Large-area Monolayer Graphene Research Articles

High mobility graphene field effect transistors on flexible EVA/PET foils

Monolayer graphene is a promising material for a wide range of applications, including sensors, optoelectronics, antennas, EMR shielding, flexible electronics, and conducting electrodes. Chemical vapor deposition (CVD) of carbon atoms on a metal catalyst is the most scalable and cost-efficient method for synthesizing high-quality, large-area monolayer graphene. The usual method of transferring the CVD graphene from the catalyst to a target substrate involves a polymer carrier which is dissolved after the transfer process is completed. Due to often unavoidable damage to graphene, as well as contamination and residues, carrier mobilities are typically 1000–3000 cm2(Vs)−1 , unless complex and elaborate measures are taken. Here, we report on a simple scalable fabrication method for flexible graphene field-effect transistors that eliminates the polymer interim carrier, by laminating the graphene directly onto office lamination foils, removing the catalyst, and depositing Parylene N as a gate dielectric and encapsulation layer. The fabricated transistors show field- and Hall-effect mobilities of 7000–10 000 cm2(Vs)−1 with a residual charge-carrier density of 2×1011 1 cm−2 at room temperature. We further validate the material quality by terahertz time-domain spectroscopy and observation of the quantum Hall effect at low temperatures in a moderate magnetic field of ∼5 T. The Parylene encapsulation provides long-term stability and protection against additional lithography steps, enabling vertical device integration in multilayer electronics on a flexible platform.

Assessing the quality of large-area monolayer graphene grown on liquid copper for size-selective ionic/molecular membrane separations

Monolayer graphene growth on liquid copper (Cu) has attracted attention due to advantages of a flat/smooth catalytic growth surface, high synthesis temperature (>1080 °C) as well as the possibility of forming graphene domains that are mobile on the liquid Cu with potential to minimize grain boundary defects and self-assemble into a continuous monolayer film. However, the quality of monolayer graphene grown on liquid copper and its suitability for size-selective ionic/molecular membrane separations has not been evaluated/studied. Here, we probe the quality of monolayer graphene grown on liquid Cu (via a metallurgical process, HSMG®) using Scanning Electron Microscope (SEM), High-resolution transmission electron microscope (HR-TEM), Raman spectroscopy and report on a facile approach to assess intrinsic sub-nanometer to nanometer-scale defects over centimeter-scale areas. We demonstrate high transfer yields of monolayer graphene (>93% coverage) from the growth substrate to polyimide track etched membrane (PITEM, pore diameter ∼200 nm) supports to form centimeter-scale atomically thin membranes. Next, we use pressure-driven transport of ethanol to probe defects > 60 nm and diffusion-driven transport of analytes (KCl ∼0.66 nm, L-Tryptophan ∼0.7–0.9 nm, Vitamin B12 ∼1–1.5 nm and Lysozyme ∼3.8–4 nm) to probe nanoscale and sub-nanometer scale defects. Diffusive transport confirms the presence of intrinsic sub-nanometer to nanometer scale defects in monolayer graphene grown on liquid Cu are no less than that in high-quality graphene synthesized via chemical vapor deposition (CVD) on solid Cu. Our work not only benchmarks quality of graphene grown on liquid copper for membrane applications but also provides fundamental insights into the origin of intrinsic defects in large-area graphene synthesized via bottom-up processes for membrane applications.

Minute-level-fast and recyclable large-area monolayer graphene transfer onto polymer membranes

Monolayer graphene-based composites are of immense importance for research and applications in the microelectronic and thermal-management fields. For the current continuous production of monolayer graphene protocols, chemical etching of the catalyst layer acts as the bottleneck, which is tedious, expensive, and polluting. Thus in this work, by combining chemical etching and mechanical exfoliation, copper foil etching has been compressed from hours to 30 s, with the foil entirely recyclable and reusable. Furthermore, the whole transfer process could be confined within 7 min, including all pre- and post-treatments, without the usage of any intermediate, glue, electricity, vacuum, heat, or pressure. And this minute-level-fast monolayer graphene transfer could be performed continuously multiple times. The key to this novel transfer method is the superficial etching and the subsequent delamination of copper foil off graphene. In-situ OM, AFM, SEM, Raman, and UV–vis spectra have been employed to carefully inspect the transfer mechanism, effectiveness, and quality. The prepared monolayer graphene/polymer composite membrane is ultrathin (∼200 nm), conductive, highly transparent (93.9%), and flexible simultaneously. Its application as an ultrathin, efficient, and conformable Joule heating membrane-form device has been demonstrated. This minute-level-fast transfer is well-adapted to continuous monolayer graphene production and could considerably improve their practicality and efficiency. Additionally, this study provides more understanding of the interface between graphene-polymer and graphene-catalyst and a minute-level-fast route to fabricate 2D-based polymeric composite materials.

In Situ Growth of Graphene on Polyimide for High-Responsivity Flexible PbS-Graphene Photodetectors.

