Large Graphene Research Articles

Growth of 12-inch uniform monolayer graphene film on molten glass and its application in PbI2-based photodetector

Direct growth of large area uniform graphene on functional insulating materials is essential for engineering versatile applications of graphene. However, the existing synthesis approaches can hardly avoid the generation of non-uniform multilayer graphene along the gas flow direction, affording huge challenges for further scaling up. Herein, by exploiting the molten state of soda-lime glass, we have accomplished the direct growth of large area uniform (up to 30 cm × 6 cm) graphene via a facile chemical vapor deposition route on low cost soda-lime glass. The use of molten glass eliminates the chemically active sites (surface corrugations, scratches, defects), and improves the mobility of carbon precursors, affording uniform nucleation and growth of monolayer graphene. Intriguingly, thus-obtained graphene acts as an ideal coating layer for the surface crystallographic modification of soda-lime glass, making it epitaxy substrates for synthesizing high-quality PbI2 nanoplates and continues films. Accordingly, a prototype photodetector was fabricated to present excellent photoelectrical properties of high responsivity (∼ 600 on/off current ratio) and fast response speed (18 µs). This work hereby paves ways for the batch production and the direct applications of graphene glass as platforms for constructing high performance electronic and optoelectronic devices.

Remote plasma-assisted low-temperature large-area graphene synthesis

Graphene is typically grown using thermal chemical vapor deposition (CVD) on metallic substrates such as copper and nickel at elevated temperatures above 1000 °C. The synthesis of large-area graphene at low temperature is highly desirable for large volume industrial production. In this paper, the authors report a remote plasma-assisted CVD graphene synthesis at a reduced temperature of 600 °C in a relatively shorter duration of 15 min. Scanning electron microscopy reveals the formation of large graphene crystal with an approximate size of 100 × 100 μm2 over the entire 2 × 10 cm2 surface of copper foil substrates. Raman spectra recorded for graphene grown at 600 °C show the presence of a graphene characteristic “2D” peak, attesting to the formation of graphene. The results show that it is possible to grow horizontal graphene at low temperatures and transfer it to flexible polyethylene terephthalate substrates. The utility of the synthesized graphene is ascertained through the successful fabrication of a flexible graphene-based electrochemical sensor for the detection of glucose concentration. The present research will have a direct impact on flexible wearable biosensors.

Controlled Electrodeposition of Gold on Graphene: Maximization of the Defect-Enhanced Raman Scattering Response.

A reliable method to prepare a surface-enhanced Raman scattering (SERS) active substrate is developed herein, by electrodeposition of gold nanoparticles (Au NPs) on defect-engineered, large area chemical vapour deposition graphene (GR). A plasma treatment strategy is used in order to engineer the structural defects on the basal plane of large area single-layer graphene. This defect-engineered Au functionalized GR, offers reproducible SERS signals over the large area GR surface. The Raman data, along with X-ray photoelectron spectroscopy and analysis of the water contact angle are used to rationalize the functionalization of the graphene layer. It is found that Au NPs functionalization of the "defect-engineered" graphene substrates permits detection of concentrations as low as 10-16 m for the probe molecule Rhodamine B, which offers an outstanding molecular sensing ability. Interestingly, a Raman signal enhancement of up to ≈108 is achieved. Moreover, it is observed that GR effectively quenches the fluorescence background from the Au NPs and molecules due to the strong resonance energy transfer between Au NPs and GR. The results presented offer significant direction for the design and fabrication of ultra-sensitive SERS platforms, and also open up possibilities for novel applications of defect engineered graphene in biosensors, catalysis, and optoelectronic devices.

