Graphene-metal Interface Research Articles

Enhanced electromagnetic shielding with ultrathin VGNs-Metal hybrid structures

Designing and preparing lightweight electromagnetic shielding materials at the micron level is crucial for aerospace and military applications. Vertical graphene nanowalls (VGNs) offer high efficiency for electromagnetic shielding due to attributes such as high carrier mobility, porosity, and low density. This study presents the preparation of ultra-thin VGNs/metal stacked films via plasma-enhanced chemical vapor deposition (PECVD) for VGNs, and combined electron beam evaporation (EBE) with cyclic PECVD for metal layers. The PECVD technique ensures homogeneity and controllable thickness of the VGNs. The experimental results demonstrate that the interface between the VGNs and the metal layer exhibits good interfacial contact, resulting in high carrier mobility. Furthermore, incorporating a metal layer with a thickness ratio of 10 % significantly enhances conductivity by an order of magnitude. The hybrid structures exhibit remarkable electromagnetic shielding effectiveness per unit thickness (∼5300 dB mm−1) evaluated in the X-band range (8–12 GHz). Stacked films as thin as 0.01 mm achieve up to 53.4 dB of electromagnetic shielding. Finite element simulations indicate that the presence of opposing electric fields at the graphene-metal interface contributes to higher ohmic loss. The coupling effect at the interface significantly contributes to the material's excellent electromagnetic shielding effectiveness.

Ultrathin damage-tolerant flexible metal interconnects reinforced by in-situ graphene synthesis

Conductive patterned metal films bonded to compliant elastomeric substrates form meshes which enable flexible electronic interconnects for various applications. However, while bottom-up deposition of thin films by sputtering or growth is well-developed for rigid electronics, maintaining good electrical conductivity in sub-micron thin metal films upon large deformations or cyclic loading remains a significant challenge. Here, we propose a strategy to improve the electromechanical performance of nanometer-thin palladium films by in-situ synthesis of a conformal graphene coating using chemical vapor deposition (CVD). The uniform graphene coverage improves the thin film’s damage tolerance, electro-mechanical fatigue, and fracture toughness owing to the high stiffness of graphene and the conformal CVD-grown graphene-metal interface. Graphene-coated Pd thin film interconnects exhibit stable increase in electrical resistance even when strained beyond 60% and longer fatigue life up to a strain range of 20%. The effect of graphene is more significant for thinner films of < 300 nm, particularly at high strains. The experimental observations are well described by the thin film electro-fragmentation model and the Coffin-Manson relationship. These findings demonstrate the potential of CVD-grown graphene nanocomposite materials in improving the damage tolerance and electromechanical robustness of flexible electronics. The proposed approach offers opportunities for the development of reliable and high-performance ultra-conformable flexible electronic devices.

Spall characteristics of three-dimensional graphene networks with embedded copper: A molecular dynamics study

The phenomenon of shock-induced spallation has garnered significant interest in both scientific and industrial circles. Laminated graphene embedded in graphene-reinforced metal-matrix composites have been shown to improve mechanical properties. This study systematically investigates the dynamic response and spall failure of a three-dimensional graphene network (3DGN) with embedded copper composites (3DGCu) and its dependence on the grain sizes (D). Firstly, increasing the wt% of graphene results in significant increase of shock wave speed due to the intrinsic high modulus of graphene, whilst leading to a reduction in shock pressure. Spallation of 3DGCu at low shock velocity is observed, which is dominated by the bonded interaction at the graphene-metal interface. However, the 3DGCu with higher graphene content (smaller D) exhibits both low density and superior spall resistance. In particular, in the sample with 60 wt% graphene, the complete spallation has never occurred and still maintains a relatively high spall strength within the impact velocity of 3.0 km/s. This is due to the remarkable capacity for 3DGN to absorb the kinetic energy during tension. In addition, during shock loading, the graphene component is responsible to the tension loading, whereas the copper metal matrix undergoes compression, leading to a stable post-shock length. These findings offer a novel approach for tailoring the dynamic properties of advanced graphene-based nano-composites with potential applications in extreme environments.

