Free-standing Graphene Sheets Research Articles

Mechanical response of monolayer graphene via a multi-probe approach

Push-to-pull (PTP) testing is employed to probe the uniaxial tensile response of freestanding monolayer graphene. Various analytical approaches are employed to estimate the elastic modulus of end-clamped graphene samples, combining in-situ Raman spectroscopy and scanning electronic microscope (SEM) measurements. The utilization of spatially resolved Raman-derived strains for assessing the elastic properties of monolayer graphene leads to results consistent with previous experimental and theoretical values of the elastic modulus (approximately 1 TPa). Molecular dynamics (MD) simulations of (pristine and defective) freestanding graphene sheets uniaxially loaded under varying clamping conditions are performed to support the experimental observations. The computational results indicate that the mechanical responses of the sheets are affected by the type, the spatial profile, and the heterogeneity of the clamping. When uniaxial pulling of end-clamped graphene is applied by a substrate adhering to the graphene sheet through van der Waals forces (as in PTP testing), the elastic modulus may be highly underestimated due to often inhomogeneous stress distribution and slippage processes. The MD simulations predict that the elastic modulus of pristine monolayer graphene is approximately 1 TPa, whereas its fracture strength can reach values of up to 110 GPa. Overall, this study underscores the limitations of traditional analyses of PTP experiments (utilizing indentation readouts and SEM imaging) and proposes new potential avenues (involving Raman measurements) for future research on the elastic properties of 2D materials.

Low temperature electrical transport in microwave plasma fabricated free-standing graphene and N-graphene sheets

In this paper, the electrical transport in free-standing graphene and N-graphene sheets fabricated by a microwave plasma-based method is addressed. Temperature–dependent resistivity/conductivity measurements are performed on the graphene/N-graphene sheets compressed in pellets. Different measurement configurations reveal directional dependence of current flow—the room-temperature conductivity longitudinal to the pellet’s plane is an order of magnitude higher than the transversal one, due to the preferential orientation of graphene sheets in the pellets. SEM imaging confirms that the graphene sheets are mostly oriented parallel to the pellet’s plane and stacked in agglomerates. The high longitudinal electrical conductivity with values on the order of 103 S/m should be noted. Further, the current flow mechanism revealed from resistivity-temperature dependences from 300K down to 10K shows non-metallic behavior manifested with an increasing resistivity with decreasing the temperature usually observed for insulating or localized systems. The observed charge transport shows variable range hopping at lower temperatures and thermally activated behaviour at higher temperatures. This allows us to attribute the charge transport mechanism to a partially disordered system in which single graphene sheets are placed predominantly parallel to each other and stacked together.

Single Metal Atom Catalysts and ORR: H-Bonding, Solvation, and the Elusive Hydroperoxyl Intermediate

The widely investigated oxygen reduction reaction (ORR) is well-known to proceed via two competing routes, involving two or four electrons, and yielding different reaction products, respectively. Both pathways are believed to share a common, elusive intermediate, namely, the hydroperoxyl radical. By exploiting a cobalt single-atom biomimetic model catalyst, based on a self-assembled monolayer of Co-porphyrins grown on an almost free-standing graphene sheet, we identify, in situ at room temperature in O2+H2O atmosphere, a hydroperoxyl-water cluster that is stabilized at the Co single-metal atom catalytic site. We show that the interplay between charge transfer, dipole and H-bonding, and water solvation behavior actually determines the hydroperoxyl-water complex stability, the Co-OOH bonding geometry, and, prospectively, opens to the engineered control of the selectivity of ORR pathways.

Screening in Graphene: Response to External Static Electric Field and an Image-Potential Problem.

We present a detailed first-principles investigation of the response of a free-standing graphene sheet to an external perpendicular static electric field E. The charge density distribution in the vicinity of the graphene monolayer that is caused by E was determined using the pseudopotential density-functional theory approach. Different geometries were considered. The centroid of this extra density induced by an external electric field was determined as = 1.048 Å at vanishing E, and its dependence on E has been obtained. The thus determined was employed to construct the hybrid one-electron potential which generates a new set of energies for the image-potential states.

