Interaction between graphene and copper substrate: The role of lattice orientation

We present a comprehensive study of graphene grown by chemical vapor deposition on copper single crystals with exposed (1 0 0), (1 1 0) and (1 1 1) faces. Direct examination of the as-grown graphene by Raman spectroscopy using a range of visible excitation energies and microRaman mapping shows distinct strain and doping levels for individual Cu surfaces. Comparison of results from Raman mapping with X-ray diffraction techniques and atomic force microscopy shows it is neither the crystal quality nor the surface topography responsible for the specific strain and doping values, but it is the Cu lattice orientation itself. We also report an exceptionally narrow Raman 2D band width caused by the interaction between graphene and metallic substrate. The appearance of this extremely narrow 2D band with full-width-at-half maximum (FWHM) as low as 16 cm⁻¹ is correlated with flat and undoped regions on the Cu(1 0 0) and (1 1 0) surfaces. The generally compressed (∼0.3% of strain) and n-doped (Fermi level shift of ∼250 meV) graphene on Cu(1 1 1) shows the 2D band FWHM minimum of ∼20 cm⁻¹. In contrast, graphene grown on Cu foil under the same conditions reflects the heterogeneity of the polycrystalline surface and its 2D band is accordingly broader with FWHM >24 cm⁻¹.     

High quality graphene sheets from graphene oxide by hot-pressing

We report a simple and effective route to convert graphene oxide sheets to good quality graphene sheets using hot pressing. The reduced graphene oxide sheets obtained from graphene oxide by low temperature thermal exfoliation are annealed at 1500 °C and 40 MPa uniaxial pressures for 5 min in vacuum. No appreciable oxygen content was observed from X-ray photoelectron spectroscopy and no D peak was detected in the Raman spectrum. The graphene sheets produced had a much higher electron mobility (1000 cm² V⁻¹ S⁻¹) than other chemically modified graphenes.     

Direct formation of graphene layers on diamond by high-temperature annealing with a Cu catalyst

The formation of high-quality graphene layers on diamond was achieved based on a high-temperature annealing method using a Cu catalyst. Typical features of monolayer graphene were observed in the Raman spectra of layers formed by annealing of Cu/diamond heterostructures at 950 °C for 90 min. The coverage ratio of these graphene layers on diamond was estimated to be on the order of 85% by Raman mapping of the 2D peak. The sheet hole concentration and mobility values of the layers were estimated to be ~ 10¹³ cm^− 2 and ~ 670 cm²/Vs, respectively. These values are comparable to those previously observed for high-quality graphene layers on SiC.     

Bomb calorimetry as a bulk characterization tool for carbon nanostructures

Multiwalled carbon nanotubes (MWCNTs) consisting of coaxial graphene cylinders (cylindrical MWCNTs), cones (herringbone MWCNTs) or carbon fibers were combusted in an isothermal bomb calorimeter. Their standard enthalpies of formation were determined to be 16.56 ± 2.76 kJ mol⁻¹(C – per carbon mol) for carbon fibers, 21.70 ± 1.32 kJ mol⁻¹(C) for herringbone MWCNTs and 8.60 ± 0.52 kJ mol⁻¹(C) for cylindrical ones. All materials were characterized by transmission electron microscopy, scanning electron microscopy, Raman spectroscopy, thermogravimetry, and elemental analysis. A linear correlation between the standard enthalpies of formation and D/G and G′/G Raman bands ratio (D – band is centered at 1350 cm⁻¹, G – 1585 cm⁻¹, G′ – 2700 cm⁻¹) demonstrates the applicability of bomb calorimetry for characterization of the “defectiveness” of the bulk carbon material in the sense Raman spectroscopy is widely used nowadays. Also, we show that the calorimetry may be used to estimate the oxygen content in the bulk carbon nanomaterials, as there is a linear correlation between the oxygen content (both total content and in carboxyl groups separately) and the standard enthalpies of formation for herringbone nanotubes oxidized by nitric acid.     

Raman scattering by defect-induced excitations in boron-doped diamond single crystals

Polarized Raman spectra of the oriented boron-doped diamond with a different content of boron (≤ 200 ppm) were obtained with 514.5 and 1064 nm excitations. The additional bands were found in the region below 1200 cm^− 1. Their intensity increased with doping. It was shown that in polarized spectra these bands were in agreement with the singularities of density of phonon states (DOS) of diamond for the A_1g, E_g and F_2g symmetries. It was assumed that the ~ 900 cm^− 1 band which does not coincide with any DOS peak and has the highest resonance character may be attributed to the localized mode of boron in a diamond lattice. The spectra were accompanied by continuum that had the same symmetry F_2g as optical phonon at 1333 cm^− 1.     