Improved mechanical and electrical properties of copper-aluminum joints with highly aligned graphene reinforcement via friction stir spot welding

This study demonstrates the potential of friction stir spot welding (FSSW) for producing highly aligned graphene configurations in copper-aluminum joints, resulting in improved mechanical and electrical properties. Pure copper and AA6061 sheet were welded with 0.5% graphene nanoparticles dispersed in ethanol drop cast at the interface. Under optimized experimental conditions, adding a graphene interlayer results in a 15.6% increase in maximum shear strength, a 46% increase in hardness value, and a 23% reduction in joint electrical resistance. Transmission electron microscopy confirmed that the laminar flow behavior of the material aligns the graphene nanoparticles parallel to the tool surface direction. The mechanical improvements are attributed to the reduction of grain size and the plastic incompatibilities that arise at the metal-graphene interface. The highly aligned graphene configuration provides more conductive paths and acts as a bridge between grains to reduce grain boundary resistance, reducing electrical resistance. These findings pave the way for exploring the possibilities of utilizing the anisotropic properties of graphene in material joining applications.

Interface mechanics of 2D materials on metal substrates

The chemical vapor deposition (CVD) of graphene on metal substrate serves as a robust method to synthesize large graphene flakes. Therefore, understanding the thermodynamics and kinetics of graphene–metal interface is an important step towards synthesizing defect-free graphene during CVD. The focus of this paper is on the phenomenon of surface reconstruction of a metal substrate during the deposition of a 2D material. Key features of surface reconstruction include the formation of straight and pyramidal facets of size ∼300 nm in width and ∼15–80 nm in height. In addition, discontinuities in facet directions under a single graphene flake distributed over two grains highlight the strong crystallographic influence on surface reconstruction.In this paper, we present an atomistically informed continuum model of a graphene–metal interface in three dimensions. Graphene is modeled as an elastic surface that is in contact with a rigid metal substrate. Due to the weak van der Waals interaction between graphene and metal, the kinematics of the model incorporates sliding of graphene. However, since we assume that graphene is always in contact with the substrate, its normal displacement is mediated by surface diffusion of metal atoms. Based on evidence from recent molecular dynamics simulations and experiments, we model the graphene-covered-metal surface energy to include in-plane elasticity and bending energy of graphene, and an interaction energy that depends on the orientation of graphene relative to the substrate. We compare the predictions of our model with surface reconstructions observed during the CVD of graphene on forged thin film palladium polycrystals. As we continue to unravel the atomic scale mechanisms responsible for surface reconstruction during CVD, we expect the current continuum framework and its generalizations will serve to bridge the atomic- and the meso-scales.

Current effect on suspended graphene nanoribbon studied using in–situ transmission electron microscopy

Graphene nanoribbons (GNRs) are expected in nanodevices and sensors but are degraded by contaminants that adsorb firmly on the surface during the transfer and lithography processes. Current annealing appears to be the most appropriate cleaning method, but its effects have not been quantitatively clarified. To this end, the cleaning processes of suspended monolayer GNRs were observed by in situ transmission electron microscopy. The gradual cleaning processes, which were successfully observed by improving the contact resistance at graphene-metal interface, showed that the GNRs was cleaned on the atomic scale in the current density range of 2.0–5.0 × 1012 A/m2. The corresponding temperature was estimated to be ∼ 700 K by finite element method. To be surprised, clean monolayer and bilayer GNRs withstood current densities of 5.1 × 1012 A/m2 and 1.7 × 1013 A/m2 (current-carrying capacities), respectively, which values were approximately three orders of magnitude higher than those of typical metals. The higher current-carrying capacity of bilayer GNR can be attributed to the formation of closed edges. This study shows that the electrical current is effective for cleaning GNR and boosting the current-carrying capacity.

