The contribution of graphene content (1 wt%) on the stability improvement of SIS is demonstrated by the increase of onset oxidation temperature from 190 °C for neat polymer to 196 °C in the presence of graphene and to 205 °C for the polymer stabilized with graphene and rosemary extract. The addition of graphene into the polymer formulations is a successful method for enlarging durability instead of the modification of receipt with synthesis antioxidants. The presumable applications of these studied materials cover the areas of medical wear, food packaging, commodities, sealing gaskets and others that may also be included through the products for nuclear power plants.This study presents the improved stabilization effects of graphene on a polymer substrate, namely a styrene-isoprene-styrene triblock copolymer (SIS) which creates opportunities for long-term applications and radiation processing. The added graphene has a remarkable activity on the protection of polymer against their oxidation due to the penetration of free macroradical fragments into the free interlayer space. The chemiluminescence procedure used for the evaluation of the progress of oxidation reveals the delaying effect of oxidative degradation by the doubling extension of oxidation induction time, when the material formulation containing graphene is oxidized at 130 °C. The pristine polymer that is thermally aged requires an activation energy of 142 kJ mol The duration of any material is directly related to its oxidation strength that is the main characteristic feature taken into account for the material delivery . The preThe preparation of graphene structures ,19,20 opThe graphene/polymer composites may be a reliable solution for several materials with an improved lifetime . These mThis paper proposes an example through which the dangerous synthesis hindered phenols may be replaced with an inorganic structure, more stable and with similar efficiency. The present study analyses the optional but relevant material with which the extension of durability does not spoil the material compatibility with the human body. The reported results complete the information communicated in a previous paper, where graphene films incorporated in SIS minimize the oxidation of polymer phase in the solar cell applications . The ideThe main desire involving polymer stabilization is related to the additive efficiency, along with the conservation or improvement in other material properties such as mechanical behavior . The benNumerous application areas of polymer/graphene composites include this crystalline carbon configuration that involves the layer structures through which molecules or molecular fragments penetrate: the scaffolds based on the honeycomb structure of graphene are ideal providers of drugs in the reconstruction of bones, cartilages, and electroactive tissues . A smallThe transport processes achieved in the degrading polymers are associated with the diffusion of outer molecular oxygen and the conversion of oxygen-containing intermediates into stable products after the former structures migrate onto the reaction spots. It was previously demonstrated  that varThe inclusion of various atoms other than carbon is described in , where tDepending on the diffusion of ammonia into the graphene structure , the sca2 hybridization maintains the integrity of graphene nanocomposites [Unfortunately, there are no reports concerning the protection of graphene against the oxidation of polymers. The stabilization mechanism is quite different from the classical activity of phenolic antioxidants. Their protection action is based on the substitution of labile protons . Becausemposites . StartinThe utilization of graphene polymer composites as drug delivery material , antifoaThis paper offers an efficient solution for the manufacture of several long-term products efficiently stabilized by an allotropic form of carbon, reduced graphene oxide (rGO), whose contribution to the improving durability is the basic feature for special applications such as electrical cable insulation and high performance capacitors, food packaging, commodities, sealing gaskets, sealants and gaskets destined for nuclear energetics, adhesive items, bags for the storage of medical wear and protective sheets for equipment with outdoor service. The reported results are possible due to the structural configuration of graphene that allows the penetration of free radicals in the material hexagonal network, where electronic cloud and oxygenated functions interact strongly with degradation intermediates, keeping them apart from the propagation stage of degradation. Thus, these multipurpose applications are possible due to the capacity of graphene to break the degradation chain by the scavenging of free molecular fragments that propagate oxidation during thermal ageing. This enumeration is a short list of products, where SIS improved with graphene powder may be used due to the functional properties available over an extended period of operation. The great advantages that recommend graphene as an antioxidant additive are: stability, efficiency, and comparable price with synthesis antioxidants.−1 @ 25 °C, average Mn 80,000 and the polydispersion index (Mw/Mn) is less than 2.The polymer material, styrene-isoprene-styrene triblock copolymer (SIS), was purchased from KRATON  as D1165 PT sort. This material was purchased as -polyoxepan-2-one pellets with an average diameter of ~3 mm. The styrene content is 30 wt%, density 1.145 g mL2SO4 (96–98%), NaNO3 (99.5%), KMnO4 (99%), HCl (37%), H2O2 (30%) and ascorbic acid (99.6%) were purchased from Chemical Company .Graphene fraction was prepared in our laboratory as reduced graphite oxide (rGO). Natural graphite powder (flakes < 30 mm) was provided from KOH-I-NOOR Grafit . The p.a. reagents H3 and 100 mL H2SO4 keeping the temperature below 5 °C by immersion in an ice bath for 4 h. After homogenization under stirring (500 rpm), 6 g KMNO4 is added, keeping the temperature of the mixture below 10 °C for 1 h. The resulting solution was warmed to 35 °C and stirred for 1 h. Afterwards, the solution is diluted with 100 mL of distilled water, homogenized by stirring and heated to 95 °C and maintained at this temperature for 2 h. The solution was allowed to cool to room temperature, then 200 mL of distilled water was added and the mixture was stirred for 1 h. Forty milliliters of 30% H2O2 were added to complete the oxidation reaction, and the mixture was stirred 1 h at room temperature. Graphene oxide was formed, and the resulting slurry was washed with 5% HCl and distilled water to neutral pH. The water is completely removed from the resulting graphene oxide by freeze drying.The GO was synthesized from natural graphite powder by the modified Hummer’s method ,60. The The resulting GO (1 g) was diluted in 200 mL distilled water and sonicated at room temperature for 2 h to homogenously disperse the GO phase in water. Fifteen grams of ascorbic acid, used as reducing agent, was added to the resulting suspension and continuous stirring for 2 h at a temperature of 90–95 °C was applied. In the next step, the mixture was stirred for 20 h at room temperature during which time the reduced graphene oxide is formed. The suspension was washed with 5% HCl and distilled water to neutral pH. The water is completely removed from the resulting reduced graphene oxide by freeze drying.−1) [−1) [−1 identified as C–O units [−1, where rGO presents a very prominent peak like our material.The FTIR spectrum of prepared rGO is presented in −1) . The oth−1) [−1)  and 1100–O units . The dis–O units ; it is rSmall SIS balls were dissolved in chloroform by vigorous shaking until a clear liquid was obtained. From this mother solution, aliquots of 10 mL were transferred into small bottles, where appropriate rGO was added to obtain the concentrations of 1, 2 and 3 wt%. For the evaluation of coupling contribution of reduced graphene oxide (rGO) and rosemary extract (RM—processed in our laboratory ), a fourThe thermal treatment was carried out in a Lenton laboratory  oven with forced air convection. The samples were thermally treated in air at a constant temperature. The oven was stabilized at 80 °C, after which the samples were introduced and maintained at this temperature for 5, 10, 15 and 20 h. At the end of the exposure time, they were immediately removed from the oven and cooled to room temperature.−1 for nonisothermal procedure was selected, while, for isothermal study, three temperatures  allow the comparison of thermal stability and the calculation of activation energies required for the thermal degradation of the studied probes.Chemiluminescence (CL) determinations were achieved by means of a LUMIPOL 3  spectrometer, whose reading error in temperature values is ±0.5 °C. The proper parameters for the optimal experimental conditions were established: the heating rate of 10 °C minThe improvement of polymer stability requires an appropriate structure through which the free radicals are withdrawn from the degradation chain . GraphenThe thermal treatment of the polymer matrix reveals several basic aspects related to the fate of free radicals that result from the scission of SIS molecules. The most relevant feature that must be taken into account is the reduction of onset oxidation temperature (OOT), the kinetic parameter which indicates the start temperature of oxidation. If it is 208 °C for neat material, the preheated probes show lower temperatures around 192 °C . This beAs it may be expected, the modification of thermal resistance due to the presence of radical scavengers, is suitably obtained. An important application of this benefit would be the recycling of plastics , when thThe stabilization efficiency of the couple consisting of graphene (1 wt%) and rosemary (0.5 wt%) is revealed in The thermal processing of the SIS matrix reveals an interesting peculiarity related to the evolution of CL emission, which follows the classical mechanism based on the radical reactions . The forThe isothermal CL investigation highlights the effect of additive concentration on the relative durability of stabilized materials in comparison to pristine polymer .The delay of oxidation is illustrated by the values of oxidation induction times (OIT) which increase as the amount of graphene is enhanced. For the SIS substrates that are not subjected to any environmental aggression, these figures are strikingly different than the value shown by the control specimen; they are minimum double and the protection activity determines an extension of the period during which the material remains undamaged. The radiation processing narrows the OIT values, but they are greater than the similar characteristic of pristine polymer. Another positive effect of the participation of graphene to the improvement of thermal stability is the significant extension of propagation stage of degradation, when the graphene nanoplates keep radicals