Structure of the silver containing diamond like carbon films: Study by multiwavelength Raman spectroscopy and XRD

In the present study structure of silver containing diamond like carbon (DLC:Ag) films deposited by reactive magnetron sputtering was investigated by X-ray diffractometry (XRD) and multiwavelength Raman spectroscopy. In the case of the DLC:Ag films containing low amount of silver, crystalline silver oxide prevails over silver. While at higher Ag atomic concentrations formation of the silver crystallites of the different orientations was observed. Surface enhanced Raman scattering (SERS) effect was detected for high Ag content in the films. For UV excited Raman spectra sp³ bonded carbon related Raman scattering T peak at ~ 1060 cm^− 1 was detected only for the films with the highest amount of silver (34.3 at.%). The dependence of the Raman scattering spectra parameters such as position of the G peak, G peak full width at half maximum (FWHM(G)), D/G peak area ratio on Ag atomic concentration in DLC:Ag film as well as Raman scattering spectra excitation wavelength were studied. The dependence on Ag amount in film was more pronounced in the case of the Raman scattering spectra excited by higher wavelength laser beam, while in the case of the spectra excited by 325 nm and 442 nm laser beams only weak dependence (or no dependence) was observed. Overall tendency of the decrease of the dispersion of the G peak with the increase of Ag atomic concentration was found. Thus sp³/sp² bond ratio in DLC:Ag film decreased with the increase of Ag atomic concentration in the films.     

Structural and optical characterization of Zn0.95−xMg0.05AlxO nanoparticles

The structural and optical properties of ZnO nanoparticles doped simultaneously with Mg and Al were investigated. XRD results revealed the hexagonal wurtzite crystalline structure of ZnO. The FE-SEM study confirmed the formation of nano-sized homogeneous grains whose sizes decreased monotonously with increasing doping concentrations of Mg and Al. The absorption spectra showed that band gap increased from 3.20 to 3.31 eV with Mg doping. As the Al concentration changed from x=0.01 to x=0.06 mol% at constant Mg concentration the band gap observed to be decreased. Particle sizes estimated from effective mass approximation using absorption data and these values are in good agreement with the crystallite sizes calculated from XRD data. Raman spectra of ZnO showed a characteristic peak at 436 cm⁻¹ correspond to a non-polar optical phonon E₂ (high). With increase of the Al doping concentrations, E₂ (high) phonon frequency shifted to 439 cm⁻¹ from to 436 cm⁻¹. The origin of E₂ (high) peak shift in ZnO nanoparticles is attributed to optical phonon confinement effects or the presence of intrinsic defects on the nanoparticles. PL spectra indicated that with increase of Al co-doping along with Mg into ZnO, intensity of the peak positioned at 395 nm was initially increased at x=0 and then decreased with increase of the Al concentrations from x=0.01 to x=0.06 mol%.     

Sp3/sp2 character of the carbon and hydrogen configuration in micro- and nanocrystalline diamond

Hot filament and microwave plasma CVD micro- nanocrystalline diamond films are analysed by visible and ultra-violet excitation source Raman spectroscopy. The sample grain size varies from 20 nm to 2 μm. The hydrogen concentration in samples is measured by SIMS and compared to the grain size, and to the ratio of sp² carbon bonds determined by Raman spectroscopy from the 1332 cm^− 1 diamond peak and the sp² 1550 cm^− 1 G band. Hydrogen concentration appears to be proportional to the sp² bonds ratio. The 3000 cm^− 1 CH_x stretching mode band intensity observed on the Raman spectra is decreasing with the G band intensity. Thermal annealing modifies the sp² phase structure and concentration, as hydrogen outdiffuses.     