Intercalation of Au Atoms into SiC(0001)/Buffer Interfaces-A First-Principles Density Functional Theory Study.

The process of Au intercalation into a SiC/buffer interface has been theoretically investigated here by using density functional theory (DFT) and the nudged elastic band (NEB) method. Energy barriers were at first calculated (using NEB) for the transfer of an Au atom through a free-standing graphene sheet. The graphene sheet was either of a nondefect character or with a defect in the form of an enlarged hexagonal carbon ring. Defects in the form of single and double vacancies were also considered. Besides giving a qualitative prediction of the relative energy barriers for the corresponding SiC/buffer interfaces, some of the graphene calculations also proved evidence of energy minima close to the graphene sheet. The most stable Au positions within the SiC/buffer interface were, therefore, calculated by performing geometry optimization with Au in the vicinity of the buffer layer. Based on these NEB and DFT calculations, two factors were observed to have a great influence on the Au intercalation process: (i) energy barrier and (ii) preferential bonding of Au to the radical C atoms at the edges of the vacancies. The energy barriers were considerably smaller in the presence of vacancies. However, the Au atoms preferred to bind to the edge atoms of these vacancies when approaching the buffer layer. It can thereby be concluded that the Au intercalation will only occur for a nondefect buffer layer when using high temperature and/or by using high-energy impacts by Au atoms. For this type of Au intercalation, the buffer layer will become completely detached from the SiC surface, forming a single layer of graphene with an intact Dirac point.

Free-standing N-Graphene as conductive matrix for Ni(OH)2 based supercapacitive electrodes

Free-standing N-doped graphene (NG2N1O) sheets with 2.3 at. % of nitrogen and residual oxygen content were synthesized using low-pressure microwave plasma.A composite made with Ni(OH)2 and NG2N1O was prepared by the hydrothermal route. Physico-chemical characterizations evidenced the formation of crystalline β-phase of Ni(OH)2 nanoplates interconnected with graphene nanosheets.The electrochemical results of N-graphene electrodes evidenced very good supercapacitive response with a high rate capability of 97%, negligible charge transfer resistance of 0.05 Ω cm2 and very low time constant of 50 ms. The specific capacity of the Ni(OH)2 + NG2N1O composite increased 20% compared to Ni(OH)2 (107 mAh g−1vs. 86 mAh g−1, respectively) and the rate capability was 75% at current density of 10 A g−1, higher than Ni(OH)2 which retained only 34.4%. The composite showed excellent stability, by retaining 92% of its initial specific capacity after 4000 charge-discharge cycles. Furthermore, electrochemical impedance spectroscopy evidenced that graphene decreased the charge transfer resistance and diffusional contributions while enhancing the capacitive behaviour and the high-frequency response of the electrodes.An asymmetric cell was assembled using activated carbon as negative electrode and the composite as positive electrode. The cell displayed good capacitive response in a potential window of 1.8 V, in aqueous electrolyte, stored a maximum energy density of 38.64 W h kg−1 at a power density of 450 W kg−1 and retained 16 W h kg−1 at a power density of 4.7 kW kg−1.

Shaping and Edge Engineering of Few-Layered Freestanding Graphene Sheets in a Transmission Electron Microscope.