tightly, and their further oxidation is blocked over a long time period. It can be significantly compared with the previous time, when the free radicals (the active intermediates that promote the progress of oxidation) are accumulated.The failure of SIS products is directly dependent on several factors: the applied technology, the duration and intensity of energetic transfer, the material composition, especially the presence of stabilizing structures, the history of the item. The destructive actions of all these elements are minimized by the contribution of additives, whose efficiencies must be checked before their addition into new products.The structural modifications during thermal degradation are revealed by the appropriate evaluation of stability, when the measured characteristics are correlated with the evolution of the state of the material. Chemiluminescence, an accurate analytical procedure, allows the counting of emitted photons by the conversion of carbonyl intermediates from the exited state into their background energetic level . The sciThe stabilization activity of graphene is sustained by the intercalation of radicals through the carbon layers. The electronic distribution over the planar carbon atoms allows the formation of enough strong bridges with the penetrated fragments. Thus, the delay of oxidation is achieved by the immobilization of degradation intermediates, which is proved by the extension of experimental times.The reduced graphene oxide is a graphene where polar units containing oxygen are grafted on the constituent carbon atoms . These uThe association of graphene with a natural antioxidant such as rosemary extract is an excellent version of stabilization formulation. In The prediction of material durability based on the values of activation energy is a good method for the description of the competition between oxidation and temperature-dependent deterioration. When an investigated material presents a certain oxidation induction time (OIT), the effect of additives on the accumulation of oxygen-containing product is depicted by the duration of stabilized structures. The longer the steady state period on the start of oxidation, the more the participation of the protector delays the degradation . If the The values of OOT shown in the last column of The availability of reduced graphene oxide is illustrated by the values of activation energies that are involved in the delay of oxidative degradation, when the patterns contain increasing loadings of graphene additive .The protection activity is enhanced by the stacking effect , throughIn the action against oxidative degradation, the presence of several groups resulting from the oxidation of graphene  is assocThe rGO powder interfering in the mechanism of polymer degradation is an ecological component of products allowing long periods of operation because it is not damaged or consumed. Its inclusion as a stabilization component provides structural integrity to the material and the conservation of features.The results presented in this paper prove protective action of reduced graphene oxide against oxidation by the penetration of polymer fragment into the free interlayer space. The presence of oxygen containing groups brings about supplementary stabilization strength. The optimization of stabilization efficiency is obtained by the suitable activity of graphene as the scavenger of degradation intermediates even during the operation on high temperature range exceeding 200 °C. The presence of graphene in the formulation of SIS based materials brings about a significant slowing down of the oxidation rate by the reduction of local concentration of free radicals, as well as the breaking degradation chain by the integration of degradation promoters into the lamellar configuration of additive. The increase in the graphene loading leads to the higher material stability extending the oxidation induction time from 130 min obtained for the ageing of pristine SIS at 130 °C to 380 min at the same temperature, when the oxidation takes place in the polymer containing 3 wt% graphene. The activation energies that increase as the rGO concentration become 1.2 times higher in the presence of 3 wt% graphene represent the reliable proofs for the selection of this additive as an efficient antioxidant for polymers. The polymer support, SIS copolymer, is an example for its class of materials, which may be improved when convenient loading of rGO is present. This study demonstrates that the classical antioxidants, hindered phenols or amine, may be replaced by this rGO due to the proper action as the chain breaker in the degradation mechanism. The availability for the material protection recommends it to be used in several applications in material manufacturing for food handling, medical wear, conductive sheets and bars, anticorrosive layers and commodities. Several applications of packaging materials, including food handling, are possible by the manufacturing of products based on the formulations that include a graphene/rosemary extract couple.


Friction force microscopy measurements reveal a dramatic difference of a factor 3 between the friction forces experienced on single-monolayer graphene on top of oxidized and unoxidized copper substrates. We associate this difference with the strong and weak adhesion that the graphene experiences on these two substrates, respectively, but argue that it is too large to be ascribed either to a difference in contact area or to a difference in contact commensurability or even to a combination of these two effects. We use density functional theory to show a significant increase in the chemical reactivity of graphene when it is curved. The increase of chemical reactivity in curved graphene explains the reduction in friction for monolayer graphene over oxidized/unoxidized copper substrates. It is the lightest, thinnest and strongest material known to date1 and new, extreme properties continue to be added to the growing list for this special, two-dimensional material. As graphene forms a natural constituent of graphite and graphite is commonly used as a solid lubricant, the frictional properties of graphene have attracted attention too and even a single layer of graphene is known to reduce dry friction and wear in sliding contact.2–11 Nanoscale friction experiments with friction force microscopy (FFM) have demonstrated that friction on graphene can depend sensitively on the number of graphene layers12–14 and approximately reaches the value of bulk graphite only at a thickness of four layers.15Graphene is a single monolayer of sp15 This effect was observed specifically on silicon oxide and copper substrates, both of which have weak adhesion to graphene. The puckering was found to be sensitive to the number of graphene layers and to become less pronounced, as the number of layers increases. The latter effect is associated with the stronger adhesion of the graphene top layer to underlying graphene layers, which reduces the puckering and is accompanied by a reduction in the friction force. Correspondingly, the puckering is suppressed on substrates to which graphene experiences strong adhesion, such as mica,15 on which the friction force is observed not to depend on the number of graphene layers.Since graphene has a relatively low out-of-plane bending stiffness, it can easily bulge (pucker) in front of the sliding AFM tip.15–17 Later, the same authors showed that the friction increase was substantially larger than the change in contact area that they estimated due to the puckering of the graphene. Therefore, they proposed an alternative explanation, in which a key role was given to the freedom of the graphene to locally deform in order to optimize its commensurability with those regions on the tip, to which it made intimate contact. This contact-commensurability effect could indeed be recognized in classical MD simulations.18 Recent friction force microscopy experiments on free-standing graphene provided further evidence for this scenario.19The increase of friction associated with the puckering effect was originally attributed to the increase in contact area between the AFM tip and the fold that it introduces in the graphene in front of itself.20 These differences have a direct impact on the tribological behavior of graphene on the two different substrates.21,22Recent density functional theory (DFT) calculations revealed that the adhesion of a single layer of graphene to a metallic substrate is highly sensitive to the electronic structure of that substrate. For example, graphene binds more strongly on iron than on copper because the d states of iron, which are partially occupied, rehybridize with π orbitals of graphene, thereby promoting chemisorption instead of physisorption.In this article, we present a combined experimental and theoretical study of the interaction between a graphene monolayer and its support and the influence of this interaction on friction. As supports for the graphene, we used clean copper and oxidized copper, and we find that the friction forces experienced on the graphene on these supports differ by a factor 3 – by far the largest friction contrast found on graphene to date. This difference exceeds the combined effect of the puckering-induced increase in contact area and the deformation-assisted improvement of contact commensurability, mentioned above, and suggests the contribution of an even stronger source of influence on the friction force. We applied density functional theory (DFT) calculations to reveal that the puckering induces a re-hybridization of the carbon bonds within bent graphene regions, which transforms them from inert to ‘reactive’. A tip sliding on such a region therefore experiences higher adhesion and friction.e.g., in MEMS or NEMS devices.23,24Our study may be relevant in the context of potential applications of single-layer graphene in dry sliding contacts, especially in cases where local fine tuning of friction forces may be required, 2.2.1.