Multilayer Graphene Films Research Articles

Multilayer graphene for long-term corrosion protection of stainless steel bipolar plates for polymer electrolyte membrane fuel cell

Motivated by similar investigations recently published (Pu et al., 2015), we report a comparative corrosion study of three sets of samples relevant as bipolar plates for polymer electrolyte fuel cells: stainless steel, stainless steel with a nickel seed layer (Ni/SS) and stainless steel with Ni seed layer coated by a multi-layered graphene thin film (G/Ni/SS). The graphene film, synthesized by chemical vapour deposition (CVD), has a moderate amount of defects according to Raman spectroscopy. Short/medium-term corrosion test shows no significant advantage of using G/Ni/SS rather than Ni/SS, both samples exhibiting a similar trend, thus questioning the short-term positive effect of graphene coatings. However, partial immersion in boiling seawater for three weeks reveals a clear superiority of the graphene coating with respect to steel just protected by Ni. After the test, the graphene film is still intact with unchanged defect density. Our results show that even non-perfect multilayer graphene films can considerably increase the lifetime of future-generation bipolar plates for fuel cells.

Realization of Large-Area Wrinkle-Free Monolayer Graphene Films Transferred to Functional Substrates

Structural inhomogeneities, such as the wrinkles and ripples within a graphene film after transferring the free-standing graphene layer to a functional substrate, degrade the physical and electrical properties of the corresponding electronic devices. Here, we introduced titanium as a superior adhesion layer for fabricating wrinkle-free graphene films that is highly applicable to flexible and transparent electronic devices. The Ti layer does not influence the electronic performance of the functional substrates. Experimental and theoretical investigations confirm that the strong chemical interactions between Ti and any oxygen atoms unintentionally introduced on/within the graphene are responsible for forming the clean, defect-free graphene layer. Our results accelerate the practical application of graphene-related electronic devices with enhanced functionality. The large-area monolayer graphenes were prepared by a simple attachment of the Ti layer with the multi-layer wrinkle-free graphene films. For the first time, the graphene films were addressed for applications of superior bottom electrode for flexible capacitors instead of the novel metals.

46.2: A Novel Laminated Organic Light‐Emitting Diodes with a Multi‐Layered Graphene Top Anode

We report on a novel fabrication method for organic lightemitting diodes (OLEDs) by using a multilayered graphene (MLG) film and a lamination technique. The OLED structure with the MLG top electrode has higher transmittance and lower reflectance than the OLED structure with the conventional thin metal electrode. Moreover, higher luminous current efficiency of the device with the MLG top electrode has been achieved compared with that of the conventional bottom‐emission OLEDs with aluminum metal electrode.

Synthesis of ultra-thin carbon layers on SiC substrate by ion implantation

Direct synthesis of uniform thickness and large-size monolayer or multilayer graphene films on insulating/semiconducting substrates are desirable for future nanoelectronics. In this work, ion implantation technique is used to produce ultra-thin carbon layers directly on the (0001) Si-face of a 6H-silicon carbide surface. C ions are implanted in Si-face of SiC followed by thermal annealing. Upon cooling, C atoms, either implanted or supplied by the substrate, segregate to the surface and self-assemble into thin carbon layers. Compared with by thermal decomposition to grow epitaxial graphene on SiC, the ion implantation-annealing method requires much lower graphitization temperature. To understand the growth mechanism, a characterization of our thin carbon layers has been performed using comprehensive structural and chemical analyses. This synthesis method is promising for industrial production, deserving considerable attention in the future.

Formation and field emission properties of multilayer graphene hybrid films grown on laser pretreated Ni layer

A series of petaloid few-layer graphene–aggregated state multilayer graphene hybrid films with large areas (∼500 mm2) were made on Ni-coated Al2O3 ceramic substrate utilising a microwave plasma-assisted chemical vapour deposition method. Nickel layers were pretreated by laser etching technology. A Raman spectra, field emission scanning electron microscope and field emission I–V measurements were used for the detection of the film properties. Test results indicated that the laser etched region (groove region) and the un-etched region (prominent ridge region) surface topography had significant differences between the two, and a petaloid few-layer graphene film was found in the laser-etched region while in the un-etched region, an aggregated state multilayer graphene film was formed. The influence of the laser etching pretreatment on surface microstructure and morphologies of the graphene films was investigated. Moreover, synthesis mechanisms of the films were discussed. Field emission results showed a turn on the field was 4·5 V μm−1 and the current density was 0·1 mA cm−2 in an electric field of 6·1 V μm−1.

