Large Graphene Research Articles

Nanomechanical Strain Concentration on a Two-Dimensional Nanobridge within a Large Suspended Bilayer Graphene for Molecular Mass Detection

The recent emergence of strain gradient engineering directly affects the nanomechanics, optoelectronics and thermal transport fields in 2D materials. More specifically, large suspended graphene under very high stress represents the quintessence for nanomechanical mass detection through unique molecular reactions. Different techniques have been used to induce strain in 2D materials, for instance by applying tip indentation, pressure or substrate bending on a graphene membrane. Nevertheless, an efficient way to control the strain of a structure is to engineer the system geometry as shown in everyday life in architecture and acoustics. Similarly, we studied the concentration of strain in artificial nanoconstrictions (~100 nm) in a suspended epitaxial bilayer graphene membrane with different geometries and lengths ranging from 10 to 40 micrometer. We carefully isolated the strain signature from micro-Raman measurements and extracted information on a scale below the laser spot size by analyzing the broadened shape of our Raman peaks, up to 100 cm-1. We potentially measured a strong strain concentration in a nanoconstriction up to 5percent, which is 20 times larger than the native epitaxial graphene strain. Moreover, with a bilayer graphene, our configuration naturally enhanced the native asymmetric strain between the upper and lower graphene layers. In contrast to previous results, we can achieve any kind of complex strain tensor in graphene thanks to our structural approach. This method completes the previous strain-induced techniques and opens up new perspectives for bilayer graphene and 2D heterostructures based devices.

Secondary-Transferring Graphene Electrode for Stable FOLED

In this work, sharp wrinkles on graphene films, caused by graphene duplicating the grain boundary cracks of copper foil during the preparation process, were carefully explored. A secondary-transferring graphene film process was proposed to re-transform the “Peak” morphology of graphene surface into “Valley” form. The process we have developed is highly effective and almost nondestructive to the graphene through testing the surface morphology and photo-electric properties before and after the secondary-transferring process. Flexible organic light-emitting device (FOLED) with PEDOT:PSS/SLG/NOA63 framework as a targeted application was fabricated to illustrate the value of our proposed method in fabricating stable devices, the maximum luminance can reach about 35000 cd/m2, and the maximum current efficiency was 16.19 cd/A. This method can also be applied to the roll-to-roll preparation of large area high-quality graphene.

Graphene-based papers as substrates for cell growth: Characterisation and impact on mammalian cells

Graphene based material (GBM) substrates are promising candidates for tissue engineering and as cell scaffolds owing to their outstanding properties that favour cell adhesion and growth. In this study, we report the fabrication and use of different oxidised graphene-based ‘papers’ as substrates for cell culture, using starting materials of different thickness and lateral dimensions. Graphite oxide (GTO) consisted of thicker sheets than the thinner large graphene oxide (l-GO) and small graphene oxide (s-GO) sheets. The ‘papers’ were prepared by vacuum filtration of the material suspensions and characterised by an array of experimental techniques. The substrates were then evaluated for cellular adhesion and proliferation with two epithelial cell lines that differ in their morphologies, the human lung cell culture (A549) model and the human neural cell culture (SH-SY5Y) model using microscopic analysis. Release of lactate dehydrogenase (LDH) in cell supernatants was measured to assess any cytotoxic responses. Our results revealed that the GO papers had similar physicochemical properties but differing topographies. The papers supported cell growth and no cytotoxicity was observed. These results suggest that substrates based on graphite and graphene oxide papers are suitable biocompatible cellular supports for anchorage-dependent cell growth and can be further explored for tissue engineering, regenerative medicine, substrates for cell growth and bionic applications.

Strength of graphene grain boundaries under arbitrary in-plane tension

Understanding the tensile strength of graphene grain boundaries (GBs) is crucial for correlating the mechanical properties of two dimensional polycrystalline graphene with its atomic defect structure, a key to the success of large area graphene in many promising applications. Existing modeling studies mainly focus on the deformation and fracture of graphene GBs under tension that is perpendicular to the GBs. In reality, however, when a polycrystalline graphene is subject to a simple tension, random distribution of GBs in the graphene leads to arbitrary in-plane loading conditions of the GBs that cannot be fully understood with existing knowledge. To this end, we carry out systematic molecular dynamics (MD) simulations and also delineate a continuum mechanics model to investigate the failure strength of graphene GBs under tension in all possible loading directions. Particular focus is placed on quantitatively deciphering the interplay between GB misorientation angle and loading angle, and their effects on the failure strength of graphene GBs. Prediction from the continuum mechanics model based on a disclination dipole theory agrees well with the results from MD simulations. In this sense, the present study offers important insights on a better understanding of the mechanical properties of large area polycrystalline graphene.

