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In recent years, liquid metal catalysts have emerged as a compelling choice for the controllable, large-scale, and high-quality synthesis of two-dimensional materials. At present, there is little mechanistic understanding of the intricate catalytic process, though, of its governing factors or what renders it superior to growth at the corresponding solid catalysts. Here, we report on a combined experimental and computational study of the kinetics of graphene growth during chemical vapor deposition on a liquid copper catalyst. By monitoring the growing graphene flakes in real time using in situradiation-mode optical microscopy, we explore the growth morphology and kinetics over a wide range of CH4-to-H2pressure ratios and deposition temperatures. Constant growth rates of the flakes’ radius indicate a growth mode limited by precursor attachment, whereas methane-flux-dependent flake shapes point to limited precursor availability. Large-scale free energy simulations enabled by an efficient machine-learning moment tensor potential trained to density functional theory data provide quantitative barriers for key atomic-scale growth processes. The wealth of experimental and theoretical data can be consistently combined into a microkinetic model that reveals mixed growth kinetics that, in contrast to the situation at solid Cu, is partly controlled by precursor attachment alongside precursor availability. Key mechanistic aspects that directly point toward the improved graphene quality are a largely suppressed carbon dimer attachment due to the facile incorporation of this precursor species into the liquid surface and a low-barrier ring-opening process that self-heals 5-membered rings resulting from remaining dimer attachments.

The development of nanocomposites relies on structure–property relations, which necessitate multiscale modeling approaches. This study presents a modeling framework that exploits mesoscopic models to predict the thermal and mechanical properties of nanocomposites starting from their molecular structure. In detail, mesoscopic models of polypropylene (PP)- and graphene-based nanofillers (graphene (Gr), graphene oxide (GO), and reduced graphene oxide (rGO)) are considered. The newly developed mesoscopic model for the PP/Gr nanocomposite provides mechanistic information on the thermal and mechanical properties at the filler–matrix interface, which can then be exploited to enhance the prediction accuracy of traditional continuum simulations by calibrating the thermal and mechanical properties of the filler–matrix interface. Once validated through a dedicated experimental campaign, this multiscale model demonstrates that with the modest addition of nanofillers (up to 2 wt %), the Young’s modulus and thermal conductivity show up to 35 and 25% enhancement, respectively, whereas the Poisson’s ratio slightly decreases. Among the different combinations tested, the PP/Gr nanocomposite shows the best mechanical properties, whereas PP/rGO demonstrates the best thermal conductivity. This validated mesoscopic model can contribute to the development of smart materials with enhanced mechanical and thermal properties based on polypropylene, especially for mechanical, energy storage, and sensing applications.

We report that few graphene flakes embedded into polymer matrices can be mechanically stretched to relatively large deformation (>1%) in an efficient way by adopting a particular ladder-like morphology consisting of consecutive mono-, bi-, tri-, and four-layer graphene units. In this type of flake architecture, all of the layers adhere to the surrounding polymer inducing similar deformation on the individual graphene layers, preventing interlayer sliding and optimizing the strain transfer efficiency. We have exploited Raman spectroscopy to quantify this effect from a mechanical standpoint. The finite element method and molecular dynamics simulations have been used to interpret the above experimental findings. The results suggest that a step pyramid-like architecture of a flake can be ideal for efficient loading of layered materials embedded into a polymer and that there are two prevailing mechanisms that govern axial stress transfer, namely, interfacial shear transfer and axial transmission through the ends. This concept can be easily applied to other two-dimensional materials and related van der Waals heterostructures fabricated either by mechanical exfoliation or chemical vapor deposition by appropriate patterning. This work opens new perspectives in numerous applications, including high volume fraction composites, flexible electronics, and straintronic devices.

