Wrinkles are out-of-plane deformations, commonly seen in chemical vapor deposition (CVD) grown graphene, that mostly arise from thermal expansion mismatches. These one-dimensional corrugations are believed to be centers of altered electrical and electronic properties for graphene. Herein, high-resolution electrical modes of Atomic Force Microscopy (AFM) were employed to measure the nanoscale current and work function distribution of graphene wrinkles. Tapping current measurements showcased an unforeseen increase up to two orders of magnitude compared to flat regions. As revealed by extensive first principles calculations (density functional theory employing CAM-B3LYP and PBE0 functionals) and experimental data (AFM advanced electrical modes), an interplay of mechanisms between polarization, separation from the substrate and strain gradients result in an impressive increase in vertical conductivity with possible implications for analogous surge also along wrinkle axes. Furthermore, characteristic variations in the work function of wrinkles, with respect to various substrates, could be clearly identified. The implications of our findings may pave the way for fine-regulation of conductivity via wrinkle engineering for the previously unexplored out-of-plane direction, as well as for the controlled formation of conductive channels.

The axial tensile deformation of exfoliated monolayer graphene in air is always accompanied by the formation of lateral wrinkles, or buckles, due to the extremely low bending stiffness. When supported or embedded in polymer matrices, the resistance to buckling significantly increases, but for large irregular flakes, out-of-plane wrinkling occurs at axial strains below similar to 1 %, leading to interfacial failure. However, Poisson's driven lateral strains are generated by shear and, therefore, for small widths (less than twice the transfer length) the material is expected to remain flat under axial deformation. In this work, we have tested this assertion by performing uniaxial tensile testing combined with in-situ Atomic Force Microscopy, in order to observe the onset of lateral wrinkling in simply-supported monolayer graphene flakes having different widths. We have provided evidence that the onset of wrinkle formation can be eliminated or pushed back to high strains (i.e. >2 %), if graphene is shaped as a narrow micro-ribbon. We also implemented a theoretical model based on shear-lag theory to predict the critical tensile strain for the initiation of lateral wrinkling, and demonstrated that for ribbons of width <800 nm the lateral wrinkling is fully eliminated. We argue that this is the only route possible for the exploitation of graphene as a strong and tough material in a multitude of applications.

Failure of polymers frequently initiates at discontinuities in the material, such as holes and notches, as they constitute points of increased stress concentration. Herein, we propose the use of monolayer graphene produced via Chemical Vapour Deposition to monitor the stress distribution close to a defect in poly(methyl methacrylate). Combining in-situ Raman spectroscopic mapping with tensile tests, the stress/strain distribution around the defect can be probed via monitoring the wavenumber shift of the spectroscopic features of graphene. The measured stress concentration factor of 2.41 is remarkably close to the value derived from Finite Element Analysis, and agrees with other studies in the literature, thus demonstrating that the proposed technique is reliable, and that graphene can accurately sense stress concentration close to a defect, with a sub-micron spatial resolution and a strain resolution of approximate to 60 mu epsilon.

The development of bio-based primary packaging is imperative to improve food safety and quality, along with mitigating the environmental pollution resulting from the increasing production of packaging wastes. Whey protein concentrate (WPC) has received significant attention as an alternative raw material for edible films production, whereas an area of many ongoing researches has been the inclusion of bio-based compounds to formulate functional edible films. This study elaborated the development of WPC edible films fortified with different essential oils (EO) to assess antimicrobial activity against gram-positive (Staphylococcus aureus and Listeria monocytogenes) and gram-negative (Salmonella enteritidis and Pseudomonas aeruginosa) food-borne pathogens. Subsequently, the effect of grafting bacterial cellulose nanowhiskers (BCNW) in film composites was evaluated for WPC/oregano EO (WPC/OEO) films, which showed the highest antibacterial activity. Notably, increasing the concentration of grafted BCNW entailed a proportional increase on the inhibitory activity. In specific, antibacterial activity increased up to 80% when 10% BCNW was grafted in WPC/OEO films. Inclusion of BCNW and homogenous distribution of OEO into the film blend was pointed out through Raman analysis. In addition, AFM and SEM analysis revealed that inclusion of BCNW and OEO into the film matrix led to the formation of rough films with pores. This structural change enhanced the release capacity of OEO, that was further confirmed through a release kinetic experiment. This study indicated that BCNW addition positively affected whey protein films properties, thereby envisaging the potential development of sustainable food packaging materials with tailor-made properties.

