoS2 flake is normally subjected to residual stresses resulting from the transfer procedure or the roughness of the underlying substrate along with substrate induced doping. 11 Therefore, the unperturbed phonon frequency is difficult to be obtained experimentally. However, the mean value of the recorded frequencies in a detailed Raman mapping all over the area of the examined flake can be used as a reference.…”

“…It is well documented for graphene that crystalline monolayer materials experience uncontrollable mechanical fields via the preparation (e.g. exfoliation 11 or chemical vapor deposition 12 ) or the transfer processes along with charge doping mainly induced by the interaction of the material with the substrate. 13 These residual mechanical fields affect considerably the electronic properties and, therefore, the quantification of strain and doping levels in MoS2 is a prerequisite for its proper characterization and implementation in nanoscale devices.…”

“…During the deposition and peeling procedures it is assumed that the flake is subjected to biaxial strain 11,21,22 .…”