Few-layer Graphene Structures Research Articles

Surface structure of few layer graphene

Understanding surface structure of graphene is important for its integration into composite materials. Here, we have used synchrotron X-ray reflectivity (XRR) to study the structure of commercially available graphene samples (prepared via chemical vapor deposition, and marketed as graphene monolayers) on SiO2/Si at different temperatures. X-ray photoelectron spectroscopy, photoemission electron microscopy and atomic force microscopy (AFM) were employed to evaluate the composition and morphology of the graphene layer. Our results indicate that the samples we characterized consisted of 3–4 layers of graphene, which should thus be more accurately described as few layer graphene (FLG). Furthermore, a “contaminant” layer, comprising polymethylmethacrylate and graphene multilayers, was found present atop FLG. We also report tentative results on the effect of temperature on the graphene sample thickness. At 25 °C, the FLG thickness from XRR measurements was 13.0 ± 1.0 Å, in agreement with that obtained from AFM (13.9 ± 0.7 Å). Upon heating to 60 °C, the FLG thickness expanded to 13.8 Å, which further increased to 14.3 Å upon cooling to 25 °C. We attribute this temperature dependent thickness to the out-of-plane rippling of graphene as previously reported. These unprecedented results on the FLG surface structure are valuable to its potential bioanalytical applications.

Electrical Conductivity of Films Formed by Few-Layer Graphene Structures

Current–voltage characteristics of few-layer graphene structures produced by electrochemical exfoliation of graphite in an electrolyte solution were measured. It was shown that few-layer graphene structures possess an electronic conductivity, which is indicative of the small degree of surface functionalization of the structures. The resistance of films formed by these structures grows with increasing relative humidity of the medium because of the shielding of fl akes of the few-layer graphene structures by a film of water.

Rippling of graphitic surfaces: a comparison between few-layer graphene and HOPG.

The surface structure of Few-Layer Graphene (FLG) epitaxially grown on the C-face of SiC has been investigated by TM-AFM in ambient air and upon interaction with dilute aqueous solutions of bio-organic molecules (l-methionine and dimethyl sulfoxide, DMSO). Before interaction with molecular solutions, we observe nicely ordered, three-fold oriented rippled domains, with a 4.7 ± 0.2 nm periodicity (small periodicity, SP) and a peak-to-valley distance in the range 0.1-0.2 nm. Upon mild interaction with the molecular solution, the ripple periodicity "relaxes" to 6.2 ± 0.2 nm (large periodicity, LP), while the peak-to-valley height increases to 0.2-0.3 nm. When additional energy is transferred to the system through sonication in solution, graphene planes are peeled off, as shown by quantitative analysis of Raman spectroscopy and X-ray photoelectron spectroscopy which indicate a neat reduction of thickness. Upon exfoliation rippled domains are no longer observed. In comparative experiments on cleaved HOPG, we could not observe ripples on pristine samples in ambient air, while LP ripples develop upon interaction with the molecular solutions. Recent literature on similar systems is not univocal regarding the interpretation of rippling. The ensemble of our comparative observations on FLG and HOPG can be hardly rationalized solely on the basis of the surface assembly of molecules, either organic molecules coming from the solution or adventitious species. We propose to consider rippling as the manifestation of the free-energy minimization of quasi-2D layers, eventually affected by factors such as interplanar stacking, and interactions with molecules and/or with the AFM tip.

Современные способы получения малослойных графеновых структур методом электрохимической эксфолиации графита

Современные способы получения малослойных графеновых структур методом электрохимической эксфолиации графита

Improved tribological properties of Si3N4/GCr15 sliding pairs with few layer graphene as oil additives

Abstract The tribological behaviours of two types of tribo-pairs (Si3N4/GCr15, GCr15/GCr15) lubricated with 4010 aviation lubricants (4010AL) containing different concentrations (0–0.3 wt%) of few layer graphene (FG) were investigated using a four-ball testing machine. Atomic force microscopy (AFM) and Raman spectroscopy were employed to characterise the structure and morphology of FG. Laser scanning confocal microscopy (LSCM), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and Raman spectroscopy were employed to investigate the worn surface. The test results demonstrated that the frictional properties of the Si3N4/GCr15 pairs were significantly improved after adding FG into the base lubricating oil. With the lubricating oil containing 0.075 wt% FG, the friction coefficient and wear scar diameter were reduced by 27% and 43%, respectively. On rubbing surfaces, disordered graphene was obtained as examined by EDS and Raman measurements. The lubrication mechanism was investigated to explain the enhancement in the friction-reduction and anti-wear properties, which can be attributed to shearing, tearing, and filling of FG as well as its adhesion to rubbing surfaces, and the mechanical contact between frictional interfaces was avoided.