Chemical Vapor Deposition Growth of Bernal-Stacked Bilayer Graphene by Edge-Selective Etching with H2O

Bernal-stacked bilayer graphene is uniquely suited for application in electronic and photonic devices because of its tunable band structure. Even though chemical vapor deposition (CVD) is considered to be the method of choice to grow bilayer graphene, the direct synthesis of high-quality, large-area Bernal-stacked bilayer graphene on Cu foils is complicated by overcoming the self-limiting nature of graphene growth on Cu. Here, we report a facile H2O-assisted CVD process to grow bilayer graphene on Cu foils, where graphene growth is controlled by injecting intermittent pulses of H2O vapor using a pulse valve. By optimizing CVD process parameters fully covered large area graphene with bilayer coverage of 77 ± 3.6% and high AB stacking ratio of 93 ± 3% can be directly obtained on Cu foils, which presents a hole concentration and mobility of 4.5 × 1012 cm–2 and 1100 cm2 V–1 s–1, respectively, at room temperature. The H2O selectively etches graphene edges without damaging graphene facets, which slows down the ...

Highly Conductive and Fracture-Resistant Epoxy Composite Based on Non-oxidized Graphene Flake Aerogel

Graphene aerogel (GA) has shown great promise as reinforcement of polymeric composites with exceptional electrical and mechanical characteristics. Although there has been significant progress in controlling the structure of GAs, no studies have appeared on the enhanced properties of GAs by employing high-quality precursor graphene flakes (GFs). However, the assembly of high-quality GFs is particularly challenging due to their highly hydrophobic and agglomerative nature in aqueous media, and of the few methods available to synthesize high-quality GFs, most produce flakes with very small lateral sizes. Herein, we report the fabrication of highly crystalline GAs using large nonoxidized graphene flakes (NOGFs) prepared by a novel graphite intercalation compound-based method. Bidirectional freeze casting is utilized for aligning NOGFs in two orthogonal directions, vertically and laterally, where the NOGF walls individually function as effective conductive pathways. The as-prepared nonoxidized graphene aerogel (NOGA) exhibits a defect concentration as low as 1.4% of impurity oxygen with an excellent electrical conductivity of 202.9 S/m at a low density of 5.7 mg/cm3. The corresponding NOGA-epoxy composite shows a remarkable electrical conductivity of 122.6 S/m and a fracture toughness of 1.74 MPa·m1/2 at a low filler content of 0.45 vol %.

Graphene growth through a recrystallization process in plasma enhanced chemical vapor deposition

Thermal chemical vapor deposition (TCVD) is the current method of choice to fabricate high quality, large area graphene films on catalytic copper substrates. In order to obtain sufficiently high growth rates at reduced growth temperatures an efficient dissociation of the precursor molecules already in the gas phase is required. We used plasma enhanced chemical vapor deposition (PECVD) to fabricate high quality graphene films at various temperatures. The efficient, plasma-induced dissociation of the precursor molecules results in an activation energy of 2.2 eV for the growth rate in PECVD, which is reduced by almost a factor of 2 compared to TCVD growth in the same reactor. By varying the growth time, we demonstrate that crystalline graphene grains surrounded by amorphous carbon formed during the early stage of growth merge into an almost defect-free graphene film with growth time via a recrystallization process. Almost defect-free graphene is prepared with negligible (ID/IG < 0.1) contributions of the D peak in Raman spectroscopy and with a sheet resistance down to 470 Ω/sq.

