Bulk Graphene Research Articles

Synthesis of Graphene Oxide (GO) by Modified Hummers Method and Its Thermal Reduction to Obtain Reduced Graphene Oxide (rGO)*

Over the span of years, improvements over various synthesis methods of graphene are constantly pursued to provide safer and more effective alternatives. Though the extraction of graphene through Hummers method is one of the oldest techniques yet it is one of the most suitable methods for the formation of bulk graphene. Graphene can be obtained in the form of reduced Graphite oxide, sometimes also referred as Graphene oxide. The effectiveness of this oxidation process can be evaluated by the magnitude of carbon/oxygen ratio of the obtained graphene. Here, graphene oxide (GO) was prepared by oxidizing the purified natural flake graphite (NFG) by a modified Hummers method. The attempts have been made to synthesize GO having few layers by using a modified Hummers method where the amount of NaNO3 has been decreased, and the amount of KMnO4 is increased. The reaction has been performed in a 9:1 (by volume) mixture of H2SO4/H3PO4. This modification is successful in increasing the reaction yield and reducing the toxic gas evolution while using a varied proportion of KMnO4 and H2SO4 as those required by Hummers method. A new component of K2S2O8 has been introduced to the reaction system to maintain the pH value. Reduced graphene oxide (rGO) was thereafter extracted by thermal modification of GO. Here, GO has been used as a precursor for graphene synthesis by thermal reduction processes. The results of FTIR and Raman spectroscopy analysis show that the NFG when oxidized by strong oxidants like KMnO4 and NaNO3, introduced oxygen atoms into the graphite layers and formed bonds like C=O, C-H, COOH and C-O-C with the carbon atoms in the graphite layers. The structure and morphology of both GO and rGO were analyzed using X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HR-TEM), Fourier transform infrared spectroscopy (FTIR), ultraviolet-visible spectroscopy, Raman spectroscopy, Brunauer-Emmett-Teller (BET) surface area analysis and differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA).

Graphene pn junction in a quantizing magnetic field: Conductance at intermediate disorder strength

In a graphene {\em pn} junction at high magnetic field, unidirectional "snake states" are formed at the {\em pn} interface. In a clean {\em pn} junction, each snake state exists in one of the valleys of the graphene band structure, and the conductance of the junction as a whole is determined by microscopic details of the coupling between the snake states at the {\em pn} interface and quantum Hall edge states at the sample boundaries [Tworzydlo {\em et al.}, Phys. Rev. B {\bf 76}, 035411 (2007)]. Disorder mixes and couples the snake states. We here report a calculation of the full conductance distribution in the crossover between the clean limit and the strong disorder limit, in which the conductance distribution is given by random matrix theory [Abanin and Levitov, Science {\bf 317}, 641 (2007)]. Our calculation involves an exact solution of the relevant scaling equation for the scattering matrix, and the results are formulated in terms of parameters describing the microscopic disorder potential in bulk graphene.

Transmission spectra and valley processing of graphene and carbon nanotube superlattices with inter-valley coupling

We numerically investigate the electronic transport properties of graphene nanoribbons and carbon nanotubes with inter-valley coupling, e.g., in and superlattices. By taking the graphene superlattice as an example, we show that tailoring the bulk graphene superlattice results in rich structural configurations of nanoribbons and nanotubes. After studying the electronic characteristics of the corresponding armchair and zigzag nanoribbon geometries, we find that the linear bands of carbon nanotubes can lead to the Klein tunnelling-like phenomenon, i.e., electrons propagate along tubes without backscattering even in the presence of a barrier. Due to the coupling between K and valleys of pristine graphene by supercells, we propose a valley-field-effect transistor based on the armchair carbon nanotube, where the valley polarization of the current can be tuned by applying a gate voltage or varying the length of the armchair carbon nanotubes.

Chebyshev polynomial filtered subspace iteration in the discontinuous Galerkin method for large-scale electronic structure calculations.