Characteristics of hybrid surface plasmon polaritons at a chiral graphene metal interface in cylindrical waveguides

A detailed theoretical analysis of the hybrid surface plasmon mode generation and propagation at a chiral graphene metal (CGM) interface for a cylindrical structure is presented. The conductivity of graphene is modeled using the Kubo formalism and the dispersion relation for the hybrid surface waves is computed on the basis of the Kubo formalism and impedance matching boundary conditions. The hybrid mode consisting of lower and upper plasmon modes is witnessed due to the presence of the chiral medium. The frequency band gap between the lower and upper plasmon modes is found to be sensitive and tunable with respect to the chiral strength, radial distance of the waveguide, and chemical potential of graphene. The propagation length and effective refractive index can also be modulated by varying the chiral strength and chemical potential. For very high values of chirality and biasing voltage, the Goos–Hänchen effect is observed at the CGM interface. The cutoff values of chiral strength as a function of normalized frequency make the proposed structure applicable for chiroptical and chemical sensing and enantiomeric detection in the THz frequency regime.

Modeling damage evolution of graphene/aluminum composites considering crystal cracking and interface failure

Graphene is a significant reinforcement in metal matrix composites by virtue of its superior mechanical properties. The cracking of metal crystal and the failure of graphene-metal interface are the main reasons for the decrease of mechanical properties of graphene/metal composites, but the damage mechanism of that is not clear. In this paper, a novel two-dimensional microstructure model of graphene/polycrystalline metal composites is established by a self-developed structure modeling procedure. According to the actual structure of graphene/metal composites, the numbers, sizes, orientations, arrangements of graphene and grain can be controlled respectively. Moreover, by employing the method of combining the crystal plasticity finite element method (CPFEM) and the cohesive zone model (CZM), the damage mechanism of graphene/aluminum (Al) composites on polycrystalline Al matrix, graphene-reinforcement and graphene-Al interface under tensile load is revealed from the mesoscale for the first time, then the effects of graphene morphology and initial microcracks on the failure behavior and overall mechanical properties of graphene/Al composites are fully captured. This study provides a strong theoretical support and inspiration for the construction of graphene/Al composites with excellent properties.

Ultrahigh-strength multi-layer graphene-coated Ni film with interface-induced hardening

Graphene-reinforced metal matrix composites exhibit excellent mechanical properties owing to dislocation impedance at the metal-graphene interface. Graphene coated on metal with composites fabricated using powder sintering can be applied as high-strength thin films across various fields (e.g., microelectromechanical systems, flexible electronics). In this study, a bilayer composite of multilayer graphene (MLG)-coated Ni is synthesized using the chemical vapor deposition (CVD) and transfer methods; mechanical properties are investigated using nanoindentation methods. MLG-coated Ni synthesized by CVD exhibits 195% and 470% increases in hardness and Young’s modulus, respectively, compared with single-layer Ni. In contrast, the Young modulus and hardness of MLG-coated Ni synthesized via the transfer method can be estimated using the rule of mixture for composite materials. Transmission electron microscopy (TEM) shows that in MLG-coated Ni synthesized by CVD, dislocations are dense and evenly distributed compared with that synthesized by the transfer method, leading to its high mechanical strength. Molecular dynamics (MD) simulations demonstrate that interface-induced hardening is effective in graphene-coated Ni(111) with a strongly coupled interface. Therefore, ultrahigh-strength MLG-coated metal films can be obtained by manipulating the interface property between the MLG and metal, offering the potential for use as a thin film resistor against external force.

First principles study of behavior of helium at Fe(110)–graphene interface* *Project supported by the Nuclear Power Technology Innovation Center Program, National Defense Science & Technology Industry, China (Grant No. HDLCXZX-2019-ZH-028).

Recently, metal–graphene nanocomposite system has aroused much interest due to its radiation tolerance behavior. However, the related atomic mechanism for the metal–graphene interface is still unknown. Further, stainless steels with Fe as main matrix are widely used in nuclear systems. Therefore, in this study, the atomic behaviors of point defects and helium (He) atoms at the Fe(110)–graphene interface are investigated systematically by first principles calculations. The results indicate that graphene interacts strongly with the Fe(110) substrate. In comparison with those of the original graphene and bulk Fe, the formation energy values of C vacancies and Fe point defects decrease significantly for Fe(110)–graphene. However, as He atoms have a high migration barrier and large binding energy at the interface, they are trapped at the interface once they enter into it. These theoretical results suggest that the Fe(110)–graphene interface acts as a strong sink that traps defects, suggesting the potential usage of steel–graphene with multiply interface structures for tolerating the radiation damage.

