Synthesis and characterization of large-area graphene and graphite films on commercial Cu-Ni alloy foils

Graphene film growth on sputtered thin Cu–Ni alloy film by inductively coupled plasma chemical vapor deposition

Graphene film growth on a Cu–Ni alloy thin film with various alloy compositions is reported in this paper.

Highly dispersed graphene ribbons produced from ZnO–C core–shell nanorods and their use as a filler in polyimide composites

Long and few-layer thickness graphene ribbons (GRs) were fabricated through an efficient process from a well-ordered array of ZnO–C core–shell hexagonal nanorods that were formed by thermally heating zinc acetate dihydrate in a sealed chamber.

Conversion of multilayer graphene into continuous ultrathin sp³-bonded carbon films on metal surfaces

The conversion of multilayer graphenes into sp(3)-bonded carbon films on metal surfaces (through hydrogenation or fluorination of the outer surface of the top graphene layer) is indicated through first-principles computations. The main driving force for this conversion is the hybridization between sp(3) orbitals and metal surface dz(2) orbitals. The induced electronic gap states and spin moments in the carbon layers are confined in a region within 0.5 nm of the metal surface. Whether the conversion occurs depend on the fraction of hydrogenated (fluorinated) C atoms at the outer surface and on the number of stacked graphene layers. In the analysis of the Eliashberg spectral functions for the sp(3) carbon films on a metal surface that is diamagnetic, the strong covalent metal-sp(3) carbon bonds induce soft phonon modes that predominantly contribute to large electron-phonon couplings, suggesting the possibility of phonon-mediated superconductivity. Our computational results suggest a route to experimental realization of large-area ultrathin sp(3)-bonded carbon films on metal surfaces.

Rapid-thermal-annealing surface treatment for restoring the intrinsic properties of graphene field-effect transistors

Graphene field-effect transistors (GFETs) were fabricated by photolithography and lift-off processes, and subsequently heated in a rapid-thermal-annealing (RTA) apparatus at temperatures (T(A)) from 200 to 400 °C for 10 min under nitrogen to eliminate the residues adsorbed on the graphene during the GFET fabrication processes. Raman-scattering, current-voltage (I-V), and sheet resistance measurements showed that, after annealing at 250 °C, graphene in GFETs regained its intrinsic properties, such as very small intensity ratios of D to G and G to 2D Raman bands, a symmetric I-V curve with respect to ~0 V, and very low sheet resistance. Atomic force microscopy images and height profiles also showed that the surface roughness of graphene was almost minimized at T(A) = 250 °C. By annealing at 250 °C, the electron and hole mobilities reached their maxima of 4587 and 4605 cm(2) V(-1) s(-1), respectively, the highest ever reported for chemical-vapor-deposition-grown graphene. Annealing was also performed under vacuum or hydrogen, but this was not so effective as under nitrogen. These results suggest that the RTA technique is very useful for eliminating the surface residues of graphene in GFETs, in that it employs a relatively low thermal budget of 250 °C and 10 min.

Synthesis of three-dimensional reduced graphene oxide layer supported cobalt nanocrystals and their high catalytic activity in F-T CO2 hydrogenation

The reduced graphene oxide (rGO) supported cobalt nanocrystals have been synthesized through an in situ crystal growth method using Co(acac)2 under solvothermal conditions by using DMF as the solvent. By carefully controlling the reaction temperature, the phase transition of the cobalt nanocrystals from the cubic phase to the hexagonal phase has been achieved. Moreover, the microscopic structure and morphology as well as the reduction process of the composite have been investigated in detail. It is found that oxygen-containing functional groups on the graphene oxide (GO) can greatly influence the formation process of the Co nanocrystals by binding the Co(2+) cations dissociated from the Co(acac)2 in the initial reaction solution at 220 °C, leading to the 3D reticular structure of the composite. Furthermore, this is the first attempt to use a Co/rGO composite as the catalyst in the F-T CO2 hydrogenation process. The catalysis testing results reveal that the as-synthesized 3D structured composite exhibits ideal catalytic activity and good stability, which may greatly extend the scope of applications for this kind of graphene-based metal hybrid material.