−1 confirms that this is a single graphene layer, while the absence of a D-peak at 1350 cm−1 demonstrates the absence of crystal defects within the graphene grains and should be taken as the signature of the good quality of the material.25,26 Also the intensity ratio between the 2D–G-peaks, I2D/IG, which is slightly larger than unity, indicates the presence of only monolayer graphene. Note, that this ratio strongly depends on the laser excitation energy and that it is sensitive to the details of the binding between the graphene layer and the substrate.27 It was demonstrated that in the green laser range (excitation energy of 2.2 ÷ 2.4 eV) the intensity of the Raman resonance profile becomes stronger than at higher energies (above 2.8 eV). This effect is related to the photoluminescence band of the Cu substrate. In our experiments, we employed a laser wavelength of 514 nm, which corresponds to an excitation energy of 2.41 eV. The acquired spectrum is in good agreement with the literature.25–27 For completeness, we mention that the background of the luminescence from the copper substrate was subtracted from the spectrum in We start by characterizing the freshly deposited graphene-on-copper system, prepared as described in Section 3. In addition to the Raman spectra, we inspected the graphene-on-copper samples at length with AFM, as illustrated by 28 This gives rise to a slow, diffusion-limited oxidation of the copper substrate, that sets in at the GGB and spreads out sideways to oxidize the copper underneath the graphene.After deposition and initial characterization, the graphene-on-copper samples were stored at room temperature in air at a relative humidity of approximately 50%, for a total duration of two months. We found that not only bare copper surfaces changed under the influence of the atmosphere, but also the surface topography of our graphene-on-copper samples slowly evolved. This can be recognized by comparing the AFM image in 28For completeness, we mention that the optical image also contains an extended defect that is clearly different in shape and contrast from the oxide-decorated GGBs. We associate it with the copper film and interpret it as a copper grain boundary (GB). The orientations of the GGBs seem not to depend on the orientation of the copper steps and the copper GB . The bla−1 are the signature of Cu2O,29 and support our interpretation that the copper substrate is oxidized at the location of the protrusion. These peaks are absent in the lower spectrum, in accordance with the idea that the graphene protected most of the copper substrate from oxidation. We find that on the oxidized copper, the G- and 2D-peaks of the graphene are both present, indicating that the graphene is still there and is not oxidized. The G- and 2D-peaks are both ‘red-shifted’ with respect to the corresponding peaks for ‘regular’ graphene on unmodified copper  in an oxygen atmosphere.28,30 Interestingly, a red-shift of the characteristic peaks is known to originate from tensile strain in the graphene. In our case, there is a natural reason for such strain in view of the increased volume below the graphene due to the oxidation of the copper.30 Using the observations of ref. I2D/IG = 2.3 on the oxidized copper . The AFM measurements were conducted in ambient at a normal force FN in the range from 23.7 to 33.0 nN, using a cantilever with a normal spring coefficient of 0.09 N m−1 and a lateral spring coefficient of 38 N m−1. The graphs in FL in panel (b) indicate that the friction force experienced by the AFM tip on the graphene on oxidized copper is significantly lower than that on the unoxidized copper. This can be recognized from the contrast in the friction force image, the brighter color corresponding to a lower absolute value of the lateral force, and from the changes in both the trace and retrace curves in corresponding lateral force graph, the absolute value for the lateral force for both curves being minimal over the oxidized region. Note, that a local maximum is observed in the lateral force across the GGB  substrate, in particular, the under-coordinated copper atoms tend to form chemical bonds with the carbon atoms above, which introduces minor out-of-plane deformations in the graphene layer and increases its binding energy by more than a factor two with respect to pure copper.The optimized structures of graphene adsorbed on the four considered substrates are shown in The results reported in 15 a reduction of graphene puckering results in a friction decrease. Here, we will inspect the relation between puckering and friction in further detail. As mentioned in the introduction, two main hypotheses were proposed in this context, namely the increase of contact area caused by the formation of a bulge in front of the tip and the increase of commensurability between the tip and graphene, enabled by the flexibility of graphene on weakly attracting substrates. It seems unrealistic that either of these effects or even a combination of the two could lead to the observed reduction in friction by nearly a factor three.The difference in binding energies that our calculations revealed for graphene on elemental and oxidized Cu, can be related to the difference in frictional properties observed in our experiments. In particular, we should expect that the increase in binding energy occurring upon oxidation of the Cu substrate suppresses the puckering effect. Indeed, as was suggested to explain the different frictional behavior of multilayer graphene films on different substrates,2 to sp3-like, with a corresponding increase of reactivity due to the appearance of dangling bonds. This hypothesis is in agreement with the observation that water molecules dissociatively chemisorb on curved graphene regions, while they weakly physisorb on flat graphene, as revealed by QM/MM dynamic simulations performed by our group.40Here we introduce a new explanation, based on the analysis of the chemical reactivity of graphene as a function of its curvature. In particular, we propose that graphene puckering is accompanied by a change in the hybridization of the carbon atoms from sp2O(111) substrate, does not result in a significant increase of surface energy with respect to flat graphene  is quite different from that on curved graphene (−0.36 eV per H2 molecule) and certainly more in favor of the latter. Therefore, our calculations and experimental data suggest a new hypothesis regarding the puckering effect present in nanotribology experiments. Instead of increasing the contact area between the tip and the substrate, we propose that it is the increased charge transfer of curved graphene that leads to an increased reactivity that can explain the larger friction between the tip and the sample observed in the experiments.The higher reactivity of curved graphene is not a new concept in the literature: this system has already been proposed as an efficient configuration for hydrogen storage.3.Based on our combined experimental and theoretical study, we conclude that the local oxidation of the Cu substrate underneath a monolayer of graphene leads to a significant increase in the adhesion of the graphene to the substrate and an according reduction in the puckering effect. The accompanying decrease in measured friction force is so large that it cannot be ascribed solely to changes in contact area or local commensurability. As the dominant contribution to the large difference in friction we identify the change in electronic structure that the curves locally introduce in the puckered graphene and the corresponding changes in the reactivity and, thus, the adhesion and friction between the graphene and the tip of the friction force microscope. Since the geometry in these single-asperity experiments may be similar to the local configurations in extended, dry contacts that are lubricated by a single monolayer of graphene, we should expect a similarly strong dependence of the lubricating qualities of such contacts on the oxidation state of the substrate on which the graphene resides. This should be of relevance in the context of advanced MEMS and NEMS devices where graphene lubrication is applied or considered.4.4.1.43 The graphene was grown by chemical vapor deposition (CVD) on thin polycrystalline copper films. These copper films were formed by sputter deposition of copper on sapphire (111) wafers . The thickness of the copper film was 1 μm. The copper grains in the film showed a strong preference for the (111) surface orientation. The graphene-on-copper samples were inspected at various stages in the experiment with optical microscopy and with Renishaw Raman spectroscopy with an excitation wavelength of 514 nm. These Raman spectroscopy measurements were conducted with a spot size of approximately 1 μm that could be located with respect to the features in the optical microscopy images with a precision of several micrometers. More precise topography measurements and lateral force measurements were done with a Bruker Icon AFM, using V-shaped Si3N4 probes (DNP-10). The normal and lateral force calibration was carried out according to the method discussed in ref. For our experiments, we used high-quality, single-monolayer graphene samples obtained from Applied Nanolayers B.V. (ANL).4.2.46 considering different approximations for the exchange-correlation functional, namely the local density approximation (LDA), the generalized gradient approximation due to Perdew, Burke and Ernzerhof (PBE),47 and the PBE-D scheme. The latter includes van-der-Waals interactions within the semi-empirical approach proposed by Grimme.48We performed density functional theory (DFT) calculations within the plane-waves/pseudopotential scheme,2O we considered three different surface orientations, in order to analyze the effects of different surface stoichiometries. In particular, we considered the Cu2O(100) : Cu surface, terminating with a copper layer; the Cu2O(100) : O surface, terminating with an oxygen layer, and the Cu2O(111) surface, terminating with a layer that contains both copper and oxygen in the stoichiometric ratio 2 : 1. The relative stabilities of these surfaces change as a function of the chemical potential of oxygen.39We studied graphene adsorption both on copper and on copper oxide. In the case of copper, we considered the (111) surface, as the most stable surface of this material and the preferred one, as found in the experiments, while for Cu2O(100), and a (2 × 2) cell for the Cu2O(111) surface. For all considered systems, the residual lattice mismatch between graphene and the substrates was around 2.5%. We calculate the graphene binding energy on a substrate as Eb = (Esubstrate + Egrap − Etot)/A, where A is the in-plane area of the considered supercell, Esubstrate (Egrap) is the total energy of a supercell containing the isolated substrate (graphene) and Etot is the total energy of the same supercell containing the adsorbed graphene on the considered substrate. Further computational details are reported in the ESI,Graphene adsorption on these copper and copper oxide substrates was studied using periodic supercells containing a substrate slab and a vacuum region of 1 nm and 1.5 nm thickness, respectively. The in-plane size of the supercells was chosen according to the lattice mismatch between the graphene layer and the substrate: the (1 × 1) cell was used for Cu(111), a (3 × 3) cell for CuP. V. Antonov: investigation, conducting experimental work, writing-original draft. P. Restuccia: investigation, performing theoretical calculations, writing-review & editing. M. C. Righi: conceptualization of the theoretical work, supervision, writing-review & editing. J. W. M. Frenken: conceptualization of the experimental work, supervision, writing-review & editing.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.NA-004-D2NA00283C-s001