Enhancing interface doping in graphene-metal hybrid devices using H2 plasma clean

Synthetic graphene suffers from surface contamination after the removal of transfer polymer. This impacts the chemical and electronic properties at the interface of graphene-metal structures. Hydrogen plasma treatment is promising to effectively clean, dope and pattern graphene. However, it also causes irreversible damage to the graphene lattice or reversible graphene hydrogenation which primarily depends on the flux and energy of plasma radicals/ions. Here, we optimize a downstream Ar/H2 plasma with moderate flux and low energy thermal H radicals to eliminate polymer residues and enhance the charge transfer at the graphene-metal interface. A systematic study is carried out to evaluate the plasma-induced doping and defectivity in ruthenium capped graphene devices. Single layer graphene is found to be n-doped after plasma treatment accompanied by an increase in defect density. Similarly, bilayer graphene (BLG) also undergoes n-doping but does not incur significant plasma-induced damage. Sheet resistance of Ru capped plasma cleaned BLG is found to decrease by 18%. After device fabrication, a final plasma clean reduces the surface roughness of the capping metal, and further improves the electrical conductivity. This study provides a viable basis to effectively clean and dope graphene-metal interface with a fab-compliant plasma technique while preserving the high quality of graphene.

Electronic interface and charge carrier density in epitaxial graphene on silicon carbide. A review on metal–graphene contacts and electrical gating

For over 15 years, the number of studies on graphene electronics has not ceased growing. The rich physics, a set of outstanding properties, and the envisioned range of potential applications have consolidated graphene as a research field in its own. In this Research Update, we address a specific case of graphene for electronics, epitaxial graphene on silicon carbide (SiC) substrates. This paper mainly focuses on the electronic interface of graphene with metals. The first part of this paper describes the most characteristic aspects of the growth of epitaxial graphene on SiC wafers, and the main techniques for graphene material characterization are presented first. The main objective of this paper is to gather and discuss the most representative studies on the graphene–metal interface and the strategies employed to obtain low values for the contact resistances, which is a key feature for achieving the best performance of any graphene electronic devices. To benchmark developments in specifically epitaxial graphene on SiC, we include the results on mechanically exfoliated graphene from HOPG, as well as chemical vapor deposition graphene. In the last part of this paper, relevant device architectures for electrically gating graphene are briefly discussed.

Microstructure-Electrochemical Behaviour Correlation for Nico-GO and Nico-Mwcnts Composite Coatings

NiCo coatings are widely used in sensors, actuators and as protective coatings. Recent studies have shown that addition of foreign particles like Al2O3, SiC and TiO2 promotes grain randomization and refinement resulting in improved corrosion resistance performance. In the present work, NiCo coating containing varying amount of graphene oxide (GO) and multi-walled carbon nanotubes (MWCNTs) were electrodeposited over mild steel substrate. Microstructure and corrosion behavior of these composite coatings were then studied as a function of the volume fraction of the secondary additives. NiCo-GO & NiCo-CNT composite coating were characterized using x-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDS) for phase identification, morphology and composition respectively. Potentiodynamic polarization studies and electrochemical impedance spectroscopy (EIS) studies in 3.5 wt. % NaCl solution revealed that the corrosion behavior of the coatings was very sensitive to the concentration of GO/ MWCNTs in the coating matrix. With continuous increase in the amount of GO/CNT in the coatings, the corrosion resistance first increased followed by a decrease, indicating possibility of an optimum with respect to the amount of the additives for achieving high corrosion resistance performance. On analyzing the electrolytic bath solution, we observed that nickel ions (Ni+2) showed preferential deposition onto the graphene oxide sheets resulting in a compositional heterogeneity near and far from the graphene-metal interface in the electrodeposited coating microstructure. This compositional segregation led to a change in the relative abundance of low angle and high angle boundaries in the coating microstructure. Electron backscattered diffraction (EBSD) technique was used to study the type of grain boundaries, strain in the grain and average grain size (area) of the samples (prepared using focused ion beam FIB). The data revealed that the samples exhibiting maximum corrosion resistance (optimum CNT/ GO concentrations) also had the highest fraction of low angle grain boundaries (LAGBs) with no additional strain in the matrix. Further increase in GO concentration resulted in more grains with high misorientation angle. In case of CNTs addition, the coatings were found to be hydrophobic in nature with a grain growth along low energy (111) direction. Figure 1

