Catalytic conversion of Kraft lignin to bio-multilayer graphene materials under different atmospheres

Kraft lignin was catalytic graphitized by iron at 1000 °C in argon, hydrogen, CO₂, methane, and natural gas atmospheres, respectively. The effect of atmospheric agent types on product distribution (gas, liquid, and solid carbon yields) was analyzed. The solid products were characterized by scanning electron microscopy, Raman, high-resolution transmission electron microscopy, and X-ray diffraction. Experimental results have shown that the degree of graphitization of Kraft lignin depends not only on the highest temperature, but also the type of ambient gas phase during heat treatment. Methane and natural gas in the ambient gas phase seem to accelerate the formation of multilayer graphene materials with a range of 2–30 layers, and hydrogen and carbon dioxide have an etching effect on solid carbon species during the catalytic graphitization process, while multilayer graphene-encapsulated iron nanoparticles were the main products in the case of argon.     

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

Controlling the thickness and uniformity during growth of multilayer graphene is an important goal. Here we report the synthesis of large-area monolayer and multilayer, particularly bilayer, graphene films on Cu-Ni alloy foils by chemical vapor deposition with methane and hydrogen gas as precursors. The dependence of the initial stages of graphene growth rate on the substrate grain orientation was observed for the first time by electron backscattered diffraction and scanning electron microscopy. The thickness and quality of the graphene and graphite films obtained on such Cu-Ni alloy foils could be controlled by varying the deposition temperature and cooling rate and were studied by optical microscopy, scanning electron microscopy, atomic force microscopy, and micro-Raman imaging spectroscopy. The optical and electrical properties of the graphene and graphite films were studied as a function of thickness.     

Few-layer graphene films prepared from commercial copper foil tape

We present a facile, versatile and cost-effective method for the synthesis of mono- and bilayer graphene films on copper substrate using as carbon feedstock the pyrolysis products of the conductive adhesive polymer of a commercial copper tape commonly used in electron microscopy. A copper tape with adhesive on both sides is subjected to a heat treatment during 15 min at temperatures of 900, 1000, and 1050 °C under the flow of an Ar + 3%H₂ gas mixture. With this treatment, the tape adhesive polymer is pyrolized and the interaction of its decomposition products with the copper substrate gives rise to a graphene film of good structural quality mixed with amorphous carbon residues of the pyrolysis. For a temperature of 1050 °C (few degrees below the melting point of Cu), mono- and bilayer coexisting domains of graphene are obtained with almost 100% area coverage of the Cu substrate. For lower heat treatment temperatures, area coverage is reduced to 60–70% and the graphene film becomes predominantly bilayer. The treatment at the lowest temperature of 900 °C results in isolated hexagonal domains of graphene intermixed with a large amount of amorphous carbon residues and large uncovered areas of oxidized copper substrate. These results indicate that the number of active species for the formation of graphene films increases with increasing temperature, nevertheless limited by the copper melting point. Characterization of the obtained samples was performed with scanning electron microscopy, Raman scattering, and high-resolution transmission electron microscopy.     

Substrate-free gas-phase synthesis of graphene sheets

We present a novel method for synthesizing graphene sheets in the gas phase using a substrate-free, atmospheric-pressure microwave plasma reactor. Graphene sheets were synthesized by passing liquid ethanol droplets into an argon plasma. The graphene sheets were characterized by transmission electron microscopy, electron energy loss spectroscopy, Raman spectroscopy, and electron diffraction. We prove that graphene can be created without three-dimensional materials or substrates and demonstrate a possible avenue to the large-scale synthesis of graphene.     