Raman spectroscopy is employed to investigate the temperature dependence of the E2gphonon mode of single-layer, few-layer (FL), and bulk hexagonal boron nitride (hBN) sheets, situated over Si/SiO2(90 nm). Depending on the sample, two temperature regimes are recorded. In the low to mid range of the examined temperatures, the monolayer and FL samples exhibit significantly higher slopes compared to the thicker ones because of the thermal expansion of the underlying substrate. In the high-temperature region, all the examined samples show almost the same temperature slope, indicating slippage of the hBN sheets relative to the substrate and providing strong evidence that the slopes of the monolayer, FL, and bulk hBN are quite similar of about −0.020 cm–1/K. This is further justified from the full width at half-maximum versus Tof the studied samples and the observed similarities of the thermal expansion coefficient (TEC) of one to three layers and bulk hBN, revealing a comparable level of anharmonicity for the E2gmode from the monolayer up to bulk hBN. Moreover, using a finite element method, we have determined the TEC of the underling substrate and the strain induced by TEC mismatch between the single-layer hBN and the substrate. Consequently, the intrinsic frequency shift of the E2gmode for the monolayer hBN upon temperature change is extracted. Finally, the vibrational response of monolayer hBN upon temperature is also examined by means of computational simulations, and satisfactory agreement with the experiment is obtained.

This work reports on advances in capacitive humidity and strain sensor technologies through the development of state-of-the-art 3D-printed Interdigitated Electrodes (IDEs) coated with a unique GO/ PVA coating. These IDEs are constructed using a novel composite filament of MWCNTs and polylactic acid (PLA) that offer superior flexural strength and electrical properties compared to conventional polymer matrices.

Chemical vapor deposition (CVD) is regarded as a promising fabrication method for the automated, large-scale, production of graphene and other two-dimensional materials. However, its full commercial exploitation is limited by the presence of structural imperfections such as folds, wrinkles, and even cracks that downgrade its physical and mechanical properties. For example, as shown here by means of Raman spectroscopy, the stress transfer from an epoxy matrix to CVD graphene is on average 30% of that of exfoliated monolayer graphene of over 10 μm in dimensions. However, in terms of electrical response, the situation is reversed; the resistance has been found here to decrease by the imposition of mechanical deformation possibly due to the opening up of the structure and the associated increase of electron mobility. This finding paves the way for employing CVD graphene/epoxy composites or coatings as conductive “networks” or bridges in cases for which the conductivity needs to be increased or at least retained when the system is under deformation. The tuning/control of such systems and their operative limitations are discussed here.

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 (2D) 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 2D 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.

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 ∼1.6GPa, 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 ∼3.8GParequired 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.

The mechanical behavior of a prototype touch panel display, which consists of two layers of CVD graphene embedded into PET films, is investigated in tension and under contact-stress dynamic loading. In both cases, laser Raman spectroscopy was employed to assess the stress transfer efficiency of the embedded graphene layers. The tensile behavior was found to be governed by the “island-like” microstructure of the CVD graphene, and the stress transfer efficiency was dependent on the size of graphene “islands” but also on the yielding behavior of PET at relatively high strains. Finally, the fatigue tests, which simulate real operation conditions, showed that the maximum temperature gradient developed at the point of “finger” contact after 80 000 cycles does not exceed the glass transition temperature of the PET matrix. The effect of these results on future product development and the design of new graphene-based displays are discussed.

Graphene oxide (GO) has shown great potential as a component in various devices due to its excellent solution processability and two-dimensional structure. However, the oxygenated form of graphene has a moderate charge-transport capability. The latter parameter may be enhanced through controlled deoxygenation of GO with subsequent tuning of its work function (WF). Various reduction approaches were employed to investigate the effect of the oxygen content on the work function of GO derivatives as thin films on an indium tin oxide substrate. Such films were reduced by stepwise thermal annealing in ultrahigh vacuum up to 650 °C, by chemical reduction with hydrazine, or by a combination of chemical and thermal reduction processes. The effect of the GO film thickness and the flake size on the WF was also investigated. UV photoelectron spectroscopy and X-ray photoelectron spectroscopy were used to correlate the WF of GO derivatives with their oxygen content. The results showed that the WF is strongly dependent on the oxygen content, reaching a ∼1 eV difference between GO and highly reduced GO, under the specific reduction conditions. The film thickness affects the work function, since in thin films interaction with the substrate is pronounced. Finally, the WF of reduced GO after combination of chemical and thermal reduction reaches its lowest value of 4.20 eV, due to the presence of heteroatoms which doped the surface.