Effect of interface configuration on the mechanical properties and dislocation mechanisms in metal graphene composites

The effect of graphene-metal interface on dislocation gliding and generation is a key element in understanding the mechanical properties of graphene metal-matrix composites. Graphene-metal interfaces can impede the propagation of dislocations, which enhances the mechanical properties, particularly the strength. On the other hand, the presence of such interfaces can also act as the dislocation source in the metal matrix, which can significantly weaken the material properties. This implies that graphene will not always improve the mechanical properties of metals, but alternatively, it can even significantly weaken the properties. Whether graphene acts as a dislocation sink or source primarily depends on the metal-graphene interface and also other pre-existing defects in the matrix. In this paper, we investigate these two opposing effects at the atomistic scale, using Molecular Dynamics (MD) simulations. To study the effect of the interface and to isolate the effects of dislocations from defects, we have reinforced a perfect crystal of nickel with graphene. We found that in the perfect crystal, the graphene in any interface structure acts as a defect and weakens the metal matrix. Each interface structure in the composite has a different effect on the system, and the topfcc structure of the graphene-nickel interface results in the least strength reduction. For graphene particles, apart from the interface, edges can also act as a dislocation nucleation site. We investigated the two edges –Zigzag and Armchair edge of graphene– and found that both the edges cause decrease in mechanical strength. Experimental results (always reported an increase in mechanical strength) are in contradiction to what we found in this work. The reason for this contradiction is that in the real metallic system, defects are already present, and those acts as a dislocation source more readily than the graphene. To verify this, we studied the metallic systems having defects in nickel crystal and the graphene is reinforced. Through these studies, we investigate the conditions in which the strengthening mechanisms overcome the weakening mechanisms, and improvement is observed in the mechanical properties, similar to some of the reported experimental works. Our results shed light on the deformation mechanisms in metal-graphene composites and can be used as a guide in designing and fabricating more efficient metal-matrix composites.

Confinement of Hydrogen Molecules at Graphene–Metal Interface by Electrochemical Hydrogen Evolution Reaction

Confinement of hydrogen molecules at graphene–substrate interface has presented significant importance from the viewpoints of development of fundamental understanding of two-dimensional material in...

Extraordinary magnetoresistance in encapsulated monolayer graphene devices

We report a proof-of-concept study of extraordinary magnetoresistance (EMR) in devices of monolayer graphene encapsulated in hexagonal boron nitride having metallic edge contacts and a central metal shunt. Extremely large EMR values, MR=(R(B)−R0)/R0∼105, are achieved in part because R0 approaches or crosses zero as a function of the gate voltage, exceeding that achieved in high mobility bulk semiconductor devices. We highlight the sensitivity, dR/dB, which in two-terminal measurements is the highest yet reported for EMR devices and in particular exceeds previous results in graphene-based devices by a factor of 20. An asymmetry in the zero-field transport is traced to the presence of pn-junctions at the graphene-metal shunt interface.