Contactless millimeter wave method for quality assessment of large area graphene

We demonstrate that microwave absorption experiments offer a route for efficient measurements of transport properties for fast and accurate quality control of graphene. This conctactless characterization method can be used to quickly evaluate transport properties over large areas without recourse to complex lithographic methods making it suitable as a probe of quality during wafer scale fabrication. In particular, we demonstrate that absorption measurement of transport properties is sensitive to inhomogeneities in sample transport properties. This is in contrast to traditional methods using electrical contacts which tend to overestimate transport properties due to the formation of preferential conducting channels between the electrodes. Here we compare Shubnikov–de Haas oscillations simultaneously detected by microwave absorption and by conventional contact Hall bar measurements in fields up to 15 T on quasi-free standing, large area (25 mm2) monolayer graphene. We find that although the evaluated charge carrier densities from both measurements are similar, the mobility differs considerably due to electronic transport inhomogeneity.

Poly-melamine sponge derived N-doped carbon/Fe3O4/graphene synthesized for lithium-ion anode

Nitrogen-doped pyrolytic carbon coated Fe3O4 nanocrystals supported by large sheet-size graphene (N-pC/Fe3O4/G) were synthesized by a self-assembly process followed by heating to obtain a high-performance anode material. The 3D porous poly-melamine sponge (PMS) acted as a template for the assembly of graphene to form curved and hollow foam structure, and as a source of N-doped carbon coating of Fe3O4 nanocrystals. The N-pC/Fe3O4/G composite exhibits good reversible capacity of 825 mAh/g after 350 cycles at a high current density of 1 A/g, showing much better performance than either one of its two components. Its performance benefits from the long-range conductivity of the graphene foam and the short-range local conductivity of the N-doped carbon coating.

High graphene fillers content for improving the tribological performance of silicon nitride-based ceramics

The effect of large graphene nanoplatelets (GNPs) additions on the friction and wear of silicon nitride (Si3N4) ceramics is evaluated under isooctane lubrication. All Si3N4/GNPs composites, with up to 20.6 vol% of graphene fillers, exhibit better tribological response than Si3N4, in particular, a 50% reduction in the friction that continuously decreases with the GNPs content is attained, jointly with an improvement in the wear resistance of up to 63%. The survival of a self-lubricant carbon-rich tribolayer formed on the worn surfaces controls the tribological properties of the materials. Si3N4/GNPs composites are promising materials to enhance the performance of gasoline direct injection systems.

Gate electrostatics and quantum capacitance in ballistic graphene devices

We experimentally investigate the charge induction mechanism across gated, narrow, ballistic graphene devices with different degrees of edge disorder. By using magnetoconductance measurements as the probing technique, we demonstrate that devices with large edge disorder exhibit a nearly homogeneous capacitance profile across the device channel, close to the case of an infinitely large graphene sheet. In contrast, devices with lower edge disorder (< 1 nm roughness) are strongly influenced by the fringing electrostatic field at graphene boundaries, in quantitative agreement with theoretical calculations for pristine systems. Specifically, devices with low edge disorder present a large effective capacitance variation across the device channel with a nontrivial, inhomogeneous profile due not only to classical electrostatics but also to quantum mechanical effects. We show that such quantum capacitance contribution, occurring due to the low density of states (DOS) across the device in the presence of an external magnetic field, is considerably altered as a result of the gate electrostatics in the ballistic graphene device. Our conclusions can be extended to any two dimensional (2D) electronic system confined by a hard-wall potential and are important for understanding the electronic structure and device applications of conducting 2D materials.

Direct growth of large area uniform double layer graphene films on MgO(100) substrates by chemical vapor deposition

Direct growth of large area uniform high quality graphene films on insulating substrates is very important for graphene-based devices. Here we report a facile approach to directly fabricate uniform and large area bi-layer graphene films on MgO(100) substrates by chemical vapor deposition, without the aid of any metal catalysts. Raman spectroscopy, X-ray photoelectron spectroscopy, atomic force microscopy are used to characterize the synthesized graphene films. Raman mapping indicates that the graphene films are large area uniformity. The mobility of these directly grown graphene films is 130 cm2/(VS) and they may have potential applications in electronic devices, sensors, conductive films, etc.

