The transformation of a gold film on few-layer graphene to produce either hexagonal or triangular nanoparticles during annealing

The shape transformation of gold directly on graphene has been well studied by thermally annealing gold-deposited graphene samples at the temperature range from 600 to 800 °C. We find that few-layer graphene can be served as a platform to transform a gold film into mainly hexagonal gold nanoparticles (AuNPs) at 600 or 700 °C, or coexistence of hexagonal and triangular AuNPs at 800 °C. Especially, the size and density of these AuNPs are dependent on the number of graphene layers, indicating the strong relationship between gold shape transformation and the number of graphene layers on the substrate. We propose that annealing-induced growth of gold islands and the layer-dependent interactions among Au and n-layer graphene are the two main causes for this shape transformation. Meanwhile, Raman enhancing effects of these AuNPs are also investigated. These faceted AuNPs exhibit excellent SERS effects on Raman spectra of few-layer graphene with the enhancement factors up to several hundreds. Combined with n-layer graphenes, these faceted AuNPs can be used as graphene-based SERS substrates for increasing Raman signals of adsorbed rhodamine 6G molecules with a larger scale than those based on fresh graphenes.     

Thermal chemical vapor deposition grown graphene heat spreader for thermal management of hot spots

Graphene of different layer numbers was fabricated using thermal chemical vapor deposition (TCVD), and it was demonstrated as a heat spreader in electronic packaging. Platinum thermal evaluation chips were used to evaluate the thermal performance of the graphene heat spreaders. The temperature of a hot spot driven at a heat flux of up to 430 W cm⁻² was decreased from 121 °C to 108 °C (ΔT ≈ 13 °C) with the insertion of the monolayer graphene heat spreader, compared with the multilayer (n = 6–10) ones’ temperature drop of ∼8 °C. Various parameters affecting the thermal performance of graphene heat spreaders were discussed, e.g. layer numbers of graphene, phonon scattering, thermal boundary resistance. We demonstrate the potentials of using a complementary metal oxide semiconductor compatible TCVD process to utilize graphene as a heat spreader for heat dissipation purposes.     

Non-metal catalytic synthesis of graphene from a polythiophene monolayer on silicon dioxide

Direct synthesis of graphene without metal catalysts on a dielectric substrate is a major goal in graphene-based electronics and is an increasingly popular nanotechnology alternative to metal oxide semiconductor technology. However, current methods for the synthesis of these graphenes have many limitations, including the use of metal catalyst. Herein, we report a facile approach to the direct synthesis of graphene sheets based on the self-assembled monolayers (SAMs) technique. The new method for metal catalyst-free direct synthesis of a graphene sheet is through a solution-processable, inexpensive, easy, and reproducible cross-linked polythiophene self-assembled monolayer (SAM) that is formed via the [4 + 2] π cycloaddition reaction of π-electron conjugated thiophene layer self-assembled on the dielectric silicon dioxide substrate. The bifunctional molecules were carefully designed to create an SAM via silanization of alkoxy silane groups on the SiO₂ substrate, and at the other end, a thin cross-linked polythiophene layer via a [4 + 2] π-electron cycloaddition reaction of π-electron conjugated thiophene SAM. By heating the cross-linked polythiophene SAM up to 1000 °C under a high vacuum, single-layered or few-layered graphene sheets were successfully prepared on the dielectric silicon oxide substrate.     

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.     

A high density of vertically-oriented graphenes for use in electric double layer capacitors

The growth, capacitance and frequency response of vertically-oriented graphenes grown by radio frequency plasma-enhanced chemical vapor deposition on nickel substrates and used as electrodes in electric double layer capacitors (EDLCs) are presented. The graphenes grown on the grain boundary of substrates show a faster growth rate, but less ordered structure than in the center of the nickel grain. At a few nanometers away from the grain boundaries the graphenes grow vertically at the rate of 70–80 nm per minute. The film height increased linearly with growth time from 700 nm (10 min sample) to 3.1 μm (40 min sample). Raman spectra show that the intensity ratio of the D band to G band gradually decreased with growth time to a value of 0.5, indicating that the crystalline order of the graphene increases with height. The specific capacitance of symmetric, parallel plate EDLC devices fabricated with these films was found to increase in a linear fashion with growth time up to values greater than 120 μF/cm² at 1 kHz. An impedance phase angle of ?45° was reached at 30 kHz. Specific capacitance normalized to growth height suggests that mechanisms other than double layer charge storage on planar surface area were operative.     