Analysis of graphene films grown on copper foil at 845 °C by intermediate pressure chemical vapor deposition

Chemical vapor deposition (CVD) is the widely accepted method for synthesis of large area graphene from thermal decomposition of hydrocarbons. However, the high temperature (above 1000 °C) requirement is the major drawback of this technique. The present study is an effort towards growing graphene on copper foil using methane at a temperature of 845 °C without incorporation of any additional energy sources. This is achieved by tailoring the growth parameters at 8 mbar in thermal chemical vapor deposition. The effect of total pressure and the effect of growth-supporting gases on the growth and number of layers of graphene were analyzed to select a favorable deposition condition. Graphene formation and its uniformity were examined by Raman spectroscopy, SEM, optical microscopy, AFM and TEM analysis. No studies till date, have reported a growth temperature as low as 845 °C for complete graphene coverage on copper by thermal CVD using methane as the precursor. The low temperature can reduce the thermal budget for graphene production to a certain extent.

Sensing ion channel in neuron networks with graphene field effect transistors

Graphene, the atomically-thin honeycomb carbon lattice, is a highly conducting 2D material whose exposed electronic structure offers an ideal platform for chemical and biological sensing. Its biocompatible, flexible and chemically inert nature associated with the lack of dangling bonds, offers novel perspectives for direct interfacing with biological molecules. Combined with its exceptional electronic and optical properties, this promotes graphene as a unique platform for bioelectronics. Among the successful bio-integrations of graphene, the detection of action potentials in numerous electrogenic cells including neurons has paved the road for the high spatio-temporal and wide-field mapping of neuronal activity. Ultimate resolution of sensing ion channel activity can be achieved with neural interfaces, and it was shown that macroscale electrodes can record extracellular current of individual ion channels in model systems, by charging the quantum capacitance of large graphene monolayer (mm2). Here, we show the field effect detection of ion channel activity within neuron networks, cultured during several weeks above graphene transistor arrays. Dependences upon drugs, reference potential gating and device geometry confirm the field effect detection of individual ion channel and suggest a significant contribution of grain boundaries, which provide highly sensitive nanoscale-sized sensing sites. Our theoretical analysis and simulations demonstrate that the ion gating of a single grain boundary in liquid affects the electronic transmission of the whole transistor channel, resulting in significant conductance variations. Monitoring the ion channels activity is of great interest as most of neurodegenerative diseases relied on channelopathies, which rely on ion channel abnormal activity. Thus, such highly sensitive and biocompatible neuro-electronics which open the way to FET detection at the sub-cell precision should be useful for a wide range of fundamental and applied research areas, including brain-on-chip, pharmacology, and in vivo monitoring or diagnosis.

Mass production of tunable multicolor graphene quantum dots from an energy resource of coke by a one-step electrochemical exfoliation

Herein, we originally propose a simple and facile one-step electrochemical exfoliation approach to synthesize in large scale graphene quantum dots (GQDs) from low-cost energy resource of coke. We found that the GQDs emerged multi-color fluorescence by adjusting the water content of electrolyte solution and applying current density, which effectually caused green-GQDs (G-GQDs), yellow-GQDs (Y-GQDs), and orange-GQDs (O-GQDs) of regular micro-variation in size from 3.02 to 4.61 nm with the fluorescence emissions at 500, 530 and 560 nm, respectively. Furthermore, blue-GQDs (B-GQDs) with emission at 440 nm can be obtained by reducing G-GQDs with NaBH4 at room temperature for 4 h. The quantum yields of B-, G-, Y- and O-GQDs are obtained up to 19.27%, 8.47%, 7.90% and 9.24%, respectively. The B-, G-, Y- and O-GQDs with high yields severally corresponding to 13.04 wt%, 17.88 wt%, 42.86 wt% and 31.13 wt%, possess scalable and industrial production prospect. All four types of GQDs were fully characterized and the electrochemical production mechanism of these GQDs has been also investigated in full. Additionally, GQDs were also available to fabricate their solid-state GQDs/epoxy composites for potential applications in multicolor light-emitting diode devices.

Simple Synthesis of Large Graphene Oxide Sheets via Electrochemical Method Coupled with Oxidation Process.