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 situ radiation-mode optical microscopy, we explore the growth morphology and kinetics over a wide range of CH4-to-H2 pressure 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.

AbstractGraphene is a perfect 2D crystal of covalently bonded carbon atoms and constitutes the building block for all graphitic structures. Its superior properties make it an attractive material for a variety of technological applications. However, mass production does not meet the initial expectations. Chemical Vapor Deposition (CVD) is currently the only available method for large‐scale automated production, but the produced graphene sheets suffer from structural and morphological defects that degrade considerably the mechanical and other physical properties of synthesized graphene. Recently, the use of liquid metal catalysts (LMCat) has been proposed as an alternative platform for facile and high‐quality synthesis of single‐crystal graphene. Herein, simultaneous Raman spectroscopy combined with mechanical testing is adopted confirming that the reinforcing efficiency of the LMCat graphene is greatly improved. In fact, the effective Young's modulus of LMCat graphene has been found ≈630 GPa, which is significantly higher than the graphene grown on solid Cu substrate due to differences in the morphology of Cu substrate. Overall, this work paves the way for the development of defect‐free graphene of quality comparable to exfoliated flakes, and this will have a major technological impact for many applications.

Push-to-pull (PTP) testing is employed to probe the uniaxial tensile response of freestanding monolayer graphene. Various analytical approaches are employed to estimate the elastic modulus of end-clamped graphene samples, combining in-situ Raman spectroscopy and scanning electronic microscope (SEM) measurements. The utilization of spatially resolved Raman-derived strains for assessing the elastic properties of monolayer graphene leads to results consistent with previous experimental and theoretical values of the elastic modulus (approximately 1 TPa). Molecular dynamics (MD) simulations of (pristine and defective) freestanding graphene sheets uniaxially loaded under varying clamping conditions are performed to support the experimental observations. The computational results indicate that the mechanical responses of the sheets are affected by both the type, the spatial profile, and the heterogeneity of the clamping. When uniaxial pulling of end-clamped graphene is applied by a substrate adhering to the graphene sheet through van der Waals forces (as in PTP testing), the elastic modulus may be highly underestimated due to often inhomogeneous stress distribution and slippage processes. The MD simulations predict that the elastic modulus of pristine monolayer graphene is approximately 1 TPa, whereas its fracture strength can reach values of up to 110 GPa. Overall, this study underscores the limitations of traditional analyses of PTP experiments (utilizing indentation readouts and SEM imaging) and proposes new potential avenues (involving Raman measurements) for future research on the elastic properties of 2D materials.

Graphene, with its superior physical properties, has been considered as the perfect candidate for the production of lightweight, high-strength composite materials with interesting multi-functionalities. The use of large-sized, high-quality CVD graphene monolayers alternated to ultra-thin polymer films in a laminate configuration has been recently proposed as an efficient route to overcome many of the limitations faced by the use of discontinuous sheets of graphene in nanocomposites. Here we report on the production of CVD graphene/polyetherimide (Gr/PEI) nanolaminates with very low graphene volume fractions (up to 0.165 vol%), using a modified iterative and automatic lift-off/float-on procedure. The produced freestanding Gr/PEI nanolaminates present not only a significant enhancement of mechanical and electrical properties but, very interestingly, show impressive Joule heating efficiency. In fact, upon the application of an electrical potential, they can reach temperatures higher than 250 degrees C, with heating rates up to 325 degrees C/s. The produced heaters show a very uniform distribution of the temperature even when bend and are characterized by low power consumptions (up to 16 Watt) and high areal power densities (up to ca. similar to 1.28 W/cm(2)), thus suggesting their possible application in thermal management.

