Continuous Graphene Films Research Articles

Liquid phase growth of graphene on silicon carbide

We developed a novel method to produce graphene on silicon carbide (SiC) at a temperature as low as 1000°C. The method is based on liquid phase growth (LPG) of graphene mediated by liquid gallium, which acts not only as a flux to store carbon dissolved from a surface of SiC when heated, but also as a catalyst to promote the formation of graphene on SiC when cooled. Our experimental results revealed that gallium-treated SiC substrates are coated with uniform and continuous graphene films. The LPG method is able to supply graphene films consisting of one to several hundreds of layers, depending on heating temperatures. Our approach can not only provide an alternative way to form graphene natively on SiC, but will also bring a technological breakthrough in industrial applications of graphene, e.g. the realization of graphene-on-insulator substrates.

Reply to Harutyunyan: Continuous and uniform graphene film grown on liquid Cu surface

We would like to thank Harutyunyan (1) for his interest in our work (2). One of the conclusions from our work is that the use of liquid Cu can eliminate the grain boundaries found in solid Cu and results in the production of continuous and uniform graphene film. Such a conclusion has been drawn from the following experimental results. First, well-dispersed hexagonal graphene flakes (HGFs) can finally be grown into a continuous and uniform graphene film on liquid Cu surface with longer growth time, and these features of graphene films have been confirmed by extensive scanning electron microscopy, optical microscopy, and Raman spectroscopy techniques. Second, compared with nonuniform graphene film (i.e., mixed single and multilayer) grown on solid Cu surface (3), liquid Cu surface results in much improved uniform monolayer-dominated graphene film. Therefore, we think, there exists no misleading conclusion.

Effects of ambient conditions on the quality of graphene synthesized by chemical vapor deposition

In this work, we investigated the effects of methane concentration and gas flow rate ratio between hydrogen and methane on the quality of graphene synthesized by chemical vapor deposition. It is found that a critical concentration of methane is needed to grow continuous graphene films, while discontinuous graphene flakes are formed at low methane concentrations. Under the condition without hydrogen, a graphene film in which monolayer areas are predominant is grown, whereas a great proportion of hydrogen causes thick graphene, which reduces the transmittance of the film. Our results present an instructive reference to the large-area synthesis of graphene for the potential applications in electronics.

Low-Temperature Chemical Vapor Deposition Growth of Graphene from Toluene on Electropolished Copper Foils

A two-step CVD route with toluene as the carbon precursor was used to grow continuous large-area monolayer graphene films on a very flat, electropolished Cu foil surface at 600 °C, lower than any temperature reported to date for growing continuous monolayer graphene. Graphene coverage is higher on the surface of electropolished Cu foil than that on the unelectropolished one under the same growth conditions. The measured hole and electron mobilities of the monolayer graphene grown at 600 °C were 811 and 190 cm(2)/(V·s), respectively, and the shift of the Dirac point was 18 V. The asymmetry in carrier mobilities can be attributed to extrinsic doping during the growth or transfer. The optical transmittance of graphene at 550 nm was 97.33%, confirming it was a monolayer, and the sheet resistance was ~8.02 × 10(3) Ω/□.

Direct growth of few-layer graphene films on SiO2substrates and their photovoltaic applications

We first demonstrate the use of few layer graphene films directly grown on SiO2 substrates obtained by ambient pressure chemical vapor deposition (APCVD) as counter electrodes in dye-sensitized solar cells (DSSCs). The layer number and crystal size of graphene films can be tuned by changing growth temperature, growth time and gas flow ratio (CH4 : H2). The continuous graphene films exhibit extremely excellent electrical transport properties with a sheet resistance of down to 63.0 Ω sq−1 and extremely high mobility of up to 201.4 cm2 v−1 s−1. The highly conductive graphene films as counter electrodes of DSSCs achieve a photovoltaic efficiency of 4.25%, which is comparable to the DSSC efficiency (4.32%) based on FTO counter electrodes. Our work indicates the great potential of CVD graphene films directly grown on dielectric substrates for photovoltaic and electronic applications.