Full exploitation of graphene's superior properties requires the ability to precisely control its morphology and edge structures. We present such a structure-tailoring approach via controlled atom removal from graphene edges. With the use of a graphitic-carbon-capped tungsten nanoelectrode as a noncontact "milling" tool in a transmission electron microscope, graphene edge atoms approached by the tool tip are locally evaporated, thus allowing a freestanding graphene sheet to be tailored with high precision and flexibility. A threshold for the tip voltage of 3.6 ± 0.4 V, independent of polarity, is found to be the determining factor that triggers the controlled etching process. The dominant mechanisms involve weakening of carbon-carbon bonds through the interband excitation induced by tunneling electrons, assisted with a resistive-heating effect enhanced by high electric field, as elaborated by first-principles calculations. In addition to the precise shape and size control, this tip-based method enables fabrication of graphene edges with specific chiralities, such as "armchair" or "zigzag" types. The as-obtained edges can be further "polished" to become entirely atomically smooth via edge evaporation/reconstruction induced by in situ TEM Joule annealing. We finally demonstrate the potential of this technique for practical uses through creating a graphene-based point electron source, whose field emission characteristics can effectively be tuned via modifying its geometry.

Microwave N2-Ar plasmas applied for N-graphene post synthesis

High level of nitrogen doping (∼25%) of free-standing graphene sheets was achieved using the remote region of a Ar-N2 microwave plasmas. The relation between the percentage of nitrogen in the plasma and the characteristics of the treated samples has been investigated. Structural changes, being mainly created at the surface of graphene, increase significantly with the percentage of nitrogen in the gas mixture. For a mixture containing 5% of N2 about 2.7 at. % of nitrogen was incorporated in the form of imide group and graphitic bonds (70%), and as N bound to sp3 carbon (30%). Increasing the nitrogen amount to 40% results in the production of samples with up to 25% of incorporated nitrogen. Formation of new types of carbon-nitride structures has also been observed. Detailed x-ray Photoelectron Spectroscopy and Transmission Electron Microscopy analysis support the hypothesis that a structure similar to β-C3N4 has been synthesized.

Facile tool for green synthesis of graphene sheets and their smart free-standing UV protective film

Green, cost-effective and scalable method was developed for synthesis of graphene sheets from sugar beet leaves. Considerable graphene sheets were synthesized through drying, and carbonization of sugar beet leaves then, exfoliation from the obtained graphite flakes using ultrasonication in presence of polyvinyl alcohol (PVA). High quality less defected and large dimensions graphene sheets of 50% yield were obtained. The mass ratios of PVA and their effect on exfoliation were studied. Free-standing transparent graphene sheets thin film was developed of an average thickness of 136 µm. The transparent free-standing graphene sheets thin film transparent to visible rays and blocked harmful UV rays by ∼70% which is among the highest reported value for UV protective thin film. To the best of our knowledge this the first time of free-standing transparent graphene sheets film prepared from renewable source and used as UV filter. This work will open new avenues for using renewable and cost-effective UV protective materials and filters.

Nanoparticle activated and directed assembly of graphene into a nanoscroll

Graphene nanoscrolls (GNS) have emerged as an important class of materials with applications in the field of lubrication, energy storage devices, and catalysis. However, the formation of GNS even with existing advanced experimental techniques could be very challenging. Here, we investigate the ability of diamond, nickel (Ni), platinum (Pt), and gold (Au) nanoparticles (NPs) in activating, guiding, and stabilizing the GNS by performing reactive molecular dynamics (RMD) simulations. NPs of certain diameters, when placed on a free-standing graphene sheet, can activate either wrapping or scrolling of the graphene. This activation is due to non-bonded interactions between graphene and NPs. However, for a complete wrap and scroll formation and its stabilization both graphene-graphene and NPNP interactions were also critical. Surface atoms of Ni and Pt NPs undergo reconstruction during the wrapping or scrolling of graphene, suggesting that the NP-graphene interactions are more favorable as compared to NPNP and graphene-graphene interactions. We have identified new energy criteria which must be satisfied to facilitate a complete GNS formation for the graphene-NP systems investigated in the present study. Overall, our study can provide a new method to design metal-graphene hybrid materials that can be used for a variety of applications.