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Due to their outstanding electrical and thermal properties, graphene and related materials have been proposed as ideal candidates for the development of lightweight systems for thermoelectric applications. Recently, the nanolaminate architecture that entails alternation of continuous graphene monolayers and ultrathin polymer films has been proposed as an efficient route for the development of composites with impressive physicochemical properties. In this work, we present a novel layer-by-layer approach for the fabrication of highly ordered, flexible, heat-resistant, and electrically conductive freestanding graphene/polymer nanolaminates through alternating Marangoni-driven self-assembly of reduced graphene oxide (rGO) and poly(ether imide) (PEI) films. The microstructure, the mechanical behavior, and the electrical conductivity of the produced Marangoni rGO/PEI nanolaminates are studied as a function of rGO content (up to 5.2 vol %). These nanolaminate thin films show excellent heating properties, with fast heating responses at high temperatures to maximum temperatures at ca. 325 degrees C due to the Joule heating effect, at maximum rates of 444 degrees C/s, thus bringing forward an impressive potential of these materials for electrothermal applications. The areal power density was found to be 30 kW/m2 for the 5.20% volume fraction of rGO and 325 degrees C temperature. The robust highly flexible heaters developed in this research hold great promise for a whole range of applications.

The interaction zone between the matrix and the reinforcing component within carbon fibre (CF) composites plays a crucial role in influencing their performance characteristics, as extensively documented in scientific literature. Building on this fundamental knowledge, this research focuses to the investigation of the interface of fibrous composites and the effect of the incorporation of graphene nanoplatelets (GNPs) into the epoxy matrix. Through the utilization of laser Raman spectroscopy, a significant increase of over 60 % in interfacial strength with just a 2 % weight fraction of GNPs, as expressed by the enhanced shear-lag parameter observed in single fibre model composites, has been observed on model composites indicating a stronger interfacial interaction between the matrix and fibres. The effect of the incorporation of GNPs on CF-graphene interaction has been also investigated on a macroscopic level. Hybrid carbon fibre epoxy resin composite produced, by the introduction of GNPs into the polymer matrix. A series of mechanical tests were conducted, including extensive tensile testing of epoxy/graphene nanocomposite films, thorough examination of bending characteristics, and meticulous measurement of interlaminar shear strength for hybrid GNP/carbon fibre composites. These evaluations provided a clearer understanding of how the GNP integration affects the interface parameters that govern the mechanical behaviour of both the resin and carbon fibre composites.

Graphene and other two-dimensional materials (2DMs) have been shown to be promising candidates for the development of flexible and highly-sensitive strain sensors. However, the successful implementation of 2DMs in practical applications is slowed down by complex processing and still low sensitivity. Here, we report on a novel development of strain sensors based on Marangoni self-assemblies of graphene and of its hybrids with other 2DMs that can both withstand very large deformation and exhibit highly sensitive piezoresistive behaviour. By exploiting the Marangoni effect, reference films of self-assembled reduced graphene oxide (RGO) are first optimized, and the electromechanical behaviour has been assessed after deposition onto different elastomers demonstrating the potential of producing strain sensors suitable for different fields of application. Hybrid networks have been then prepared by adding hexagonal boron nitride (hBN) and fluorinated graphene (FGr) to the RGO dispersion. The hybrid integration of 2D materials is demonstrated to become a potential solution to increase substantially the sensitivity of the produced resistive strain sensors without compromising the mechanical integrity of the film. In fact, for large quasi-static deformations, a range of gauge factor values up to 2000 were demonstrated, while retaining a stable performance under cyclic deformations.

Agglomerations effects of graphene-based nanofillers are often reported in the literature to be the main reason on the deterioration of the mechanical properties, especially at high filler loadings. In our study, we focused on the correlated effects of plasma-treated graphene nanofillers on the curing reaction and mechanical properties of an epoxy matrix. Specifically, we explored the effect of dispersion state, planar size, filler content, surface functionalization and stoichiometric ratio on the epoxy curing process. The surface of the treated graphene nanofillers were studied in detail by X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and X-ray diffraction (XRD). The results indicated greater presence of oxygen containing groups with the crystallinity to be unaffected after the plasma process. Dynamic Mechanical Analysis (DMA) was used to assess the changes in both the T-g and the mechanical properties of graphene-epoxy nanocomposites. Rheological and microscopic data showed that a well-dispersed material was achieved at high filler loadings with the use of calendaring and plasma functionalization. Although, a well-dispersed material was achieved on the bulk composite, no further mechanical reinforcement was observed at high filler loadings. The adsorption of epoxy groups onto the graphene nanofillers' surface, leading to a stoichiometric imbalance between the epoxy chains and hardener molecules, was proposed to explain the results.