The Discontinuous Galerkin (DG) electronic structure method employs an adaptive local basis (ALB) set to solve the Kohn-Sham equations of density functional theory in a discontinuous Galerkin framework. The adaptive local basis is generated on-the-fly to capture the local material physics and can systematically attain chemical accuracy with only a few tens of degrees of freedom per atom. A central issue for large-scale calculations, however, is the computation of the electron density (and subsequently, ground state properties) from the discretized Hamiltonian in an efficient and scalable manner. We show in this work how Chebyshev polynomial filtered subspace iteration (CheFSI) can be used to address this issue and push the envelope in large-scale materials simulations in a discontinuous Galerkin framework. We describe how the subspace filtering steps can be performed in an efficient and scalable manner using a two-dimensional parallelization scheme, thanks to the orthogonality of the DG basis set and block-sparse structure of the DG Hamiltonian matrix. The on-the-fly nature of the ALB functions requires additional care in carrying out the subspace iterations. We demonstrate the parallel scalability of the DG-CheFSI approach in calculations of large-scale two-dimensional graphene sheets and bulk three-dimensional lithium-ion electrolyte systems. Employing 55 296 computational cores, the time per self-consistent field iteration for a sample of the bulk 3D electrolyte containing 8586 atoms is 90 s, and the time for a graphene sheet containing 11 520 atoms is 75 s.

Preparation of Bulk Graphene Nanoplatelets by Spark Plasma Sintering — Electrical and Thermal Properties

Consolidation of graphene nanoplatelets (GNPs) by spark plasma sintering (SPS) to study the feasibility of its structure retention at extreme temperature and pressure conditions. Structural characterization of the GNP powder and pellet were carried out by Micro-Raman, SEM, and TEM. HT-XRD. A.C. and D.C. conductivity of GNP pellet is carried out at room temperature. GNPs survived its structure in the SPS processing at an extreme temperature of 1850[Formula: see text]C and uni-axial pressure 60[Formula: see text]MPa, vacuum at [Formula: see text] Torr. Our study shows the potential for GNPs to be successfully used as a reinforcing in ceramic matrix composites using SPS. The diffraction has been accurately calibrated to waterfall the shift in 2[Formula: see text] values at elevated temperatures. The corrected lattice parameter data have been used to estimate the instantaneous and mean thermal expansion coefficients as a function of temperature. The lattice parameters “[Formula: see text]” and “[Formula: see text]” for powder and pellet GNP is found to be 0.2456(1)[Formula: see text]nm and 0.6700(2)[Formula: see text]nm, respectively. The thermal expansivity of GNP powder and pellet along “[Formula: see text]”- and “[Formula: see text]”-axis are found to be [Formula: see text][Formula: see text]K[Formula: see text], [Formula: see text][Formula: see text]K[Formula: see text] and [Formula: see text][Formula: see text]K[Formula: see text], [Formula: see text][Formula: see text]K[Formula: see text], respectively. Electrical conductivity of GNP pellet is found to be 5700[Formula: see text]S/m.

Coulomb charging energy of vacancy-induced states in graphene

Vacancies in graphene have been proposed to give rise to $\pi$-like magnetism in carbon materials, a conjecture which has been supported by recent experimental evidence. A key element in this "vacancy magnetism" is the formation of magnetic moments in vacancy-induced electronic states. In this work we compute the charging energy $U$ of a single-vacancy generated localized state for bulk graphene and graphene ribbons. We use a tight-binding model to calculate the dependency of the charging energy $U$ on the amplitudes of the localized wave function on the graphene lattice sites. We show that for bulk graphene $U$ scales with the system size $L$ as $(\ln L)^{-2}$, confirming the predictions in the literature, based on heuristic arguments. In contrast, we find that for realistic system sizes $U$ is of the order of eV, a value that is orders of magnitude higher than the previously reported estimates. Finally, when edges are considered, we show that $U$ is very sensitive to the vacancy position with respect to the graphene flake boundaries. In the case of armchair nanoribbons, we find a strong enhancement of $U$ in certain vacancy positions as compared to the value for vacancies in bulk graphene.

Electrostatically Confined Monolayer Graphene Quantum Dots with Orbital and Valley Splittings.

The electrostatic confinement of massless charge carriers is hampered by Klein tunneling. Circumventing this problem in graphene mainly relies on carving out nanostructures or applying electric displacement fields to open a band gap in bilayer graphene. So far, these approaches suffer from edge disorder or insufficiently controlled localization of electrons. Here we realize an alternative strategy in monolayer graphene, by combining a homogeneous magnetic field and electrostatic confinement. Using the tip of a scanning tunneling microscope, we induce a confining potential in the Landau gaps of bulk graphene without the need for physical edges. Gating the localized states toward the Fermi energy leads to regular charging sequences with more than 40 Coulomb peaks exhibiting typical addition energies of 7–20 meV. Orbital splittings of 4–10 meV and a valley splitting of about 3 meV for the first orbital state can be deduced. These experimental observations are quantitatively reproduced by tight binding calculations, which include the interactions of the graphene with the aligned hexagonal boron nitride substrate. The demonstrated confinement approach appears suitable to create quantum dots with well-defined wave function properties beyond the reach of traditional techniques.