Role of limited hydrogen and flow interval on the growth of single crystal to continuous graphene by low-pressure chemical vapor deposition

A method for defect-free large crystallite graphene growth remains unknown despite much research effort. In this work, we discuss the role of flow duration of H2 gas for the production of graphene as per requirement and production at a minimum flow rate considering the safety issue of hydrogen utilization. The copper substrate used for growth was treated for different time intervals (0 to 35 min) in H2 flow prior to growth. Structural and chemical changes occurring in the copper substrate surface were probed by grazing incidence x-ray diffraction and x-ray photoelectron spectroscopy. The results were correlated with the Raman spectroscopy data, which can quantify the quality of graphene. With increasing H2 flow interval, secondary nucleation sites were observed and growth favored few-layer graphene structures. The surface-adsorbed oxygen molecules and its conversion to an OH terminated surface with increasing hydrogen flow interval was found to be a key factor in enhancing nucleation density. The Stranski–Krastanov type of nucleation was observed for samples grown with different time intervals of H2 treatment, except 5 min of H2 flow prior to growth for which the Volmer–Weber type of growth favored monolayer graphene crystallite growth.

Ballistic electron propagation through periodic few-layer graphene nanostructures

We have studied electron propagation in periodic structures containing mono- and few-layer graphene regions and/or semiconducting stripes. The calculation of the transmission coefficient in all cases has been performed using transfer matrices inside regions with the same material/potential energy, as well as interface matrices between regions in which the evolution laws of charge carriers differ. Numerical simulations of the transmission coefficient, as well as of the low-temperature conductance, suggest that different periodic structures modulate differently the electrical current. The obtained results can be used to model ballistic transport in all-graphene devices, in particular in few-layer graphene structures.

Interfacial Atomic Structure of Twisted Few-Layer Graphene

A twist in bi- or few-layer graphene breaks the local symmetry, introducing a number of intriguing physical properties such as opening new bandgaps. Therefore, determining the twisted atomic structure is critical to understanding and controlling the functional properties of graphene. Combining low-angle annular dark-field electron microscopy with image simulations, we directly determine the atomic structure of twisted few-layer graphene in terms of a moiré superstructure which is parameterized by a single twist angle and lattice constant. This method is shown to be a powerful tool for accurately determining the atomic structure of two-dimensional materials such as graphene, even in the presence of experimental errors. Using coincidence-site-lattice and displacement-shift-complete theories, we show that the in-plane translation state between layers is not a significant structure parameter, explaining why the present method is adequate not only for bilayer graphene but also a few-layered twisted graphene.

Reversible Loss of Bernal Stacking during the Deformation of Few-Layer Graphene in Nanocomposites

The deformation of nanocomposites containing graphene flakes with different numbers of layers has been investigated with the use of Raman spectroscopy. It has been found that there is a shift of the 2D band to lower wavenumber and that the rate of band shift per unit strain tends to decrease as the number of graphene layers increases. It has been demonstrated that band broadening takes place during tensile deformation for mono- and bilayer graphene but that band narrowing occurs when the number of graphene layers is more than two. It is also found that the characteristic asymmetric shape of the 2D Raman band for the graphene with three or more layers changes to a symmetrical shape above about 0.4% strain and that it reverts to an asymmetric shape on unloading. This change in Raman band shape and width has been interpreted as being due to a reversible loss of Bernal stacking in the few-layer graphene during deformation. It has been shown that the elastic strain energy released from the unloading of the inner graphene layers in the few-layer material (∼0.2 meV/atom) is similar to the accepted value of the stacking fault energies of graphite and few layer graphene. It is further shown that this loss of Bernal stacking can be accommodated by the formation of arrays of partial dislocations and stacking faults on the basal plane. The effect of the reversible loss of Bernal stacking upon the electronic structure of few-layer graphene and the possibility of using it to modify the electronic structure of few-layer graphene are discussed.