Highly stable and flexible silver nanowire-graphene hybrid transparent conducting electrodes for emerging optoelectronic devices

A new AgNW-graphene hybrid transparent conducting electrode (TCE) was prepared by dry-transferring a chemical vapor deposition (CVD)-grown monolayer graphene onto a pristine AgNW TCE. The AgNW-graphene hybrid TCE exhibited excellent optical and electrical properties as well as mechanical flexibility. The AgNW-graphene hybrid TCE showed highly enhanced thermal oxidation and chemical stabilities because of the superior gas-barrier property of the graphene protection layer. Furthermore, the organic solar cells with the AgNW-graphene hybrid TCE showed excellent photovoltaic performance as well as superior long-term stability under ambient conditions.

Improving the binding characteristics of tripodal compounds on single layer graphene

Graphene is an atomically thin, transparent, and conductive electrode material of interest for sensors and energy conversion and storage devices, among others. Fully realizing its potential will require robust and general methods to anchor active functionality onto its pristine basal plane. Such strategies should not utilize covalent bond formation, which disrupts the graphene's π-electron system, from which most of its desirable properties arise. We recently introduced a tripodal binding motif, which forms robust monolayers on graphene capable of immobilizing active proteins and preventing their denaturation. Here we describe structure-property relationships for a series of tripod binding groups with "feet" of different sizes. Each derivative adsorbs strongly (ΔGads ≈ -39 kJ mol(-1)) to graphene's basal plane, yet the resulting monolayers exhibit kinetic stabilities that vary over 2 orders of magnitude and molecular densities that vary by a factor of 2. This study identifies phenanthrene as a superior anchor relative to pyrene on the basis of its increased monolayer density and similar kinetic stability. We also demonstrate that varying the length of the methylene linkers between the feet and tripodal core does not affect binding substantially. These results represent the first demonstration of structure-property relationships in the assembly of molecular adsorbates on graphene and provide a paradigm for designing effective graphene binding motifs.

Chemical vapor deposition of mesoporous graphene nanoballs for supercapacitor

A mass-producible mesoporous graphene nanoball (MGB) was fabricated via a precursor-assisted chemical vapor deposition (CVD) technique for supercapacitor application. Polystyrene balls and reduced iron created under high temperature and a hydrogen gas environment provide a solid carbon source and a catalyst for graphene growth during the precursor-assisted CVD process, respectively. Carboxylic acid and sulfonic acid functionalization of the polystyrene ball facilitates homogeneous dispersion of the hydrophobic polymer template in the metal precursor solution, thus, resulting in a MGB with a uniform number of graphene layers. The MGB is shown to have a specific surface area of 508 m(2)/g and is mesoporous with a mean mesopore diameter of 4.27 nm. Mesopores are generated by the removal of agglomerated iron domains, permeating down through the soft polystyrene spheres and providing the surface for subsequent graphene growth during the heating process in a hydrogen environment. This technique requires only drop-casting of the precursor/polystyrene solution, allowing for mass-production of multilayer MGBs. The supercapacitor fabricated by the use of the MGB as an electrode demonstrates a specific capacitance of 206 F/g and more than 96% retention of capacitance after 10,000 cycles. The outstanding characteristics of the MGB as an electrode for supercapacitors verify the strong potential for use in energy-related areas.

Thinning of large-area graphene film from multilayer to bilayer with a low-power CO2 laser

Bilayer graphene has attracted a great deal of attention for many electronic and optical applications. Although large-area bilayer graphene can be synthesized by chemical vapor deposition (CVD), multilayer growth often occurs and subsequent processes are required to obtain uniform bilayer films. We report an efficient way of thinning multilayer graphene film by low-power CO2 laser irradiation in vacuum. With a laser power density of ~10(2) W cm(-2), pristine graphene film of 4-5 layers can be thinned to a bilayer free of defects in 30 s. Contrary to previous laser-assisted graphene thinning processes, which reduced graphene layers precisely and locally with a high power density and a small beam diameter, our approach enables high-efficiency thinning of large-area graphene film whilst using a significantly reduced power density and an increased laser beam diameter.