Control of thickness uniformity and grain size in graphene films for transparent conductive electrodes

Large-scale and transferable graphene films grown on metal substrates by chemical vapor deposition (CVD) still hold great promise for future nanotechnology. To realize the promise, one of the key issues is to further improve the quality of graphene, e.g., uniform thickness, large grain size, and low defects. Here we grow graphene films on Cu foils by CVD at ambient pressure, and study the graphene nucleation and growth processes under different concentrations of carbon precursor. On the basis of the results, we develop a two-step ambient pressure CVD process to synthesize continuous single-layer graphene films with large grain size (up to hundreds of square micrometers). Scanning electron microscopy and Raman spectroscopy characterizations confirm the film thickness and uniformity. The transferred graphene films on cover glass slips show high electrical conductivity and high optical transmittance that make them suitable as transparent conductive electrodes. The growth mechanism of CVD graphene on Cu is also discussed, and a growth model has been proposed. Our results provide important guidance toward the synthesis of high quality uniform graphene films, and could offer a great driving force for graphene based applications.

Ambipolar doping in quasifree epitaxial graphene on SiC(0001) controlled by Ge intercalation

The electronic structure of decoupled graphene on SiC(0001) can be tailored by introducing atomically thin layers of germanium at the interface. The electronically inactive (6 root 3 x 6 root 3)R30 degrees reconstructed buffer layer on SiC(0001) is converted into quasi-free-standing monolayer graphene after Ge intercalation and shows the characteristic graphene pi bands as displayed by angle-resolved photoelectron spectroscopy. Low-energy electron microscopy (LEEM) studies reveal an unusual mechanism of the intercalation in which the initial buffer layer is first ruptured into nanoscopic domains to allow the local in-diffusion of germanium to the interface. Upon further annealing, a continuous and homogeneous quasifree graphene film develops. Two symmetrically doped (n- and p-type) phases are obtained that are characterized by different Ge coverages. They can be prepared individually by annealing a Ge film at different temperatures. In an intermediate-temperature regime, a coexistence of the two phases can be achieved. In this transition regime, n-doped islands start to grow on a 100-nm scale within p-doped graphene terraces as revealed by LEEM. Subsequently, the n islands coalesce but still adjacent terraces may display different doping. Hence, lateral p-n junctions can be generated on epitaxial graphene with their size tailored on a mesoscopic scale. (Less)

Step driven competitive epitaxial and self-limited growth of graphene on copper surface

The existence of surface steps was found to have significant function and influence on the growth of graphene on copper via chemical vapor deposition. The two typical growth modes involved were found to be influenced by the step morphologies on copper surface, which led to our proposed step driven competitive growth mechanism. We also discovered a protective role of graphene in preserving steps on copper surface. Our results showed that wide and high steps promoted epitaxial growth and yielded multilayer graphene domains with regular shape, while dense and low steps favored self-limited growth and led to large-area monolayer graphene films. We have demonstrated that controllable growth of graphene domains of specific shape and large-area continuous graphene films are feasible.

Harvesting Energy from Water Flow over Graphene

Water flow over carbon nanotubes has been shown to generate an induced voltage in the flow direction due to coupling of ions present in water with free charge carriers in the nanotubes. However, the induced voltages are typically of the order of a few millivolts, too small for significant power generation. Here we perform tests involving water flow with various molarities of hydrochloric acid (HCl) over few-layered graphene and report order of magnitude higher induced voltages for graphene as compared to nanotubes. The power generated by the flow of ∼0.6 M HCl solution at ∼0.01 m/sec was measured to be ∼85 nW for a ∼30 × 16 μm size graphene film, which equates to a power per unit area of ∼175 W/m(2). Molecular dynamics simulations indicate that the power generation is primarily caused by a net drift velocity of adsorbed Cl(-) ions on the continuous graphene film surface.