Oxygen Reduction Reaction Catalyzed by Pt3M (M = 3d Transition Metals) Supported on O-doped Graphene

Pt3M (M = 3d transition metals) supported on oxygen-doped graphene as an electrocatalyst for oxygen reduction was investigated using the periodic density functional theory-based computational method. The results show that oxygen prefers to adsorb on supported Pt3M in a bridging di-oxygen configuration. Upon reduction, the O–O bond breaks spontaneously and the oxygen adatom next to the metal–graphene interface is hydrogenated, resulting in co-adsorbed O* and OH* species. Water formation was found to be the potential-limiting step on all catalysts. The activity for the oxygen reduction reaction was evaluated against the difference of the oxygen adsorption energy on the Pt site and the M site of Pt3M and the results indicate that the oxygen adsorption energy difference offers an improved prediction of the oxygen reduction activity on these catalysts. Based on the analysis, Pt3Ni supported on oxygen-doped graphene exhibits an enhanced catalytic performance for oxygen reduction over Pt4.

Investigation of the transport mechanism in graphene-induced metal-oxide-semiconductor capacitors (MOSCAP)

Graphene is the most promising contender for the future generation of electronic and photonic devices, based on its extraordinary properties. The effect of the metal interface with graphene, however, which completely alters its properties, is of great importance. The effects of the substrate supporting the graphene matrix, the graphene/metal contact resistance and the overall metal oxide semiconductor capacitors (MOSCAP) for possible CMOS circuitry have been thoroughly investigated in this research work. We have fabricated a structure with pertinent deposition techniques and performed a detailed electrical analysis to obtain the transport characteristics. Nickel (Ni) is chosen as the transition metal which makes the chemisorption bonding with graphene while qualifying as an interface. We present an analysis of the metal contacts, a study of the metal resistivity at various planes, a study of the graphene (carbon) atom's resistance at the atomistic scale, the graphene based MOSCAP leakages, the necessary charge accumulation at the metal–graphene interface and the charge inversion just beneath the oxide layer.

Doped graphene and Ag(1 1 1) hybrid material as fuel cell electrode: New insights on interfacial features and oxygen adsorption from dispersion-corrected density functional theory

Graphene nanostructures are among the most interesting materials investigated in recent years for several different applications. Their activity towards Oxygen Reduction Reaction (ORR) has attracted a lot of attention for application in fuel cells and lithium-air batteries. In this, and many other applications, a graphene-metal interface is involved. In this work, we investigated the interaction between graphene and Ag(1 1 1), the effect of boron and nitrogen doping on graphene, and the combination of both factors. We applied a dispersion-corrected DFT method derived from a recent reparameterization of DFT-D2, purposely tuned for silver-molecule interfaces. With this approach, we analyse the adsorption of an oxygen molecule (the first key step of ORR) on the pristine/doped and metal-supported graphene systems. Our results highlight the role of specific dopant-substrate interactions that are key to enhance the adsorption of molecular oxygen.

Reversible doping of graphene field effect transistors by molecular hydrogen: the role of the metal/graphene interface

In this work, we present an investigation regarding how and why molecular hydrogen (H2) changes the electronic properties of graphene field effect transistors (GFETs). We demonstrate that interaction with H2 leads to local doping of graphene near of the graphene-contact heterojunction. We also show that such interaction is strongly dependent on the characteristics of the metal-graphene interface. By changing the type of metal in the contact, we observe that Ohmic contacts can be strongly or weakly electrostatically coupled with graphene. For strongly coupled contacts, the signature of the charge transfer effect promoted by the contacts results on asymmetric ambipolar conduction, and such asymmetry can be tunable under interaction with H2. On the other hand, for contacts weakly coupled with graphene, the hydrogen interaction has a more profound effect. In such a situation, the devices show a second charge neutrality point (CNP) in graphene transistor transfer curves (a double-peak response) upon H2 exposure. We propose that this double-peak phenomenon arises from the decoupling of the work function of graphene and that of the metallic electrodes induced by the H2 molecules. We also show that the gas-induced modifications at the metal-graphene interface can be exploited to create a controlled graphene p-n junction, with considerable electron transfer to graphene layer and significant variation in the graphene resistance. These effects can pave the way for a suitable metallic contact engineering providing great potential for the application of such devices as gas sensors.