The deformation of monolayer graphene, produced by chemical vapor deposition (CVD), on a polyester film substrate has been investigated through the use of Raman spectroscopy. It has been found that the microstructure of the CVD graphene consists of a hexagonal array of islands of flat monolayer graphene separated by wrinkled material. During deformation, it was found that the rate of shift of the Raman 2D band wavenumber per unit strain was less than 25% of that of flat flakes of mechanically exfoliated graphene, whereas the rate of band broadening per unit strain was about 75% of that of the exfoliated material. This unusual deformation behavior has been modeled in terms of mechanically isolated graphene islands separated by the graphene wrinkles, with the strain distribution in each graphene island determined using shear lag analysis. The effect of the size and position of the Raman laser beam spot has also been incorporated in the model. The predictions fit well with the behavior observed experimentally for the Raman band shifts and broadening of the wrinkled CVD graphene. The effect of wrinkles upon the efficiency of graphene to reinforce nanocomposites is also discussed.

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.

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.

In this communication an extended “in–out” polymerization method is presented, which leads to the synthesis of novel heteroarm star block terpolymers of the type An(B‐b‐C)n. A four step/one‐pot synthetic procedure is pursued using anionic polymerization under an inert atmosphere. The resulted star‐shaped terpolymer consists of a divinyl benzene nodule bearing pure polystyrene and poly(hexyl methacrylate)‐block‐poly(methyl methacrylate) diblock copolymer arms. It is shown that this kind of star terpolymers can self‐assemble in the bulk forming lamellae mesophase by arm and block segregation. The mechanical properties of the terpolymer have been examined in detail. Finally, the proposed synthetic procedure can be easily employed in other controlled polymerization methods.

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 ∼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 (≥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.

The Raman spectrum of isotactic polypropylene (iPP) has been found to exhibit vibrational peaks in the region of 750 to 880 cm−1that are sensitive to the degree of crystallinity. These features are broadly assigned to various modes of methyl group rocking, ρ(CH2), and there have been various attempts to assess crystallinity based on the integrated intensities of these bands. Various vibrational analyses performed in the past in combination with experimental studies have concluded that the presence of crystalline order with trans-gaucheconformation gives rise to a peak at 809 cm−1, which is assigned to a ρ(CH2) mode coupled with the skeletal stretching mode. However, the presence of additional peaks at 830 cm−1, 841 cm−1, and 854 cm−1, within the same envelope, have been the subject of controversy. In this work isotropic films of iPP derived from the same precursor of identical tacticity have been subjected to various degrees of annealing and the integrated intensities of the Raman bands were measured. The results showed that true 3d crystallinity in isotropic iPP can only be expressed by the 809 cm−1band whereas the band at 841 cm−1corresponds to an uncoupled ρ(CH2) fundamental mode and thus is a measure of the amorphous content. The less intense satellite bands at 830 cm−1and 854 cm−1of solid iPP cannot be distinguished from the 841 cm−1band in the melt and are generally considered as intermediate phases possibly related to non-crystalline components with 31-helical conformations. Independent differential scanning calorimetry (DSC) crystallinity measurements were in broad agreement with the Raman measurements based on the normalized intensity of the 809 cm−1Raman band. By comparing the Raman with the DSC data a new value for the theoretical heat of fusion for the 100% crystalline iPP has been proposed.