Relatively low temperature synthesis of graphene by radio frequency plasma enhanced chemical vapor deposition

We present a simple, low-cost and high-effective method for synthesizing high-quality, large-area graphene using radio frequency plasma enhanced chemical vapor deposition (RF-PECVD) on SiO 2/Si substrate covered with Ni thin film at relatively low temperatures (650 °C). During deposition, the trace amount of carbon (CH 4 gas flow rate of 2 sccm) is introduced into PECVD chamber and the deposition time is only 30 s, in which the carbon atoms diffuse into the Ni film and then segregate on its surface, forming single-layer or few-layer graphene. After deposition, Ni is removed by wet etching, and the obtained single continuous graphene film can easily be transferred to other substrates. This investigation provides a large-area, low temperature and low-cost synthesis method for graphene as a practical electronic material.

Continuous Reduced Graphene Oxide Film Prepared by Stitching of Nanosheets at the Interface of Two Immiscible Solutions

Figure 1. Photograph images (A) graphene oxide (GO) and RGO sheets dispersed in water solution. (B) RGO sheets in immiscible water and chloroform solutions before and after sonication, and after settlement of the sonicated solution. (C) graphene sheets in immiscible water and benzene solutions before sonication and after sonication followed by settlement. Graphene, an atom-thick two-dimensional (2D) honeycomb lattice sheet of sp-bonded carbon atoms, has recently been emerged as a new promising material in various fields. Because of its gigantic charge carrier mobility it could be applied to field-effect transistors as a substitute of silicon. Due to its transparency and high electrical conductivity, it could be used as a substitute of ITO electrode in solar cells and light-emitting diodes. Other superior properties include large surface area, flexibility, strength, stiffness, and thermal conductivity. These provides wide applications of graphene including supercapacitor, battery, sensor, storage and drug delivery. For real applications, large-scale of graphene sheets or films needs to be prepared. Large-area (orders of centimeters) graphene films have recently been fabricated using a chemical vapor deposition (CVD) method on various metal substrates. This method requires a transfer process from the metal substrates to a desired substrate such as a transparent plastic sheet. Simpler methods are a thermal reduction of graphene oxide film prepared via the filtration of graphene sheets, Langmuir-Blodgett (LB) layerby-layer assembly, and filtration of graphene sheets followed by a film transfer. Investigators have now actively pursued various simpler ways of fabricating continuous graphene films or papers. Highly motivated by this, we have challenged to synthesize large-area/scale graphene films from solution.

Efficient growth of high-quality graphene films on Cu foils by ambient pressure chemical vapor deposition

We developed an ambient pressure chemical vapor deposition (CVD) for rapid growth of high-quality graphene films on Cu foils. The quality and growth rate of graphene films are dramatically increased with decreasing H2 concentration. Without the presence of H2, continuous graphene films are obtained with a mean sheet resistance of <350 Ω/sq and light transmittance of 96.3% at 550 nm. Because of the ambient pressure, rapid growth rate, absence of H2 and readily available Cu foils, this CVD process enables inexpensive and high-throughput growth of high-quality graphene films.

Novel Liquid Precursor-Based Facile Synthesis of Large-Area Continuous, Single, and Few-Layer Graphene Films

Graphene has attracted a great deal of attention because of its unique band structure and electronic properties that make it promising for applications in next-generation electronic devices, transparent flexible conducting electrodes, and sensors. Here, we report the substrate selective growth of centimeter size (∼3.5 cm × 1.5 cm), uniform, and continuous single and few-layer graphene films employing chemical vapor deposition technique on polycrystalline Cu foils using liquid precursor hexane. Structural characterizations suggest that as-grown graphene films are mostly single and few layers over large areas. We have demonstrated that these graphene films can be easily transferred to any desired substrate without damage. A liquid-precursor-based synthesis route opens up a new window for simple and inexpensive growth of pristine as well as doped graphene films using various organic liquids containing the dopant atoms.