Tough and Thermosensitive Poly(N-isopropylacrylamide)/Graphene Oxide Hydrogels with Macroscopically Oriented Liquid Crystalline Structures.

Bulk graphene oxide (GO) nanocomposite materials with macroscopically oriented GO liquid crystalline (LC) structures exhibit interesting anisotropic properties, but their facile preparations remain challenging. This work reports for the first time the facile preparation of poly(N-isopropylacrylamide) (PNIPAM)/GO nanocomposite hydrogels with macroscopically oriented LC structures with the assistance of a flow field induced by vacuum degassing and the in situ polymerization accelerated by GO. The hydrogel prepared with a GO concentration of 5.0 mg mL(-1) exhibits macroscopically aligned LC structures, which endow the gels with anisotropic optical, mechanical properties, and dimensional changes during the phase transition. The hydrogels show dramatically enhanced tensile mechanical properties and phase transition rates. The oriented LC structures are not damaged during the phase transition of the PNIPAM/GO hydrogels, and hence their LC behavior undergoes reversible change. Moreover, highly oriented LC structures can also be formed when the gels are elongated, even for the gels which do not have macroscopically oriented LC structures. Very impressively, the oriented LC structures in the hydrogels can be permanently maintained by drying the gel samples elongated to and then kept at a constant tensile strain. The thermosensitive nature of PNIPAM and the angle-dependent nature of the macroscopically aligned GO LC structures allow the practical applications of the PNIPAM/GO hydrogels as optical switches, soft sensors, and actuators and so on.

Size quantization of Dirac fermions in graphene constrictions.

Quantum point contacts are cornerstones of mesoscopic physics and central building blocks for quantum electronics. Although the Fermi wavelength in high-quality bulk graphene can be tuned up to hundreds of nanometres, the observation of quantum confinement of Dirac electrons in nanostructured graphene has proven surprisingly challenging. Here we show ballistic transport and quantized conductance of size-confined Dirac fermions in lithographically defined graphene constrictions. At high carrier densities, the observed conductance agrees excellently with the Landauer theory of ballistic transport without any adjustable parameter. Experimental data and simulations for the evolution of the conductance with magnetic field unambiguously confirm the identification of size quantization in the constriction. Close to the charge neutrality point, bias voltage spectroscopy reveals a renormalized Fermi velocity of ∼1.5 × 106 m s−1 in our constrictions. Moreover, at low carrier density transport measurements allow probing the density of localized states at edges, thus offering a unique handle on edge physics in graphene devices.

(Invited) Plasma-Based Technology for Mass Production of Graphene and Other 2D Layered Materials

Arc discharge plasma is as one of the most practical and efficient methods to synthesize various carbon nanostructures such as single-walled carbon nanotubes (SWCNT) and graphene with minimal defects and large yield. This is achieved due to the relatively high synthesis temperature and eco-friendly growth mechanism. Moreover, by introducing a magnetic field during synthesis process, large-scale graphene and high-purity SWCNT can be obtained in one step. In addition, the yield of graphene can be controlled by external parameters, such as the type and pressure of buffer gas, the temperature of substrate, and plasma density and temperature. Bulk graphene (graphene platelets, flakes) is usually characterized by several layer (2-20 nm) graphene pieces having characteristics sizes of about microns to ten microns. Currently, the main method for synthesis of bulk graphene is chemical exfoliation, where chemicals are utilized to separate the graphene sheets from the piece of graphite. Current production capability are quite limited and estimated to be around several tens of tons of material annually worldwide. This work is focused on development of novel highly efficient and low cost plasma-based method for graphene production in industrial quantities characterized by exceptional product properties. The synthesis is based on utilization of arc discharge supported by ablation of anode material at atmospheric pressures. Pure carbon anode (several millimeters in diameter) and arc current in the range 50-100A were utilized for the synthesis. The graphene was synthesized on the metal substrate and was mechanically remove using metallic brush immediately after the synthesis. The synthesized samples were characterized using SEM, TEM and Raman spectroscopy. Typical size of the graphene platelets was found to be about several microns. The graphene platelets down to ~2 layers were obtained. We present state-of-the-art experimental data on various attempts to employ the arc plasma technique for the graphene synthesis and consider growth mechanisms including precipitation, surface-catalyzed processes and a substrate-independent approach. The potential of arc synthesis for the growth of other types of 2D materials and future prospects are discussed.