In this paper, we report a simple two-step approach for the synthesis of large graphene oxide (GO) sheets with lateral dimensions of ≈10 μm or greater. The first step is a pretreatment step involving electrochemical exfoliation of graphite electrode to produce graphene in a mixture of H2SO4 and H3PO4. The second step is the oxidation step, where oxidation of exfoliated graphene sheets was performed using KMnO4 as the oxidizing agent. The oxidation was carried out for different times ranging from 1 to 12 h at ∼60 °C. Prepared GO batches were characterized using a number of spectroscopy and microscopy techniques such as X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), Fourier-transform infrared spectroscopy (FTIR), and UV–visible spectroscopy. Raman and thermogravimetric analysis techniques were used to study the degree of oxidation in the as-synthesized GO batches. The UV–visible absorption spectrum showed an intense peak at 230 nm and an adjacent band at 300 nm corresponding to π–π* and n−π* transitions in all samples. Normalized FTIR plots were used to calculate the relative percentages of oxygen-containing functional groups, which were found to be maximum in GO (6 h). Boehm titration was used to quantify the functional groups present on the GO surface. Overall GO sheets obtained after 6 h of oxidation, GO (6 h), were found to be the best. XRD pattern of GO (6 h) revealed a characteristic peak at 2θ = 8.88°, with the corresponding interplanar spacing between the layers being 0.995 nm, which is among the best with respect to the previous methods reported in the literature. Raman spectroscopy showed that the degree of defect (ID/IG) area ratio for GO (6 h) was 1.24, which is higher than that obtained for GO (1.18) prepared by widely used Marcano’s approach.

Single layer graphene functionalized MEA for enhanced detection of neuronal network development

The exploitation of graphene for neuro-interfacing applications requires a complete, yet missing, understanding of neuron-graphene interaction. Here, we have explored the interplay between the carbon based interface and neuronal networks during the complete developmental phase at whole network scale. To this purpose, we have, first, successfully transferred large grains single layer graphene (LG-SLG) via wet etching onto commercial planar 60 electrode devices; then, we have compared to control the neuronal growth on the functionalized devices, recording the spontaneous activity up to completion of network maturation, i.e., from 7 to 25 days-in-vitro. The immunohistochemistry investigation demonstrated a comparable morphology of the neuronal network on SLG and control substrates but with a higher number of neurons on SLG. The most striking results of the electrophysiological investigation were the observation of spikes and bursts activity at an earlier developmental phase and of strongly synchronized neuronal networks on SLG-MEA versus control, suggesting an improved neuron/electrode coupling. These observations agree with our previous study of single neuron synaptogenesis by patch clamp, where earlier synaptogenesis on SLG compared to the control was detected. The results have been corroborated by the firing and bursting analysis showing higher and statistically significant values on SLG-MEA.

Highly conductive, mechanically strong graphene monolith assembled by three-dimensional printing of large graphene oxide

The manufacturing of three-dimensional (3D) graphene monolith with high mechanical and electrical performance has become an urgent issue in view of their potential applications in energy and electronics fields. Due to the structure rigidity and poor liquid-phase processing capability of graphene sheets, it is challenging to fabricate 3D graphene monolith with high mechanical performance, including strength, toughness and resiliency. Graphene oxide (GO) shows an improved dispersibility and reduction-restorable conductivity, which enables it to effectively balance the processing and comprehensive performances of graphene monolith. Here, we demonstrate a strategy to fabricate high-performance, shape-designable 3D graphene monolith through a 3D printing method based on large-sized graphene oxide (LGO) fluid ink. The concentration of the LGO ink for printing is as low as 20 mg/mL. The resulting monolith exhibits low density (12.8 mg/cm3), high electrical conductivity (41.1 S/m), high specific strength (10.7 × 103 N·m/Kg) and compressibility (up to 80% compressive strain). Such a 3D printing technique enables plenty of complicated monolith structures and broadens the application range of graphene.

Oxygen-Promoted Chemical Vapor Deposition of Graphene on Copper: A Combined Modeling and Experimental Study.

Mass production of large, high-quality single-crystalline graphene is dependent on a complex coupling of factors including substrate material, temperature, pressure, gas flow, and the concentration of carbon and hydrogen species. Recent studies have shown that the oxidation of the substrate surface such as Cu before the introduction of the C precursor, methane, results in a significant increase in the growth rate of graphene while the number of nuclei on the surface of the Cu substrate decreases. We report on a phase-field model, where we include the effects of oxygen on the number of nuclei, the energetics at the growth front, and the graphene island morphology on Cu. Our calculations reproduce the experimental observations, thus validating the proposed model. Finally, and more importantly, we present growth rate from our model as a function of O concentration and precursor flux to guide the efficient growth of large single-crystal graphene of high quality.