Continuous, Highly Flexible, and Transparent Graphene Films by Chemical Vapor Deposition for Organic Photovoltaics

We report the implementation of continuous, highly flexible, and transparent graphene films obtained by chemical vapor deposition (CVD) as transparent conductive electrodes (TCE) in organic photovoltaic cells. Graphene films were synthesized by CVD, transferred to transparent substrates, and evaluated in organic solar cell heterojunctions (TCE/poly-3,4-ethylenedioxythiophene:poly styrenesulfonate (PEDOT:PSS)/copper phthalocyanine/fullerene/bathocuproine/aluminum). Key to our success is the continuous nature of the CVD graphene films, which led to minimal surface roughness ( approximately 0.9 nm) and offered sheet resistance down to 230 Omega/sq (at 72% transparency), much lower than stacked graphene flakes at similar transparency. In addition, solar cells with CVD graphene and indium tin oxide (ITO) electrodes were fabricated side-by-side on flexible polyethylene terephthalate (PET) substrates and were confirmed to offer comparable performance, with power conversion efficiencies (eta) of 1.18 and 1.27%, respectively. Furthermore, CVD graphene solar cells demonstrated outstanding capability to operate under bending conditions up to 138 degrees , whereas the ITO-based devices displayed cracks and irreversible failure under bending of 60 degrees . Our work indicates the great potential of CVD graphene films for flexible photovoltaic applications.

Substrate-free synthesis of large area, continuous multi-layer graphene film

Large area (∼cm2) multi-layer graphene film is synthesized in the substrate-free vapor phase. The scanning electron microscopy observation reveals that the graphene nanoflakes are either six- or five-sided, and the internal angles between adjacent sides are nearly 120° and 105°, respectively. The nucleation of hexagonal and pentagonal carbon rings leads to carbonaceous nuclei, which grow into six- or five-sided nanoflakes, respectively. A well-ordered and large-area graphene monolayer is formed by the interconnection of these flakes, and stacking of these monolayers results in the formation of a multi-layer graphene film. The creation of hexagonal and pentagonal carbon rings is considered to be important for the easy production of large area multi-layer graphene at large scale.

Influence of the growth conditions of epitaxial graphene on the film topography and the electron transport properties

In this work we report on temperature-dependent magnetotransport measurements on epitaxial graphene grown on SiC ( 0 0 0 1 ) under different preparation conditions. We demonstrate that the temperature dependence of the charge carrier density and mobility is correlated to the annealing conditions during the graphitization process. As recently shown, SiC substrates annealed in an Ar atmosphere near atmospheric pressure exhibit continuous monolayer graphene films over 2 – 3 μ m wide and more than 50 μ m long terraces. For these films we determine a constant charge carrier density in the range from 1.4 K up to room temperature and an electron mobility exceeding 3000 cm 2 ( V s ) - 1 at low temperatures.

Graphene formation mechanisms on4H-SiC(0001)

Graphene is created through thermal decomposition of the Si face of $4H\text{-SiC}$ in high-vacuum. Growth temperature and time are varied independently to gain a better understanding of how surface features and morphology affect graphene formation. Growth mechanisms of graphene are studied by ex situ atomic force microscopy (AFM) and scanning tunneling microscopy (STM). On the route toward a continuous graphene film, various growth features, such as macroscale step bunching, terrace pits, and fingers, are found and analyzed. Topographic and phase AFM analysis demonstrates how surface morphology changes with experimental conditions. Step-bunched terraces and terrace pits show a strong preference for eroding along the ${11\overline{2}0}$ planes. Data from AFM are corroborated with STM to determine the surface structure of the growth features. It is shown that elevated finger structures are SiC while the depressed interdigitated areas between the fingers are comprised of at least a monolayer of graphene. Graphene formation at the bottom of terrace pits shows a dependence on pit depth. These features lend support for a stoichiometric view of graphene formation based on the number of decomposing SiC bilayers.