Thick Graphene Research Articles

Effects of reduction process and carbon nanotube content on the supercapacitive performance of flexible graphene oxide papers

The effects of the reduction process and carbon nanotube (CNT) content on the supercapacitive behavior of electrodes made from flexible, binder-free thick graphene oxide (GO) papers are studied. It is found that the supercapacitive performance depends on several factors, including the presence of oxygenated functional groups after reduction, the interlayer spacing of the GO papers and their wettability with electrolyte. A moderate reduction of GO papers using hydrazine or annealing at a low temperature of 220°C in air is proven to be more beneficial to achieve a high capacitance than the heavy reduction using a hydrazine vapor or a high temperature thermal treatment. The addition of a small amount of CNT, typically 12.5wt.%, to form thick GO/CNT sandwich papers gives rise to an excellent specific capacitance of 151Fg−1 at a current density of 0.5Ag−1, as well as a retention ratio of 86% of the initial value after 6000 charge/discharge cycles at 5Ag−1. These improvements arise from the synergistic effects of the increased electronic conductivity and effective surface area associated with large electrochemical active sites due to the presence of intercalated CNT.

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.

Direct Growth Properties of Graphene Layers on Sapphire Substrate by Alcohol-Chemical Vapor Deposition

Few nanometers thick graphene layers were directly grown on a-plane (112̄0) sapphire substrates by alcohol-chemical vapor deposition (alcohol-CVD) using ethanol as a carbon source and without any catalytic metal on the substrate surface. The growth relationship between the graphene layer and substrate was analyzed using a transmission electron microscope (TEM). The growth rate of graphene layers with different growth temperatures revealed that the Al atom act as a catalyst for synthesizing a graphitic material during the decomposition of ethanol. An optical transmittance and a sheet resistance of the graphene sheet directly grown on sapphire substrate were observed. SiO2/Si and n-6H-SiC substrates were also examined for graphene direct growth to discuss the catalytic behavior of Si atoms compared with Al atoms.

Strong Charge-Transfer Doping of 1 to 10 Layer Graphene by NO2

We use resonance Raman and optical reflection contrast methods to study charge transfer in 1-10 layer (1L-10L) thick graphene samples on which NO(2) has adsorbed. Electrons transfer from the graphene to NO(2), leaving the graphene layers doped with mobile delocalized holes. Doping follows a Langmuir-type isotherm as a function of NO(2) pressure. Raman and optical contrast spectra provide independent, self-consistent measures of the hole density and distribution as a function of the number of layers (N). At high doping, as the Fermi level shift E(F) reaches half the laser photon energy, a resonance in the graphene G mode Raman intensity is observed. We observe a decrease of graphene optical absorption in the near-IR that is due to hole-doping. Highly doped graphene is more optically transparent and much more electrically conductive than intrinsic graphene. In thicker samples, holes are effectively confined near the surface, and in these samples, a small band gap opens near the surface. We discuss the properties and versatility of these highly charge-transfer-doped, few-layer-thick graphene samples as a new class of electronic materials.

Synthesis of free standing carbon nanosheet using electron cyclotron resonance plasma enhanced chemical vapor deposition

Carbon nanosheets (CNSs) have been synthesized by electron cyclotron resonance (ECR) plasma enhanced chemical vapor deposition (PECVD) using a mixture of acetylene and argon gases on copper foil as the substrate. Micrometer-wide carbon sheets consisting of several atomic layers thick graphene sheets have been synthesized by controlled decomposition of carbon radicals in ECR-PECVD. Raman spectroscopy of these films revealed characteristics of a disordered graphitic sheet. Thick folded carbon-sheets and a semi transparent freestanding CNSs have been observed by scanning electron microscopy. This is a promising technique to synthesize free standing CNSs and can be used in the fabrication of nanoelecronic devices in future.