At a low temperature, the carrier mobility of graphene devices can reach 3 × 105 cm2/V·s, and at room temperature, the carrier mobility can still exceed 1 × 105 cm2/V·s, which is very helpful for the development of high-temperature quantum Hall effects under moderate magnetic fields. At a low temperature of 1.6 K, a series of half-integer quantum Hall plateaus are well-observed in graphene with a magnetic field of 1 T. More importantly, the ν = ±2 quantum Hall plateau clearly persists up to 150 K with only a few-tesla magnetic field. These findings show that graphite-gated high-mobility graphene devices hold great potential for high-sensitivity Hall sensors and resistance metrology standards for the new Système International d’unités.Since the discovery of the quantum Hall effect in 1980, it has attracted intense interest in condensed matter physics and has led to a new type of metrological standard by utilizing the resistance quantum. Graphene, a true two-dimensional electron gas material, has demonstrated the half-integer quantum Hall effect and composite-fermion fractional quantum Hall effect due to its unique massless Dirac fermions and ultra-high carrier mobility. Here, we use a monolayer graphene encapsulated with hexagonal boron nitride and few-layer graphite to fabricate micrometer-scale graphene Hall devices. The application of a graphite gate electrode significantly screens the phonon scattering from a conventional SiO At present, the standard is implemented in GaAs/AlGaAs heterostructure devices [The quantum Hall effect (QHE) is a perfect phenomenon that reflects the microscopic quantum physics behavior in the mesoscopic scale. It has attracted great attention since it was discovered in 1980 [ devices , which u ≈ 1.3 K ,5. This  ≈ 1.3 K ,7,8,9, t ≈ 1.3 K ,12, is a ≈ 1.3 K ,14,15. I ≈ 1.3 K . To make4 cm2/V·s can exhibit good QHE at 5 cm2/V·s shows a fractional quantum Hall effect (FQHE) at Recently, a more accurate and practical quantum Hall resistance standard has been rapidly developed by utilizing the unique electronic characteristics of graphene ,19,20,21 devices . Therefog factor . Additiog factor  with a hture QHE  is earlygraphene ,27,28. Tgraphene ,31,32,33Here, we fabricate a monolayer graphene (MLG) Hall device encapsulated with hBN, the fabrication process is shown in −1 and the characteristic G and 2D peak of monolayer graphene is at 1578 cm−1 and 2678 cm−1, respectively. We notice the G peak intensity of graphene is rather low due to the coverage of hBN, but the 2D peak is symmetrical, which accords with the Raman 2D peak of monolayer graphene. The low charged defect density in hBN can improve the carrier mobility [mobility , and themobility , which cmobility ,39. At tmobility . Comparemobility ,41,42,43mobility ,45, and mobility  and makemobility .Rxx) and Hall resistance (Rxy) and applied the graphite-gate voltage Vg to adjust the carrier density, the measurement set-up is shown in Rxx-Vg) under room temperature (290 K) and low temperature (1.6 K). The gate voltage changes the carrier concentration n and thus the resistance of graphene through the capacitance of MLG/hBN/FLG. In consideration of the thickness of the bottom hBN flake, a graphite-gate capacitance per area A of nT can be estimated as: T is temperature, vF is Fermi velocity. Therefore, the resistance at DP increases with decreasing temperature, as shown in We first measured the electronic transport properties of the device at room temperature (290 K) and low temperature (1.6 K) by using standard low-frequency locking technology under small ac bias  carrier density in ‘‘dirty’’ graphene never drops below a value Δn, referred to as inhomogeneity density. Both the thermally activated carriers and phonon scattering process increase with increasing temperature, leading to a broadened Δn and width of the Rxx-Vg peak in The width of the 4He cryostat system to n) to hole side (negative n) by apply a gate voltage (Vg). The carrier fluctuation (referred as Δn) is determined by the fitting in μ(n) may originate from different scattering efficiency for a particular type of defects in graphene. An alternative explanation consistent with the behavior of μ(n) is the renormalization of the Fermi velocity vF, as previously observed in high-mobility suspended graphene devices [The residual charge carrier fluctuation is uctivity . It can  devices ,50,51. IVg = 0.5 V) and low temperature (T = 1.6 K), the overall positive Rxy indicates that the contribution is mainly from electrons. Under a low magnetic field, the QHE shows clear Shubnikov-de Haas (SdH) oscillations. With the increase of the magnetic field, Rxy appears plateaus and Rxx is vanishing, which is the typical QHE [Rxy measurements. For the filling factors ν = 2 and ν = 6, the magnetic fields range from 1.88 T to 2.36 T (ν = 2) and from 0.77 T to 0.82 T (ν = 6), respectively. Therefore, the ν = 2 plateau has the widest range of magnetic field in all the Rxy plateaus, which can be utilized as the quantum resistance standard. In Vg at a fixed magnetic field (B = 1 T) and low temperature (T = 1.6 K) to access the quantum Hall plateaus. The lowest point of longitudinal resistance Rxx corresponds to the plateaus of the Hall resistance, and the quantum Hall plateaus of Rxy) in the hole doping regime, while it is positive for electron doping. In Rxx with the regulation of gate voltage (Vg) and magnetic field (B), in which we can see the typical fan of Landau levels. The black-colored areas indicate the vanish of Rxx, which means the Landau levels at these areas are clearly visible [B > 0.5 T, Rxy(B) exhibits plateaus and Rxx is vanishing, which are the hallmarks of the QHE. At least two well-defined plateaus with values (2e2/h) and (6e2/h), followed by a developing (10e2/h) plateau, are observed before the QHE features transform into SdH oscillations at a lower magnetic field. We observed the equivalent QHE features for holes with negative Rxy values. Alternatively, we can probe the QHE in both electrons and holes by fixing the magnetic field B = 1 T and changing Vg across the Dirac point. In this case, as Vg increases, first holes (Vg < VDP) and later electrons (Vg > VDP) fill successive Landau levels and exhibit the QHE, as shown in Rxx oscillation in carrier concentration is given by:As shown in ical QHE ,53. Mult visible . At magnT = 200 K and B = 12 T, where we can we can barely see the quantum hall plateaus, see 5 cm2/V·s at room temperature, which is proportional to scattering time τ. Additionally, the scattering time of our device can reach τ~10−12 s, so the condition of 2 supported graphene [To further support the high performance of the device, we also measured the transport properties by changing the magnetic field at relatively high temperatures of 100 K and 150 K, and we observe that the device can also show hall plateau values of au level ,23. The ructures , and SiOgraphene , which u5 cm2/V·s and 3 × 105 cm2/V·s at room temperature (290 K) and cryogenic temperature (1.6 K). The residual charge carrier fluctuation is only about 1010 cm−2, indicating the great purity and homogeneity of the graphene devices. The well-defined low carrier concentration in graphene makes it ideal for an ultra-sensitive Hall sensor with a current-related sensitivity of SI~ 500 V/(AT) at room temperature, which surpasses both silicon-based Hall sensors and the best Hall sensors based on III/V semiconductors. Further, benefiting from the high carrier mobility, the integer quantum Hall effect is finely developed in graphene under a perpendicular magnetic field below 1 T at low temperature. More surprisingly, clear quantum plateaus at ν = 2 in graphene can even persist at an elevated temperature of 150 K, far above the liquid nitrogen temperature. At the same time, the required magnetic field for the high-temperature quantum Hall effect is only about 5 T, which can be realized by commercial superconducting solenoids. The relaxed conditions of quantum Hall effect in graphene, including high temperatures and small magnetic fields, will open new routes for the development of graphene-based resistance standards for the new Système International d’unités.In summary, we report high-quality graphene field-effect devices by hBN encapsulation and local graphite bottom gate electrodes. Electrical transport experiments show that the graphene devices exhibit very high carrier mobility in both electron and hole sides. For the electron doping regime, the mobility exceeds 10


We study the crossed Andreev reflection in zigzag phosphorene nanoribbon based ferromagnet/superconductor/ferromagnet junction. Only edge states, which are entirely detached from the bulk gap, involved in the transport processes. The perfect crossed Andreev reflection, with the maximal nonlocal conductance  It is desirable to find large probability and convenient manipulation of CAR in electronic devices, as the reversed process of CAR is practical in generating nonlocal entangled-electrons4. Nowadays, the normal/superconductor/normal (N/S/N) junction had been used to detect CAR6, where the superconductivity of the junction can be induced by the proximity effect. At the same time, the discovery of graphene10 become a breakthrough in the field of two-dimensional material. Studies of CAR in graphene based superconductor junctions were reported since then. Cayssol11 proposed a graphene based N/S/N junction and point out that the nonlocal conductance acts as a function of the length of the narrow superconductor region. Wu et al.12 found that a cooper pair split into two nonlocal electrons perfectly in a superconductor junction based on bilayer graphene. Besides spin, they stressed that such two electrons are also involved in valley entanglement. Beiranvand et al.13 provided a new way to create entangled-electrons with same spin by studying the anomalous CAR in a graphene based junction, where the device is consist of a triplet pairing superconductor. In Ref.14, researchers examine the CAR in EuO-graphene/superconductor/EuO-graphene junction, where the lead and device region would be induced to a ferromagnet and superconductor by the proximity effect. They found that the pure CAR oscillates rapidly and periodically with the length of the device. Meanwhile, study of CAR in graphene nanoribbon junctions was reported. Zhang et al.15 proposed a ferromagnetic/superconductor junction based on zigzag graphene nanoribbon, and pointed out that the CAR in such junction is controllable by changing the direction of the ensemble magnet.In a superconductor junction, the nonlocal coupling between electrons from different leads produces the crossed Andreev reflection (CAR)21, a graphene-like two-dimensional material, had been successfully isolated. Like graphene, the honeycomb structure of phosphorene nanoribbon provides two distinct nanoribbons: armchair and zigzag nanoribbon. The armchair nanoribbon is a gapped semiconductor, while the zigzag one contains four degenerate quasi-flat edge bands detached from the bulk