Large Graphene Sheets Research Articles

Large energy laser pulses with high repetition rate by graphene Q-switched solid-state laser

We demonstrated that the graphene could be used as an effective saturable absorber for Q-switched solid-state lasers. A graphene saturable absorber mirror was fabricated with large and high-quality graphene sheets deprived from the liquid phase exfoliation. Using this mirror, 105-ns pulses and 2.3-W average output power are obtained from a passively Q-switched Nd:GdVO(4) laser. The maximum pulse energy is 3.2 μJ. The slope efficiency is as high as 37% approximating to 40% of the continue-wave laser, indicating a low intrinsic loss of the graphene.

Green-synthesized gold nanoparticles decorated graphene sheets for label-free electrochemical impedance DNA hybridization biosensing

Sensitive electrochemical impedance assay of DNA hybridization by using a novel graphene sheets platform was achieved. The graphene sheets were firstly functionalized with 3,4,9,10-perylene tetracarboxylic acid (PTCA). PTCA molecules separated graphene sheets efficiently and introduced more negatively-charged –COOH sites, both of which were beneficial to the decoration of graphene with gold nanoparticles. Then amine-terminated ionic liquid (NH 2-IL) was applied to the reduction of HAuCl 4 to gold nanoparticles. The green-synthesized gold nanoparticles, with the mean diameter of 3 nm, dispersed uniformly on graphene sheets and its outer layer was positively charged imidazole termini. Due to the presence of large graphene sheets and NH 2-IL protected gold nanoparticles, DNA probes could be immobilized via electrostatic interaction and adsorption effect. Electrochemical impedance value increased after DNA probes immobilization and hybridization, which was adopted as the signal for label-free DNA hybridization detection. Unlike previously anchoring DNA to gold nanoparticles, this label-free method was simple and noninvasive. The conserved sequence of the pol gene of human immunodeficiency virus 1 was satisfactorily detected via this strategy.

The Role of Chemistry in Graphene Doping for Carbon-Based Electronics

Graphene forms an important two-dimensional (2D) material class that displays both a high electronic conductivity and optical transparency when doped. Yet, the microscopic origin of the doping mechanism in single sheet or bulk intercalated systems remains unclear. Using large-scale ab initio simulations, we show the graphene surface acts as a catalytic reducing/oxidizing agent, driving the chemical disproportionation of adsorbed dopant layers into charge-transfer complexes which inject majority carriers into the 2D carbon lattice. As pertinent examples, we focus on the molecular SbCl(5) and HNO(3) intercalates, and the solid compound AlCl(3). Identifying the microscopic mechanism for the catalytic action of graphene is important, given the availability of large area graphene sheets, to spur research into new redox reactions for use in science and technology.

Femtosecond laser induced micropatterning of graphene film

Micropatterning of CVD synthesized large area graphene film is demonstrated with femtosecond laser cutting process. Homogenous microribbon or other patterned structure can be fabricated without using any resist or other material containing the graphene surface within a very short duration. Once the suitable laser beam doses are determined, sharp edge profile and clean etching are obtained. Scanning electron microscopic study shows that the patterned microribbon is having 5 μm width and mm in length. The width of the patterned microribbon can be controlled with control of laser energy and preprogramming of laser ablation process. Raman study at the edge of the microribbon shows increase in D peak and appearance of D + G mode, signifying edge defects. The defect can be explained from the breaking of sp 2 carbon hybridization with oxidation due to laser etching. The Raman study shows no amorphous carbon formation with laser cutting of the graphene film. The presented process shows a simple way to make patterned microribbon on large area graphene sheet which can be extremely necessary for microelectronics fabrication.

Driving Force for Hydrophobic Interaction at Different Length Scales

We study by molecular dynamics simulations the driving force for the hydrophobic interaction between graphene sheets of different sizes down to the atomic scale. Similar to the prediction by Lum, Chandler, and Weeks for hard-sphere solvation [J. Phys. Chem. B 1999, 103, 4570-4577], we find the driving force to be length-scale dependent, despite the fact that our model systems do not exhibit dewetting. For small hydrophobic solutes, the association is purely entropic, while enthalpy favors dissociation. The latter is demonstrated to arise from the enhancement of hydrogen bonding between the water molecules around small hydrophobes. On the other hand, the attraction between large graphene sheets is dominated by enthalpy which mainly originates from direct solute-solute interactions. The crossover length is found to be inside the range of 0.3-1.5 nm(2) of the surface area of the hydrophobe that is eliminated in the association process. In the large-scale regime, different thermodynamic properties are scalable with this change of surface area. In particular, upon dimerization, a total and a water-induced stabilization of approximately 65 and 12 kJ/mol/nm(2) are obtained, respectively, and on average around one hydrogen bond is gained per 1 nm(2) of graphene sheet association. Furthermore, the potential of mean force between the sheets is also scalable except for interplate distances smaller than 0.64 nm which corresponds to the region around the barrier for removing the last layer of water. It turns out that, as the surface area increases, the relative height of the barrier for association decreases and the range of attraction increases. It is also shown that, around small hydrophobic solutes, the lifetime of the hydrogen bonds is longer than in the bulk, while around large hydrophobes it is the same. Nevertheless, the rearrangement of the hydrogen-bond network for both length-scale regimes is slower than in bulk water.