Influence of the chemical potential on the Casimir-Polder interaction between an atom and gapped graphene or a graphene-coated substrate

We present a formalism based on first principles of quantum electrodynamics at nonzero temperature which permits to calculate the Casimir-Polder interaction between an atom and a graphene sheet with arbitrary mass gap and chemical potential, including graphene-coated substrates. The free energy and force of the Casimir-Polder interaction are expressed via the polarization tensor of graphene in (2+1)-dimensional space-time in the framework of the Dirac model. The obtained expressions are used to investigate the influence of the chemical potential of graphene on the Casimir-Polder interaction. Computations are performed for an atom of metastable helium interacting with either a free-standing graphene sheet or a graphene-coated substrate made of amorphous silica. It is shown that the impacts of the nonzero chemical potential and the mass gap on the Casimir-Polder interaction are in opposite directions by increasing and decreasing the magnitudes of the free energy and force, respectively. It turns out, however, that the temperature-dependent part of the Casimir-Polder interaction is decreased by a nonzero chemical potential, whereas the mass gap increases it compared to the case of undoped, gapless graphene. The physical explanation for these effects is provided. Numerical computations of the Casimir-Polder interaction are performed at various temperatures and atom-graphene separations.

Thermal effect in the Casimir force for graphene and graphene-coated substrates: Impact of nonzero mass gap and chemical potential

The rigorous finite-temperature QED formalism of the polarization tensor is used to study the combined effect of nonzero mass gap $m$ and chemical potential $\mu$ on the Casimir force and its thermal correction in the experimentally relevant configuration of a Au sphere interacting with a real graphene sheet or with graphene-coated dielectric substrates made of different materials. It is shown that for both a free-standing graphene sheet and for graphene-coated substrates the magnitude of the Casimir force decreases as $m$ is increased, while it increases as $\mu$ is increased, indicating that these parameters act in opposite directions. According to our results, the impact of $m$ and/or $\mu$ on the Casimir force for graphene-coated plates is much smaller than for a free-standing graphene sheet. Furthermore, computations show that the Casimir force is much stronger for graphene-coated substrates than for a free-standing graphene sample, but the thermal correction and its fractional weight in the total force are smaller in the former case. These results are applied to a differential setup that was recently proposed to observe the giant thermal effect in the Casimir force for graphene. We show that this experiment remains feasible even after taking into account the influence of the nonzero mass-gap and chemical potential of real graphene samples. Possible further applications of the obtained results are discussed.

Towards large-scale in free-standing graphene and N-graphene sheets

One of the greatest challenges in the commercialization of graphene and derivatives is production of high quality material in bulk quantities at low price and in a reproducible manner. The very limited control, or even lack of, over the synthesis process is one of the main problems of conventional approaches. Herein, we present a microwave plasma-enabled scalable route for continuous, large-scale fabrication of free-standing graphene and nitrogen doped graphene sheets. The method’s crucial advantage relies on harnessing unique plasma mechanisms to control the material and energy fluxes of the main building units at the atomic scale. By tailoring the high energy density plasma environment and complementarily applying in situ IR and soft UV radiation, a controllable selective synthesis of high quality graphene sheets at 2 mg/min yield with prescribed structural qualities was achieved. Raman spectroscopy, scanning electron microscopy, high resolution transmission electron microscopy, X-ray photoelectron spectroscopy and Near Edge X-ray-absorption fine-structure spectroscopy were used to probe the morphological, chemical and microstructural features of the produced material. The method described here is scalable and show a potential for controllable, large-scale fabrication of other graphene derivatives and promotes microwave plasmas as a competitive, green, and cost-effective alternative to presently used chemical methods.