bands, considering spin degeneracy21. Generally, the electronic properties of zigzag phosphorene nanoribbon (ZPR) are dominated by the edge bands. Because the electrons in the edge bands are approached to the edges, it can be easily controlled by an external field, especially by the electric field19. Furthermore, the edge bands are entirely detached from the bulk bands, which helps them to eliminate the trouble caused by the bulk bands. Such magnificent properties of edge bands make it good for building the electronic device. Note that the Josephson effect in phosphorene based superconductor junctions were investigated20. Researcher found the supercurrent is highly anisotropic, due to the anisotropic band structure of the system. Furthermore, the supercurrent is also affected by the electric potential and the length of the superconductor region.Recently, phosphorene22 and s-wave superconductor respectively. According to the first principle calculation, the exchange energy can reach 184 meV. Normally, the electron tunneling (ET) and local Andreev reflection (LAR) will compete with the CAR in the scattering processes inevitably23, making the nonlocal conductance weaken. In our study, the scattering processes in the junction can be consisted of CAR and electron reflection (ER), where the ET and LAR are completely blocked by setting the electric potentials of both leads and device properly. The maximal nonlocal conductance of CAR can reach However, compared to graphene, according to our investigation, the study of CAR in phosphorene based junction is still a few.In this Letter, we study the crossed Andreev reflection in zigzag phosphorene nanoribbon based ferromagnet/superconductor/-ferromagnet (F/S/F) junction. The ferromagnetism and the superconductivity of phosphorene nanoribbon can be achieved in proximity to EuOThis work is organized as follows. The structure and model of the junction are presented in “X = 3.27 Å and Y = 4.43 Å18 are the length of the cell in x and y direction respectively. The size of the device region is 24. Actually, compare with the model in Ref.24, j in one layer to atom i in another layer,shown in Fig. Figure mentclass2pt{minimmentclass2pt{minimBogliubov-de Gennas (BdG) equation is used to study the CAR in ZPR superconductor junction. The tight-binding Hamiltonian of the junction can be written as25, a software for quantum transport calculation. The tight-binding model of the system can be built by Kwant’s package, and the structure can be visualized by setting system parameters. In detail, the lattice built by Kwant is the phosphorene lattice shown in Fig. S] can be obtained. In our study, the expression of the wave function in the right lead is 27. According to the Ref.26, the nonlocal conductance can be given as25.The calculation in this study is done by KwantThe band structures of the ferromagnetic lead had been shown in Fig. entclass1pt{minimaentclass1pt{minimaThe calculation results are shown in Fig. E are fixed at Figure k and L are the wave vector and the length of the device region respectively. The mechanism of such periodic osocillation had reported in Ref.28. These properties make the CAR in the junction controllable.Then, when we change The nonlocal conductance are presented at Fig. In this work, we studied the crossed Andreev reflection in the ferromagnet/superconductor/ferromagnet junction based on zigzag phosphorene nanoribbon. By setting the electric potential of both leads differently, the scattering processes consists of electron reflection and crossed Andreev reflection, where the local Andreev reflection and electron tunneling are suppressed. The maximal nonlocal conductance can reach


Hydrogels show great potential as soft materials for biomedical applications and flexible devices. However, conventional hydrogels exhibit poor mechanical strengths owing to the presence of water in their polymer networks. Therefore, improving the mechanical properties of hydrogels by controlling the chemical and physical structures that affect their macroscopic behaviors is a challenging issue. In this study, we developed a nanocomposite (NC) hydrogel that harbors exfoliated few-layer graphene sheets through noncovalent interactions. The bifunctional polymer PImQ, which contains both aromatic and cationic groups, was found to enable the direct exfoliation of graphite to few-layer graphene through π–π interactions in 2.7% yield. The poly(acrylamide)-based NC hydrogel containing the PImQ/graphene composite as a nanofiller shows a 3.4-fold increase in tensile stress compared with the hydrogel without the nanofiller. The introduction of the PImQ/graphene nanocomposite also increases the fracture stress of the NC hydrogel through cation–π and π–π interactions. The improved mechanical properties of the NC hydrogel result from the synergistic effects of the chemical crosslinking of the polymer network and the physical crosslinking of the polymer/graphene nanofiller. Hydrogels are typical soft materials consisting of three-dimensional polymer networks that exhibit excellent biocompatibility, permeability, and tissue-like properties ,2,3. HydRecently, nanocomposite (NC) hydrogels, which are reinforced with nano-sized fillers to improve their mechanical properties, have attracted extensive research attention ,8. Two-d2 carbon atoms in the 2D lattice become sp3 hybridized. Such disturbances in the sp2-hybridized orbitals result in decreased electrical conductivity owing to disruption of the π-conjugated carbon structure.The chemical exfoliation of graphite to obtain GO by oxidation is known as the Hummers’ method . HoweverIn contrast, physical exfoliation of graphite through noncovalent interactions is a “soft” fabrication process . DispersIn this study, we synthesized a polymer dispersant, PImQ, that contains imidazole moieties as aromatic groups and imidazolium moieties as cationic groups a. This bHere, PImQ/graphene is used as a nanofiller in NC hydrogels. The tensile stress of the NC hydrogel containing 3 wt% PImQ/graphene is 3.4 times that of the hydrogel without the PImQ/graphene nanofiller. Furthermore, the fracture stress of the hydrogel increases from 250 to 440 kPa upon the introduction of PImQ/graphene. This increase in mechanical strength is attributed to the physical crosslinking associated with the cation–π and π–π interactions of PImQ/graphene in the NC hydrogel c.An important aspect of this study is that the exfoliated graphene sheets provided by PImQ change the macroscopic properties of the hydrogel depending on the assembled structure. The positively charged PImQ/graphene can be suspended in water owing to its electrostatic repulsion. In contrast, the cation–π and π–π interactions between the graphene sheets improve the mechanical properties of the NC hydrogel.N,N′-methylene bis(acrylamide) (BIS), acrylamide (AAm), and anhydrous ethanol were purchased from FUJIFILM Wako Pure Chemical Co., Ltd. . Acetic acid and diethyl ether were purchased from Junsei Chemical Co., Ltd. . 4-Cyano-4-[(dodecylsulfanylthiocarbonyl)sulfanyl]pentanol and 2-hydroxy-4′-(2-hydroxyethoxy)-2-methylpropiophenone were purchased from Sigma-Aldrich Co., Ltd. . Deuterated water was purchased from Cambridge Isotope Laboratories Inc. . Graphite (≥99% carbon basis) was gifted from Nippon Graphite Industries Co., Ltd. . Dialysis membranes were purchased from Repligen Co., Ltd. . An elastic carbon-coated copper transmission electron microscopy (TEM) grid was purchased from Okenshoji Co., Ltd. .Unless otherwise noted, compounds and solvents were purchased from commercial suppliers and used without further purification. 1-Vinylimidazole and 2-(2-chloroethoxy)ethanol were purchased from Tokyo Chemical Industry Co., Ltd. . 2,2′-Azobis(2-methylpropionitrile) (AIBN), −1, 10 mL). The mixture was sonicated in an ice bath for 60 min using a tip-type ultrasonic homogenizer . The colloidal solution was centrifuged at 4000 rpm for 1 h using a microcentrifuge . After centrifugation, homogeneous PImQ/graphene ) was obtained from the supernatant. PImQ/graphene suspensions with different component ratios up to PImQ/graphene = 1:1 (w/w) were also prepared.Graphite (5 mg) was added to deionized water containing PImQ  and y represents the percentage molar ratio of the BIS crosslinker to AAm in NC hydrogels (%). For example, G0.2H0.05 means that the NC hydrogel is prepared with the weight percentage of PImQ/graphene of 0.2 wt.% and the molar ratio of BIS to AAm of 0.05%.Poly(acrylamide) (PAAm)-based NC hydrogels containing PImQ/graphene are denoted as Gv/v, 2 mL). We fabricated a cell from two glass substrates . The cell was filled with the mixed solution by capillary action. Photopolymerization was performed by photoirradiation at 365 nm (light intensity: 4 mW cm–2) for 30 min. The resulting solid was removed from the glass cell and then washed with deionized water, yielding the graphene-doped hydrogel G0.2H0.05. NC hydrogels with different graphene ratios up to G3H0.05 were prepared using PImQ/graphene suspension = 5:1 (v/v).AAm , BIS , and 2-hydroxy-4′-(2-hydroxyethoxy)-2-methylpropiophenone  were added to a PImQ/graphene suspension . Mw and Mn were measured by gel permeation chromatography  using 0.3 M NaClaq as the eluent, following calibration with pullulan standards. Absorption spectra of the exfoliated graphene were obtained using a UV–Vis spectrophotometer . The exfoliation yields for PImQ/graphene were estimated using thermogravimetric analysis . The exfoliated graphene was investigated using X-ray diffractometry , TEM , and Raman spectrometry . Zeta potentials were determined using a zeta potential analyzer . Photopolymerization for the preparation of hydrogels was carried out using a high-pressure mercury lamp  equipped with a glass filter . The surface morphologies of the hydrogels were investigated using scanning electron microscopy . Rheological properties were determined using a rheometer  with a 25 mm diameter parallel plate attached to a transducer. Mechanical tensile-stress analyses were performed using a testing machine . Samples were evaluated for each graphene loading amount. Tensile analyses were conducted at 25 °C with different strain rates to evaluate stretchability according to the ASTM 882-09 test method using dumbbell configured specimens according to JIS K-6251-7 . Experimental values for strain-at-break and stress-at-break were obtained by uniaxial tensile testing at a strain rate of 100 mm min−1.The synthesized compounds were structurally characterized by means of 3COOH produced poly(1-vinylimidazole) (PIm) (1H NMR methylene peaks at 3.48–3.78 ppm in D2O confirmed that quaternization had occurred (PImQ was synthesized using reversible addition−fragmentation chain-transfer (RAFT) polymerization see . The RAFe) (PIm) . After poccurred .w/w); however, absorbance decreases at higher concentrations of PImQ ( of PImQ b. These αG) of exfoliated graphene was measured at 660 nm, which is commonly used