Chemical vapor deposition of graphene on copper from methane, ethane and propane: evidence for bilayer selectivity

To study the effects of hydrocarbon precursor gases, graphene is grown by chemical vapor deposition from methane, ethane, and propane on copper foils. The larger molecules are found to more readily produce bilayer and multilayer graphene, due to a higher carbon concentration and different decomposition processes. Single- and bilayer graphene can be grown with good selectivity in a simple, single-precursor process by varying the pressure of ethane from 250 to 1000 mTorr. The bilayer graphene is AB-stacked as shown by selected area electron diffraction analysis. Additionally propane is found to only produce a combination of single- to few-layer and turbostratic graphene. The percent coverage is investgated using Raman spectroscopy and optical, scanning electron, and transmission electron microscopies. The data are used to discuss a possible mechanism for the second-layer growth of graphene involving the different cracking pathways of the hydrocarbons.     

Layer-by-layer thinning of graphene by plasma irradiation and post-annealing

A simple and efficient method of thinning graphene with an accuracy of a single layer is proposed, which includes mild nitrogen plasma irradiation and annealing in Ar/O2. On the basis of our data, plasma irradiation induces damages in the top-layer graphene and the annealing removes the damaged layer by fast oxidation. The process was used to turn bilayer graphene into monolayer as well as thin multilayer graphene layer-by-layer via repeated utilization. Combined with electron beam lithography, patterns were fabricated by selectively removing graphene planes. The thinned graphene possesses good quality verified by atomic force microscopic investigation and Raman analysis. The process presented here offers a very useful post-synthesis manipulation of graphene thickness, which may find important applications for graphene-based device fabrication.     

Synthesis of few-layer graphene via microwave plasma-enhanced chemical vapour deposition

If graphene is ever going to live up to the promises of future nanoelectronic devices, an easy and cheap route for mass production is an essential requirement. A way to extend the capabilities of plasma-enhanced chemical vapour deposition to the synthesis of freestanding few-layer graphene is presented. Micrometre-wide flakes consisting of four to six atomic layers of stacked graphene sheets have been synthesized by controlled recombination of carbon radicals in a microwave plasma. A simple and highly reproducible technique is essential, since the resulting flakes can be synthesized without the need for a catalyst on the surface of any substrate that withstands elevated temperatures up to 700?°C. A thorough structural analysis of the flakes is performed with electron microscopy, x-ray diffraction, Raman spectroscopy and scanning tunnelling microscopy. The resulting graphene flakes are aligned vertically to the substrate surface and grow according to a three-step process, as revealed by the combined analysis of electron microscopy and x-ray photoelectron spectroscopy.     

Surface topography and hydrogen sensor response of APCVD grown multilayer graphene thin films

Atmospheric pressure chemical vapour deposition was employed to deposit graphene thin films on thermally oxidized p-silicon substrates. Raman spectroscopy and energy dispersive spectroscopy revealed the multilayer nature and the composition of the grown graphene films respectively. The defective nature and the defect density of the graphene films were determined from the Raman experiments. Field effect scanning electron microscopy, transmission electron microscopy and atomic force microscopy were used to study the surface morphology of the multilayer graphene films. The film topography was sensitive to temperature and time of growth. A suitable growth mechanism has been proposed to explain the topographical observations. The large surface area of the multilayer films was found to be suitable for hydrogen sensor applications and the sensing results were correlated with the morphology of the grown films.     

High-precision thickness regulation of graphene layers with low energy helium plasma implantation

In this paper we present a novel method of regulation to obtain graphene layers with homogeneous thickness by means of helium plasma implantation. The obtained graphene layers show neither large deep pits nor loss of lateral dimension. The etching rate can be precisely controlled (one to six atomic layers?min(-1) or higher) and it remains consistent regardless of the thickness of the multilayer graphene. This approach is compatible with traditional complementary metal-oxide-semiconductor fabrication technologies and has great potential to modulate the performance of graphene for device applications.     