Reconfigurable Graphene Logic Device Based on Tilted P-N Junctions

ABSTRACTIn this paper, we introduce a novel reconfigurable graphene logic based on graphene p-n junctions. In this logic device, switching is accomplished by using co-planar split gates that modulate the properties that are unique to graphene, including ambipolar conduction, electrostatic doping, and angular dependent carrier reflection. In addition, the use of these control gates can dynamically change the operation of the device, leading to reconfigurable multi-functional logic. A device model is derived from carrier transmission probability across the p-n junction for allowing quantitative comparison to CMOS logic. Based on this model, we show that the proposed graphene logic has significant advantages over CMOS gate in terms of area, delay, power, and signal restoration. Furthermore, the device utilizes a large graphene sheet with minimal patterning, allowing feasible integration with CMOS circuits, for potential CMOS-graphene hybrid circuits.

The role of hydrocarbon concentration on the synthesis of large area few to multi-layer graphene structures

Large area and few to multi-layer graphene sheets were synthesized on a bimetallic catalyst system via radio-frequency chemical vapor deposition (RF-CVD). Synthesis conditions were varied and their effects on the morphological properties of the graphene structures were observed. As the flow rate of the hydrocarbon was varied, the diameter of the graphene structures increased from a few hundred nanometers to a few micrometers. The number of graphene layers was also found to be dependent on the hydrocarbon concentration. This method shows great potential for the synthesis of large area few-layer graphene sheets, which can be used in various electronic applications.

Few layers of graphene as transparent electrode from botanical derivative camphor

Here, we report synthesis of large area graphene sheets by control pyrolysis of solid botanical derivative camphor (C 10H 16O) and fabrication of transparent electrodes. Raman study shows highly ordered graphene sheet with minimum defects. Second order Raman spectrum shows that graphene layers are more than single layer and can be controlled with amount of camphor pyrolyzed. Transmission electron microscopic images show presence of 4 layers for thinner and 13 layers for thicker graphene sheets. Transferred graphene sheets on glass substrates show very good transparency in wide range of wavelength (0.3–2 μm). Electrical measurements of the graphene sheets show thickness dependent sheet resistance. A sheet resistance of 203 Ω/sq is obtained at a transmittance of 63.5% of the graphene sheet. The technique to fabricate few layer of graphene as transparent electrode from camphor is both viable and scalable for potential large area optoelectronic applications.

Dimensional crossover of thermal transport in few-layer graphene

Graphene, in addition to its unique electronic and optical properties, revealed unusually high thermal conductivity. The fact that thermal conductivity of large enough graphene sheets should be higher than that of basal planes of bulk graphite was predicted theoretically by Klemens. However, the exact mechanisms behind drastic alteration of material's intrinsic ability to conduct heat as its dimensionality changes from 2-D to 3-D remain elusive. Recent availability of high-quality few-layer graphene materials allowed us to study dimensional crossover experimentally. Here we show that the room-temperature thermal conductivity changes from K~3000 W/mK to 1500 W/mK as the number of atomic plains in few-layer graphene increases from 2 to 4. We explained the observed evolution from 2-D to bulk by the cross-plane coupling of the low-energy phonons and corresponding changes in the phonon Umklapp scattering. The obtained results shed light on heat conduction in low-dimensional materials and may open up few-layer graphene applications in thermal management of nanoelectronics.

Reconfigurable Graphene Logic

In this paper, we introduce a novel reconfigurable graphene logic based on graphene p-n junctions. In this logic device, switching is accomplished by using co-planar split gates that modulate the properties that are unique to graphene, including ambipolar conduction, electrostatic doping, and angular dependent carrier reflection. In addition, the use of these control gates can dynamically change the operation of the device, leading to reconfigurable multi-functional logic. A device model is derived from carrier transmission probability across the p-n junction for allowing quantitative comparison to CMOS logic. Based on this model, we show that the proposed graphene logic has significant advantages over CMOS gate in terms of area, delay, power, and signal restoration. Furthermore, the device utilizes a large graphene sheet with minimal patterning, allowing facile integration with CMOS circuits, for potential CMOS-graphene hybrid circuits.