Numerical experiments on phonon properties of isotope and vacancy-type disordered graphene

We have studied phonon properties of graphene theoretically with different concentrations of ¹³C isotope and vacancy-type defects. The forced vibrational method, which is based on the mechanical resonance to extract the pure vibrational eigenmodes by numerical simulation, has been employed to compute the phonon density of states (PDOSs) and mode pattern of isotope-disordered graphene as well as a combined isotope and vacancy-type defective graphene structure. We observe a linear reduction of the E_2g mode frequencies with an increase in ¹³C concentration due to the reduced mass variation of the isotope mixture. We find a downshift of the E_2g mode of 65 cm^− 1, which is a very good agreement with the experimental results, and the phonon frequencies described by the simple harmonic oscillator model. The vacancy-type defects break down the phonon degeneracy at the Г point of the LO and TO modes, distort and shift down the phonon density of states significantly. The PDOS peaks for the combined isotope and vacancy-type defects show the remarkable increase in the low-frequency region induced by their defect formations. Due to phonon scattering by ¹³C isotope or vacancies, some graphene phonon wave functions become localized in the real space. Our numerical experiments reveal that the lattice vibrations in the defective graphene show the remarkably different properties such as spatial localization of lattice vibrations due to their random structures from those in the perfect graphene. The calculated typical mode patterns for in-plane K point optical phonon modes indicate that the features of strongly localized state depend on the defect density, and the phonon is localized strongly within a region of several nanometers in the random percolation network structures. In particular, for in-plane K point optical phonon modes, a typical localization length is on the order of ≈ 7 nm for isotope impurities, ≈ 5 nm for vacancy-type defects and ≈ 6 nm for mixed-type defects at high defect concentrations of 30%. Our findings can be useful for the interpretation of experiments on infrared, Raman, and neutron-diffraction spectra of defective graphene, as well as in the study of a wide variety of other physical properties such as thermal conductivity, specific heat capacity, and electron–phonon interaction.     

Thermal performance of vertically-aligned multi-walled carbon nanotube array grown on platinum film

Vertically-aligned carbon nanotube array is expected to inherit high thermal conductivity and mechanical compliance of individual carbon nanotube and serve as thermal interface material. In this paper, vertically-aligned multi-walled carbon nanotube arrays have been directly grown on Pt film and the thermal performance has been studied by using laser flash technique. The determined thermal diffusivity decreases from 0.187 to 0.135 cm² s⁻¹ and the thermal conductivity increases from 1.8 to 3.1 W m⁻¹ K⁻¹ as temperature increases from 243.2 to 453.2 K. The fracture surface of the array peeled off the Pt film was characterized by scanning electron microscopy. It has been illustrated that the tearing surface is not smooth but fluffy with torn carbon nanotubes, indicating strong interfacial bonding and consequent small interface resistance between carbon nanotube array and Pt film. According to Raman spectra and transmission electron microscopy image, the possible mechanisms responsible for the thermal transport degradation are low packing density, twist, and the presence of impurities, amorphous carbon, defects and flaws. The influence of intertube van der Waals interactions has been studied by comparing the phonon dispersion relations and is expected to be not significant.     

Graphene synthesis by C implantation into Cu foils

Cu foils of 2 × 2 cm² have been implanted with 70 keV C⁻ ions to nominal fluences of (2–10) × 10¹⁵ cm⁻² at room temperature (RT) and subsequently annealed at 900–1100 °C for 15 min, before being cooled to RT to form graphene layers on the Cu surfaces. Analyses with Raman spectroscopy and atomic force microscopy demonstrate that a continuous film of bi-layer graphene (BG) is produced for implant fluences as low as 2 × 10¹⁵ cm⁻², much less than the carbon content of the BG films. This suggests that the implanted carbon facilitates the nucleation and growth of graphene, with additional carbon supplied by the Cu substrate (0.515 ppm carbon content). No graphene was observed on unimplanted Cu foils subjected to the same thermal treatment. This implantation method provides a novel technique for the selective growth of graphene on Cu surfaces.     

Casimir effect demonstrated by Raman spectroscopy on trilayer graphene intercalated into stiff layered structures of surfactant

Polarized Raman scattering studies on stiff layered structure of surfactant intercalated with trilayer graphene were performed at different intensities and excitation wavelengths. The D and 2D Raman bands reach the highest and lowest intensity when the polarization of laser excitation light is oriented along and perpendicular on the edges. The 2D band discloses two lorentzian components, separated by ∼40 cm⁻¹, which result from the action of interplanar forces, of Casimir nature. The value of ∼40 cm⁻¹ is close to the energy value associated with E_2g interplanar layer shear mode evidenced so far only by neutron spectrometry. A new result regards the opposite variation of the intensities of D and 2D bands with the increase of the wavelength of the excitation light. This originates in the different origin of the D and 2D bands; the former is dependent on disorder including also the graphene edges while the latter, results from in a double resonant mechanism combined with a Casimir effect. One demonstrates that the magnitude of Casimir force, which activates interlayer vibration modes, depends on the carrier density on the graphene sheets which can be varied both by the intensity and the wavelength of the excitation laser light.     