Processing and characterization of high content multilayer graphene/epoxy composites with high electrical conductivity

A new processing method was developed to fabricate nanocomposites with a high concentration of multilayer graphene (MLG) in a highly oriented morphology. MLG was first dispersed in a water-based solution with the aid of polyethylenimine. A thin MLG film (paper) having highly in-plane aligned platelets was produced by using a vacuum-assisted self-assembly (VASA) technique. After heat treatment, the MLG paper was immersed in an epoxy/acetone bath at room temperature under vacuum to produce an epoxy impregnated composite. After removal of the acetone, nanocomposites consisting of multiple layers of the MLG paper with up to 27 wt% MLG were fabricated and thermally cured. Scanning electron microscopy (SEM) examination showed that the MLG was well dispersed and aligned, and the MLG paper was fully impregnated with epoxy resin. At 30°C, dynamic mechanical analysis (DMA) results showed that the storage modulus of the resulting nanocomposites with 27.2 wt% MLG reached 10.2 GPa, a 300% increase compared to the neat epoxy. The resulting composites also exhibited electrical conductivity as high as 35 Siemens per centimeter (S/cm). This research demonstrates that the VASA processing technique is capable of fabricating well aligned, high content MLG nanostructured polymer composites with high electrical conductivity. POLYM. COMPOS., 37:2897–2906, 2016. © 2015 Society of Plastics Engineers

Graphene grown on stainless steel as a high-performance and ecofriendly anti-corrosion coating for polymer electrolyte membrane fuel cell bipolar plates

In this study, the growth of graphene by chemical vapor deposition (CVD) on SUS304 stainless steel and on a catalyzing Ni/SUS304 double-layered structure was investigated. The results indicated that a thin and multilayered graphene film can be continuously grown across the metal grain boundaries of the Ni/SUS304 stainless steel and significantly enhance its corrosion resistance. A 3.5 wt% saline polarization test demonstrated that the corrosion currents in graphene-covered SUS304 were improved fivefold relative to the corrosion currents in non-graphene-covered SUS304. In addition to enhancing the corrosion resistance of stainless steel, a graphene coating also ameliorates another shortcoming of stainless steel in a corrosive environment: the formation of a passive oxidation layer on the stainless steel surface that decreases conductivity. After a corrosion test, the graphene-covered stainless steel continued to exhibit not only an excellent low interfacial contact resistance (ICR) of 36 mΩ cm2 but also outstanding drainage characteristics. The above results suggest that an extremely thin, lightweight protective coating of graphene on stainless steel can act as the next-generation bipolar plates of fuel cells.

Direct synthesis of multi-layer graphene film on various substrates by microwave plasma at low temperature

We introduce a possible route for vertically standing multi-layer graphene films (VMGs) on various substrates at low temperature by electron cyclone resonance microwave plasma. VMG films on various substrates, including copper sheet, glass and silicon oxide wafer, were analyzed by studying their structural, electrical, and optical properties. The density and temperature of plasma were measured using Cylindrical Langmuir probe analysis. The morphologies and microstructures of multi-layer graphene were characterized using field emission scattering electron microscope, high resolution transmission electron microscope, and Raman spectra measurement. The VMGs on different substrates at the same experimental conditions synthesized the wrinkled VMGs with different heights. In addition, the transmittance and electrical resistance were measured using ultra-violet visible near-infrared spectroscopy and 4 probe point surface resistance measurement. The VMGs on glass substrate obtained a transmittance of 68.8% and sheet resistance of 796Ω/square, whereas the VMGs on SiO2 wafer substrate showed good sheet resistance of 395Ω/square and 278Ω/square. The results presented herein demonstrate a simple method of synthesizing of VMGs on various substrates at low temperature for mass production, in which the VMGs can be used in a wide range of application fields for energy storage, catalysis, and field emission due to their unique orientation.

Study on transfer-free graphene synthesis process utilizing spontaneous agglomeration of catalytic Ni and Co metals

Transfer-free graphene synthesis process utilizing metal agglomeration phenomena was investigated by using carbon films deposited on Ni or Co catalyst metals on SiO2/Si substrates. As a result of metal agglomeration at high temperatures, multilayer graphene films appeared to be formed directly on SiO2 films. The microscopic Raman mapping study revealed that graphene films were preferentially synthesized around areas where metal films disappeared at an early stage of agglomeration, and that they finally covered almost the whole surface. It was also found that the synthesized graphene films tended to have better structural qualities and lower layer numbers with the increase in the starting metal thicknesses regardless of the kinds of catalyst metals. Raman study also showed that they had good two-dimensional uniformity in the structural quality.