The effect of growth parameters on the intrinsic properties of large-area single layer graphene grown by chemical vapor deposition on Cu

We present a comprehensive study of the parameter space for single layer graphene growth by chemical vapor deposition on Cu. The temperature is the most widely recognized control parameter in single layer graphene growth. We show that the methane-to-hydrogen ratio and the growth pressure also are critical parameters that affect the structural perfection and the cleanliness of graphene. The optimal conditions for suppressing double and multilayer graphene growth occur near 1000 °C, 1:20 methane-to-hydrogen ratio, and a total pressure in the range from 0.5 to 1 Torr. Raman mapping of a 40 × 30 μm² area shows single layer domains with 5–10 μm linear dimensions. Atomic resolution imaging of suspended graphene by aberration corrected scanning transmission electron microscopy shows that the single layer graphene consists of areas of 10–15 nm linear dimensions and smaller patches of residual contamination that was undetected by other characterization methods.     

Porous structures in stacked,crumpled and pillared graphene-based 3D materials

Graphene, an atomically thin material with the theoretical surface area of 2600 m² g⁻¹, has great potential in the fields of catalysis, separation, and gas storage if properly assembled into functional 3D materials at large scale. In ideal non-interacting ensembles of non-porous multilayer graphene plates, the surface area can be adequately estimated using the simple geometric law ∼2600 m² g⁻¹/N, where N is the number of graphene sheets per plate. Some processing operations, however, lead to secondary plate–plate stacking, folding, crumpling or pillaring, which give rise to more complex structures. Here we show that bulk samples of multilayer graphene plates stack in an irregular fashion that preserves the 2600/N surface area and creates regular slot-like pores with sizes that are multiples of the unit plate thickness. In contrast, graphene oxide deposits into films with massive area loss (2600–40 m² g⁻¹) due to nearly perfect alignment and stacking during the drying process. Pillaring graphene oxide sheets by co-deposition of colloidal-phase particle-based spacers has the potential to partially restore the large monolayer surface. Surface areas as high as 1000 m² g⁻¹ are demonstrated here through colloidal-phase deposition of graphene oxide with water-dispersible aryl-sulfonated ultrafine carbon black as a pillaring agent.     

Nanoarchitecture of variable sized graphene nanosheets incorporated into three-dimensional graphene network for dye sensitized solar cells

A novel three-dimensional (3D) nanoarchitecture consisting of hybrid graphene nanosheets (GNs)/graphene foam (GF) was fabricated on the FTO conducting substrate as a high efficient counter electrode (CE) for dye sensitized solar cells (DSSCs). The GNs with various sized such as large-sized heat-reduced graphene nanosheets (H-GNs) and small-sized laser-reduced graphene quantum dots (L-GQDs) were synthesized and used as catalytic materials incorporated into a 3D GF network, respectively. In this design, the aggregations and restacking of GNs were efficiently reduced, which is beneficial for increasing the amount of the active defective sites at the edges of graphene to the electrolyte solution. Especially, L-GQDs with smaller dimension less than 100 nm have more active defective sites at edges, providing superiority over the large-sized H-GNs in terms of electrocatalytic activity. Meanwhile, the GF network with high conductivity provides fast electron transport channels for charge injection between the GNs and FTO. The DSSC with this hybrid CE exhibited energy conversion efficiency (η) of 7.70% with an open circuit voltage (V_OC), short circuit photocurrent density (J_SC) and fill factor (FF) of 760 mV, 15.21 mA cm⁻², and 72.0%, respectively, which is comparable to that of the conventional Pt CE (7.68%).     

Graphene nanochains and nanoislands in the layers of room-temperature fluorinated graphite

Intercalated compound of graphite fluoride with n-heptane has been synthesized at room temperature using a multi-stage process including fluorination by a gaseous BrF₃ and a set of intercalant exchange reactions. It was found that composition of the compound is CF_0.40(C₇H₁₆)_0.04 and the guest molecules interact with the graphite fluoride layers through the van der Waals forces. Since the distance between the filled layers is 1.04 nm and the unfilled layers are separated by ∼0.60 nm, the obtained compound can be considered as a stack of the fluorinated graphenes. These fluorinated graphenes are large in area making it possible to study local destruction of the π conjugated system on the basal plane. It was shown that fluorine atoms form short chains, while non-fluorinated sp² carbon atoms are organized in very narrow ribbons and aromatic areas with a size smaller than 3 nm. These π electron nanochains and nanoislands preserved after the fluorination process are likely responsible for the value of the energy gap of the compound of ∼2.5 eV. Variation in the size and the shape of π electron regions within the fluorinated graphene layers could be a way for tuning the electronic and optical characteristics of the graphene-based materials.     

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.     