(Invited) 2D Materials: Crystal Growth for Future Device Structures

The isolation of graphene, now over decade ago, has given rise to the revitalization of an old full set of materials, two-dimensional materials (2DM), which have exceptional electrical, chemical and physical properties. Some of the materials under investigation in addition to graphene are hexagonal boron nitride (h-BN), semiconducting, metallic, and superconducting transition metal dichalcogenides (TMD) with a general chemical formula, MX2 where M is for example equal to Mo, W, Ta, Nb, Zr, Ti, and X = S, Se and Te, and others. While graphene is a material with many exceptional properties and h-BN is an excellent 2D insulator, TMD materials provide what neither graphene nor h-BN can, bandgap engineering that, in principle, can be used to create new devices that cannot be fabricated with h-BN and graphene alone. There is hope that 2DM can be integrated to fabricate numerous device types for many applications ranging from inkjet-printed circuits, photonic applications, flexible electronics, and high-performance electronics. However, to fully realize the benefits of these materials, the community will have to work together to define new device structures, device integration schemes, and materials growth processes to help the semiconductor industry make progress toward meeting the original goals of the International Technology Roadmap for Semiconductors (ITRS). A number of deposition/growth techniques have been used to prepare large area graphene, such as growth on SiC through the evaporation of Si at high temperatures, precipitation of carbon from metals, and catalytic chemical vapor deposition on Cu and Pt. Direct growth of good quality graphene on dielectrics/semiconductors other than SiC with reasonable properties has also been reported recently on Ge. Due to its insulating nature and compatibility with graphene and TMDs, the preparation of large area h-BN is also being developed on both metals and dielectrics. Transition metal dichalcogenides, however, present altogether different opportunities and difficulties in the preparation of low defect density large area single crystals. Vapor transport, chemical vapor deposition (CVD), and molecular beam epitaxy (MBE) are being developed to produce these materials for initial studies of materials physics and device fabrication. In addition, there is some effort in performing simulations to guide growth for both CVD and MBE growers. Therefore, there is an opportunity here to have the crystal growers and the modeling community collaborate to develop high quality materials and processes. A number of devices structures are currently under evaluation to take advantage of the basic properties of graphene, bi-layer graphene, h-BN and TMDs. Some of the devices are based on tunneling phenomena while others are based on excitonic phenomena. In this presentation I will present the state of the art results of graphene, h-BN, and a few TMD materials and their prospects for future electronic device applications.

(Invited) Large Area Ion Sensitive Graphene Field Effect Transistors for Potassium, Sodium and Chlorine Sensing

We review our work on large-area graphene based ion sensitive field effect transistors (ISFETs), employing metal oxide layers or ionophore based membranes. Unlike traditional silicon based ISFETs, our work is focused on macroscopic graphene devices fabricated using chemical vapour deposition methods. Large-area graphene ISFETs benefit from the combination of high mobility charge transport, reduced low-frequency noise with increasing transistor channel area, and facile integration with ion sensitive layers. Graphene ISFETs with an active area of approximately 1cm x 1cm are characterized by an rms current noise as low as 5 ~ nA in a 60 Hz bandwidth, field effect mobilities up to 5000 cm2 V-1 s-1 and quantum capacitance limited coupling between graphene channel and the solid/electrolyte interface. We have demonstrated graphene ISFETs selectively sensitive to H+, K+, Na+ and Cl-. In the case of H+ sensing, metal oxides such as Ta2O5 and Al2O3 deposited by atomic layer deposition can be used to achieve Nernstian limited sensitivity with a 0.1 mpH resolution. Ion sensitive membranes based on ionophores can be used for selective sensing of other ionic species. For example, in the specific case of the K+, real-time sensing was achieved using potassium ionophore III with a detection limit of 10-9 M [K+], equivalent to 39~ng/L, and a resolution of 1.5×10-3 log[K+]. Spiking experiments reveal good reversibility and stability. The cross-sensitivity has been measured to be: 2.5 mV/decade for Na+, 4.2 mV/decade for Ca2+, 1.5 mV/decade for Mg2+ and 9.0 mV/decade for NH4 +. Graphene ISFETs are sufficiently robust to measure the K+ content of common beverages and blood samples. We will conclude our talk with a discussion of open questions concerning graphene ISFET performance and their potential applications.