Wet chemical synthesis of graphene

A suitable technology for the preparation of graphene based on versatile wet chemistry is presented for the first time. The protocol allows the wet chemical synthesis of graphene from a new form of graphene oxide that consists of an intact hexagonal σ-framework of C-atoms. Thus, it can be easily reduced to graphene that is no longer dominated by defects.

A platform for large-scale graphene electronics--CVD growth of single-layer graphene on CVD-grown hexagonal boron nitride

Direct chemical vapor deposition (CVD) growth of single-layer graphene on CVD-grown hexagonal boron nitride (h-BN) film can suggest a large-scale and high-quality graphene/h-BN film hybrid structure with a defect-free interface. This sequentially grown graphene/h-BN film shows better electronic properties than that of graphene/SiO2 or graphene transferred on h-BN film, and suggests a new promising template for graphene device fabrication.

The effects of oxygen on controlling the number of carbon layers in the chemical vapor deposition of graphene on a nickel substrate

While oxygen is typically considered undesirable during the chemical vapor deposition (CVD) of graphene on metal substrates, we demonstrate that suitable amounts of oxygen in the CVD system can in fact improve the uniformity and thickness control of the graphene film. The role of oxygen on the CVD of graphene on a nickel substrate using a propylene precursor was investigated with various surface analytical techniques. It was found that the number of carbon layers in the deposited graphene sample decreases as the concentration of oxygen increases. In particular, single-layer graphene can be easily obtained with an oxygen/propylene ratio of 1/9. In the presence of oxygen, a thin layer of nickel oxide will form on the substrate. The oxide layer decreases the concentration of carbon atoms dissolved in the nickel substrate and results in graphene samples with a decreasing number of carbon layers.

Millimeter-size single-crystal graphene by suppressing evaporative loss of Cu during low pressure chemical vapor deposition

Millimeter-size single-crystal monolayer graphene is synthesized on polycrystalline Cu foil by a method that involves suppressing loss by evaporation of the Cu at high temperature under low pressure. This significantly diminishes the number of graphene domains, and large single crystal domains up to ∼2 mm in size are grown.

Growth of adlayer graphene on Cu studied by carbon isotope labeling

The growth of bilayer and multilayer graphene on copper foils was studied by isotopic labeling of the methane precursor. Isotope-labeled graphene films were characterized by micro-Raman mapping and time-of-flight secondary ion mass spectrometry. Our investigation shows that during growth at high temperature, the adlayers formed simultaneously and beneath the top, continuous layer of graphene and the Cu substrate. Additionally, the adlayers share the same nucleation center and all adlayers nucleating in one place have the same edge termination. These results suggest that adlayer growth proceeds by catalytic decomposition of methane (or CH(x), x < 4) trapped in a "nano-chemical vapor deposition" chamber between the first layer and the substrate. On the basis of these results, submillimeter bilayer graphene was synthesized by applying a much lower growth rate.