Twinning mechanism and habit lines in monolayer-thick free-standing graphene: Theoretical predictions

Starting from the symmetry breaking stretches of the hexagonal and the rhombohedral graphite viewed as 2-lattices, we study martensite/martensite interfaces (twins) and martensite/austenite interfaces (habit lines) for a monolayer-thick free-standing graphene sheet. We do that by adopting the theoretical framework of Bhattacharya and James (1999). The outcome of our analysis consists of inequalities that should be satisfied by the components of the symmetry breaking stretches in order the twinning equation to have a solution. We also evaluate the vector describing the twin interface and the habit line. This is done for some specified film orientation and for the first breaking of symmetry for hexagonal and rhombohedral graphite which leads to orthorombic and face-diagonal monoclinic configurations, respectively. In 2 dimensions, this transformation corresponds to the hexagonal to rhombic phase transition for graphene. The second symmetry breaking for both the hexagonal and the rhombohedral graphite results to the triclinic configuration and corresponds, in 2 dimensions, to the hexagonal to oblique transformation for graphene. For the second symmetry breaking, we only report inequalities that should be satisfied by the components of the symmetry breaking stretches in order the twinning equation to have a solution, for some specific cases. Constraints necessary for the formation of tunnels and tents are also included in our analysis. Connection with what one may expect to see during experiments or when using molecular calculations is done using some representative figures.

Mapping unoccupied electronic states of freestanding graphene by angle-resolved low-energy electron transmission

We report angle-resolved electron transmission measurements through freestanding graphene sheets in the energy range of 18 to 30 eV above the Fermi level. The measurements are carried out in a low-energy electron point source microscope, which allows simultaneously probing the transmission for a large angular range. The characteristics of low-energy electron transmission through graphene depend on its electronic structure above the vacuum level. The experimental technique described here allows mapping the unoccupied band structure of freestanding two-dimensional materials as a function of energy and probing angle, respectively in-plane momentum. Our experimental findings are consistent with theoretical predictions of a resonance in the band structure of graphene above the vacuum level [V. U. Nazarov, E. E. Krasovskii, and V. M. Silkin, Physical Review B 87, 041405 (2013)].

In Situ Transmission Electron Microscopy Modulation of Transport in Graphene Nanoribbons.

In situ transmission electron microscopy (TEM) electronic transport measurements in nanoscale systems have been previously confined to two-electrode configurations. Here, we use the focused electron beam of a TEM to fabricate a three-electrode geometry from a continuous 2D material where the third electrode operates as side gate in a field-effect transistor configuration. Specifically, we demonstrate TEM nanosculpting of freestanding graphene sheets into graphene nanoribbons (GNRs) with proximal graphene side gates, together with in situ TEM transport measurements of the resulting GNRs, whose conductance is modulated by the side-gate potential. The TEM electron beam displaces carbon atoms from the graphene sheet, and its position is controlled with nanometer precision, allowing the fabrication of GNRs of desired width immediately prior to each transport measurement. We also model the corresponding electric field profile in this three-terminal geometry. The implementation of an in situ TEM three-terminal platform shown here further extends the use of a TEM for device characterization. This approach can be easily generalized for the investigation of other nanoscale systems (2D materials, nanowires, and single molecules) requiring the correlation of transport and atomic structure.

Production of N-Graphene By Microwave N2-Ar Plasmas (invited)

Nitrogen doped graphene sheets, have attracted much attention due to their exceptional performance as parts of fuel cells, lithium-ion batteries and supercapacitors. Free-standing Nitrogen-doped graphene sheets where produced in a low pressure microwave N2-Ar plasma. Graphene sheets were treated by plasma at different exposure times and doped with nitrogen atoms, which incorporated the hexagonal carbon lattice, mainly, in pyridinic, pyrrolic and quaternary functional groups. Optical emission spectroscopy were applied to monitor the nitrogen atoms, gas temperature and electron density at the substrate position. Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) techniques were applied to characterize the produced N-graphene, along with transmission electron microscopy (TEM) to study the morphology and structure of the samples. Doping levels as high as 5.6 % were achieved and an increase in the sp2/sp3ratio was observed for a relatively short exposure times. This study shows that by incorporating nitrogen atoms into graphene, its physico-chemical properties are considerably altered. Acknowledgements: This work was supported by Fundação para a Ciência e a Tecnologia, under Project Pest-OE/UID/FIS/50010/2013 and grant SFRH/BD/52413 (PD-F APPLAuSE) .

