Large Graphene Sheets Research Articles

Origin of high storage capacity in N-doped graphene quantum dots

Study of high performance supercapacitor using doping and edge states in graphene is an interesting area of research, however, the storage capacity and understanding of the origin of enhancement are still in its infant stage. In the present work, nitrogen doped graphene quantum dot (N_GQD) is synthesized to enhance the storage capacity exploiting both the doping level as well as edge states. The results are compared with undoped graphene quantum dot and doped large graphene sheet. It is seen that the highest value of average specific capacitance (509Fg−1) is achieved in case of N_GQD. The origin of this high performance is explained on the basis of trap states created by both dopants and edge states which can adsorb charge carriers to enhance the storage capacity .The charge transport in N_GQD sample is also studied on the basis of trap induced electron transport.

Effect of grain boundaries on electrical properties of polycrystalline graphene

Since it was discovered, synthesizing large area graphene sheets by chemical or epitaxial methods is the conventional method for the application of graphene's outstanding properties. Unfortunately, CVD grown graphene is polycrystalline in nature with grain boundaries (GBs) unlike exfoliated single crystal graphene flakes. These GBs are known as the main source of degrading mechanical, chemical and electrical properties of CVD graphene. Nematic liquid crystal was employed to visualize grains and boundaries and to calculate the atomic orientation of adjacent grains. Electrical performance of various GB devices was observed, and the role of grain boundaries on the ultimate electrical performance was investigated. The same work was carried out at low temperature (∼7 K) to minimize the perturbation of two-dimensional electron which originated from the graphene lattice vibration. Interestingly, it was observed that there was no significant effect of the grain boundaries on the overall electrical performance of devices even at low temperature. These results lead to the conclusion that the grain boundaries in polycrystalline graphene sheets are not the cause for degradation in the electrical performance, if the grains were stitched well at the boundary in the growth process.

A graphene–carbon nanotube hybrid material for photovoltaic applications

Large area graphene sheets grown by chemical vapor deposition can potentially be employed as a transparent electrode in photovoltaics if their sheet resistance can be significantly lowered, with minimal loss in transparency. Here, we report the fabrication of a graphene-conducting-carbon-nanotube (CCNT) hybrid material with a sheet resistance considerably lower than neat graphene, and with the requisite small reduction in transparency. Graphene is deposited on top of a a self-assembled CCNT monolayer which creates parallel conducting paths on the graphene surface. The hybrid thereby circumvents electron scattering due to defects in the graphene sheet, and reduces the sheet resistance by a factor of two. The resistance can be further reduced by chemically doping the hybrid. Moreover, the chemically doped hybrid is more stable than a standalone chemically doped graphene sheet, as the CCNT network enhances dopant binding. In order to understand the results, we develop a 2D resistance network model in which we couple the CCNT layer to the graphene sheet and demonstrate the model accounts quantitatively for the resistance decrease. Our results show that a graphene-CCNT hybrid system has high potential for use as a transparent electrode in photovoltaic applications.

Preparation and electrochemical properties of LiFePO4/graphene composites from tailoring graphene oxides

LiFePO4/graphene composites have been prepared by using tailoring graphene oxide (GO) nanosheets as precursors. The structure and electrochemical properties of the composites were investigated by X-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM), Raman microscopy, and a variety of electrochemical testing techniques. The decrease in graphene size reduces the contact resistance between activated materials, and enhances the lithium-ion transport in LiFePO4/graphene composites. With low weight fractions of small-size graphene sheets, the composites show better electrochemical performance than those with large size graphene sheets.

Large-area single layer exfoliation of graphene oxide via Couette-Taylor flow reactor

L graphene sheets can reduce the contact resistance caused by the number of graphene-graphene contacts and enhance the electrical conductivity. Furthermore, larger graphene sheets can be more effective in load transfer while graphene is used as reinforcement filler in composites. In previous studies, graphite oxide synthesized by the conventional Hummers’ method was exfoliated through sonication process to obtain single layer graphene oxide. However, its lateral size was less than 10μm and additionally, omnidirectional mechanical energy input, such as sonication, reduces the lateral size of graphene oxide sheets. This lateral size reduction of single layer graphene oxide attributes to the further weakening of graphene sheets resulting from the hydroxyl and epoxy sites functionalized edge and plane of graphene sheets. Here, we describe a facile method to prepare large-area single layer graphene oxide, which uses the exfoliation of graphite oxide in a Couette-Taylor flow reactor. We found that the formation of Taylor vortex flow with shearing stress in the reactor is effective for exfoliation of graphite oxide, which allows for the production of a more than 100μm in lateral size single or few-layer graphene oxide platelets at a high yield of 90% within 60 min of reaction time. The properties of fabricated graphene oxide were examined by field emission scanning electron microscope, Raman spectra, atomic force microscope, X-ray photoelectron spectroscope and X-ray diffraction.