Lightweight Composite Microwave Absorbing Materials Based on Graphene Aerogels with Honeycomb Structure

Porous graphene‐based composites offer immense potential for microwave absorption because of their light weight and high performance. In this work, a honeycomb structure filled with porous graphene is demonstrated with excellent radar‐absorbing properties. This graphene‐based composite reaches the maximum reflection loss (RL) of −15 dB at 8.8 GHz with only a thickness of 4.0 mm and an areal density of 0.26 kg m−2. Raman spectroscopy confirms the existence of graphene and scanning electron microscopy images reveal how these large graphene flakes are arranged to a three‐dimensional porous structure. In addition, a monolithic porous graphene aerogel, without the honeycomb core, and a RL simulation of it are also investigated for comparison, which reveals antipodal properties. This work shows that the insulated honeycomb structure increases the surface characteristic impedance of the whole composite material, which mainly contributes to the reduction of total reflection from the interface. Various structural influence factors have been analyzed, the results provide a new idea for exploring high‐performance microwave absorbers of graphene‐based materials.

Dispersion and Stabilization of Exfoliated Graphene in Ionic Liquids.

The liquid-phase exfoliation of graphite is one of the most promising methods to increase production and commercial availability of graphene. Because ionic liquids can be easily obtained with chosen molecular structures and tuneable physicochemical properties, they can be use as media to optimize the exfoliation of graphite. The understanding of the interactions involved between graphite and various chemical functions in the solvent ions will be helpful to find liquids capable of dissociating and stabilizing important quantities of large graphene layers. After a step of sonication, as a mechanical precursor, samples of suspended exfoliated graphene in different ionic liquids have been characterized experimentally in terms of flake size, number of layers, total concentration and purity of the exfoliated material. Nine different ionic liquids based on imidazolium, pyrrolidinium and ammonium cations and on bis(trifluoromethylsulfonyl)imide, triflate, dicyanamide, tricyanomethanide, and methyl sulfate anions have been tested. UV-vis, Raman and X-ray photoelectron in addition to high resolution transmission electron and atomic force microscopy have been selected to characterize suspended exfoliated graphene in ionic liquids. The number of layers in the flakes exfoliated, the size and concentration depend of the structure of the ionic liquid selected. In order to obtain large flake sizes, ionic liquids with bis(trifluoromethylsulfonyl)imide anions and a cation with an alkyl chain of medium length should be selected. Smaller cation and anion favors the exfoliation of graphene. The exfoliation caused the formation of C-H bonds and the oxidation of the graphitic surface.

Water-based and inkjet printable inks made by electrochemically exfoliated graphene

Inkjet printable graphene inks are very attractive for applications in flexible and foldable electronics, such as wearable electronics and the Internet of Things. However, the ink preparation is still very time consuming as high concentrations can be achieved only with prolonged sonication (>24 h) or with expensive setups. Here we demonstrate a water-based inkjet printable ink made from electrochemically exfoliated graphene. A printable and stable (>1 month) ink with concentration of ∼2.25 mg mL−1 was formulated in less than 5 h, using two successive steps: first exfoliation and dispersion of large graphene flakes (>5 μm) followed by 1 h tip-sonication to reduce the flake size below 1 μm, as required for the material to be ejected by the nozzle. The formulated ink contains more than 75% single- and few-layers (i.e. less than 2 nm in thickness) graphene flakes with an average lateral size of 740 nm. Thermal annealing allows to achieve high C/O ratio (>10), which translates into one of the highest electrical conductivity (≈3.91 × 104 S m−1) reported so far for solution-processed graphene, without the use of any harsh chemical processing.