Molecular dynamics simulations of hydrogen storage capacity of few-layer graphene

The adsorption of molecular hydrogen on few-layer graphene (FLG) structures is studied using molecular dynamics simulations. The interaction between graphene and hydrogen molecules is described by the Lennard-Jones potential. The effects of pressure, temperature, number of layers in a FLG, and FLG interlayer spacing are evaluated in terms of molecular trajectories, binding energy, binding force, and gravimetric hydrogen storage capacity (HSC). The simulation results show that the effects of temperature and pressure can offset each other to improve HSC. An insufficient interlayer spacing (0.35 nm) largely limits the HSC of FLG because hydrogen adsorbed at the edges of the graphene prevents more hydrogen from entering the structure. A low temperature (77 K), a high pressure, a large number of layers in a FLG, and a large FLG interlayer spacing maximize the HSC.

Magnetoelectric effect in functionalized few-layer graphene

We show that the spin moment induced by $s{p}^{3}$-type defects created by different covalent functionalizations on a few-layer graphene structure can be controlled by an external electric field. Based on ab initio density functional calculations, including van der Waals interactions, we find that this effect has a dependence on the number of stacked layers and concentration of point defects, but the interplay of both with the electric field drives the system to a half-metallic state. The calculated magnetoelectric coefficient $\ensuremath{\alpha}$ has a value comparable to those found for ferromagnetic thin films (e.g., Fe, Co, Ni) and magnetoelectric surfaces (e.g., CrO${}_{2}$). The value of $\ensuremath{\alpha}$ also agrees with the universal value predicted for ferromagnetic half-metals and also points to a novel route to induce half-metallicity in graphene using surface decoration.

Laboratory-based real and reciprocal space imaging of the electronic structure of few layer graphene on SiC(0001¯) using photoelectron emission microscopy

We present real and reciprocal space photoelectron emission microscopy (PEEM) results on few layer graphene using laboratory based He I and II radiation. The combination of a focused high-intensity source and high transmission PEEM electron optics provides good signal to noise ratios for the different modes of acquisition. We demonstrate work function mapping and secondary electron analysis, related to the graphene layer thickness, band structure imaging from micron scale regions by wave vector resolved PEEM (k-PEEM) and local secondary electron spectroscopy, giving information on the valence and conduction band states and the dispersion relations of the π bands. Dark field PEEM is done by selecting the Dirac cone corresponding to the specific rotation of each graphene layer and allows spatial mapping of the commensurate rotation angles. The use of He II radiation increases the volume of reciprocal space accessible to k-PEEM and improves signal to background. The preferential linear polarization of the light source is used to investigate aspects of the electronic chirality near the Dirac cone. Recent developments in sample manipulation and cooling are presented.

Synthesis of Ag-decorated, few-layer graphene structures over a novel Ag/MgO catalytic system by radio-frequency chemical vapor deposition

We report the synthesis, by radio-frequency chemical vapor deposition with methane as the carbon source, of multicomponent nanostructural materials based on Ag nanoparticles embedded into graphene layers and discuss their properties. We found that the Ag concentration plays an important role in controlling the morphology and the structure of the resulting graphene sheets. Raman analysis was used to calculate the in-plane crystallites (La) from the (IG/ID) ratio, which were found to range between 12.1 and 13.1 nm. Such composites could find applications in a number of areas ranging from energy harvesting and plasmonics to sensing and nanomedicine.

Evolution of surface morphology and electronic structure of few layer graphene after low energy Ar+ ion irradiation

We report on co-existing dual anisotropy ripple formation, sp bonding transformation, and variation in the delocalized π electron system in 1 keV Ar+ ion irradiated few-layer graphene surfaces. Ripples in directions, perpendicular and parallel to the ion beam were found. The irradiation effect and the transition from the sp2-bonding to sp3-hybridized state were analyzed from the deconvolution of the C (1s) peak and from the shape of the derivative of the Auger transition spectra. The results suggest a plausible mechanism for tailoring of few-layer graphene electronic band structure with interlayer coupling tuned by the ion irradiation.

Few-layer graphene under high pressure: Raman and X-ray diffraction studies

The effect of pressure on the structure of few-layer graphene has been investigated to 50GPa in both quasi-hydrostatic and non-hydrostatic conditions, using X-ray diffraction and Raman spectroscopy. The results indicate that few-layer graphene loses its long-range order at the critical interlayer distance of ∼2.8Å (or above ∼18GPa), while maintaining the local sp2 hybridization in the layer to 50GPa. This suggests that graphene not only has the highest stability of all graphitic layer structures, but also becomes one of the most healable structures under large stress.