Mussel-inspired,ultralight, multifunctional 3D nitrogen-doped graphene aerogel

Inspired by marine mussel, an ultralight, three dimensional (3D), and nitrogen doped graphene aerogel (NGA) is developed by virtue of dopamine (DA). DA undergoes self-polymerization that can functionalize the graphene surface and also embeds nitrogen atoms onto the graphene sheets upon pyrolysis. The morphology and textural properties were investigated by scanning and transmission electron microscopy, X-ray photoelectron spectroscopy and Raman spectrum. By adjusting the amount of graphene oxide and DA in the precursor mixture, NGA with a density as low as 1.9 mg cm⁻³ can be prepared. The resultant NGA is macroscopic-assembled, mechanically stable, and fire-resistant. It has very high adsorption capacity and efficient space utilization towards oil uptake, thereby prompting practical applications as a suction skimmer in marine oil spillage or chemical leakage recovery. The high conductivity and 3D interconnected porous structure of the NGA permit good performance as electrode material for electrochemical sensors.     

Spatial dependence of Raman frequencies in ordered and disordered monolayer graphene

A micro-Raman investigation was carried out on several flakes of monolayer (1 L) graphene obtained by the micro-mechanical exfoliation technique and, then, put on a c-Si wafer coated by a SiO₂ thin film. Some of the 1 L zones show a remarkable dispersion of the 2D-overtone wavenumber as a function of the position within the graphene sheet, and, in some case, a significant broadening of the E_2g phonon (G band) is associated to this wavenumber shift of 2D-band. Such effects were studied, in particular, for a 1 L zone characterized by a rather strong lattice disorder, as revealed by the strong D/G band intensity ratio, and for other zones quite ordered, showing a vanishing intensity of the D band. Moreover, by moving along different directions within 1 L graphene sheets, different trends for 2D wavenumber and E_2g phonon bandwidth vs. position were observed. All these reported behaviours are explained in terms of different distributions of intrinsic uniaxial strain occurring within the 1 L graphene sheets.     

Reduction of graphite oxide to graphene with laser irradiation

Reduction of graphite oxide (GO) to graphene induced by picosecond pulsed laser irradiation has been studied by Raman spectroscopy, scanning electron microscopy together with modeling of temperature dynamics in the materials. Dependence of the D, G, and 2D Raman band parameters on the laser pulse energy and the irradiation dose was evaluated. The exponential decline of the full width at half maximum of the Raman lines with increasing product of the pulse energy and irradiation dose was observed indicating ordering in the film and reduction in the number of graphene layers during the laser treatment. The minimum concentration of structural defects and the largest relative intensity of the 2D peak were found for the 50 mW mean laser power and the 30 mm/s scanning speed. Modeling of temperature dynamics revealed that the temperature of the GO film irradiated with a single laser pulse at a fluence of 0.04 J/cm² (50 mW) increased up to 1400 °C for a few nanoseconds, which was sufficient for the effective reduction of GO to graphene with successive laser pulses.     

Effective synthesis of well graphitized high yield bamboo-like multi-walled carbon nanotubes on copper loaded α-alumina nanoparticles

A high-yield bamboo like multiwalled carbon nanotubes (CNTs) were successfully synthesized on copper substituted alumina nanoparticles by thermal chemical vapor deposition (CVD) technique under atmospheric pressure. The obtained products were characterized by various techniques like FESEM with EDX, HRTEM and Raman spectroscopy, which reveals the formation of CNTs and are of bamboo shaped (stacking arrangement) multiwalled type with graphene layers having a diameter between 4 and 9 nm. The appearance of two peaks at 1597 cm^− 1 and 1302 cm^− 1 in Raman spectra are noticed as G-band and D-band for graphitic nature and defects due to bending & curvature of bamboo like carbon nanotubes (b-CNTs), respectively. The influence of reaction parameters such as time, temperature and flow rate was also studied to increase the carbon yield.     