for graphene analysis [αG. A660) for PImQ/graphene composites with different component ratios. According to the Beer–Lambert law, A660 = αGCGl, the concentration of exfoliated graphene (CG) is proportional to the absorbance per pass length l (A660l−1). The αG value was calculated to be 1216 mL mg−1 m−1 using the following parameters: CG = 0.0162 mg mL−1 and A660l−1 = 19.7 m−1. This value is similar to the αG value of 1390 mL mg−1 m−1 reported in a previous study [A660 for the PImQ/graphene (0.5:1 (w/w)) suspension remains unchanged during one month of storage at 25 °C, indicating that this sample exhibits stable suspension ability ) . The exfoliation yield is 2.7% for PImQ/graphene (0.5:1 (w/w)) . This vaζ) analysis was used to investigate the dispersion behavior of the PImQ/graphene suspension. We found that the suspension exhibits dispersion stability in water. The ζ value of the PImQ solution is +38 mV owing to the cationic groups of the imidazolium units in the polymer side chains  reflection owing to the 3.34 Å separation between graphene layers ; the (G) band (1567 cm−1), which corresponds to the vibration of sp2 carbons in the graphitic layers; and the 2D band, which is derived from two-photon modes of graphene sheets (2652 cm−1) (ID/IG) is an important parameter that indicates the degree of structural defect in graphene. ID/IG for the exfoliated graphene is 0.14, which is lower than those for graphene chemically exfoliated using sodium hydride (ID/IG: 1.08) [ID/IG: 1.44) [Raman spectroscopy was used to assess the quality of the graphene sheets provided by noncovalent exfoliation b. The ch−1 . The C–HG: 1.08)  and hydrG: 1.44)   index through (1–210)–(0–110)–(–1010)–(–2110) diffractions [αG value derived from the Beer–Lambert law was obtained from few-layer graphene sheets.TEM was used to observe an exfoliated graphene sheet directly. It is important to understand the exfoliated graphene that remains dispersed after centrifugation. First, the initial state of the pristine graphite was examined a. TEM obractions . We alsoractions . These rFirst, we investigated the rheological properties of PAAm gels without PImQ/graphene. The storage modulus (G′) increases with increasing concentration of the crosslinker in the frequency dispersion curve a. This r−1  was clamped in a tensile testing machine and stressed at a rate of 100 mm min−1 c. The H0−1 d. The elThe SEM images show that the microstructures of freeze-dried NC hydrogels are highly dependent on the concentration of PImQ/graphene . To confWe expected that the physical crosslinking of the PImQ/graphene would alter the viscoelasticity of the NC hydrogel. Rheological tests showed that the NC hydrogel with PImQ/graphene exhibits enhanced viscoelasticity, confirming the crosslinked structure of the network. The G′ value for G3H0.05 is ~3.6 times higher than that for H0.05, indicating that the cation–π and π–π interactions in PImQ/graphene promote physical interactions within the polymer network a. The chxH0.05 NC hydrogels are higher than that for the H0.05 hydrogel owing to the physical interactions formed between the crosslinked polymer chains (xH0.05 hydrogels with different concentrations of PImQ/graphene, the Young’s modulus increases with increasing graphene content at 300% tensile strain (xH0.05 NC hydrogels. We also found that the mechanical strength of NC hydrogels with PImQ/graphene is better than that of NC hydrogels with GO. The stress of the G3H0.05 NC hydrogel is 440 kPa at 610% strain, while that of a previously reported NC hydrogel containing GO was approximately 180 kPa at 60% strain [The tensile stresses for the Gr chains c. For Gxe strain . This su% strain . This inWe prepared NC hydrogels containing few-layer graphene sheets exfoliated from graphite by noncovalent interactions. PImQ/graphene acts as a physical crosslinker in the hydrogel based on cation–π and π–π interactions. These physical interactions are affected by the concentration of PImQ/graphene and significantly change the tensile strength of the NC hydrogels. We also found that the mechanical strength of the NC hydrogels can be controlled by altering the concentration of PImQ/graphene. This mechanical behavior results from the synergistic effects of the chemical and physical crosslinking in the hydrogel.


The spacing of layer, surface chemical composition, structural defects, thermal conductivity and electrical property of the GOP before and after irradiation was measured. The results indicated that the spacing of layer decreased after irradiation. The ratio of total carbon atom and total oxygen atom increased from 2.40 to 4.31 as well as the sp2 hybridized carbons obviously increased after 2 × 1015 cm−2 irradiation. The XPS analysis suggested the occurrence of reduction, and the Raman spectra indicated that the defects were produced after proton irradiation. Furthermore, the thermal conductivity of GOP decreased, and then increased smoothly as the irradiation fluences were increased, and the electrical property showed the similar trend. The change in the thermal and electrical properties for GOP could be attributed to the defects and the removal of oxygen-containing functional groups, which lead to the phonon conduct path and scattering centers changed under proton irradiation. This study could promote the application of GOP in future space expeditions.A graphene oxide paper (GOP) was irradiated with 500 keV proton for total fluence of 2 × 10 The 500 keV proton irradiaiton effect on a graphene oxide paper was studied in a ground-based irradiation simulator. NASA proposed that graphene can be used as nanocomposites, charge dissipation layers and multifunctional electronic components in spacecraft.3 Zhang et al. has demonstrated that graphene can be used as protection coatings under atomic oxygen erosion.4,5 The study of ultraviolet exposure effects on graphene and its field effect transistors are important keys on the design of electronic and optoelectronic devices for space missions. Emilie J. Siochi pointed out that the graphene used in aerospace mainly depends on reliable functionality like any other new material,3 so it is necessary to study the irradiation effect, such as gamma ray, energetic particles from space environment before they will be used in aeronautics and space applications.The extraordinary physical and mechanical properties have raised concern about two dimensional graphene materials, which exhibit outstanding characteristics in the aspect of thermal conductivity, high carrier mobility and ballistic electron transport, good fracture and mechanical strength, chemical inertness and large specific surface area.6 The majority of research for energetic particles, including ion and electron effects on graphene, includes modification in property and structure.7–12 Chen et al. found that graphene oxide (GO) can be partially reduced by electron beam irradiation,12 and the adsorptive performance of the reduced graphene oxide (rGO) also can be improved.13 The research indicates that ion irradiation is a useful way for the modification of GO and also for tailoring its structure,14–16 and the electron emission properties can be enhanced by irradiation.14 Tyagi et al. has exhibited in their published study that the maximum lattice temperature of a GO film (GOF) can be changed by C ion irradiation.17 The changes brought in structure and property was defects, orders of magnitude and amorphization induced by irradiation. These researches have an undeniable role in promoting the space application of graphene, though the initial intention was not based on this aim. There are also some studies based on the space radiation effect on graphene materials, in which the materials in the experiments are usually suspended graphene, with size as the limitation.11,18The existing irradiation effect research on graphene is mainly focused on materials preparation and property modification, and the experiment materials used are small-sized or thin films. For instance, some studies show that the gamma ray irradiation is an effective and green approach for the reduction, functionalization, the synthesis of nanocomposites and the self-assembly of oxide graphene sheets.18 The research by Chen et al. shows that the structural defects and electrical characteristics of graphene transistors change after different energy proton irradiations.19 Mathew et al. have also found that the stability and the layer number of graphene on a SiO2/Si substrate fabricated by micro-mechanical exfoliation are influenced by focused proton irradiation.20 Therefore, there is lack of irradiation effect on the bulk graphene materials synthesized by a chemical method, which is considered to be a promising method for mass production. For this, the proton effect on bulk GO materials has been studied in this paper.The charged particles from near-Earth space and solar cosmic rays, including high-speed protons, are a threat to astronauts and satellites. The most damaging effects are known to be the charging of the surfaces and electrostatic discharge in the spacecraft. The study of proton irradiation damage in a mono-layer graphene suggests that the energy/dose has a direct relationship on defect formation.9,10,15,21 In the AP-8max trapped proton environment of the Earth's radiation belt, the proton irradiation energy is in the range of 0.1–400 MeV.22 The current research focussed on the 5–20 MeV proton irradiation on a monolayer or a thin film graphene,18,23,24 and Korkut et al. have studied the 500 keV proton irradiation on supported bilayer graphene by a simulation method.25 In this study, we have adopted the AP-8max mode to discuss the 500 keV proton effect irradiation on bulk GOP. The total purpose is to promote the bulk graphene in harsh space applications.A free standing GO paper (GOP) is a fascinating material possessing excellent mechanical properties and chemical stability, and its potential applications cover isotropic ionic conductors, thermal conductive films, mechanically reinforced composites or transparent materials.2.2.1−3, respectively, which was obtained by the Hummers' method. The materials were vacuumed for 24 h, which were maintained at 0.1 Pa to prevent contamination and ensure that the desorption processes finished.GOP was purchased from JCNANO company in China with the thickness of about 50 ± 10 μm, size 6 cm × 6 cm and the density of 1.30 g cm2.210 cm−2 s−1 for ion fluences 2 × 1013, 2 × 1014 and 2 × 1015 cm−2. The pressure was kept in vacuum of 10−5 Pa at room temperature in an irradiation chamber.The proton irradiation experiment was performed using a ground-based irradiation simulator at Lanzhou Institute of Physics (LIP), China Academy of Space Technology (CAST) 730000. . The irradiation of the samples was conducted by 500 keV proton with the flux of 1 × 102.3λ = 0.154 nm) with a rate of 5° min−1 at room temperature. The X-ray photoelectron spectroscopy  was performed to determine