Tailoring patterns of graphene wrinkles by circular torsion

Wrinkled graphene has been emerging as a hot topic of interest due to its easily induced physical changes accompanied by changes in its material behavior. However, the wrinkling pattern of graphene and its relevant properties remain poorly understood. Here we employ molecular dynamics simulations to model the behavior of graphene under periodic, torsional wrinkling and elucidate the effect of torsion pattern, torsion velocity, and hole size on the wrinkling characteristics of a large graphene sheet. Simulation results show that gross control over the wrinkling pattern is feasible via manipulation of torsion direction and relative hole size, with fine-tuning of the wrinkle formation possible by control of the relative torsion speed of each hole.

Synthesis and characterization of porous, mixed phase, wrinkled, few layer graphene like nanocarbon from charcoal

A technique to synthesis wrinkled graphene like nano carbon (GNC) from charcoal is reported in the current study. The charcoal produced by thermal decomposition and is intercalated by Hummers method. It is separated by centrifugation and sonication to get few layer graphene sheets. The structural and chemical changes of the nanostructure is elucidated by Raman spectroscopy, TEM, SEM-EDS and XPS. Raman spectra revealed the existence of highly graphitized amorphous carbon, which is confirmed by the appearance of five peaks in the deconvoluted first order Raman spectra. The SEM analysis reveals the formation of large area graphene sheets with nano-porous structure in it. The TEM/SAED analysis exhibits the presence of short range few layer graphene.

Organic Radical-Assisted Electrochemical Exfoliation for the Scalable Production of High-Quality Graphene.

Despite the intensive research efforts devoted to graphene fabrication over the past decade, the production of high-quality graphene on a large scale, at an affordable cost, and in a reproducible manner still represents a great challenge. Here, we report a novel method based on the controlled electrochemical exfoliation of graphite in aqueous ammonium sulfate electrolyte to produce graphene in large quantities and with outstanding quality. Because the radicals (e.g., HO(•)) generated from water electrolysis are responsible for defect formation on graphene during electrochemical exfoliation, a series of reducing agents as additives (e.g., (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO), ascorbic acid, and sodium borohydride) have been investigated to eliminate these radicals and thus control the exfoliation process. Remarkably, TEMPO-assisted exfoliation results in large graphene sheets (5-10 μm on average), which exhibit outstanding hole mobilities (∼405 cm(2) V(-1) s(-1)), very low Raman I(D)/I(G) ratios (below 0.1), and extremely high carbon to oxygen (C/O) ratios (∼25.3). Moreover, the graphene ink prepared in dimethylformamide can exhibit concentrations as high as 6 mg mL(-1), thus qualifying this material for intriguing applications such as transparent conductive films and flexible supercapacitors. In general, this robust method for electrochemical exfoliation of graphite offers great promise for the preparation of graphene that can be utilized in industrial applications to create integrated nanocomposites, conductive or mechanical additives, as well as energy storage and conversion devices.

A superior mix of big and small

Materials Science Graphene is often described as an unrolled carbon nanotube. However, although nanotubes are known for their exceptional mechanical and conductivity properties, the same is not true of graphene-based fibers. Xin et al. intercalated small fragments of graphene into the gaps formed by larger graphene sheets that had been coiled into fibers. Once annealed, the large sheets provided pathways for conduction, while the smaller fragments helped reinforce the fibers. The result? Superior thermal and electrical conductivity and mechanical strength. Science , this issue p. [1083][1] [1]: /lookup/doi/10.1126/science.aaa6502

Highly thermally conductive and mechanically strong graphene fibers

Graphene, a single layer of carbon atoms bonded in a hexagonal lattice, is the thinnest, strongest, and stiffest known material and an excellent conductor of heat and electricity. However, these superior properties have yet to be realized for graphene-derived macroscopic structures such as graphene fibers. We report the fabrication of graphene fibers with high thermal and electrical conductivity and enhanced mechanical strength. The inner fiber structure consists of large-sized graphene sheets forming a highly ordered arrangement intercalated with small-sized graphene sheets filling the space and microvoids. The graphene fibers exhibit a submicrometer crystallite domain size through high-temperature treatment, achieving an enhanced thermal conductivity up to 1290 watts per meter per kelvin. The tensile strength of the graphene fiber reaches 1080 megapascals.