Graphene film growth on polycrystalline metals

Graphene, a true wonder material, is the newest member of the nanocarbon family. The continuous network of hexagonally arranged carbon atoms gives rise to exceptional electronic, mechanical, and thermal properties, which could result in the application of graphene in next generation electronic components, energy-storage materials such as capacitors and batteries, polymer nanocomposites, transparent conducting electrodes, and mechanical resonators. With one particularly attractive application, optically transparent conducting electrodes or films, graphene has the potential to rival indium tin oxide (ITO) and become a material for producing next generation displays, solar cells, and sensors. Typically, graphene has been produced from graphite using a variety of methods, but these techniques are not suitable for growing large-area graphene films. Therefore researchers have focused much effort on the development of methodology to grow graphene films across extended surfaces. This Account describes current progress in the formation and control of graphene films on polycrystalline metal surfaces. Researchers can grow graphene films on a variety of polycrystalline metal substrates using a range of experimental conditions. In particular, group 8 metals (iron and ruthenium), group 9 metals (cobalt, rhodium, and iridium), group 10 metals (nickel and platinum), and group 11 metals (copper and gold) can support the growth of these films. Stainless steel and other commercial copper-nickel alloys can also serve as substrates for graphene film growth. The use of copper and nickel currently predominates, and these metals produce large-area films that have been efficiently transferred and tested in many electronic devices. Researchers have grown graphene sheets more than 30 in. wide and transferred them onto display plastic ready for incorporation into next generation displays. The further development of graphene films in commercial applications will require high-quality, reproducible growth at ambient pressure and low temperature from cheap, readily available carbon sources. The growth of graphene on metal surfaces has drawbacks: researchers must transfer the graphene from the metal substrate or remove the metal by etching. Further research is needed to overcome these transfer and removal challenges.

Graphene synthesis: relationship to applications

Graphene is a true wonder material that promises much in a variety of applications that include electronic devices, supercapacitors, batteries, composites, flexible transparent displays and sensors. This review highlights the different methods available for the synthesis of graphene and discusses the viability and practicalities of using the materials produced via these methods for different graphene-based applications.

A facile method to observe graphene growth on copper foil

A novel scanning electron microscope (SEM) method is presented for high contrast identification of each layer of pyramidal graphene domains grown on copper. We obtained SEM images by combining the advantages of the high resolution property of the secondary electron signal and the elemental sensitivity of the backscattering electron signal. Through this method, we investigated the difference in the growth mechanisms of mono-layer and few-layer graphene. Due to different lattice mismatches, both the surface adsorption process and the epitaxial growth process existed under the atmospheric growth conditions. Moreover, the copper oxidation process can be easily discovered. It is obvious from the SEM images that the graphene greatly delayed the oxidation process of the copper surface. Finally, the nucleation and growth speed of graphene domains was found to depend on the linear array distribution of surface ledges and terraces of annealed rolled copper foil. This result explains the linear rows of graphene during the growth process and accords with theoretical results.

Large-area Bernal-stacked bi-, tri-, and tetralayer graphene

Few-layer graphene, with Bernal stacking order, is of particular interest to the graphene community because of its unique tunable electronic structure. A synthetic method to produce such large area graphene films with precise thickness from 2 to 4 layers would be ideal for chemists and physicists to explore the promising electronic applications of these materials. Here, large-area uniform Bernal-stacked bi-, tri-, and tetralayer graphene films were successfully synthesized on a Cu surface in selective growth windows, with a finely tuned total pressure and CH(4)/H(2) gas ratio. On the basis of the analyses obtained, the growth mechanism is not an independent homoexpitaxial layer-by-layer growth, but most likely a simultaneous-seeding and self-limiting process.

Graphene: an emerging electronic material

Graphene, a single layer of carbon atoms in a honeycomb lattice, offers a number of fundamentally superior qualities that make it a promising material for a wide range of applications, particularly in electronic devices. Its unique form factor and exceptional physical properties have the potential to enable an entirely new generation of technologies beyond the limits of conventional materials. The extraordinarily high carrier mobility and saturation velocity can enable a fast switching speed for radio-frequency analog circuits. Unadulterated graphene is a semi-metal, incapable of a true off-state, which typically precludes its applications in digital logic electronics without bandgap engineering. The versatility of graphene-based devices goes beyond conventional transistor circuits and includes flexible and transparent electronics, optoelectronics, sensors, electromechanical systems, and energy technologies. Many challenges remain before this relatively new material becomes commercially viable, but laboratory prototypes have already shown the numerous advantages and novel functionality that graphene provides.