the chemical state of the GOP. The laser Raman spectroscopy was performed using an HR Raman spectrometer 532 nm laser with Nd filter 0.1% to get the information about the structure composition. The thermal diffusion coefficient of GOP was measured using a thermal conductivity meter  in plane. The fracture surface of a pristine GOP was observed using a field emission scanning electron microscope . The electrical property of GOP was investigated by square resistance test with a four-point probe (RTS-9 KEITHLEY 2400). The XRD and XPS experiments were performed at Lanzhou university, and the Raman spectroscopy and SEM tests were performed at Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, and the thermal conductivity test were performed at NETZSCH Scientific Instruments Trading , respectively. The test results of GOP before and after proton irradiation were evaluated and compared.To study the proton irradiation on GOP, the X-ray diffraction (XRD) were conducted using Cu Kα radiation (3.3.1d-spacing (the distance of layer-to-layer) is 0.48 nm according to the Scherrer's equation as show in expression (1). The diffraction peak gradually shifts to 10.14° with the d-spacing 0.44 nm after proton irradiation with the fluences 2.0 × 1013 and 2.0 × 1015 cm−2. The intensity of the domain peak decreased dramatically, which may be due to the removal of oxygen-containing functional groups accompanied with the edges and basal plane of materials, and the increase in broadness of diffraction peaks with irradiation fluences indicates a disordered structure.7,12,26,27d is the layer-to-layer distance, n is the order, λ is the wavelength of X-ray and θ is the corresponding angle of XRD peak.The XRD patterns of GOP before and after irradiation are shown in the inset of 3.22 hybridized carbon  scale" fill="currentColor" stroke="none"><path d="M0 440 l0 -40 320 0 320 0 0 40 0 40 -320 0 -320 0 0 -40z M0 280 l0 -40 320 0 320 0 0 40 0 40 -320 0 -320 0 0 -40z"/></g></svg>C bond) as well as sp3 hybridized carbons , and other peaks, including C–O bond (hydroxyl and epoxy), CO bond (carbonyl) and C–OOH bond (carboxyl) appears around 286.7 eV, 287.7 eV and 288.7 eV, respectively. A similar result has been reported for others.10 It can be seen from 14 cm−2 irradiated GOP show a smooth change in functional compositions, while the total carbon to total oxygen (C/O) atomic ratio increased to 2.77. When the irradiation fluences increase to 2 × 1015 cm−2, the content of oxygen-containing functional groups decreased from 39.58% for the pristine GOP to 23.06% for the irradiated GOP, while value of C/O atomic ratio was increased from 2.30 to 4.31. Moreover, the ratio of the sp3 hybridized atoms and the sp2 hybridized atoms was decreased by 34.35%. The C–O bond and the CO content was decreased appreciably, whereas the COOH bond content gradually increased. The decrease in hydroxyl, epoxide and carbonyl indicates the reduction action that happened in GO,6,28–31 whereas we speculated that the GOP was highly reduced under higher irradiation fluences. Meanwhile, it is suggested that the C–O components was converted to sp2 hybridized carbon under energetic proton knocking on carbon atoms. Moreover, the sp2 hybridized carbon was easily formed than sp3 hybridized carbon under proton irradiation.To further study the proton effect on GOP, the changes in the functional compositions are analyzed by XPS, and the C 1s peaks are divided into different curves according to symmetric Gaussian. 3.310,29,31–36 It can be used to confirm the vibrational modes of surface functional groups and structural damage. −1 and a G peak positioned at 1590 cm−1. The G band indicates the origin of the stretching vibration of the sp2 carbon atoms at the Brillouin zone center and its existence is considered neglecting the order or disorder of sp2 carbons in the material.37–40 In a perfect graphite structure, the vibrational mode associated with D peak is not permitted and only appears for six-fold carbon rings located near structural defects. Therefore, the observation of D peak is a common method for the detection of defect formation in carbon materials, and the D band is representative of defects, such as bond-angle disorder, bond-length disorder, vacancies, edge defects, amorphous, involving the transverse optical phonons near the κ-point phonons. Therefore, the ratio of the peak intensities ID/IG (D peaks and G peaks) is a typical analysis method to evaluate defect formation and the structure order in a qualitative way. The strong D peak refers to the defects produced during fabrication and the influence of structure edges in the pristine GOP.31 For pristine GOP irradiated with different fluences, the value ID/IG was first increased from 1.22 to 1.44 and then decreased to 1.32. Furthermore, the FWHM of the D and G peaks for pristine is 95 cm−1 and 66 cm−1 but then increases to 106 cm−1 and 70 cm−1 after the irradiation of 2 × 1015 cm−2 fluences, respectively. The results indicate that the large numbers of defects are created in the lower 2 × 1013 cm−2 and 2 × 1014 cm−2 fluences irradiated GOP. The D and G peaks are wide in higher 2 × 1015 cm−2 fluence irradiated GOP, and it can be concluded that the disorder structure is formed, and the higher value ID/IG suggest the CC bond (sp2 carbon) distortion, and the non-six ring carbon rings production.18,29,31,35,38Raman spectroscopy is used as a common tool for studying the structural composition of GOP, which is an invaluable method in graphene studies.ID/IG value is very sensitive to defects and six-ring structure,26 while the broadening of the FWHM of D and G peaks always represent a structure disorder.41,42 Many previous studies suggest that there is a positive correlation relation between the number of defects and ID/IG ratio in graphene.18,43 In this study, the ID/IG value increases obviously after 2 × 1013 cm−2 irradiation fluences in GOP, which indicates that the proton irradiation lead to a high defect density. In the 2 × 1015 cm−2 fluence irradiated GOP, the decline in the ID/IG value suggests the lower defect density compared to lower fluence irradiated materials, which is attributed to the complete removal of oxygen-containing functional groups. Combined with the above XRD and XPS analysis, the findings indicate that although the amount of oxygen-containing functional groups is reduced compared to that in pristine GOP, many defects are retained after the high fluence irradiation process.10,18,23,29,32–36,41,43In general, the 3.4k W (m K)−1 is the thermal conductivity that needs to be calculated, α is the thermal diffusivity, Cp J (g K)−1 is the specific heat capacity assumed as value 0.708,42 and ρ (g cm−3) denotes the volume density of the samples. In order to get the ρ, weight of the GOP is measured in a microbalance, and the thickness is measured by SEM. The average value of the thickness of GOP is obtained from the cross-section structure that can be observed in Recently the thermal properties of lower dimensional graphene have attracted considerable interest among scientific and engineering communities. The heat transport is a key factor for the graphene in the electronic industry. Therefore, the thermal performance of GOP has been studied in this study. The thermal diffusivity in plane of the sample can be directly obtained by LFT transient method. The circle shaped sample with the diameter of 2.5 mm is heated from one side, and then the temperature change is obtained on the opposite side. The thermal conductivity is obtained according to the −1 , which may be caused by the lower quality of a layer structure and due to the influence of the defects. The thermal property of irradiated GOP shows that the thermal conductivity is decreased mildly and then increased to 2.982 W (m K)−1with the irradiation fluences of up to 2 × 1015 cm−2. The thermal transport in carbon materials is mainly determined by acoustic phonons, and the phonon can be further influenced by the defects and structure disorder.44–49 The defects, disorder structure and the oxygen functional group are influenced by proton irradiation, leading to the change in the thermal properties of GOP.3.513, 2 × 1014 and 2 × 1015 cm−2, respectively. The resistance of GOP materials show remarkable increase and then decrease, while the value also exceeds that of pristine 53.9% after irradiation fluences are made to the highest, similar to thermal conductivity. It is demonstrated that the proton irradiation effect caused the changes in electrical conductivity. The increment in the square resistance after irradiation can be explained by the number of defects. A point defect is the primary effect on electrical conduction degradation in graphene, and the defects increase as well as cause scattering of the charge carriers.7,10,19,49–52 The resistance increase in the initial stage may be due to the high defect density, and the decrease may be related to the removal of oxygen-containing functional groups.Now the study of electrical properties for graphene is emphasized on reinforcing the conductivity of polymers, and there are limited studies on pure bulk materials, particularly GO. Here, we have tested the electrical resistance of GOP before and after proton irradiation, as show in 3.6d-spacing for irradiated GOP with highest fluences 2 × 1015 cm−2, which is possibly put down to the removal of oxygen-containing functional groups in a significant reduction eventually.11,53–55 The presence of a more disordered structure imputes the damage after high fluence proton irradiation. The surface compositions of the GOP show the reduction action event under proton irradiation. It can be explained by the oxygen-containing functional groups decrease for the highest fluence irradiated samples, while the sp2 hybridized carbon increase remarkably. The hydroxyl, epoxy and carbonyl groups may be more easily turned to sp2 carbon because of better electronic transfers under reduction process induced by proton irradiation. The peak area ratio of ID/IG for 2 × 1013 cm−2 fluence irradiated GOP increases significantly compared to that of pristine, which indicates the defects introduced under energetic proton irradiation. The ion irradiation can induce polymorphic atomic defects in graphene,56,57 which generally contain vacancies, adatom–vacancy, and reconstructed.18,23,24 The possible models of vacancies involve the formation of single vacancies (SVs) and double vacancies (DVs), and the adatom–vacancy pair models are constituted by vacancy and adatom, and reconstructions are by Stone–Wales (SW) that take place due to rotating or breaking and the recombination of the CC bonds.18 The energetic ion irradiation damage effect caused on graphene is due to the displacement and excitation of atoms.29,31,33,34,56,57 In this experiment, when the 500 keV proton hit against the surface of GOP, the energy of the colliding particles could transform to the target lattice carbon atoms, which caused