Electrochemical perspective on the size-dependent density of states at single graphene flake

To uncover the distinction in the electronic density of states (DOS) near Fermi level at the graphene and metal, we compare the standard rate constant k0 values for quinone species at graphene and gold nanoelectrodes, which might be affected by the electronic DOS near Fermi level. The experimental result indicates that the k0 value obtained at the individual graphene flake of nanometer size is very close to that obtained on gold nanoelectrode, and higher than that at large graphene flake. Unlike the high electronic DOS in gold nanoelectrodes, the difference of k0 values at graphene nanoelectrodes of different sizes will reveal the size-dependent DOS at single graphene flake.

Facile fabrication of large 3D graphene filler modified epoxy composites with improved thermal conduction and tribological performance

A simple method to fabricate three-dimensional graphene fillers (3DGFs) modified epoxy resin (EP) composite was developed in this study. This method offers a flexible fabrication process and the polymer composites obtained preserve to some extent the advantages of the traditional 3D graphene foam structure. The thermal conductivity of 3DGF/EP composites was increased successfully to 0.52 W/mK with only 0.14 vol% (0.27 wt%) graphene loading which is 1.74 times higher than that of the neat epoxy. This remarkable improvement normalized by graphene loading (i.e., the thermal conductivity enhancement efficiency) is amongst the highest reported values of carbon-based nanofiller-epoxy composites. The unique hollow 3D structure, defect-free feature and large length-thickness ratio of 3DGF compared to conventional graphene sheets are responsible for the high thermal conductivity of the epoxy composite. Further, finite element analyses also confirmed the positive effect of the filler length-thickness ratio on the thermal conductivity of 3DGF/EP composites which agreed well with experimental results. Moreover, the tribological performance of the composites was also improved by incorporation of 3DGFs.

Plasma Enhanced CVD Growth of Graphene on Cu and Ge

Thermal chemical vapor deposition (T-CVD) is currently the process of choice to grow large area graphene with good quality on metal surfaces like copper (Cu) or nickel (Ni) due to their catalytic properties. However the typical growth temperatures are still around 1000°C, and the process cannot be easily adapted to alternative, i.e. non-metallic, substrates. Regarding these issues we used a commercially available 4” Aixtron Black Magic system for establishing a plasma enhanced CVD process (PE-CVD) for graphene growth. A pulsed DC plasma was applied to dissociate the precursor material –methane- already in the gas phase and hence reduce the growth temperature. This process facilitates graphene fabrication with good quality down to a growth temperature of 600°C on Cu substrates. By studying the growth rate for different temperatures we have been able to extract a characteristic activation energy of 1.8 eV in PE-CVD, which is reduced by 2.2 eV as compared to T-CVD owing to the dissociation of methane already in the gas phase. In order to understand the plasma enhanced growth process, we analyzed the graphene formed for different growth times at a growth temperature of 700°C. After 45 min (Fig. 1a, top) we observe amorphous carbon around high quality graphene flakes. This is supported by Raman measurements (Fig. 1b), where Raman signatures of both, amorphous carbon and crystalline graphene are found. By increasing the growth time to 3 h, a continuous graphene film is obtained (Fig. 1a, bottom). In Raman spectroscopy, the amorphous signal disappears and Raman signatures of monolayer graphene with strongly reduced defect peak are detected. This indicates that through a recrystallization process the amorphous carbon turns into a graphene film around the crystalline graphene nuclei. The fabricated graphene film has a sheet resistance down to < 0.5 kΩ/sq. when transferred onto a Si/SiO2 substrate. Establishing a PE-CVD process at growth temperatures down to 600°C indicates that the need for catalytical substrates become superfluous. We thus adapted our approach to non-metallic substrates. As an example, we chose crystalline Ge(100), a CMOS compatible material that is used for example in broadband photodetectors in combination with graphene [1]. Graphene growth on Ge is typically performed by T-CVD at temperatures of 900°C or above. Such high temperatures might be critical for practical applications because of the temperature dependent dopant diffusion. [2] We demonstrate that our PE-CVD process can be applied to reduce the growth temperature of graphene on Ge(100) down to 800°C. We observed that by increasing the growth time from minutes to hours a 2D peak appears and the D and G bands isolate indicating growth is initiated from amorphous carbon and proceed to nanocrystalline graphene. In contrast to PE-CVD growth on Cu, a significant defect peak is observed for graphene grown on Ge(100), which we attribute to the numerous nucleation sites arising from the 100 nm large terraces as reported by Luskosius et al. [3] Fig. 1: Graphene growth in PE-CVD at 700°C on Cu foil at different growth times. a) Exemplary SEM images of the graphene flakes at growth times of 45 min (top) and 3 h (bottom). b) Average Raman spectra of the samples presented in a), extracted from the corresponding Raman maps.