Liquid-phase exfoliation of fluorinated graphite to produce high-quality graphene sheets

Chemical exfoliation of graphite as a facile method is used to synthesize graphene sheets on a large scale. However, the preparation process including oxidation of graphite and reduction of graphene oxide causes many defects and unnecessary oxygen-containing groups in obtained graphene sheets. In this study, a simple and facile method is proposed to create graphene starting from fluorinated graphite (FGi). Fourier transform infrared spectroscopy, x-ray photoelectron spectroscopy, and transmission electron microscopy were employed to analyze the components and structure of the graphene sheets. The results showed that large and flat graphene sheets with the polycrystalline structure were obtained by the chemical exfoliation of FGi. The F/C ratio can be facilely tuned just by adjusting the refluxing time, and the ratio is almost not decreased when reaction time is more than 8 h. Furthermore, the electrical conductivity and electrochemical properties of the as-prepared graphene have been systematically investigated and exhibited regularity with a variation of the F/C ratio.

Controlled electrophoretic deposition of electrochemically exfoliated graphene sheets on Ag nanowires network

Electrochemical exfoliation of graphite has recently attracted a big attention as a simple, fast and scalable method for the preparation of high quality graphene, but there are some drawbacks that hinder its application. Direct deposition is one of the most critical challenges that makes it difficult to deposit uniform, compact and large scale graphene thin films. This work develops a facile electrophoretic deposition route to fabricate exfoliated graphene (EG) film on Ag nanowires (NWs) networks with a controllable film thickness in nanometers scale. EG thin films are deposited with different applied potentials and times from an EG dispersion in N, N -dimethylformamide solvent. Since confirmed by light transmittance results, the increase in deposition time or applied potential lead to an increase in the number of deposited EG layers. The scanning electron microscopy results confirm the uniformity and compactness of EG films deposited on Ag NWs network. Furthermore, transparency and conductivity of Ag NWs films before and after the deposition of EG are investigated. It shows significant improvement in its performance, as a result, it leads to Ag NWs/EG hybrid films exhibiting a sheet resistance of R s = 30 Ω sq -1 , and 85% light transmittance, comparable to conventional transparent conductive oxide films.

Conformation dependence of tyrosine binding on the surface of graphene: Bent prefers over parallel orientation

Binding of tyrosine (Tyr) with two finite size graphene sheets of 62 and 186 carbon atoms was investigated using M06-2X/6-31G(d) level. Thorough conformational analysis was done for tyrosine to select few conformers to explore their complexation by considering different orientations on the surface of graphene. The binding energy values are not significantly changed while moving from small to large graphene. Tyrosine exhibits strong π-π interactions that support recent experiments on binding of aromatic amino acids on the surface of graphene and modified graphene. The binding energies of 9.4–9.9 kcal/mol were obtained for different conformers of tyrosine binding with graphene via parallel orientation (having π-π and C(β)-H…π interactions). Owing to multiple interactions including C-H…π, N-H/O-H…π and π-π, the bent prefers over parallel orientation (by 1 kcal/mol) for the high energy conformer of Tyr adsorption on graphene. The binding energies in the aqueous medium are slightly lower than those obtained in the gas phase. HOMO-LUMO energy gaps calculated at the TPSSh/6-31G(d) level suggest that adsorption of tyrosine conformers with different orientations on the surface of graphene does not affect the band gap value of graphene (1.43 and 0.08 eV correspondingly for small and large graphene). Intermolecular orbital interactions support the π-π interactions in bent and parallel orientations of tyrosine with graphene.