the carbon atoms to deviate from the initial position, and the defects are formed when these atoms are knocked away. Under the lower 2 × 1013 cm−2 irradiation fluences, the defects may originate from the vacancy modal in GOP. In the higher fluence irradiated samples, the XPS results indicate the creation of a sp2 carbon, which originate from the strong removal of the oxygen-containing functional group through the bond rupture. It can be conjectural that the defects came from the non-six carbon ring. Therefore, it can be concluded that the proton irradiation effect on GOP are defects formation, reduction and structure modification.For the above analysis, the proton irradiation effect on GOP shows the change of structure and property. The XRD results represent a small decrease of 2 carbon and lots of defects. The heat transport in all bulk carbon allotropes is mostly carried by acoustic phonons, which are attributed to the strong covalent sp2 bonding, leading to efficient heat transfer with lattice vibrations.44–48 Moreover, the impurities and defects are the powerful primary factors that affect the phonon scatterings in graphene.49 We consider that the small thermal conductivity in GOP before and after lower fluence irradiation is attributed to defects and lesser amounts of sp2 carbons, respectively. When the irradiation fluences up to 2 × 1015 cm−2 are used, the slight increase in the thermal conductivity may be induced by two factors. First, the irradiation introduces a higher sp2 carbon content, which can improve the efficiency of scattering for phonon transport. Second, the thermal conductivity slightly increases may be due to the removal of large number of oxygen-containing functional groups. The study suggests that a well interface can reduce the interfacial thermal, and good orientation can provide energetic pathways for phonon conduction.42 From this point, the removal of oxygen-containing functional groups may be the key factor to improve the quality of interface. The ion irradiation have limited the depth of penetration in graphene,25,29,31 and we have calculated the stopped range of 500 keV protons in GOP to be about 8.29 μm, so the change in defects and chemical composition may not obviously be attributed to the increase in depth, which can give a reasonable explanation for thermal properties before and after irradiation GOP. Furthermore, the GOP irradiated with 2 × 1013 cm−2 exhibits the highest electrical resistance, which is correlated with plenty of oxygen functional groups remaining in the material and the increase in defects. The oxygen-containing functional groups are strong scattering centers, which can affect the electrical transport in graphene, and the defect density also can impact the scattering of charge carriers.10,50–52 When the irradiation fluences are increased to the highest 2 × 1015 cm−2, its more complete reduction action led to the increase in content of sp2 and sp3 carbon, still retaining many defects, so the resistance is higher than that of pristine GOP but lower than other irradiated ones.In this study, the thermal property of the pristine GOP shows a lower thermal conductivity, which may be caused by the low content of sp4.d-spacing caused by relatively complete reduction in GOP after the highest fluences of 2 × 1015 cm−2. The chemical compositions on the surface of the materials show a decrease in the oxygen-containing functional group and remarkable increase in the sp2 carbon, which further confirms the induced reduction by proton irradiation under higher fluence irradiation. The increase in the ID/IG value indicates that a large number of defects are formed under lower 2 × 1013 cm−2 irradiation fluences, which may be caused by the carbon atoms displacement under collisions of proton. In summary, the defect formation and the reduction action are the main effects in proton effect process. The irradiation effect on thermal conductivity and electrical properties is related to the number of defects, the sp2 carbon and oxygen-containing functional group content. The study illustrate that these bulk GO materials can be used as thermal or electrical conductivity coating, or electronic devices in future space missions.We have demonstrated the 500 keV proton irradiation effect on GOP produced by Hummers' method. The irradiation brought about the reduction process and the defects. The analysis of XRD indicates the small There are no conflicts to declare.RA-009-C9RA05389A-s001


Thermal management is a critical task for highly integrated or high-power semiconductor devices. Low dimensional materials including graphene and single-layer hexagonal boron nitride (BN) are attractive candidates for this task because of their high thermal conductivity, semi-conductivity and other excellent physical properties. The similarities in crystal structure and chemistry between graphene and boron nitride provide the possibility of constructing graphene/BN heterostructures bearing unique functions. In this paper, we investigated the interfacial thermal transport properties of graphene/BN nanosheets via non-equilibrium molecular dynamics (NEMD) simulations. We observed a significant thermal rectification behavior of these graphene/BN nanosheets, and the rectification ratio increased with the system length increases up to 117%. This phenomenon is attributed to the mismatch of out-of-plane phonon vibration modes in two directions at the interface. In addition, we explored the underlying mechanism of the length dependence of the thermal transport properties. The results show promise for the thermal management of this two-dimensional heterostructure in an actively tunable manner. In the past decade, thermal diodes have been widely studied in many fields, including new energy , sensors2N/graphene were 21% and 13%, respectively. Another piece of work on the thermal energy transfer at the graphene/C3N interface using reverse nonequilibrium molecular dynamics (RNEMD) was carried out by Song et al. [Considerable works have been carried out around one-dimensional, two-dimensional and hybrid one- and two-dimensional nanomaterials ,15,16,17g et al. . They inBoron nitride crystals belong to the hexagonal crystal system and are very similar to graphene in structure. At this point, graphene/BN heterostructures are considered to be a potential candidate for the thermal diode . Zhuang In this work, we proposed to design an efficient thermal diode based on 2D graphene/BN. We used NEMD to investigate the interfacial heat transport properties of 15–150 nm graphene/BN nanosheets based on the ReaxFF potential. Our group has been working on molecular dynamics simulations based on the ReaxFF potential ,28,29,30x-axis) of each system is 4.919 nm and the length (y-axis) varies from 15 nm to 100 nm. The first and last unit of atoms in the y-direction are fixed to avoid the spurious global rotation. Next to the fixed atoms, two units of atoms serve as the heating and cooling areas. The periodic boundary conditions are applied in x- and z-directions, while the fixed boundary conditions are applied in the y-direction. To eliminate the effect of periodic action on the simulation results, empty space (size of three units of atoms) is left in both x- and z-directions.The simulation models, which consist of graphene and boron nitride (BN), are constructed using similar methods described in our previous work . The edgy-axis). The temperature of each segment is calculated by averaging the kinetic energies of the atoms in each segment:jT is the local temperature of segment j, jN is the number of atoms in the segment, im is the atomic mass, iV is the atomic velocity, and Bk is the Boltzmann constant. We place the system in a heat bath at 300 K until the average temperature difference All NEMD simulations are performed by the LAMMPS package . The ReaK, across the graphene/BN is defined by: A = W * δ. Here, A is the cross-section area of graphene/BN, W is the width of the system and δ is the thickness of the graphene, which is 0.335 nm. The thermal rectification ratio, R, is defined as The interfacial thermal conductance, We calculate the phonon density of states (PDOS) for different systems to probe the mechanism of the thermal rectification effect at the graphene/BN interface. A unit of atoms at the interface  is used to calculate PDOS. The PDOS are extracted from the discrete Fourier transform of the velocity autocorrelation function:To quantify the degree of overlap of the PDOS, we calculate the cumulative correlation factors (CCF) to describe the matching properties of the phonon modes of atoms at the interface. The CCFs were used by Dong et al.  and Diao−2K−1 and 5547 MWm−2K−1, respectively, which is similar to those calculated by others, as summarized in First, we calculate the thermal conductance K of the 75 nm system to verify the reliability of our approach. The results show that K(forward) and K(backward) equal 4554 MWmThe phonon density of states (PDOS) is a powerful tool for describing phonon information in materials. It has been used in many studies to deeply trace the phonon activity, and thus, to understand the thermal transport properties of materials ,39,40,41We can see from −2K−1 to 3199 MW m−2K−1, while in the backward direction, K first gradually decreases from 7263 MW m−2K−1 to 5547 MW m−2K−1 and then increases to 6972 MW m−2K−1. These changes lead to the increase in R from 3% to 117% as the system length increases from 15 nm to 150 nm, which is in contrast to that found by Chen et al. [Although Chen et al.  and Zhuan et al.  that theNext, we investigated the relationship between the thermal rectification ratio and length from the perspective of phonon vibration modes. Taking systems of 25 nm and 100 nm in length as examples, we perform a phonon power spectrum analysis and plot the PDOS curves of out-of-plane phonons in Then, we further calculate the CCF of PDOS between C and BN atoms in the for-ward and backward direction in frequency ≤ 25 THz. Compared with the system length of 25 nm, the system with a length of 100 nm has a larger matching degree difference . This exIn summary, we used non-equilibrium molecular dynamics simulations to investigate the interfacial thermal transport properties of graphene/BN nanosheets based on ReaxFF potential. We found that the thermal rectification phenomenon and the thermal rectification ratio increases with the increase in graphene/BN length. This phenomenon can be well explained by the mismatch of out-of-plane phonon vibration modes in two directions at the interface. Then, we explored the underlying mechanism of the length dependence of the thermal transport properties. For instance, the difference between the length dependence of the heat conduction in two directions reflects the change in the thermal rectification ratio. The yielded result shows the potential of graphene/BN heterostructure to act as an active thermal management element for semiconductor devices.