Role of structure of C-terminated4H-SiC(0001¯) surface in growth of graphene layers: Transmission electron microscopy and density functional theory studies

Principal structural defects in graphene layers, synthesized on a carbon-terminated face, i.e. the SiC(000) face of a 4H-SiC substrate, are investigated using microscopic methods. Results of high-resolution transmission electron microscopy (HRTEM) reveal their atomic arrangement. Mechanism of such defects creation, directly related to the underlying crystallographic structure of the SiC substrate, is elucidated. The connection between the 4H-SiC(000) surface morphology, including the presence of the single atomic steps, the sequences of atomic steps, and also the macrosteps, and the corresponding emergence of planar defective structure (discontinuities of carbon layers and wrinkles) is revealed. It is shown that disappearance of the multistep island leads to the creation of wrinkles in the graphene layers. The density functional theory (DFT) calculation results show that the diffusion of both silicon and carbon atoms is possible on a Si-terminated SiC surface at a high temperature close to 1600{\deg}C. The creation of buffer layer at the Si-terminated surface effectively blocks horizontal diffusion, preventing growth of thick graphene layer at this face. At the carbon terminated SiC surface, the buffer layer is absent leaving space for effective horizontal diffusion of both silicon and carbon atoms. DFT results show that excess carbon atoms converts a topmost carbon layer to sp2 bonded configuration, liberating Si atoms in barrierless process. The silicon atoms escape through the channels created at the bending layers defects, while the carbon atoms are incorporated into the growing graphene layers. These results explain growth of thick graphene underneath existing graphene cover and also the creation of the principal defects at the C-terminated SiC(0001) surface

A simple way of improving graphite nanoplatelets (GNP) for their incorporation into a polymer matrix

A simple method of solvent exfoliation/refining of direct-graphite nanoplatelets for their better incorporation into a polymer matrix is presented. We demonstrate the method for polystyrene. The method relies on sonication in N-methyl-2-pyrrolidone solvent, with surfactant assistance. A small amount of polystyrene is added to the solvent in order to increase the viscosity, this enhancing the exfoliation process and resulting in formation of a polymeric layer on graphene for its better incorporation in the polymer matrix. Polystyrene-coated thin graphene stacks form a stable dispersion, while thicker graphite nanoplatelets settle out. Thus bulk separation of thin and thick graphene stacks takes place. The polystyrene-coated thin graphene stacks are studied using Transmission Electron Microscopy in two ways: (i) we calculate the number of graphene layers forming thin graphene stacks, and (ii) we employ Selected Area Diffraction to confirm our image analysis results by checking the intensity ratio (1100) and (2100) deflections in the diffraction patterns. Five layers is found to be the cut-off number, that is there are no stacks >5 layers, and 3 layer stacks are dominantly present. The average largest in-plane dimension is found to be approximately 2.5 mu m (reduction by 50%).

Graphene sheets as anode materials for Li-ion batteries: preparation, structure, electrochemical properties and mechanism for lithium storage

Varied graphene sheets were prepared from the graphite oxide (GO) with different degrees of oxidation and furthermore their structural characteristics and electrochemical properties as anode materials for Li-ion batteries were investigated. From the expandable graphite with a low oxidation level, the obtained graphene sheets had a thick and intact sheet structure with good crystallinity. Its specific surface area was quite low and no porous structure was detected. The graphene sheets prepared from the GO precursor with a high degree of oxidation were quite thin and disordered, along with high specific surface area and plenty of pores. These ultrathin graphene sheets demonstrated high reversible capacity mainly in the way of lithium absorption, where the specific surface area was the key structural parameter. The thick graphene sheets prepared from the expandable graphite had good crystallinity with few defects and pores, and had a similar lithium storage mechanism to graphite, whereby lithium storage is carried out by intercalation reactions.

Layer Number Determination and Thickness-Dependent Properties of Graphene Grown on SiC

The electronic properties of few-layer graphene grown on the carbon-face of silicon carbide (SiC) are found to be strongly dependent on the number of layers. The carrier mobility is larger in thicker graphene because substrate-related scattering is reduced in the higher layers. The carrier density dependence of the mobility is qualitatively different in thin and thick graphene, with the transition occurring at about 2 layers. The mobility increases with carrier density in thick graphene, similar to multi-layer graphene exfoliated from natural graphite, suggesting that the individual layers are still electrically coupled in spite of reports recording non-Bernal stacking order in C-face grown graphene. The Hall coefficient peak value is reduced in thick graphene due to the increased density of states. A reliable and rapid characterization tool for the layer number is therefore highly desirable. To date, AFM height determination and Raman scattering are typically used since the optical contrast of graphene on SiC is weak. However, both methods suffer from low throughput. We show that the scanning electron microscopy (SEM) contrast can give similar results with much higher throughput.