Characterization of multilayer graphene prepared from short-time processed graphite oxide flake

Multilayer graphene has been prepared by thermal reduction of graphene oxide film. The graphite oxide flake was first synthesized by using modified Hummers method with a relatively small amount of oxidizing agent and short-time processing at ambient temperature. The graphite oxide flake was dispersed in deionized water and deposited on quartz substrates to form graphene oxide film. The red shift of absorption peak and decrease of interlayer distance as interpreted from the X-ray diffraction spectrum indicate the formation of multilayer graphene. The resultant multilayer graphene has been successfully used as counter electrode in FTO/ZnO nanorods/electrolyte/multilayer graphene dye sensitized solar cell.     

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 sp² 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.     

Few-layer graphene direct deposition on Ni and Cu foil by cold-wall chemical vapor deposition

We report an alternative synthesis process, cold-wall thermal chemical vapor deposition (CVD), is replied to directly deposit single-layer and few-layer graphene films on Ar plasma treated Ni and Cu foils using CH4 as carbon source. Through optimizing the process parameters, large scale single-layer graphene grown on Ni foil is comparable to that grown on Cu foil. The graphene films were able to be transferred to other substrates such as SiO2/Si, flexible transparent PET and verified by optical microscopy, Raman microscopy and scanning electron microscopy. The sheet resistance and transmission of the transferred graphene films on PET substrate were also discussed.     

Ultra-thin graphene edges at the nanowire tips: a cascade cold cathode with two-stage field amplification

A multistage field emitter based on graphene-linked ZnO nanowire array is realized by means of spin-coating a graphene dispersion (reduced graphene oxide) over a nanostructured platform followed by plasma modification. Spin-coating leads to interlinking of graphene sheets between the neighboring nanowires whereas plasma etching in the subsequent step generates numerous ultra-sharp graphene edges at the nanowire tips. The inherent tendency of graphene to lay flat over a plane substrate can easily be bypassed through the currently presented nanostructure platform based technique. The turn-on and threshold field significantly downshifted compared to the individual components in the cascade emitter. Through the facile electron transfer from nanowires to graphene due to band bending at the ZnO-graphene interface together with multistage geometrical field enhancement at both the nanowire and graphene edges remain behind this enriched field emission from the composite cold cathode. This strategy will open up a new direction to integrate the functionalities of both the graphene array and several other inorganic nanostructure array for practical electronic devices.     

Ultrastrong adhesion of graphene membranes

As mechanical structures enter the nanoscale regime, the influence of van der Waals forces increases. Graphene is attractive for nanomechanical systems because its Young's modulus and strength are both intrinsically high, but the mechanical behaviour of graphene is also strongly influenced by the van der Waals force. For example, this force clamps graphene samples to substrates, and also holds together the individual graphene sheets in multilayer samples. Here we use a pressurized blister test to directly measure the adhesion energy of graphene sheets with a silicon oxide substrate. We find an adhesion energy of 0.45±0.02 J m(-2) for monolayer graphene and 0.31±0.03 J m(-2) for samples containing two to five graphene sheets. These values are larger than the adhesion energies measured in typical micromechanical structures and are comparable to solid-liquid adhesion energies. We attribute this to the extreme flexibility of graphene, which allows it to conform to the topography of even the smoothest substrates, thus making its interaction with the substrate more liquid-like than solid-like.     

Layer number and stacking sequence imaging of few-layer graphene by transmission electron microscopy

A method based on dark field transmission electron microscopy is developed to quantitively investigate the layer number and stacking order of multilayer graphene, demonstrated here on multilayer crystalline graphene synthesized by chemical vapor deposition. Our results show that the relative intensities of first- and second-order diffraction spots and contrast in corresponding dark field images are sufficient to identify the layer number and stacking order of graphene with layer number up to seven (7) or more with few-nanometer spatial resolution.     

Facile one-step transfer process of graphene

Chemical vapour deposition (CVD) is emerging as a popular method for growing large-area graphene on metal substrates. For transferring graphene to other substrates the technique generally used involves deposition of a polymer support with subsequent etching of the metal substrate. Here we report a simpler one-step transfer process. Few-layer graphene (FLG) grown on a Cu substrate were transferred to a silanized wafer by just pressing them together. Hydrogen bonding between the hydroxyl group on FLG and the amine group on silane molecules facilitate the transfer.     