Effect of Milling Parameters on Microstructure and Selected Mechanical Properties of Si<sub>3</sub>N<sub>4</sub> – Graphene Composites

In the presented work the effect of graphene platelets (GPLs) grade and mixture homogenization parameters on the microstructure and selected properties of Si3N4 – graphene composites have been investigated. Three different commercial grades of the multilayer graphene platelets were used as a filler for ceramic matrix. The mixtures of silicon nitride powder with 2wt% of GPLs were prepared using planetary mill at 200 – 400 rpm for 1 – 16 h. The water, acetone and isopropanol have been tested as a wetting medium. The composites were sintered using Spark Plasma Sintering – SPS at 1650 °C under 35 MPa of uniaxial pressure during the whole cycle. Microstructure, density, Young modulus, hardness and indentation fracture toughness of the sintered samples have been investigated. The microstructure homogeneity of the composites depends mainly on the milling duration. Milling process should last for about 4 – 8 h at 200 rpm for optimum results. Mechanical properties of the composites are slightly varied depending on the mixture preparation parameters. The homogenization time has a little stronger influence on the Young's modulus and hardness in the case of composites containing larger graphene sheets, in connection with the fragmentation of its microstructure. The best properties have composites reinforced by the GPLs characterised both by the smallest thickness and lateral size of nanosheets.

Graphene chemically synthesized from benzene at liquid–liquid interfaces

Herein, we report the total chemical synthesis of graphene assembled at a liquid–liquid interface through a chemical reaction starting from benzene as a monomer. The reaction occurs at the benzene/water liquid interface, taking advantage of the insoluble material (the growing graphene structures) assembling as a film, which allows a heterogeneous reaction with the precipitated material and a molecular construction of large graphene sheets (ca. 800nm2 or larger). A two-step polymerization mechanism has been proposed, involving the initial polymerization of benzene to poly-paraphenylene (PPP) and/or its oligomers, followed by subsequent Scholl reaction between disconnected aryl rings to graphene.

(Invited) Microwave Chemistry Enabled Controlled Fabrication of Graphene with Tailored Structures for Designed Applications

Due to its excellent electronic, optical, thermal and mechanical properties, and its large surface area and low mass, graphene holds great potential for a wide range of applications. It seems that the research in graphene has now proceeded from the initial phase of developing myriad strategies for the synthesis of graphene sheets to the use of graphene in various research fields. Mass production of reduced graphene oxide and graphene nanoplatets has recently been achieved. However, it is still challengingto controllably produce solution processable highly conductive graphene sheets in large quantity, at low cost, via eco-friendly and energy saving process, with optimal sheet size, layer thickness, defective (vacancies and holes) and molecular structures (oxygen containing groups and non-defective graphene domains). All these structural parameters determine their electronic, thermal and mechanical properties of graphene, which are the key warrants for their practical application in various devices. As examples, fundamental studies and high-frequency electronics require pristine graphene. However, “bulk” applications such as flexible macroelectronics, and mechanically and electronically reinforced composites, require large quantities of solution-processible highly conductive large graphene sheets manufactured at low cost. On the other hand, holey graphene, referred to graphene with nanoholes in their basal plane, demonstrates much better performance in their application as metal free catalysts and in energy storage. Finally, there is a surge of interests in nanosized graphene sheets for solar cells and various biological applications due to their unique size effects, edge effects, and even quantum confinement effects. Most importantly, most of the commercial available solution processible graphene sheets were fabricated via a long multiple- step process (several hours to 5 days). By exploiting the unique thermal and kinetic effects induced by microwave heating and the right selection of graphite oxidation chemistry, we developed series of quick and scalable strategies to produce a family of “clean” graphene sheets (without surfactants and without metal ions and/or toxic residues) with controlled porous structures, and controlled sizes from a few nanometers to tens of micrometers. By choosing a series of cheap biomass molecules, also with the help of microwave heating, bottom-up fabrication approaches are developed to produce heteroatom (N, B and/or P) doped graphene sheets and graphene quantum dots with bright emission. The applications of these graphene sheets and graphene quantum dots in Lithium battery, metal free-catalysts, and multifunctional drug delivery will be discussed.