EELS study of the epitaxial graphene/Ni(1 1 1) and graphene/Au/Ni(1 1 1) systems

We have performed electron energy-loss spectroscopy (EELS) studies of Ni(1 1 1), graphene/Ni(1 1 1), and the graphene/Au/Ni(1 1 1) intercalation-like system at different primary electron energies. A reduced parabolic dispersion of the π plasmon excitation for the graphene/Ni(1 1 1) system is observed compared to that for bulk pristine and intercalated graphite and to linear for free graphene, reflecting the strong changes in the electronic structure of graphene on Ni(1 1 1) relative to free-standing graphene. We have also found that intercalation of gold underneath a graphene layer on Ni(1 1 1) leads to the disappearance of the EELS spectral features which are characteristic of the graphene/Ni(1 1 1) interface. At the same time the shift of the π plasmon to the lower loss-energies is observed, indicating the transition of initial system of strongly bonded graphene on Ni(1 1 1) to a quasi free-standing-like graphene state.     

Thermal release of hydrogen retained in multilayer graphene films prepared by mist-chemical vapor deposition

In this study, we investigated the absorption and thermal desorption processes of H and H₂O and the thickness of multilayer graphene films deposited on Cu foils using a mist-chemical vapor deposition method. Ion beam analysis techniques such as nuclear reaction analysis (NRA), elastic recoil detection (ERD), and Rutherford backscattering spectrometry (RBS) were employed. The RBS measurements revealed that the thickness of the multilayer graphene films was approximately 8 ± 3 nm (24 ± 9 layers). The depth distribution of H was analyzed using NRA and ERD. Based on these measurements, the residual H/C ratio for multilayer graphene was estimated to be approximately 0.03 in the bulk and 0.88 on the top-most surface. Additionally, the thermal desorption temperature for H from the multilayer graphene film was less than 373 K, which was much lower than that from isotropic graphite bulk (approximately 673 K). These results suggest that the thermal release of H did not occur because of desorption from sp²- and sp³-hybridized C atoms, such as intercalation and defect sites. Instead, it occurred owing to the desorption of H₂O adsorbed near the surface.     

Formation of high-quality quasi-free-standing bilayer graphene on SiC(0 0 0 1) by oxygen intercalation upon annealing in air

We report on the conversion of epitaxial monolayer graphene on SiC(0 0 0 1) into decoupled bilayer graphene by performing an annealing step in air. We prove by Raman scattering and photoemission experiments that it has structural and electronic properties that characterize its quasi-free-standing nature. The (6√3 × 6√3)R30° buffer layer underneath the monolayer graphene loses its covalent bonding to the substrate and is converted into a graphene layer due to the oxidation of the SiC surface. The oxygen reacts with the SiC surface without inducing defects in the topmost carbon layers. The high-quality bilayer graphene obtained after air annealing is p-doped and homogeneous over a large area.     

Synthesis of large area carbon nanosheets for energy storage applications

We report the large area growth of highly conductive carbon nanosheets (CNS) composed of few layer graphene on 200 mm diameter Si substrates using conventional radio frequency plasma-enhanced chemical vapour deposition. Raman spectroscopy is used to characterise the evolution of the CNS nucleation and growth with time in conjunction with TEM revealing the nano-sized graphene-like nature of these films and the intimate contact to the substrate. An individual sheet can have edges as thin as 3 graphene layers. The influence of the growth support layer is also discussed as film growth is compared on titanium nitride (TiN) and directly on Si. Electrochemical cyclic voltammogram (CV) measurements reveal these layers to form an excellent electrical contact to the underlying substrate with excellent stability towards oxidation whilst having a large electrochemical surface area. The resistance of a 150 nm film was measured to be as low as 20 μohm cm. The high percentage of narrow few layer graphene edge sites exposed allows for faster electrochemical reaction rates compared to carbon nanotubes (CNTs) and other electrode materials (glassy carbon and Pt).     

Raman Spectroscopy and Kelvin Probe Force Microscopy characteristics of the CVD suspended graphene

In this work we present combined Kelvin probe force microscopy and Raman spectroscopy studies of supported and suspended structures formed out of chemical vapor deposition (CVD) grown graphene. Work function of both suspended and supported graphene was -4.81 ± 0.06eV and -4.92 ± 0.06eV respectively. By G and 2D modes correlation we showed, that CVD graphene was influenced by biaxial strain. Increased contact potential difference (CPD) on the suspended graphene in comparison with the areas of the supported graphene was the sign of increased strain (from 0.05% to ~ 0.12%) rather than decreased doping (p-doping decreased from ~ 5.5 × 10¹²cm^-2 to ~ 4.5 × 10¹²cm^-2).     