Large area tunable arrays of graphene nanodots fabricated using diblock copolymer micelles

Nanostructured graphenes such as nanoribbons, nanomeshes, and nanodots have attracted a great deal of attention in relation to graphene-based semiconductor devices. The block copolymer micellar approach is a promising bottom-up technique for generating large area nanostructures of various materials without using sophisticated electron-beam lithography. Here we demonstrate the fabrication of an array of graphene nanodots with tunable size and inter-distance with the utilization of a monolayer of diblock copolymer micelles. Au nanoparticles were synthesized in the micellar cores and effectively worked as shielding nanostructures in generating graphene nanodots by oxygen plasma etching. We also controlled the radius and inter-distance of graphene nanodots simply through the molecular weight of the copolymers.     

Synthesis and characterization of electrochemically-reduced graphene

Graphene has superior electrical conductivity than graphite and other allotropes of carbon because of its high surface area and chemical tolerance. Electrochemically processed graphene sheets were obtained through the reduction of graphene oxide from hydrazine hydrate. The prepared samples were heated to different temperatures such as 673 and 873 K. X-ray diffraction (XRD), fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDXS), transmission electron microscopy (TEM), Raman spectra and conductivity measurements were made for as-prepared and heat-treated graphene samples. XRD pattern of graphene shows a sharp and intensive peak centred at a diffraction angle (2θ) of 26·350. FTIR spectra of as-prepared and heated graphene were used to confirm the oxidation of graphite. TEM results indicated that the defect density and number of layers of graphene sheets were varied with heating temperature. The hexagonal sheet morphology and purity of as-prepared and heat treated samples were confirmed by SEM–EDX and Raman spectroscopy. The conductivity measurements revealed that the conductivity of graphene was decreased with an increase in heating temperature. The present study explains that graphene with enhanced functional properties can be achieved from the as-prepared sample.     

Ice-assisted electron beam lithography of graphene

We demonstrate that a low energy focused electron beam can locally pattern graphene coated with a thin ice layer. The irradiated ice plays a crucial role in the process by providing activated species that locally remove graphene from a silicon dioxide substrate. After patterning the graphene, the ice resist is easily removed by sublimation to leave behind a clean surface with no further processing. More generally, our findings demonstrate that ice-assisted e-beam lithography can be used to pattern very thin materials deposited on substrate surfaces. The procedure is performed in situ in a modified scanning electron microscope. Desirable structures such as nanoribbons are created using the method. Defects in graphene from electrons backscattered from the bulk substrate are identified. They extend several microns from the e-beam writing location. We demonstrate that these defects can be greatly reduced and localized by using thinner substrates and/or gentle thermal annealing.     

Gas sensing properties of graphene synthesized by chemical vapor deposition

The gas sensing properties of graphene synthesized by a chemical vapor deposition (CVD) method are investigated. Synthesis of graphene is carried out on a copper substrate using a methane and hydrogen gas mixture by a CVD process at the atmospheric pressure. The graphene films are transferred to different substrates after wet etching of the copper substrates. The Raman spectra reveal that the graphene films made on SiO₂/Si substrates are of high quality. The reflectance spectra of graphene were measured in UV/Visible region of the spectrum. Theoretically calculated reflectance spectra based on Fresnel's approach indicates that the CVD graphene has a single layer. The gas sensing properties of graphene were tested for different reducing gasses as a function of measurement temperature and gas concentration. It is found that the gas sensing characteristics such as response time, recovery time, and sensitivity depend on the target gas, gas concentration, test temperature, and the ambient gas composition. The cross sensitivity of few combinations of reducing gasses such as, NH₃, CH_4, and H₂ was also investigated.