Quasi-Continuously Tuning the Size of Graphene Quantum Dots via an Edge-Etching Mechanism

Graphene quantum dots (GQDs), a nano version of graphene whose interesting properties that distinguish them from bulk graphene, have recently received significant scientific attention. The quantum confinement effect referring to the size-dependence of physical and chemical properties opens great possibility in the practical applications of this material. However, tuning the size of graphene quantum dots is still difficult to achieve. Here, an edge-etching mechanism which is able to tune the size of GQDs in a quasi-continuous manner is discovered. Different from the ‘unzipping’ mechanism which has been adopted to cut bulk graphitic materials into small fragments and normally cut through the basal plane along the ‘zig-zag’ direction where epoxy groups reside, the mechanism discovered in this research could gradually remove the peripheral carbon atoms of nano-scaled graphene (i.e. GQDs) due to the higher chemical reactivity of the edge carbon atoms than that of inner carbon atoms thereby tuning the size of GQDs in a quasi-continuous fashion. It enables the facile manipulate of the size and properties of GQDs through controlling merely the reaction duration. It is also believed the as discovered mechanism could be generalized for synthesizing various sizes of GQDs from other graphitic precursors (e.g. carbon fibres, carbon nanotubes, etc).

The Electronic Thermal Conductivity of Graphene.

Graphene, as a semimetal with the largest known thermal conductivity, is an ideal system to study the interplay between electronic and lattice contributions to thermal transport. While the total electrical and thermal conductivity have been extensively investigated, a detailed first-principles study of its electronic thermal conductivity is still missing. Here, we first characterize the electron-phonon intrinsic contribution to the electronic thermal resistivity of graphene as a function of doping using electronic and phonon dispersions and electron-phonon couplings calculated from first-principles at the level of density-functional theory and many-body perturbation theory (GW). Then, we include extrinsic electron-impurity scattering using low-temperature experimental estimates. Under these conditions, we find that the in-plane electronic thermal conductivity κe of doped graphene is ∼300 W/mK at room temperature, independently of doping. This result is much larger than expected and comparable to the total thermal conductivity of typical metals, contributing ∼10% to the total thermal conductivity of bulk graphene. Notably, in samples whose physical or domain sizes are of the order of few micrometers or smaller, the relative contribution coming from the electronic thermal conductivity is more important than in the bulk limit, because lattice thermal conductivity is much more sensitive to sample or grain size at these scales. Last, when electron-impurity scattering effects are included we find that the electronic thermal conductivity is reduced by 30 to 70%. We also find that the Wiedemann-Franz law is broadly satisfied at low and high temperatures but with the largest deviations of 20-50% around room temperature.

Highly Reversible and Recyclable Absorption under Both Hydrophobic and Hydrophilic Conditions using a Reduced Bulk Graphene Oxide Material.

A 3D crosslinked reduced bulk graphene oxide material with switchable absorption capability between hydrophobicity and hydrophilicity is achieved by a simple O3 and annealing treatment.

Reply to 'Do thermal effects cause the propulsion of bulk graphene material?'

Reply to 'Do thermal effects cause the propulsion of bulk graphene material?'

Do thermal effects cause the propulsion of bulk graphene material?

In a recent article, Zhang et al.1 observed the direct propulsion of a bulk graphene sponge when exposed to laser light. They attributed this to the momentum of the light-induced ejected electrons. However, the force provided by the ejected electrons is about 2.7 × 10−11 N (supposing that the average current and the kinetic energy of the ejected electrons are 9 × 10−7 A and 70 eV, respectively), which is far smaller than the gravitational force of a 0.86 mg graphene sponge. From our knowledge of rarefied gas dynamics, the horizontal, vertical and rotational motion of the laser-illuminated graphene sponge could be due to the radiometric force.