Ultrathin few-layered molybdenum selenide/graphene hybrid with superior electrochemical Li-storage performance

Ultrathin MoSe2/graphene hybrids have been successfully synthesized through a facile ionic liquid-assisted hydrothermal approach. The resultant hybrids are characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM). It is revealed that the MoSe2 nanosheets in the hybrids exhibit a graphene-like few-layered structural characteristic of 2–3 layers and are well dispersed and anchored on the large flexible graphene sheets. When evaluated as an anode for LIBs, the obtained MoSe2/graphene hybrid electrode demonstrates preferable electrochemical performances with high reversible lithium-storage capacity of ∼1100 mAh g−1, superior cyclic stability and excellent rate capability. The extraordinary electrochemical properties can be ascribed to the novel hybrid structures composed of ultrathin few-layered MoSe2 nanosheets and highly conductive graphene as well as the resultant maximized synergistic effect between MoSe2 and graphene.

Fabrication of functionalized polysulfide reservoirs from large graphene sheets to improve the electrochemical performance of lithium-sulfur batteries.

The effect of graphene lateral size on the electrochemical performance of lithium-sulfur (Li-S) batteries is often ignored. In this study, the thermally exfoliated large lateral-sized graphene (denoted LTG) was employed as the conductive matrix to support sulfur, and its performance was then compared with that of a smaller lateral-sized graphene (denoted STG) for Li-S batteries. The results showed that the LTG-S composite exhibited much higher capacity retention (53%) versus the STG-S (29%) and better rate capabilities. Because they were both identical in morphology, in terms of sulfur content and sulfur distribution, the improved properties probably resulted from the potential prevention of polysulfide diffusion upon cycling due to the larger graphene-based network and higher aspect ratio of the LTG matrix, referred as better polysulfide reservoirs. To further improve the cell performance, a reduced graphene oxide-coated carbon fiber paper (RCF) was inserted between the LTG-S cathode and the separator by a simple drop-coat method, which provided an increased conductive surface area for polysulfides to be oxidized/reduced and buffered volume expansion. As expected, the discharge capacities of 1143 and 622 mA h g(-1) at first use and after 100th cycles were obtained with an average Coulombic efficiency of 99.7%, which were higher than 847 and 455 mA h g(-1) for the cathode without the RCF, respectively. This study highlights the significance of large graphene sheets and interlayers on the inhibition of polysulfide diffusion and offers a new way to solve the problems of Li-S batteries.

Tip-Enhanced Raman Scattering of the Local Nanostructure of Epitaxial Graphene Grown on 4H-SiC (0001̅)

Step, ridge, and crack submicro/nanostructures of epitaxial graphene on 4H-SiC (0001̅) were characterized using tip-enhanced Raman scattering (TERS) spectroscopy. The nanostructures were created during graphene synthesis due to a difference in the thermal expansion coefficient of graphene and SiC. These structures are a distinctive property of epitaxial graphene, together with other desirable properties, such as large graphene sheet and minimal defects. The results of this study illustrate that the exceptional spatial resolution of TERS allows spectroscopic measurements of individual nanostructures, a feat which normal Raman spectroscopy is not capable of. By analyzing TERS spectra, the change of local strain on the nanoridge and decreased graphene content in the submicrometer crack were detected. Using G′ band positions in the TERS spectra, the strain difference between the ridge center and flat area was calculated to be 1.6 × 10–3 and 5.8 × 10–4 for uniaxial and biaxial strain, respectively. This confirms the proposed mechanism in previous researches that nanoridges on epitaxial graphene form as a relief against compressive strain. With this study, we demonstrate that TERS is a powerful technique for the characterization of individual local nanostructures on epitaxial graphene.