Control of refractive index by annealing to achieve high figure of merit for TiO2/Ag/TiO2 multilayer films

We modified the refractive index (n) of TiO₂ by annealing at various temperatures to obtain a high figure of merit (FOM) for TiO₂/Ag/TiO₂ (45 nm/17 nm/45 nm) multilayer films deposited on glass substrates. Unlike the as-deposited and 300 °C-annealed TiO₂ films, the 600 °C-annealed sample was crystallized in the anatase phase. The as-deposited TiO₂/Ag/as-deposited TiO₂ multilayer film exhibited a transmittance of 94.6% at 550 nm, whereas that of the as-deposited TiO₂/Ag/600 °C-annealed TiO₂ (lower) multilayer film was 96.6%. At 550 nm, n increased from 2.293 to 2.336 with increasing temperature. The carrier concentration, mobility, and sheet resistance varied with increasing annealing temperature. The samples exhibited smooth surfaces with a root-mean-square roughness of 0.37–1.09 nm. The 600 °C-annealed multilayer yielded the highest Haacke's FOM of 193.9×10⁻³ Ω⁻¹.     

Vertical graphene gas- and bio-sensors via catalyst-free,reactive plasma reforming of natural honey

A rapid reforming of natural honey exposed to reactive low-temperature Ar + H₂ plasmas produced high-quality, ultra-thin vertical graphenes, without any metal catalyst or external heating. This transformation is only possible in the plasma and fails in similar thermal processes. The process is energy-efficient, environmentally benign, and is much cheaper than common synthesis methods based on purified hydrocarbon precursors. The graphenes retain the essential minerals of natural honey, feature reactive open edges and reliable gas- and bio-sensing performance.     

Influence of processing on fracture toughness of Si3N4 + graphene platelet composites

Silicon nitride + 1 wt% graphene platelet composites were prepared using various graphene platelets (GPL) and two processing routes; hot isostatic pressing (HIP) and gas pressure sintering (GPS). The influence of the processing route and graphene platelets’ addition on the fracture toughness has been investigated. The matrix of the composites prepared by GPS consists of Si₃N₄ grains with smaller diameter in comparison to the composites prepared by HIP. The indentation fracture toughness of the composites was in the range 6.1–9.9 MPa m^0.5, which is significantly higher compared to the monolithic silicon nitride 6.5 and 6.3 MPa m^0.5. The highest value of K_IC was 9.9 MPa m^0.5 in the case of composite reinforced by the smallest multilayer graphene nanosheets, prepared by HIP. The composites prepared by GPS exhibit lower fracture toughness, from 6.1 to 8.5 MPa m^0.5. The toughening mechanisms were similar in all composites in the form of crack deflection, crack branching and crack bridging.     

Alkylated graphene nanosheets for supercapacitor electrodes: High performance and chain length effect

Single electrode materials capable of both electric double-layer and Faradic redox-based pseudo capacitance can be used for fabrication of high performance supercapacitors in an easy way and thus are highly desirable in the energy storage field. This contribution reports a new kind of such materials based on alkylated graphene materials (C_nrGO, n is the carbon number of their alkyl side chains) having different alkyl side chains (n = 4, 8, and 16). These materials were prepared via esterification of KOH-treated GO with the corresponding alkyl bromides in the presence of a phase transfer catalyst. More importantly, water was used as the reaction medium, and thus endowing the preparation method an eco-friend feature. The so-prepared graphene materials displayed chain length-dependent specific surface area and the population of residue CO functionalities, and thus affording vast differences in their supercapacitor behaviors. C₄rGO, the product having butyl side chains, showed the best supercapacitor performance with a capacitance up to 242.2 F g⁻¹ at a scan rate of 100 mV s⁻¹ and a good cycling stability.     

Grand canonical Monte Carlo simulations of nitrogen adsorption on graphene materials with varying layer number

Fabrication of monolayer graphene is a challenge and many processes yield few-layer or multi-layer graphene materials instead. The layer number is an important property of those materials and a quality control variable in graphene manufacture. We demonstrated that N₂ adsorption on graphene materials was used to distinguish its layer number. We performed grand canonical Monte Carlo simulation of N₂ adsorption on graphene materials with 1–10 layers to indicate the possibility of distinction of layer number by evaluating the dependence of N₂ adsorption characteristics on the layer number of graphene materials as well as the adsorption mechanism. The threshold relative pressures of monolayer adsorption of N₂ on monolayer and two-layer graphene were 1 × 10⁻³ and 2 × 10⁻⁴, respectively, while those of the others were 1 × 10⁻⁴. In contrast, the threshold pressures of second layer adsorption of N₂ were similar to each other. The difference of threshold pressures is attributed to stabilized energies induced by interactions with graphene materials. Therefore, the layer number of graphene materials could be evaluated from the threshold pressures of adsorption, providing a guide to aid fabrication of graphene materials.