Growth of dense CNT on the multilayer graphene film by the microwave plasma enhanced chemical vapor deposition technique and their field emission properties

Catalyst assisted carbon nanotubes (CNTs) were grown on multilayer graphene (MLG) on copper and silicon substrates by the microwave plasma enhanced chemical vapor deposition technique.

Layer-by-Layer Self-Assembled Graphene Multilayer Films via Covalent Bonds for Supercapacitor Electrodes

To maximize the utilization of its single-atom thin nature, a facile scheme to fabricate graphene multilayer films via a layer-by-layer self-assembled process was presented. The structure of multilayer films was constructed by covalently bonding graphene oxide (GO) using p-phenylenediamine (PPD) as a covalent cross-linking agent. The assembly process was confirmed to be repeatable and the structure was stable. With the π-π conjugated structure and a large number of spaces in the framework, the graphene multilayer films exhibited excellent electrochemical performance. The uniform ultrathin electrode exhibited a capacitance of 41.71 μF/cm2 at a discharge current of 0.1 μA/cm2, and displayed excellent stability of 88.9 % after 1000 charge-discharge cycles.

Unusual optical properties of graphene on an Rh surface

Single- and multilayer graphene films formed on the surface of a number of metals (Rh, Pt, Ir, and Re) at the thermal decomposition of benzene molecules have been studied. A surprising phenomenon has been revealed: these films with thicknesses up to 15 layers do not absorb light emitted by the surface and affect neither brightness nor true temperatures of the surface, which contradicts the fundamental results obtained by A. Geim and K. Novoselov. At larger thicknesses, absorption occurs in agreement with the commonly accepted concepts.

Controllable poly-crystalline bilayered and multilayered graphene film growth by reciprocal chemical vapor deposition.

We report the selective growth of large-area bilayered graphene film and multilayered graphene film on copper. This growth was achieved by introducing a reciprocal chemical vapor deposition (CVD) process that took advantage of an intermediate h-BN layer as a sacrificial template for graphene growth. A thin h-BN film, initially grown on the copper substrate using CVD methods, was locally etched away during the subsequent graphene growth under residual H2 and CH4 gas flows. Etching of the h-BN layer formed a channel that permitted the growth of additional graphene adlayers below the existing graphene layer. Bilayered graphene typically covers an entire Cu foil with domain sizes of 10-50 μm, whereas multilayered graphene can be epitaxially grown to form islands a few hundreds of microns in size. This new mechanism, in which graphene growth proceeded simultaneously with h-BN etching, suggests a potential approach to control graphene layers for engineering the band structures of large-area graphene for electronic device applications.

Synthesis of Multilayer Graphene Films on Copper by Modified Chemical Vapor Deposition

In this paper, large-area uniform multilayer graphene films were synthesized on copper in one growth route by modified low pressure chemical vapor deposition (LPCVD) method by introducing an assembly into the conventional LPCVD method. Scanning electronic microscopy, optical microscopy, Raman spectroscopy, ellipsometry, and transmission electron microscopy were used to characterize the graphene films. The results showed that the graphene films were multilayer. And there are about six layers with good continuity and uniformity. Meanwhile, the growth mechanism was illustrated by a growth model based on the analysis of the effects of the introduced assembly on the generation of the activated carbon atoms and on the catalysis of Cu molecules.

Fabrication of thickness controllable free-standing sandwich-structured hybrid carbon film for high-rate and high-power supercapacitor.

Hybrid carbon films composed of graphene film and porous carbon film may give full play to the advantages of both carbon materials, and have great potential for application in energy storage and conversion devices. Unfortunately, there are very few reports on fabrication of hybrid carbon films. Here we demonstrate a simple approach to fabricate free-standing sandwich-structured hybrid carbon film composed of porous amorphous carbon film and multilayer graphene film by chemical vapor deposition in a controllable and scalable way. Hybrid carbon films reveal good electrical conductivity, excellent flexibility, and good compatibility with substrate. Supercapacitors assembled by hybrid carbon films exhibit ultrahigh rate capability, wide frequency range, good capacitance performance, and high-power density. Moreover, this approach may provide a general path for fabrication of hybrid carbon materials with different structures by using different metals with high carbon solubility, and greatly expands the application scope of carbon materials.