The present work demonstrates the ability of graphene nanoplatelets (GNPs) and other two-dimensional materials (2DMs) like tungsten disulfide (WS2), molybdenum disulfide (MoS2) and hexagonal boron nitride (hBN) to act as protective barriers against the fading of architectural paints and also inks/paints used in art. The results present a new approach for improving the lightfastness of colours of artworks and painted indoor/outdoor wall surfaces taking advantage of the remarkable properties of 2DMs. As shown herein, commercial inks and architectural paints of different colours doped with graphene nanoplatelets (GNPs), graphene oxide (GO), reduced graphene oxide (rGO) and other 2DMs, exhibit a superior resistance to fading under ultraviolet radiation or even under exposure to visible light. A spectroscopic study on these inks and dyes reveals that the peaks which are characteristic of the colour pigments are less affected from aging/fading when the GNPs and the other 2DMs are present. The protection mechanism for the GNPs and the other 2DMs differs. For GNPs, mainly their high surface area which leads to free radicals scavenging (especially hydroxyl radicals), and secondarily their UV absorption, are responsible for their protection effects, while for GO, a transition to rGO structures and consequently to 'smart' paints can be observed after the performed aging routes. In this way, the paint gets improved by time preventing or slowing its own fading and decolorization. For the other 2DMs, the transition-metal dichalcogenides performed better than hBN, even though they all absorb in the UV region. This can be ascribed to the facts that the formers also absorb in the visible, while hBN does not, while most importantly, they can trap reactive oxygen species (ROS) and corrosive gases in their structure as opposed to hBN. By conducting colorimetric measurements, we have discovered that the lifetime of the as-developed 2DM-doped inks and paints can be extended by up to ∼40%.

The preparation of gold nanoparticles with multimodal properties such as near-infrared absorption, high surface-enhanced Raman scattering, cell internalization and low cytotoxicity is a challenging task. In this study, we developed a sustainable protocol to develop gold nanoparticles, using trisodium citrate as reducing, stabilizing, and shape-modulating agent at ambient conditions (25 degrees C). Reduction of gold salt at room temperature at a peculiar ratio R(Ccitrate/CHAuCl4) and concentration of reactants resulted in the formation of non-spherical, homogenous mixture of gold nanoparticles with polyhedral shapes. The protocol is extremely simple and does not even require stirring or mechanical shaking. The as-synthesized gold nanoparticles, even though multi-shaped, displayed a single monomodal peak in dynamic light scattering with polydispersity index of 0.098, representative of a fairly good monodisperse system. We investigated the optical properties of the nanoparticles both experimentally and by two-dimensional Finite-Difference-Time-Domain modeling. Due to the presence of shapes such as nanorods, nanotriangles, and prismatic, these nanoparticles exhibited a high surface-enhanced Raman scattering activity and a wide absorption range extending up to the near-infrared region, which makes them useful candidate for photothermal therapy too. We characterized the nanoparticles by electron microscopy, UV -vis-NIR spectroscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy. We have also developed an enhanced numerical diffusion limited aggregation model to simulate the growth of the particles, including the particle interfacial energy as a parameter of the system. Numerical results matched with the experimental data, and the model revealed being effective in reproducing size, shape, and morphological characteristics of non-spherical nanoparticles obtained under real experimental conditions. Finally, in vitro studies and nanoparticles cellular uptake were performed on a model cell line of mouse brain endothelium (bEnd.3) to assess biocompatibility. The main advantage of the proposed method lies in its simplicity with no other requirement like refluxing at elevated temperature, mechanical stirring, electromagnetic radiations, ultrasound, toxic chemicals, or seed mediation.(c) 2022 Elsevier Ltd. All rights reserved.