Tuning three-dimensional textures with graphene aerogels for ultra-light flexible graphene/texture composites of effective electromagnetic shielding

For extending graphene aerogels for broad applications, here we demonstrate a simple and universal approach for scalable fabricating novel dual carbon three-dimensional (3D) hybrid structures, where the interspace of a 3D carbon texture has been modified by in situ generating graphene aerogels. Owing to the unique exceptional 3D carbon bi-frameworks of enhanced electrical conductivity and flexibility, the as-prepared graphene aerogel–carbon texture hybrid presents an ultra-light feature (0.07 g cm⁻³ in density), with highly effective electromagnetic interference (EMI) shielding performance up to 27 dB and 37 dB (in the X band region) at thicknesses of 2 and 3 mm, respectively. According to the mechanisms in EMI shielding, the fundamental criteria for evaluating a shielding material has been discussed and the excellent shielding performance coupled with the ultra-low density allows such 3D all-carbon hybrids to show more advantageous than the other carbon-based shielding composites. Implication of the results suggests that the strategy of various advantages could be widely extended to a variety of applications, promising a great platform for large-scale fabricating porous graphene-based materials into high-performance products.     

High-yield graphene exfoliation using sodium dodecyl sulfate accompanied by alcohols as surface-tension-reducing agents in aqueous solution

The exfoliation of graphite was investigated in aqueous solutions containing sodium dodecyl sulfate (SDS) as a surfactant. The exfoliation was greatly enhanced near the surface aggregation concentration (SAC) of SDS, 2.6 mM, and then decreased for higher SDS contents. However, the flakes exfoliated near the SAC were graphite, whereas graphene was obtained above the critical micelle concentration (CMC). The effect of the use of alcohols as surface-tension-reducing agents (STRAs) on the exfoliation was then investigated. With ethyl alcohol, a dispersion of 2.1 mg ml⁻¹ graphene was achieved from 2.6 mM SDS after only 1 h of sonication, whereas a dispersion of 0.2 mg ml⁻¹ was obtained above the CMC in the absence of STRAs. The results demonstrate that the SDS content near the SAC is highly beneficial for exfoliation as long as the surface tension is maintained near 41.0 mN ml⁻¹. This finding supports the notion that the structure of the adsorbed SDS, depending on its concentration, strongly affects the exfoliation of graphite into graphene.     

Synthesis and properties of an atomically thin carbon nanosheet similar to graphene and its promising use as an organic thin film transistor

We synthesize an atomically thin carbon nanosheet (CNS) analogous to graphene with properties suitable for an organic thin film transistor (OTFT). The synthesis of graphene by chemical vapor deposition has serious drawbacks such as wrinkles, grain boundaries, and defects due to catalyst removal and transfer process. Here the CNS is directly synthesized on a silicon wafer by heat-treatment of spin-coated polyacrylonitrile and shows a higher electrical conductivity (>1600 S cm⁻¹) than that of chemically converted graphene. The CNS on glass, transferred from a silicon wafer, exhibits approximately 92% optical transmittance. We have used our CNS as the electrodes of OTFTs, and recorded a mobility (0.25–0.35 cm² V⁻¹ s⁻¹) that exceeds that of gold electrodes (0.2–0.25 cm² V⁻¹ s⁻¹).     

Investigation of the thermal behaviors of folded graphene by non-equilibrium molecular dynamics simulation

The thermal conductivities (TCs) of graphene and folded graphene (FG) were calculated by non-equilibrium molecular dynamics (NEMD) simulation with the Condensed-phase Optimized Molecular Potential for Atom Simulation Studies (COMPASS) force field. The heat flux of FG model is along the direction perpendicular to the graphene sheet fragments connected by two nearest kinked regions. By extrapolation of the NEMD TC data in order to obtain the corresponding macroscopic TC values, the calculated graphene TC is about 2212 W/mK, which is located within the previous simulation data. For FG, the predicted TC value is about 71.4 W/mK, which is only 3.3% that of the graphene TC, indicating the TC of graphene can be effectively reduced by folding the graphene. The phonon vibrational spectrums of graphene and FG were also calculated by the Fourier transform of the velocity autocorrelation function. The spectrum ranging from 1150 to 1650 cm⁻¹ and at peaks of 620, 750, 1250, 1750 cm⁻¹ of graphene are higher than those of FG. The decrease of the amplitudes of the long-wave phonon mode is the main reason for the reduction in the TC of FG.