(Invited) Microwave Plasmas Applied for Synthesis of Free-Standing Carbon Nanostructures at Atmospheric Pressure Conditions

A microwave plasma-assisted method for synthesis of advanced carbon nanostructures is presented. The method is based on the injection of gas/liquid carbon precursors into a surface-wave sustained argon plasma environment, where the decomposition of molecules takes place. Gas-phase carbon atoms and molecules created in the "hot" plasma diffuse into colder zones of plasma reactor and aggregate into solid carbon nuclei. The main part of the solid carbon is gradually withdrawn from the "hot" plasma region in the outlet plasma stream where flowing carbon nanostructures assemble and grow. Selective synthesis of free-standing graphene sheets and diamond-like nanostructures was achieved. The engineering of the graphene sheets’ structural qualities and the control of the number of atomic layers per sheet is achieved via synergistic tailoring of the "hot"plasma environment and thermodynamic conditions in the post-discharge zone. The produced sheets (1-5 atomic layers) are stable and present much better qualities than those of graphene assembled via conventional methods, while being synthesized without the need of metal/crystal substrates. The synthesized structures have been analyzed by Raman spectroscopy, scanning electron microscopy, high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy. It is worth noting that milligrams of high quality self-standing graphene sheets in a readily dispersive form can be obtained within a minute, through a single step process at atmospheric pressure conditions. The method is widely open for scale-up by using large-scale configurations of wave driven plasmas. Acknowledgements: This work was supported by Fundação para a Ciência e a Tecnologia, under 2015 Strategic Project.

Enhanced microwave-to-terahertz absorption in graphene

Fresnel equations predict that an ultrathin free standing conductive film, thousands times thinner than skin depth, is capable to absorb up to 50% of incident electromagnetic radiations. In the microwave range, the same holds true for a free standing graphene sheet. We demonstrate theoretically and prove experimentally that microwave absorptance of graphene can be enhanced considerably by depositing graphene on a dielectric substrate. On the experimental side, we obtain 80% and 65% absorptance at 30 GHz and 1 THz, respectively. Theory predicts that higher absorptance can be achieved with a suitable choice of the dielectric permittivity and the thickness of the substrate. Absorption can also be maximized by choosing the optimum incidence angle for s-polarized waves in free space or by working in the vicinity of the cut-off frequency of the transverse electric mode in waveguide configuration. The polarization sensitivity of the transmittance and reflectance of graphene layers can be used to tune the polarization state of the transmitted and reflected radiations.

SIMS of transfer ribonucleic acid molecules encapsulated between free-standing graphene sheets.

In this study, the authors used cluster-secondary ion mass spectrometry method to investigate the preserved transfer ribonucleic acid (tRNA) encapsulated between two free-standing graphene sheets. Single impacts of 50 keV C60 (2+) projectiles generated the emission of tRNA fragment ions in the transmission direction for mass selection and detection in a time-of-flight mass spectrometer. Ribonucleic acid (RNA) is extremely unstable and prone to rapid enzymatic degradation by ribonucleases. Employing graphene to isolate RNA from the environment, the authors prevent the aforementioned process. Encapsulation was achieved by drop casting a solution of tRNA, prepared using deuterated water, onto one graphene sheet and covering it with another. The event-by-event bombardment/detection mode allowed us to use colocalization analysis method to characterize the tRNA and its immediate environment. The authors found that upon drying, tRNA agglomerated into nanostructures ∼60 nm in diameter via formation and subsequent drying of aqua cells. The tRNA nanoagglomerates had a density of ∼42 structures per μm(2) with coverage of ∼12% of the surface area. In addition, trace amounts of water remained mostly around the tRNA nanoagglomerates, probably in the form of hydration.