Armchair-edged nanoribbon as a bottleneck to electronic total transmission through a topologically nontrivial graphene nanojunction

It is currently a promising approach to experimentally realize the topological insulator phase transition of graphene by introducing the extrinsic spin–orbit coupling (SOC). Then, electronic total transmission through various topological nontrivial graphene nanojunctions (GNJs) is obtainable, if the electronic transport is supported by the helical edge states. Though the bulk graphene is a gapless semiconductor, the inter-valley scattering could introduce a topological trivial gap in semiconducting armchair-edged graphene nanoribbon (GNR). The SOC should be strong enough to reopen a topological nontrivial gap before close such a trivial gap. Therefore, our theoretical study indicates that a semiconducting armchair-edged graphene nanoribbon (GNR) can not develop the helical edge states when the SOC strength is lower than a threshold, though the bulk phase is topological nontrivial. This implies a competition between the SOC and the inter-valley scattering. However, for a metallic armchair-edged GNR, a small SOC can always open a nontrivial gap. Nevertheless, the helical edge state is much less localized than that in a zigzag-edged GNR of the same width. As a result, and by numerically calculating the electronic transmission spectrum of step- and L-shaped GNJs, we conclude that when an armchair-edged GNR is a part of a GNJ, it is the weak point to realize the electronic total transmission even though the bulk phase of graphene is topologically insulating.

Mixed conduction properties of pristine bulk graphene oxide

We investigated the conduction properties of pristine bulk graphene oxide (GO) with different oxidation degrees. GO with a low oxidation degree shows the electron/hole conduction behavior. Interestingly, however, GO with high oxidation degree exhibits mixed conduction behaviors (electron/hole conduction and proton conduction), depending on the water content. For GO with a high oxidation degree, electron/hole conduction dominates the electrical transport in the water-lean state, while the proton conduction dominates the electrical transport in the water-rich state. Among different oxygen-containing functional groups, the epoxide groups, which could influence the band gap, interlayer distance and water intercalation process, play a critical role in the conduction properties of GO. In electrical transport process, epoxide groups act as conduction barriers for electron/hole conduction while serving as active sites for proton conduction. Both graphite and GOs exhibit a clear anisotropic behavior in electrical transport.. These observations will provide insightful information for the applications of GO in fuel cells, humidity sensors and supercapacitors.

Supercapacitors Based on Three-Dimensional Hierarchical Graphene Aerogels with Periodic Macropores.

Graphene is an atomically thin, two-dimensional (2D) carbon material that offers a unique combination of low density, exceptional mechanical properties, thermal stability, large surface area, and excellent electrical conductivity. Recent progress has resulted in macro-assemblies of graphene, such as bulk graphene aerogels for a variety of applications. However, these three-dimensional (3D) graphenes exhibit physicochemical property attenuation compared to their 2D building blocks because of one-fold composition and tortuous, stochastic porous networks. These limitations can be offset by developing a graphene composite material with an engineered porous architecture. Here, we report the fabrication of 3D periodic graphene composite aerogel microlattices for supercapacitor applications, via a 3D printing technique known as direct-ink writing. The key factor in developing these novel aerogels is creating an extrudable graphene oxide-based composite ink and modifying the 3D printing method to accommodate aerogel processing. The 3D-printed graphene composite aerogel (3D-GCA) electrodes are lightweight, highly conductive, and exhibit excellent electrochemical properties. In particular, the supercapacitors using these 3D-GCA electrodes with thicknesses on the order of millimeters display exceptional capacitive retention (ca. 90% from 0.5 to 10 A·g(-1)) and power densities (>4 kW·kg(-1)) that equal or exceed those of reported devices made with electrodes 10-100 times thinner. This work provides an example of how 3D-printed materials, such as graphene aerogels, can significantly expand the design space for fabricating high-performance and fully integrable energy storage devices optimized for a broad range of applications.

Interfacial Shear Strength of Multilayer Graphene Oxide Films.

Graphene oxide (GO) is considered as one of the most promising layered materials with tunable physical properties and applicability in many important engineering applications. In this work, the interfacial behavior of multilayer GO films was directly investigated via GO-to-GO friction force microscopy, and the interfacial shear strength (ISS) was measured to be 5.3 ± 3.2 MPa. Based on high resolution atomic force microscopy images and the available chemical data, targeted molecular dynamics simulations were performed to evaluate the influence of functional structure, topological defects, and interlayer registry on the shear response of the GO films. Theoretical values for shear strength ranging from 17 to 132 MPa were predicted for the different structures studied, providing upper bounds for the ISS. Computational results also revealed the atomic origins of the stochastic nature of friction measurements. Specifically, the wide scatter in experimental measurements was attributed to variations in functional structure and topological defects within the sliding volume. The findings of this study provide important insight for understanding the significant differences in strength between monolayer and bulk graphene oxide materials and can be useful for engineering topological structures with tunable mechanical properties.

Electrochemical reduction of bulk graphene oxide materials

1D–3D reduced graphene materials can be prepared by means of a simple electrochemical reduction process.