Properties of pentacene-based films prepared using a heated tungsten mesh

A heated tungsten (W) mesh, set between a pentacene source and a substrate in a vacuum chamber, was used to prepare a bulk-phase pentacene film and a pentacene-based organic semiconductor film. Since the pentacene molecules come into contact with the heated W mesh before reaching the substrate, their thermal energy is increased prior to deposition. As the mesh temperature was increased from 23 to 1200°C, the intensity ratio of bulk to thin-film phases increased from 0 to 9.7. Above 1300°C there is a notable decomposition reaction, the products of which were identified as dihydropentacene, p-distrylbenzene, and 2,2′-dimethyl-1,1′-binaphthalene. These decomposed precursors are expected to provide a potential source of large graphene sheets and graphene nanoribbons.

Fabrication of graphene-based flexible devices utilizing a soft lithographic patterning method

There has been considerable interest in soft lithographic patterning processing of large scale graphene sheets due to the low cost and simplicity of the patterning process along with the exceptional electrical or physical properties of graphene. These properties include an extremely high carrier mobility and excellent mechanical strength. Recently, a study has reported that single layer graphene grown via chemical vapor deposition (CVD) was patterned and transferred to a target surface by controlling the surface energy of the polydimethylsiloxane (PDMS) stamp. However, applications are limited because of the challenge of CVD-graphene functionalization for devices such as chemical or bio-sensors. In addition, graphene-based layers patterned with a micron scale width on the surface of biocompatible silk fibroin thin films, which are not suitable for conventional CMOS processes such as the patterning or etching of substrates, have yet to be reported. Herein, we developed a soft lithographic patterning process via surface energy modification for advanced graphene-based flexible devices such as transistors or chemical sensors. Using this approach, the surface of a relief-patterned elastomeric stamp was functionalized with hydrophilic dimethylsulfoxide molecules to enhance the surface energy of the stamp and to remove the graphene-based layer from the initial substrate and transfer it to a target surface. As a proof of concept using this soft lithographic patterning technique, we demonstrated a simple and efficient chemical sensor consisting of reduced graphene oxide and a metallic nanoparticle composite. A flexible graphene-based device on a biocompatible silk fibroin substrate, which is attachable to an arbitrary target surface, was also successfully fabricated. Briefly, a soft lithographic patterning process via surface energy modification was developed for advanced graphene-based flexible devices such as transistors or chemical sensors and attachable devices on a biocompatible silk fibroin substrate. Significantly, this soft lithographic patterning technique enables us to demonstrate a simple and efficient chemical sensor based on reduced graphene oxide (rGO), a metallic nanoparticle composite, and an attachable graphene-based device on a silk fibroin thin film.

Effect of annealing of graphene layer on electrical transport and degradation of Au/graphene/n-type silicon Schottky diodes

We have investigated the effect of annealing of graphene sheets on the electrical properties of Au/graphene/n-type silicon Schottky diode. Large scale graphene sheets were grown by chemical vapor deposition and then annealed at 300, 400, and 500°C; one sheet was left un-annealed as the control. The diodes were fabricated by transferring the graphene sheets directly onto n-type Si substrates and the current–voltage (I–V) and capacitance–voltage (C–V) characteristics were evaluated. The average values of the Schottky barrier height (SBH) and ideality factor (η) for the as-fabricated Au/graphene/n-type silicon Schottky diode from I–V measurements were determined to be ∼0.8±0.01eV and ∼1.79±0.05, respectively, whereas the SBH from C–V measurements was ∼0.89±0.01eV. The electrical transport characteristics measured at room temperature indicated that annealing of graphene sheet prior to the transfer of the graphene onto the n-Si substrates significantly reduces the electric degradation of the Schottky diodes, even though no distinct differences in other electric properties, including ideality factors and SBHs, before or after annealing of the graphene sheets were observed. Thus, by simply annealing the graphene sheets at 500°C, we found that the Au/graphene/n-type silicon Schottky diode showed an approximately 3.3-fold lower series resistance as compared with the un-annealed Schottky diode under air exposure of up to 7days. These annealed diodes showed significantly reduced electrical degradation by removing the potentially trapped H2O and/or O2 at the interface between the graphene layer and the n-Si substrate.

Freestanding aligned carbon nanotube array grown on a large-area single-layered graphene sheet for efficient dye-sensitized solar cell.

A novel carbon nanomaterial with aligned carbon nanotubes (CNTs) chemically bonded to a single-layered, large area graphene sheet is designed and fabricated, showing remarkable electronic and electrocatalytic properties. When the carbon nanomaterial is used as a counter electrode, the resulting dye-sensitized solar cell exhibits ≈11% enhancement of energy conversion efficiency than aligned CNT array.