Controllable Synthesis of Circular Graphene Domains By Atmosphere Pressure Chemical Vapor Deposition

Graphene, as a perfect two-dimensional crystalline structure, has attracted much attention in the research community, owing its unusual mechanical strength (>1Pa), optical transparency and high carrier mobility for various promising applications. Several methods have been used to develop to obtain graphene, such as mechanical exfoliation, thermal decomposition of SiC and chemical vapor deposition (CVD). Among these methods, CVD is considered to be the most promising way for graphene industrial development and scale-up production due to its convenience and economic efficiency. In a CVD process, growth on Cu substrates is particularly attractive since the growth occurs via a self-limiting mechanism, attributed to the negligible carbon solubility in copper. Yet the CVD graphene shows a lower carrier mobility of a few hundred to a few thousand cm2/(Vs) compared with exfoliated graphene obtained from natural graphite, because of the presence of growth defects and boundaries. In order to attain the predicted theoretical values for graphene, it is critical to improve graphene film quality. Previous studies have shown that seamless merging of graphene domains during CVD is an ideal route to reducing defects and domain boundaries, and produce single crystal graphene to enable further applications. Furthermore, the morphology and edge structure of individual graphene domains strongly influence its properties. It is very important to understand the nucleation stage and the expansion mechanism of the graphene domains to control the morphology of graphene films. Recent advances in the CVD method yields various interesting graphene domain shapes. Pentagonal, triangular, rectangular, snow-like graphene, twelve-pointed and four-lobed, graphene domains have been successfully obtained. The various shapes of these graphene domains can be achieved by optimizing growth parameters, such as precursor composition, flux, and choice of substrate. However, all of these methods either require liquid-state copper as the substrate or yield irregular circular or other shapes with one or very few separate circular graphene domain. Hence, it would be quite advantageous to grow graphene on a solid-state copper substrate via the simultaneous large scale growth and seamless merging of circular-shaped graphene domains. This provides a very good platform to observe and investigate the growth mechanism leading to large area single crystal graphene sheets. In this presentation, I will report the first successful large-scale growth of single-crystal “circular” graphene domains on a solid-state copper substrate during the early stage of CVD operated under atmosphere pressure. The individual circular graphene domains have smooth edges as well as axisymmetric and centrosymmetric shapes which permit seamless merging of adjacent graphene without the effect of orientation. Figure 1

Ambient-pressure CVD of graphene on low-index Ni surfaces using methane: A combined experimental and first-principles study

The growth of large area single-layer graphene (1-LG) is studied using ambient pressure chemical vapor deposition on single-crystal Ni(111), Ni(110), and Ni(100). By varying both the furnace temperature in the range of 800--1100 \ifmmode^\circ\else\textdegree\fi{}C and the gas flow through the growth chamber, uniform, high-quality 1-LG is obtained for Ni(111) and Ni(110) single crystals and for Ni(100) thin films. Surprisingly, only multilayer graphene growth could be obtained for single-crystal Ni(100). The experimental results are analyzed to determine the optimum combination of temperature and gas flow. Characterization with optical microscopy, Raman spectroscopy, and optical transmission support our findings. Density-functional theory calculations are performed to determine the energy barriers for diffusion, segregation, and adsorption, and model the kinetic pathways for formation of different carbon structures on the low-index surfaces of Ni.

Large area ultra-thin graphene films for functional photovoltaic devices

Abstract