High-yield chemical vapor deposition growth of high-quality large-area AB-stacked bilayer graphene

Bernal-stacked (AB-stacked) bilayer graphene is of significant interest for functional electronic and photonic devices due to the feasibility to continuously tune its band gap with a vertical electric field. Mechanical exfoliation can be used to produce AB-stacked bilayer graphene flakes but typically with the sizes limited to a few micrometers. Chemical vapor deposition (CVD) has been recently explored for the synthesis of bilayer graphene but usually with limited coverage and a mixture of AB- and randomly stacked structures. Herein we report a rational approach to produce large-area high-quality AB-stacked bilayer graphene. We show that the self-limiting effect of graphene growth on Cu foil can be broken by using a high H(2)/CH(4) ratio in a low-pressure CVD process to enable the continued growth of bilayer graphene. A high-temperature and low-pressure nucleation step is found to be critical for the formation of bilayer graphene nuclei with high AB stacking ratio. A rational design of a two-step CVD process is developed for the growth of bilayer graphene with high AB stacking ratio (up to 90%) and high coverage (up to 99%). The electrical transport studies demonstrate that devices made of the as-grown bilayer graphene exhibit typical characteristics of AB-stacked bilayer graphene with the highest carrier mobility exceeding 4000 cm(2)/V · s at room temperature, comparable to that of the exfoliated bilayer graphene.

Growth mechanism and controlled synthesis of AB-stacked bilayer graphene on Cu-Ni alloy foils

Strongly coupled bilayer graphene (i.e., AB stacked) grows particularly well on commercial "90-10" Cu-Ni alloy foil. However, the mechanism of growth of bilayer graphene on Cu-Ni alloy foils had not been discovered. Carbon isotope labeling (sequential dosing of (12)CH(4) and (13)CH(4)) and Raman spectroscopic mapping were used to study the growth process. It was learned that the mechanism of graphene growth on Cu-Ni alloy is by precipitation at the surface from carbon dissolved in the bulk of the alloy foil that diffuses to the surface. The growth parameters were varied to investigate their effect on graphene coverage and isotopic composition. It was found that higher temperature, longer exposure time, higher rate of bulk diffusion for (12)C vs(13)C, and slower cooling rate all produced higher graphene coverage on this type of Cu-Ni alloy foil. The isotopic composition of the graphene layer(s) could also be modified by adjusting the cooling rate. In addition, large-area, AB-stacked bilayer graphene transferrable onto Si/SiO(2) substrates was controllably synthesized.

Industrial graphene metrology

Graphene is an allotrope of carbon whose structure is based on one-atom-thick planar sheets of carbon atoms that are densely packed in a honeycomb crystal lattice. Its unique electrical and optical properties raised worldwide interest towards the design and fabrication of future electronic and optical devices with unmatched performance. At the moment, extensive efforts are underway to evaluate the reliability and performance of a number of such devices. With the recent advances in synthesizing large-area graphene sheets, engineers have begun investigating viable methodologies for conducting graphene metrology and quality control at industrial scales to understand a variety of reliability issues including defects, patternability, electrical, and physical properties. This review summarizes the current state of industrial graphene metrology and provides an overview of graphene metrology techniques. In addition, a recently developed large-area graphene metrology technique based on fluorescence quenching is introduced. For each metrology technique, the industrial metrics it measures are identified--layer thickness, edge structure, defects, Fermi level, and thermal conductivity--and a detailed description is provided as to how the measurements are performed. Additionally, the potential advantages of each technique for industrial use are identified, including throughput, scalability, sensitivity to substrate/environment, and on their demonstrated ability to achieve quantified results. The recently developed fluorescence-quenching metrology technique is shown to meet all the necessary criteria for industrial applications, rendering it the first industry-ready graphene metrology technique.