Role of carbon surface diffusion on the growth of epitaxial graphene on SiC

We have observed the formation of graphene on SiC by Si sublimation in an Ar atmosphere using low-energy electron microscopy, scanning tunneling microcopy, and atomic force microscopy. This work reveals unanticipated growth mechanisms, which depend strongly on the initial surface morphology. Carbon diffusion governs the spatial relationship between SiC decomposition and graphene growth. Isolated bilayer SiC steps generate narrow ribbons of graphene by a distinctive cooperative process, whereas triple bilayer steps allow large graphene sheets to grow by step flow. We demonstrate how graphene quality can be improved by controlling the initial surface morphology to avoid the instabilities inherent in diffusion-limited growth.

Microwave synthesis of large few-layer graphene sheets in aqueous solution of ammonia

Few-layer graphene (FLG) sheets with sizes exceeding several micrometers have been synthesized by exfoliation of expanded graphite in aqueous solution of ammonia under microwave irradiation, with an overall yield approaching 8 wt.%. Transmission electron microscopy (in bright-field and dark-field modes) together with electron diffraction patterns and atomic force microscopy confirmed that this graphene material consisted mostly of mono-, bi- or few-layer graphene (less than ten layers). The high degree of surface reduction was confirmed by X-ray photoelectron and infrared spectroscopies. In addition, the high stability of the FLG in the liquid medium facilitates the deposition of the graphene material onto several substrates via low-cost solution-phase processing techniques, opening the way to subsequent applications of the material.

Graphitization at interface between amorphous carbon and liquid gallium for fabricating large area graphene sheets

The authors have found that liquid gallium exhibits as a good graphitizing catalyst for a large area graphene sheet. While gallium and carbon are known to be an insoluble system, the authors have found that the catalytic reaction occurs at a very narrow interfacial region between amorphous carbon and liquid gallium. Amorphous carbon film was transformed into graphite layer composed of a few layers of graphene sheet. This thin graphene film can be easily transferred into silicon substrate through the intermediation of poly-dimethyl-siloxane rubber stamping.

Refluorination of pyrocarbon prepared from fluorine–GIC

The reaction mechanism of elemental fluorine with the carbon material prepared by the pyrolysis of a fluorine–graphite intercalation compound (fluorine–GIC; C x F) has been discussed in detail. In spite of its large graphene sheets with the graphite-like stacking order, the pyrocarbon prepared from stage-1 C 2.5F (ex-C 2.5F pyrocarbon) exhibits a remarkably high reactivity against elemental fluorine even at room temperature which is comparable to that of carbon black. The small crystallite size along the c-axis of this pyrocarbon facilitates the expansion of every interlayer space and the formation of CF bonds on both sides of the carbon sheets at the early stage of the fluorination reaction. Ex-C 2.5F pyrocarbons after the fluorination at 373 and 473 K are typical stage-1 C x F with semiionic CF bonds while those fluorinated at 573, 673 and 773 K are poly(carbon monofluoride) ((CF) n ) with covalent CF bonds. The (CF) n with fairly high crystallinity along the c-axis is obtained by the fluorination of ex-C 2.5F pyrocarbon above 673 K.

Microstructural evolution during charcoal carbonization by X-ray diffraction analysis

X-ray diffraction (XRD) of monolithic pieces of carbonized medium-density fiberboard (c-MDF) provided new insights on the microstructural evolution occurring during charcoal carbonization. Classical XRD theory was used to correlate the {002} peak intensity to the amount of carbon in large turbostratic crystallites and to bulk dimensional changes. This new analytical technique could be used to study the microstructural evolution of other monolithic carbon materials (including soft carbons) or of specific processes (including chemical activation). The quasipercolation model, a new ‘percolation-like’ model, was created based on XRD analysis of monolithic c-MDF. As the carbonization temperature ( T carb) increased above 600 °C, the large turbostratic crystallites grew very little, but the graphene sheets grew substantially. Volumetric shrinkage suggested that turbostratic crystallites were drawn closer together as the low-density disordered carbon was converted into high-density graphene sheets. At approximately 900 °C, the large graphene sheets and the large turbostratic crystallites significantly impinged on each other. The increased impingement of conductive phases with increasing carbonization temperature would cause the commonly observed nonmetal-metal transition of hard carbon materials. The quasipercolation model also suggested the source of the nanoporosity that is critical in activated carbons.