Insight into the rapid growth of graphene single crystals on liquid metal via chemical vapor deposition

Previous reports about the growth of large graphene single crystals on polycrystalline metal substrates usually adopted the strategy of suppressing the nucleation by lowering the concentration of the feedstock, which greatly limited the rate of the nucleation and the sequent growth. The emerging liquid metal catalyst possesses the characteristic of quasi-atomically smooth surface with high diffusion rate. In principle, it should be a naturally ideal platform for the low-density nucleation and the fast growth of graphene. However, the rapid growth of large graphene single crystals on liquid metals has not received the due attention. In this paper, we firstly purposed the insight into the rapid growth of large graphene single crystals on liquid metals. We obtained the millimeter-size graphene single crystals on liquid Cu. The rich free-electrons in liquid Cu accelerate the nucleation, and the isotropic smooth surface greatly suppresses the nucleation. Moreover, the fast mass-transfer of carbon atoms due to the excellent fluidity of liquid Cu promotes the fast growth with a rate up to 79 µm s–1. We hope the research on the growth speed of graphene on liquid Cu can enrich the recognition of the growth behavior of two-dimensional (2D) materials on the liquid metal. We also believe that the liquid metal strategy for the rapid growth of graphene can be extended to various 2D materials and thus promote their future applications in the photonics and electronics.

Large Area Graphene Deposition on Hydrophobic Surfaces, Flexible Textiles, Glass Fibers and 3D Structures

Graphene and its derivatives have many superior electrical, thermal, mechanical, chemical, and structural properties, and promise for many applications. One of the issues for scalable applications is the lack of a simple, reliable method that allows the deposit of a well-ordered monolayer using low-cost graphene flakes onto target substrates with different surface properties. Another issue is the adhesion of the deposited graphene thin film, which has not been well investigated yet. Following our former finding of a double self-assembly (DSA) process for efficient deposition of a monolayer of graphene flakes (MGFs), in this work we demonstrate that the DSA process can be applied even on very challenging samples including highly hydrophobic polytetrafluoroethylene (PTFE), flexible textiles, complex 3D objects, and thin glass fibers. Additionally, we tested adhesion of the graphene flakes on the flat glass substrate by scotch tape peel test of the MGFs. The results show that the graphene flakes adhere quite well on the flat glass substrate and most of the graphene flakes stay on the glass. These findings may trigger many large-scale applications of low-cost graphene feedstocks and other 2D materials.

Reversible defect engineering in graphene grain boundaries

Research efforts in large area graphene synthesis have been focused on increasing grain size. Here, it is shown that, beyond 1 μm grain size, grain boundary engineering determines the electronic properties of the monolayer. It is established by chemical vapor deposition experiments and first-principle calculations that there is a thermodynamic correlation between the vapor phase chemistry and carbon potential at grain boundaries and triple junctions. As a result, boundary formation can be controlled, and well-formed boundaries can be intentionally made defective, reversibly. In 100 µm long channels this aspect is demonstrated by reversibly changing room temperature electronic mobilities from 1000 to 20,000 cm2 V−1 s−1. Water permeation experiments show that changes are localized to grain boundaries. Electron microscopy is further used to correlate the global vapor phase conditions and the boundary defect types. Such thermodynamic control is essential to enable consistent growth and control of two-dimensional layer properties over large areas.

Review on Large-Area Synthesis of Some Graphene Films, Characterizations and Applications

For two decades and beyond, remarkable progress has been made in the use of chemical vapour deposition methods for comprehending 2D materials growth. This review is aimed at providing an overview of several synthesis, characterizations and applications of large area graphene film on some substrates. First, the major growth methods are explained. Next, the mechanism of the graphene growth is discussed, focusing on reaction of precursors, diffusion of adatoms, energetics and kinetics of growth fronts, and effects of substrates. Then, the advantage of growing graphene film on some substrate over another is explained in some of the synthesis. Furthermore, the characterization of the various synthesized graphene films are explained, which suggests a suitable application to it.