Wavelength-tunable, passively mode-locked fiber laser based on graphene and chirped fiber Bragg grating

We demonstrate a wavelength-tunable, passively mode-locked erbium-doped fiber laser based on graphene and chirped fiber Bragg grating. The saturable absorber used to enable passive mode-locking in the fiber laser is a section of microfiber covered by graphene film, which allows light-graphene interaction via the evanescent field of the microfiber. The wavelength of the laser can be continuously tuned by adjusting the chirped fiber Bragg grating, while maintaining mode-locking stability. Such a system has high potential in tuning the mode-locked laser pulses across a wide wavelength range.

van der Waals epitaxy of InAs nanowires vertically aligned on single-layer graphene

Semiconductor nanowire arrays integrated vertically on graphene films offer significant advantages for many sophisticated device applications. We report on van der Waals (VDW) epitaxy of InAs nanowires vertically aligned on graphene substrates using metal-organic chemical vapor deposition. The strong correlation between the growth direction of InAs nanowires and surface roughness of graphene substrates was investigated using various graphene films with different numbers of stacked layers. Notably, vertically well-aligned InAs nanowire arrays were obtained easily on single-layer graphene substrates with sufficiently strong VDW attraction. This study presents a considerable advance toward the VDW heteroepitaxy of inorganic nanostructures on chemical vapor-deposited large-area graphenes. More importantly, this work demonstrates the thinnest epitaxial substrate material that yields vertical nanowire arrays by the VDW epitaxy method.

Thermal stability of multilayer graphene films synthesized by chemical vapor deposition and stained by metallic impurities

Thermal stability is an important property of graphene that requires thorough investigation. This study reports the thermal stability of graphene films synthesized by chemical vapor deposition (CVD) on catalytic nickel substrates in a reducing atmosphere. Electron microscopies, atomic force microscopy, and Raman spectroscopy, as well as electronic measurements, were used to determine that CVD-grown graphene films are stable up to 700 °C. At 800 °C, however, graphene films were etched by catalytic metal nanoparticles, and at 1000 °C many tortuous tubular structures were formed in the film and carbon nanotubes were formed at the film edges and at catalytic metal-contaminated sites. Furthermore, we applied our pristine and thermally treated graphene films as active channels in field-effect transistors and characterized their electrical properties. Our research shows that remnant catalytic metal impurities play a critical role in damaging graphene films at high temperatures in a reducing atmosphere: this damage should be considered in the quality control of large-area graphene films for high temperature applications.

Effects of polycrystalline cu substrate on graphene growth by chemical vapor deposition

Chemical vapor deposition of graphene on Cu often employs polycrystalline Cu substrates with diverse facets, grain boundaries (GBs), annealing twins, and rough sites. Using scanning electron microscopy (SEM), electron-backscatter diffraction (EBSD), and Raman spectroscopy on graphene and Cu, we find that Cu substrate crystallography affects graphene growth more than facet roughness. We determine that (111) containing facets produce pristine monolayer graphene with higher growth rate than (100) containing facets, especially Cu(100). The number of graphene defects and nucleation sites appears Cu facet invariant at growth temperatures above 900 °C. Engineering Cu to have (111) surfaces will cause monolayer, uniform graphene growth.

Direct growth of bilayer graphene on SiO₂ substrates by carbon diffusion through nickel

Here we report a transfer-free method of synthesizing bilayer graphene directly on SiO(2) substrates by carbon diffusion through a layer of nickel. The 400 nm nickel layer was deposited on the top of SiO(2) substrates and used as the catalyst. Spin-coated polymer films such as poly(methyl methacrylate), high-impact polystyrene or acrylonitrile-butadiene-styrene, or gas-phase methane were used as carbon sources. During the annealing process at 1000 °C, the carbon sources on the top of the nickel decomposed and diffused into the nickel layer. When cooled to room temperature, bilayer graphene was formed between the nickel layer and the SiO(2) substrates. The nickel films were removed by etchants, and bilayer graphene was then directly obtained on SiO(2), eliminating any transfer process. The bilayer nature of the obtained graphene films on SiO(2) substrates was verified by Raman spectroscopy and transmission electron microscopy. The Raman spectroscopy mapping over a 100 × 100 μm(2) area indicated that the obtained graphene is high-quality and bilayer coverage is approximately 70%.