Nanoscale friction properties of graphene and graphene oxide

Achieving superlow friction and wear at the micro/nano-scales through the uses of solid and liquid lubricants may allow superior performance and long-lasting operations in a range of micromechanical system including micro-electro mechanical systems (MEMS). Previous studies have indicated that conventional solid lubricants such as highly ordered pyrolitic graphite (HOPG) can only afford low friction in humid environments at micro/macro scales; however, HOPG is not suitable for practical micro-scale applications. In this study, we explored the nano-scale frictional properties of multi-layered graphene films as a potential solid lubricant for such applications. Atomic force microscopy (AFM) measurements have revealed that for high-purity multilayered graphene (7–9 layers), the friction force is significantly lower than what can be achieved by the use of HOPG, regardless of the counterpart AFM tip material. We have demonstrated that the quality and purity of multilayered graphene plays an important role in reducing lateral forces, while oxidation of graphene results in dramatically increased friction values. Also, for the first time, we demonstrated the possibility of achieving ultralow friction for CVD grown single layer graphene on silicon dioxide. This confirms that the deposition process insures a stronger adhesion to substrate and hence enables superior tribological performance than the previously reported mechanical exfoliation processes.

Optical and electronic structure of quasi-freestanding multilayer graphene on the carbon face of SiC

We report the optical and electronic properties of multilayer graphene films grown epitaxially on the carbon face (C-face) of 4H-SiC probed using spectroscopic ellipsometry (SE) and angle-resolved photoemission spectroscopy (ARPES). The optical conductivity in the energy range from 1.0 to 5.3 eV extracted from SE reveals two important features: the presence of universal conductivity at the near-infrared region and asymmetric resonant excitons at 4.5 eV. Furthermore, ARPES shows the presence of independent linear electronic dispersion. These features resemble quasi-freestanding properties of multilayer graphene grown on the C-face of SiC.

Direct synthesis of few- and multi-layer graphene films on dielectric substrates by “etching-precipitation” method

A novel “etching-precipitation” method is proposed and developed for the direct synthesis of graphene on dielectric substrates. In this method, graphene precipitates from the Fe–C solid solution film during selective etching of Fe using Cl2 gas. Few- and multi-layer graphene is fabricated directly on quartz glass and SiO2/Si substrates without Fe residue at a growth temperature of 500–650°C, which is a significantly lower temperature than used in the conventional chemical vapor deposition method. The 6- to 7-layer graphene synthesized at 650°C shows a volume resistivity of 80–140μΩcm. The average number of layers can be easily controlled in a linear fashion with the initial carbon feed, which is proportional to the thickness of the starting Fe–C films. Line-patterned multi-layer graphene is also fabricated by simply pre-patterning the starting Fe–C film although its structure is somewhat different from typical graphene ribbons. “Etching-precipitation” will be a practical route to synthesize graphene with micro-patterns directly onto device substrates of arbitrary sizes.

Optical transmittance of multilayer graphene

We study the optical transmittance of multilayer graphene films up to 65 layers thick. By combing large-scale tight-binding simulation and optical measurement on CVD multilayer graphene, the optical transmission through graphene films in the visible region is found to be solely determined by the number of graphene layers. We argue that the optical transmittance measurement is more reliable in the determination of the number of layers than the commonly used the Raman spectroscopy. Moreover, the optical transmittance measurement can be applied also to other 2D materials with weak van der Waals interlayer interaction.

Using a layer-by-layer assembly method to fabricate a uniform and conductive nitrogen-doped graphene anode for indium-tin oxide-free organic light-emitting diodes.

Highly conductive, uniform, and transparent nitrogen-doped graphene multilayer films were produced by a layer-by-layer (LbL) assembly method. Such a technique was realized by alternate deposition of graphene oxide modified with the cationic surfactant N,N,N-trimethyl-1-dodecanaminium bromide (CTAB) and the anionic surfactant sodium dodecylbenzenesulfonate. In this way, we can achieve a highly conductive (900 S/cm), uniform, and controllable graphene film in terms of thickness, transmittance, and sheet resistance after high-temperature reduction. The improved conductivity is attributed to better graphitization and nitrogen-doping introduced by CTAB. The organic light-emitting diode using such a multilayer graphene film fabricated by the LbL method as an anode obtains higher current density and luminance at low voltage compared to that with an indium-tin oxide (ITO) anode. Moreover, the current efficiency of graphene-based device is comparable to that of an ITO-based device. It is proved that such a nitrogen-doped multilayer graphene film developed by the LbL assembly technique is a promising candidate for a transparent electrode in organic electronics.