The stress transfer properties of the fibre/matrix interface in the single fibre fragmentation test were investigated. Two carbon fibre-resin systems involving epoxysized and unsized fibres, were examined. Axial fibre stress data at resolutions of the order of one micron, were obtained with the technique of Remote Laser Raman Microscopy. Subsequent analytical modeling of the data was performed using a Bessel-Fourier Series stress analysis approach. The analysis provides a nearly exact solution for the stress field in the fragmentation test and simultaneously accounts for damaged or imperfect interfaces through the use of an interface parameter D,All data were fit using a two zone model in order to account for the propagation of interfacial damage as a function of applied strain. The fitting process was used to determine D(),the interface parameter in undamaged zones, Did),the interface parameter in the damage zones near fibre breaks, and Id,the length of the damage zones, all as a function of applied strain. The interface parameter in the undamaged zones, D('),was independent of applied strain. We claim D(')is a good property for comparing fibre/matrix interfaces and the most relevant property for predicting the role to the interface in real laminates.

The effect of specimen geometry upon the parameters that govern the stress transfer (transfer length, interfacial shear strength, positively affected length and stress concentration factor) in a carbon fibre/epoxy composite was examined in detail. Five frequently employed composite geometries were considered: single fibre composite coupons incorporating discontinuous and continuous carbon fibres, multi-fibre composite tapes with a controlled inter-fibre separation, four-ply unidirectional coupons, and, finally, impregnated fibre tows. The chemistry and the curing characteristics of the matrix were kept unaltered regardless of specimen geometry. All fibre stress measurements were conducted by means of Raman microscopy. The experimental results showed that the transfer length, positively affected length (PAL) and the interfacial shear stress obtained at the location of a fibre fracture are not considerably affected by specimen geometry. In contrast, the residual fibre stress of the unloaded specimens and the stress concentration factors obtained in fibres adjacent to a fibre fracture site were found to be significantly dependent upon fibre volume fraction and specimen geometry. Copyright © 1999 John Wiley & Sons, Ltd.

The micromechanics of two-dimensional carbon fibre/epoxy microcomposites incorporating seven equally spaced fibres, has been investigated. The fibre stress at each level was monitored with the technique of remote laser Raman microscopy (ReRaM). The ineffective or transfer lengths (Lt) for reinforcement and the corresponding positively affected lengths (PALs) were measured for various interfibre distances. Finally, the interfacial shear stress distribution along the fibre was derived through a balance of forces analysis. The ‘large’ interfibre distance microcomposite showed no stress redistribution at the vicinity of a fibre break. In contrast, composites of ‘zero’ interfibre distance showed a distinct redistribution of stresses in fibres adjacent to fibre breaks. A stress concentration value of the order of 1.5 was measured at the vicinity of two adjacent fibre fractures.

In this paper the failure mechanisms of unidirectional aligned carbon fibre/epoxy composites are investigated. Experimental results are presented for the strength of carbon/epoxy composite tows, as well as for single carbon fibres supplied in the sized and unsized condition. Laser Raman spectroscopy was used in this study to assess the effect of fibre breaks on the stress distribution within a composite. Fibre stress mapping of composite tows using laser Raman spectroscopy showed redistribution due to fibre failure and a value of the stress concentration factor, Kr, was obtained. The results were analysed using a Weibull distribution for the strength of the reinforcing fibres and composite.

The interfacial and fracture characteristics of highmodulus carbon-fibre/epoxy-resin composites have been investigated. Three different coupon geometries were employed, namely 2D microcomposite tapes, fibre tows and full composite tensile coupons. In all cases, the point-by-point stress in the fibre was measured by the technique of remote laser Raman microscopy (ReRaM). The composite specimens were loaded incrementally in tension and the stress transfer profiles emanating from the fibre, were closely monitored. At each applied stress level, the interfacial shear stress (ISS) distribution was derived by means of a balance of shear-to-axial forces argument. The redistribution of stress in fibres adjacent to a filament break in all geometries was determined as a function of distance from the fibre fracture. The values of stress concentration were found to depend upon the number of nearest neighbours and the radial distance from the fibre fracture. Thus, the apparent discrepancy between measurements obtained from 2D tapes and those from full unidirectional composites was resolved. A phenomenological equation was derived to relate the fibre stress concentration to interfibre distance for both geometries. This equation has also been employed to estimate the stress concentration in the bulk of a composite for a hexagonal array of fibres. Finally, the interfacial shear stress in the neighbouring fibres as a result of the shear perturbation induced by an adjacent fibre fracture has been quantified for the first time.