Observation and interpretation of adjacent Moire patterns of different shapes in bilayer graphene

In transmission electron micrography of few-layer thick graphene samples, two distinct regions, a region of superlattice and an adjacent region of parallel straight bands, are seen. These two features are explained as Moire patterns produced by (1) rotation of top part of one of the graphene layers and (2) a small change in the shape of the bottom part of the same layer. It is interesting to note that for the first time, Moire pattern of parallel straight bands is observed and satisfactorily explained.

Graphene — An exciting two-dimensional material for science and technology

One atom thick graphene is derived from graphite and is a new material; however, graphite has been a part of human history for centuries. In this article we discuss why it generates so much excitement in a wide variety of scientific disciplines. We emphasize its electronic and mechanical properties with an eye towards applications that may impact our lives sooner, rather than later. We also review methods to make this wonder material, including the famous ‘scotch-tape’ technique that led to the Nobel-Prize winning research.

Fabrication of multiwalled carbon nanotubes in the channels of iron loaded three dimensional mesoporous material by catalytic chemical vapour deposition technique

The growth of multiwalled carbon nanotubes (MWNTs) was successfully achieved in the channels of three dimensional (3D) iron loaded mesoporous matrices (KIT-6) by employing catalytic chemical vapour deposition (CCVD) technique. The synthesised MWNTs, which were characterised by SEM, TEM and Raman spectroscopy, consist of thick graphene layers of about 10nm composed of 29 graphene sheets with inner and outer diameter of ∼17nm and ∼37nm, respectively. The Raman spectrum showed the formation of well-graphitised MWNTs with significantly higher IG/ID ratio of 1.47 compared to commercial MWNTs. Comparatively, 2wt% Fe loaded KIT-6 material produced a better yield of 91%, which is also highest compared with the report of MWNTs synthesis using mesoporous materials reported so far.

Transparent conductive graphene electrode in GaN-based ultra-violet light emitting diodes

We report a graphene-based transparent conductive electrode for use in ultraviolet (UV) GaN light emitting diodes (LEDs). A few-layer graphene (FLG) layer was mechanically deposited. UV light at a peak wavelength of 368 nm was successfully emitted by the FLG layer as transparent contact to p-GaN. The emission of UV light through the thin graphene layer was brighter than through the thick graphene layer. The thickness of the graphene layer was characterized by micro-Raman spectroscopy. Our results indicate that this novel graphene-based transparent conductive electrode holds great promise for use in UV optoelectronics for which conventional ITO is less transparent than graphene.

DNA Translocation through Graphene Nanopores

We report on DNA translocations through nanopores created in graphene membranes. Devices consist of 1-5 nm thick graphene membranes with electron-beam sculpted nanopores from 5 to 10 nm in diameter. Due to the thin nature of the graphene membranes, we observe larger blocked currents than for traditional solid-state nanopores. However, ionic current noise levels are several orders of magnitude larger than those for silicon nitride nanopores. These fluctuations are reduced with the atomic-layer deposition of 5 nm of titanium dioxide over the device. Unlike traditional solid-state nanopore materials that are insulating, graphene is an excellent electrical conductor. Use of graphene as a membrane material opens the door to a new class of nanopore devices in which electronic sensing and control are performed directly at the pore.

Transmission electron microscopy investigations of epitaxial graphene on C-terminated 4H–SiC

Transmission electron microscopy (TEM) investigations of epitaxial graphene, grown on on-axis and 8° off-axis C-terminated 4H–SiC (0001¯) surfaces are presented. The TEM results provide evidence that the first carbon layer is separated by 3.2 Å from the C-terminated SiC surface. It was also found that thick graphene layers grown on on-axis SiC (0001¯) are loosely bound to the SiC substrate. Moreover, the structural observations reveal a certain degree of disorder between the graphene planes, which manifests itself in a rotation of the layers and in an increase in the interplanar spacing between certain carbon layers from 3.35 Å, which is characteristic for graphite, up to 3.7 Å. Graphene grown on 8° off-axis SiC (0001¯) substrates covers the steps of SiC and as a result disorder seems to be not as pronounced as it is on the on-axis SiC (0001¯) substrate.