Successful ways of fully exploiting the excellent structural and multifunctional performance of graphene and related materials are of great scientific and technological interest. New opportunities are provided by the fabrication of a novel class of nanocomposites with a nanolaminate architecture. In this work, by using the iterative lift-off/float-on process combined with wet depositions, we incorporated cm-size graphene monolayers produced via Chemical Vapour Deposition into a poly (methyl methacrylate) (PMMA) matrix with a controlled, alternate-layered structure. The produced nanolaminate shows a significant improvement in mechanical properties, with enhanced stiffness, strength and toughness, with the addition of only 0.06 vol% of graphene. Furthermore, oxygen and carbon dioxide permeability measurements performed at different relative humidity levels, reveal that the addition of graphene leads to significant reduction of permeability, compared to neat PMMA. Overall, we demonstrate that the produced graphene-PMMA nanolaminate surpasses, in terms of gas barrier properties, the traditional discontinuous graphene-particle composites with a similar filler content. Moreover, we found that the gas permeability through the nanocomposites departs from a monotonic decrease as a function of relative humidity, which is instead evident in the case of the pure PMMA nanolaminate. This work suggests the possible use of Chemical Vapour Deposition graphene-polymer nanolaminates as a flexible gas barrier, thus enlarging the spectrum of applications for this novel material.

Edible films were developed using whey protein concentrate (WPC) and a natural bio-polymer, namely bacterial cellulose (BC). BC was produced via fermentation from orange peels and subsequently acid-hydrolyzed to obtain BC nanowhiskers (BCNW) with high crystallinity (XRD analysis). Morphology of BCNW was analyzed by SEM, TEM, and AFM. WPC/BCNW film composites, containing different amounts of BCNW (0.5-15%, w/w) were developed and characterized. WPC/BCNW film composite was analyzed by Raman spectroscopy, indicating the successful incorporation and the homogenous distribution of BCNW into the WPC film matrix. Mechanical characterization showed that BCNW behaved as a reinforcing filler in the WPC film, increasing tensile strength and Young's modulus by 32% and 80%, respectively. In addition, water vapor permeability was reduced by 33.9% upon the addition of 0.5% BCNW. This study presented a sustainable approach towards the production of WPC films with improved tensile and water barrier properties, suggesting its potential application as a packaging material.

Controllable large-scale synthesis of two-dimensional materials (2DMs) such as graphene is a prerequisite for industrial applications. Chemical vapor deposition (CVD) is currently the most widespread synthesis method as it is efficient and easy to automatize. The process itself is quite complex and poorly understood, but it is generally believed to involve a number of distinct steps such as hydrocarbon decomposition into surface-bound intermediates, diffusion on the catalytic substrate, generation of nucleation points and, finally, graphene growth. In situ monitoring and tailoring of such a complex procedure is beneficial for understanding the growth kinetics and, eventually, for controlling the graphene growth. Herein, we report on a novel metrology system based on in situ reflectance spectroscopy that has been developed for real-time monitoring of surface changes during graphene growth on Cu foils at high operating temperatures. The implementation of this technique for extracting kinetic parameters of the growth process is presented. Furthermore, a microkinetic model of graphene growth based on density-functional theory (DFT) and the hindered translator / rotator model for enthalpy and entropy corrections is constructed and used to obtain a microscopic understanding of the apparent activation energy and related rate-determining steps in graphene growth.

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 situ techniques enabling direct feedback on the growth has limited our understanding of the process dynamics and primarily led to empirical growth recipes. Thus, an in situ multiscale 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) via four complementary in situ methods: 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.

Modern and contemporary art materials are generally prone to irreversible colour changes upon exposure to light and oxidizing agents. Graphene can be produced in thin large sheets, blocks ultraviolet light, and is impermeable to oxygen, moisture and corrosive agents; therefore, it has the potential to be used as a transparent layer for the protection of art objects in museums, during storage and transportation. Here we show that a single-layer or multilayer graphene veil, produced by chemical vapour deposition, can be deposited over artworks to protect them efficiently against colour fading, with a protection factor of up to 70%. We also show that this process is reversible since the graphene protective layer can be removed using a soft rubber eraser without causing any damage to the artwork. We have also explored a complementary contactless graphene-based route for colour protection that is based on the deposition of graphene on picture framing glass for use when the direct application of graphene is not feasible due to surface roughness or artwork fragility. Overall, the present results are a proof of concept of the potential use of graphene as an effective and removable protective advanced material to prevent colour fading in artworks.