arXiv subjects

Emmanuel N. Koukaras

Publications and source records attributed to Emmanuel N. Koukaras.

9 recordsLinked to original sources

arXiv2020

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

Achieving structural superlubricity in graphitic samples of macro-scale size is particularly challenging due to difficulties in sliding large contact areas of commensurate stacking domains. Here, we show the presence of macro-scale structural superlubricity between two randomly stacked graphene layers produced by both mechanical exfoliation and CVD. 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 achiral 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 in graphene.

cond-mat.mtrl-sci↗

arXiv2019

Non-Eulerian behavior of graphitic materials under compression

The mechanical behavior of graphitic materials is greatly affected by the weak interlayer bonding with van der Waals forces for a range of thickness from nano to macroscale. Herein, we present a comprehensive study of the effect of layer thickness on the compression behavior of graphitic materials such as graphene which are fully embedded in polymer matrices. Raman Spectroscopy was employed to identify experimentally the critical strain to failure of the graphitic specimens. The most striking finding is that, contrary to what would be expected from Eulerian mechanics, the critical compressive strain to failure decreases with increase of flake thickness. This is due to the layered structure of the material and in particular the weak cohesive forces that hold the layers together. The plate phenomenology breaks down for the case of multi-layer graphene, which can be approached as discrete single layers weakly bonded to each other. This behavior is modelled here by considering the interlayer bonding (van der Waals forces) as springs in series, and very good agreement was found between theory and experiment. Finally, it will be shown that in the post failure regime multi-layer graphenes exhibit negative stiffness and thus behave as mechanical metamaterials.

cond-mat.mtrl-sci↗

arXiv2018

Compression behavior of simply-supported and fully embedded monolayer graphene: theory and experiment

Single layer graphene simply-supported on a polymer substrate was subjected to axial compression and its behavior upon loading was monitored with laser Raman spectroscopy (LRS). The graphene was found to fail by wrinkling (buckling) at a critical strain of -0.30% and at a compressive stress of ~1.6 GPa, as revealed by the conversion of the spectroscopic data to actual stress-strain curves. This contrasts with the value of -0.60% and stress of ~3.8 GPa required for failure initiation in the fully embedded case. To elucidate the failure mechanisms in the two cases examined, molecular dynamics simulations employing the AIREBO potential were performed. We assess the impact of surface roughness, graphene-polymer interaction, and of thermal (phonon) ripples on the onset of wrinkle formation. Overall good agreement was found between theory and experiment. As argued herein, the understanding and control of out-of-plane phenomena upon mechanical loading of graphene are important prerequisites for the design and function of new graphene-based devices.

cond-mat.mtrl-sci↗

arXiv2017

Wrinkled few-layer graphene as highly efficient load bearer

Multilayered graphitic materials are not suitable as load-bearers due to their inherent weak interlayer bonding (for example, graphite is a solid lubricant in certain applications). This situation is largely improved when two-dimensional (2-D) materials such as a monolayer (SLG) graphene are employed. The downside in these cases is the presence of thermally or mechanically induced wrinkles which are ubiquitous in 2-D materials. Here we set out to examine the effect of extensive large wavelength/ amplitude wrinkling on the stress transfer capabilities of exfoliated simply-supported graphene flakes. Contrary to common belief we present clear evidence that this type of "corrugation" enhances the load bearing capacity of few-layer graphene as compared to 'flat' specimens. This effect is the result of the significant increase of the graphene/polymer interfacial shear stress per increment of applied strain due to wrinkling and paves the way for designing affordable graphene composites with highly improved stress-transfer efficiency.

cond-mat.mtrl-sci↗

arXiv2016

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

The stress transfer mechanism from a polymer substrate to a nano-inclusion, 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 nano-inclusion 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 which is simply supported onto an epoxy based photoresist (SU8)/poly(methyl methacrylate) (PMMA) 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 about 2 um 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 (ISS) of 0.4 MPa is obtained prior to flake slipping.

cond-mat.mtrl-sci↗

arXiv2016

Graphenes flakes under controlled biaxial deformation

Thin membranes, such as monolayer graphene of monoatomic thickness, are bound to exhibit lateral buckling under uniaxial tensile loading that impairs its mechanical behaviour. In this work, we have developed an experimental device to subject 2D materials to controlled equibiaxial strain on supported beams that can be flexed up or down to subject the material to either compression or tension, respectively. Using strain gauges in tandem with Raman spectroscopy measurements, we monitor the G and 2D phonon properties of graphene under biaxial strain and thus extract important information about the uptake of stress under these conditions. The experimental shift over strain for the G and 2D Raman peaks were found to be in the range of 62.3 \pm 5 cm^-1/%, and 148.2 \pm 6 cm^-1/%, respectively, for monolayer but also bilayer graphenes. The corresponding Gruneisen parameters for the G and 2D peaks were found to be between 1.97 \pm 0.15 and 2.86 \pm 0.12, respectively. These values agree reasonable well with those obtained from small-strain bubble-type experiments. The results presented are also backed up by classical and ab initio molecular dynamics simulations and excellent agreement of Γ-E2g shifts with strains and the Gruneisen parameter was observed.

cond-mat.mtrl-sci↗

arXiv2016

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.

cond-mat.mtrl-sci↗

arXiv2016

Systematic spatial and stoichiometric screening towards understanding the surface of ultrasmall oxygenated silicon nanocrystal

In most of the realistic ab initio and model calculations which have appeared on the emission of light from Si nanocrystals, the role of surface oxygen has been usually ignored, underestimated or completely ruled out. We investigate theoretically, by density functional theory (DFT/B3LYP) possible modes of oxygen bonding in hydrogen terminated silicon quantum dots using as a representative case of the Si29 nanocrystal. We have considered Bridge-bonded oxygen (BBO), Doubly-bonded oxygen (DBO), hydroxyl (OH) and Mix of these oxidizing agents. Due to stoichiometry, all comparisons performed are unbiased with respect to composition whereas spatial distribution of oxygen species pointed out drastic change in electronic and cohesive characteristics of nanocrytals. From an overall perspective of this study, it is shown that bridge bonded oxygenated nanocrystals accompanied by Mix have higher binding energies and large electronic gap compared to nanocrystals with doubly bonded oxygen atoms. In addition, it is observed that the presence of OH along with BBO, DBO and mixed configurations further lowers electronic gaps and binding energies and trends. It is also demonstrated that oxidizing constituent besides their spatial distribution significantly alters binding energy and highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) gap (HOMO-LUMO gap up to 1.48 eV) within same composition.

cond-mat.mes-hall↗

arXiv2015

Long-lived discrete breathers in free-standing graphene

Intrinsic localized modes or discrete breathers are investigated by molecular dynamics simulations in free-standing graphene. Discrete breathers are generated either through thermal quenching of the graphene lattice or by proper initialization, with frequencies and lifetimes sensitively depending on the interatomic potential describing the carbon-carbon interaction. In the most realistic scenario, for which temperature-dependent molecular dynamics simulations in three dimension using a graphene-specific interatomic potential are performed, the breather lifetimes increase to hundreds of picoseconds even at relatively high temperatures. These lifetimes are much higher than those anticipated from earlier calculations, and may enable direct breather observation in Raman spectroscopy experiments.

cond-mat.mes-hall↗