In situ synthesis of graphene oxide and its composites with iron oxide

A single step method was developed for the preparation of graphene oxide/Fe 2O 3 composites by exfoliation of graphite oxide with an oxygen-rich ferric acetyl acetonate complex. The synthesized materials were characterized by Fourier transform infrared spectroscopy, X-ray diffraction, vibrating sample magnetometry, atomic force microscopy, and low temperature dc conductivity measurements. After exfoliation, the Fourier transform infrared studies reveal the decomposition of epoxy groups and the simultaneous formation of composites of iron oxide particles within the graphene oxide. Atomic force microscopy reveals the formation of an ∼5nm thick graphene oxide stack with layered morphology. The layered morphology of graphene oxide degrades at higher concentrations of iron oxide. The vibrating sample magnetometer studies show ferromagnetic behavior of all composites in the range of 0.13–5.5 emu/g at room temperature. The conductivity of these composites is found to be governed by a quasi 1D hopping mechanism.

Massive Dirac fermion transport in a gapped graphene-based magnetic tunnel junction

The spin transport in a graphene-based magnetic (NG/ferromagnetic barrier (FB)/NG) tunnel junction with the graphene sheet being grown on a SiC substrate is investigated. Zhou et al. [Nat. Mater. 6 (2007) 770] has shown that in these epitaxial grown graphene sheets, the electrons behave like massive relativistic particles with an energy gap of 2 Δ∼260 meV opening up in the energy spectrum of the massive relativistic electron. Basing on assumption that gap in graphene can occur under the influence of the magnetic field, we find that in the case of thick ferromagnetic graphene barriers, the electronic gap causes the barrier to behave as a strong insulator when the gate potential is in the range 400−130 meV< V G <400+130 meV (and the energy level E f∼400 meV above Dirac point). For these values of V G , the spin polarization of the junction is P(%)∼100% except for V G = E f , where it is P(%)=0%. The current of the junction, for thick FB, can be rapidly switched from a 100% spin up current to a 100% spin down current by small variation of V G from V G < E f to V G > E f , the features of a perfect spin filtering electronic junction.

Optical constants of graphene layers in the visible range

We show that the optical constants of graphene in the visible range can be estimated by means of a very simple procedure involving their consistence with universal optical conductivity and experimentally measured optical spectra, within the framework of Fresnel coefficients calculation. The obtained complex refractive index allows for accurate prediction of the optical behavior of graphene in the visible range, from the two-dimensional limit (single atomically thick graphene layer) to the bulk limit (graphite). Therefore, it may result very useful for quantitative optical analysis of graphene layers and graphitic structures in general.

Synthesis of high-quality graphene with a pre-determined number of layers

A simple and effective strategy is proposed to tune the number of graphene layers by selecting suitable starting graphite, using a chemical exfoliation method. It is found that both the lateral size and the crystallinity of the starting graphite play important roles in the number of graphene layers obtained. Using artificial graphite, flake graphite powder, Kish graphite, and natural flake graphite as starting materials, ∼80% of the final products are single-layer, single- and double-layer, double- and triple-layer, and few-layer (4–10 layers) graphene, respectively, while a mixture of few-layer (4–10 layers) and thick graphene (>10 layers) is obtained when highly-oriented pyrolytic graphite is used. The smaller the lateral size and the lower the crystallinity of the starting graphite, the fewer the number of graphene layers obtained. Moreover, the graphenes obtained are of high-quality with an electrical conductivity of ∼1 × 10 3 S/cm. These findings open up the possibility for controlled production of high-quality graphene with a selected number of layers in a large quantity.

Graphene Layers on Silicon Carbide Studied by Raman Spectroscopy

We present a micro-Raman spectroscopy study on single- and few layer graphene (FLG) grown on the silicon terminated surface of 6H-silicon carbide (SiC). On the basis of the 2D-line (light scattering from two phonons close to the K-point in the Brillouin zone) we distinguish graphene mono- from bilayers or few layer graphene. Monolayers have a 2D-line consisting of only one component, whereas more than one component is observed for thicker graphene layers. Compared to the graphite the monolayer graphene lines are shifted to higher frequencies. We tentatively ascribe the corresponding phonon hardening to strain in the first graphene layer.