Laser Raman microscopy has been used to assess the effects of single and double breaks on the load redistribution within a fibre tow. Two types of specimen incorporating sized and unsized carbon fibres impregnated by an epoxy resin were examined. Fibre stress mapping in the neighbouring fibres to a filament break showed stress redistribution due to fibre failure and values of the stress concentration factor, Kr, have been calculated. The stress transfer profiles around fibre breaks for the sized and unsized fibre/resin systems were distinctly different. The maximum interfacial shear stress was of the order of 20 M Pa and 30 MPa, respectively, for the sized and unsized impregnated tows.

The synthesis of large, defect-free two-dimensional materials (2DMs) such as graphene is a major challenge toward industrial applications. Chemical vapor deposition (CVD) on liquid metal catalysts (LMCats) is a recently developed process for the fast synthesis of high-quality single crystals of 2DMs. However, up to now, the lack of in situtechniques enabling direct feedback on the growth has limited our understanding of the process dynamics and primarily led to empirical growth recipes. Thus, an in situmultiscale monitoring of the 2DMs structure, coupled with a real-time control of the growth parameters, is necessary for efficient synthesis. Here we report real-time monitoring of graphene growth on liquid copper (at 1370 K under atmospheric pressure CVD conditions) viafour complementary in situmethods: synchrotron X-ray diffraction and reflectivity, Raman spectroscopy, and radiation-mode optical microscopy. This has allowed us to control graphene growth parameters such as shape, dispersion, and the hexagonal supra-organization with very high accuracy. Furthermore, the switch from continuous polycrystalline film to the growth of millimeter-sized defect-free single crystals could also be accomplished. The presented results have far-reaching consequences for studying and tailoring 2D material formation processes on LMCats under CVD growth conditions. Finally, the experimental observations are supported by multiscale modeling that has thrown light into the underlying mechanisms of graphene growth.

Wrinklons, the transition zone where two wrinkles are merged to one, have been observed in various thin films across all scales. For suspended films the evolution of the wrinklon wavelength from the edge can be described by λ∼xm, where mis universal. Herein we show experimentally that for graphene wrinklons on a compliant polymer, mis not universal, but varies with the graphene thickness. An analytical model is developed based on energy principles, which shows that the influence of the substrate has to be taken into account. The present analysis can be applied further in all thin films on substrate and provides significant physical insight for the wrinklon phenomenology.

Carbon nanotubes are one-dimensional materials found in various forms, the most important of which are bulk unordered carbon nanotubes of lengths of few microns and aligned tubes regularly a few millimeters long. Differences in length, orientation, alignment, nanotube graphitization and purity will influence the eventual performance of the material. Herein four characterization methods are used to characterize and compare bulk and aligned carbon nanotubes. Results show that the diameters of the millimeter-long aligned carbon nanotubes are approximately 60 ∼ 80 nm with thick nanotube walls of about 25–30 nm, while the diameters of the bulk carbon nanotubes are about 15–20 nm with much thinner walls. The aligned carbon nanotubes have significantly larger graphitization degrees, higher purity and greater orientation than the bulk carbon nanotubes that tend to self-agglomerate under no external stimulus. Silicon carbide matrix nanocomposites reinforced by the aligned carbon nanotubes were found to be denser that those reinforced by the bulk carbon nanotubes and also exhibit extensive, uniform, and long pullout of carbon nanotubes.