Electrical and Structural Feature of Monolayer Graphene Produced by Pulse Current Unzipping and Microwave Exfoliation of Carbon Nanotubes

The electrical conductivity and structural properties of monolayer graphene (MLG) have been studied. MLG is fabricated from single-walled carbon nanotubes (SWCNTs) using high direct current pulse and microwave irradiation. Transformed MLG initially obtained from SWCNTs was not a stable planar structure because of its high surface energy. Therefore, they became stacked together to form the few-layer graphene structure. With the exfoliation process using an alkali metal, the stacked structure changes to a monolayer structure. A variety of characterizing techniques, such as a field emission scanning electron microscopy, high resolution transmission electron microscopy, high resolution Raman spectroscopy, and X-ray photoelectron spectroscopy were used for the analysis of MLG structures.

Structure of few-layer epitaxial graphene on 6H-SiC(0001) at atomic resolution

Using directly interpretable atomic-resolution cross-sectional scanning transmission electron microscopy, we have investigated the structure of few-layer epitaxial graphene (EG) on 6H-SiC(0001). We show that the buried interface layer possesses a lower average areal density of carbon atoms than graphene, indicating that it is not a graphenelike sheet with the 63×63R30° structure. The EG interlayer spacings are found to be considerably larger than that of the bulk graphite and the surface of the SiC(0001) substrate, often treated as relaxed, is found to be strained. Discontinuity of the graphene layers above the SiC surface steps is observed, in contradiction with the commonly believed continuous coverage.

The evolution of electronic structure in few-layer graphene revealed by optical spectroscopy

The massless Dirac spectrum of electrons in single-layer graphene has been thoroughly studied both theoretically and experimentally. Although a subject of considerable theoretical interest, experimental investigations of the richer electronic structure of few-layer graphene (FLG) have been limited. Here we examine FLG graphene crystals with Bernal stacking of layer thicknesses N = 1,2,3,...8 prepared using the mechanical exfoliation technique. For each layer thickness N, infrared conductivity measurements over the spectral range of 0.2-1.0 eV have been performed and reveal a distinctive band structure, with different conductivity peaks present below 0.5 eV and a relatively flat spectrum at higher photon energies. The principal transitions exhibit a systematic energy-scaling behavior with N. These observations are explained within a unified zone-folding scheme that generates the electronic states for all FLG materials from that of the bulk 3D graphite crystal through imposition of appropriate boundary conditions. Using the Kubo formula, we find that the complete infrared conductivity spectra for the different FLG crystals can be reproduced reasonably well within the framework a tight-binding model.

Electronic Structure of Few-Layer Graphene: Experimental Demonstration of Strong Dependence on Stacking Sequence

The electronic structure of few-layer graphene (FLG) samples with crystalline order was investigated experimentally by infrared absorption spectroscopy for photon energies ranging from 0.2-1 eV. Distinct optical conductivity spectra were observed for different samples having precisely the same number of layers. The different spectra arise from the existence of two stable polytypes of FLG, namely, Bernal (AB) stacking and rhombohedral (ABC) stacking. The observed absorption features, reflecting the underlying symmetry of the two polytypes and the nature of the associated van Hone singularities, were reproduced by explicit calculations within a tight-binding model. The findings demonstrate the pronounced effect of stacking order on the electronic structure of FLG.

Atomic and electronic structure of few-layer graphene on SiC(0001) studied with scanning tunneling microscopy and spectroscopy

Epitaxial growth of graphene on SiC surfaces by solid state graphitization is a promising route for future development of graphene based electronics. In the present work, we have studied the morphology, atomic scale structure, and electronic structure of thin films of few-layer graphene (FLG) on SiC(0001) by scanning tunneling microscopy and spectroscopy (STS). We show that a quantitative evaluation of the roughness induced by the interface is a tool for determining the layer thickness of FLG. We present and interpret thickness dependent tunneling spectra, which can serve as an additional fingerprint for the determination of the layer thickness. By performing spatially resolved STS, we find evidence that the charge distribution in bilayer graphene is inhomogeneous.