Graphene is an ideal material for flexible optoelectronic devices due to its excellent electrical and optical properties. However, the extremely high growth temperature of graphene has greatly limited the direct fabrication of graphene-based devices on flexible substrates. Here, we have realized in situ growth of graphene on a flexible polyimide substrate. Based on the multi-temperature-zone chemical vapor deposition cooperated with bonding a Cu-foil catalyst onto the substrate, the growth temperature of graphene was controlled at only 300 °C, enabling the structural stability of polyimide during growth. Thus, large-area high-quality monolayer graphene film was successfully in situ grown on polyimide. Furthermore, a PbS-graphene flexible photodetector was fabricated using the graphene. The responsivity of the device reached 105 A/W with 792 nm laser illumination. The in-situ growth ensures good contact between graphene and substrate; therefore, the device performance can remain stable after multiple bending. Our results provide a highly reliable and mass-producible path for graphene-based flexible devices.

Two-step method for growth of adlayer-free large-area monolayer graphene on Cu foil

Chemical vapor deposition is the most promising approach for synthesis of large-area monolayer graphene on Cu foil. However, numerous factors can result in formation of adlayers, such as the morphology of the Cu foil, methane concentration, and growth temperature. Here, we report atmospheric pressure chemical vapor deposition growth of large-area adlayer-free monolayer graphene by the two-step ‘bottom-up-etching’ method. The experimental results showed that a temperature increase in the second step can dramatically accelerate etching of the bottom graphene layer. A growth model for adlayer-free monolayer graphene on Cu foil is proposed.

Controlled growth of large-area monolayer graphene on Ni (110) facet: Insight from molecular dynamics simulation

To achieve the controlled synthesis of high-quality graphene/nickel heterostructure in practice, the study of the atomistic mechanism of graphene growth on the Ni (110) facet whose lattice mismatches with graphene is indispensable. A series of molecular dynamics simulations based on the modified interatomic force field were performed to provide an insightful understanding of the process of graphene growth by chemical vapor deposition. Herein, the dissolution and precipitation mechanism of graphene in the initial stage of growth on Ni (110) has been demonstrated, followed by the dynamic evolution of graphene growth was elucidated. The effects of carbon deposit rate and annealing temperature on graphene growth were investigated in-depth, and the optimum carbon deposit rate and temperature in theory for homogeneous monolayer graphene growth on Ni (110) facet were determined. Furthermore, two self-healing routes of defects of embedded C-chain and C-heptagon for high-temperature catalyzed were analyzed. Lastly, the relevance of the chemical vapor deposition technique was discussed in terms of the actual deposition process. The theoretical investigations and practical discussions can provide an instructive reference for optimizing chemical vapor deposition processing conditions in preparing homogeneous monolayer graphene on the Ni (110) facet.

Controlled Growth of Large-Area Monolayer Graphene on Ni (110) Facet: Insight from Molecular Dynamics Simulation

Controlled Growth of Large-Area Monolayer Graphene on Ni (110) Facet: Insight from Molecular Dynamics Simulation

Unconventional Reaction Phase Diagram for the Penetration Etching/Growth of Graphene Adlayers

Cu has shown an advantage in growing monolayer graphene due to the very low C solubility and surface-mediated self-limiting growth, which hinders the growth of multilayer graphene. This work reports an unconventional penetration etching/growth of graphene adlayers tuned by oxygen beyond the self-limiting growth, supported by the C isotope labeling results. The effect of oxygen is nonmonotonic, i.e., with the increase of oxygen, graphene adlayers are etched without damaging the top layer, then shift to growth, and finally, all layers are etched. In addition, the reaction did not seem to reach equilibrium in the time range of the experiment but continued as if oxygen was increasing with respect to time. An oxygen-assisted exchange penetration model is proposed to interpret the growth mechanism. Oxygen etches the top shield layer, which is simultaneously healed by consuming the C species around adlayers and results in adlayer decomposition due to the break of equilibrium. Additionally, oxygen assists penetration of C from the gaseous agents into the shield layer for adlayer growth. The domination of one over the other depends on the concentration of oxygen, resulting in overall etching or growth. Finally, the synthesis of large-area monolayer and bilayer graphene films with good uniformity is demonstrated.

Enhancement of the adhesion energy between monolayer graphene and SiO2 by thermal annealing

Because graphene is placed on a rigid or flexible substrate for future applications such as transparent or flexible electronic devices, the interfacial characteristics between graphene and the underlying substrate are of great importance for developing mechanically and electrically reliable devices. In this work, the interaction strength of large-area monolayer graphene transferred on SiO2/Si was adjusted by thermal treatments, and its adhesion energy was measured by mode I fracture tests using a double cantilever beam (DCB) configuration with a laminated composite (Si/SiO2/graphene/epoxy/SiO2/Si). Mechanical delamination of monolayer graphene from SiO2 showed an adhesion energy of 1.5 ± 0.15 J/m2. It was found that thermal annealing of graphene on SiO2 at 300 and 450 °C increased the adhesion energy to 2.2 ± 0.20 and 4.6 ± 0.31 J/m2, respectively. Characterization of graphene on SiO2 using atomic force microscopy and Raman spectroscopy revealed that the enhanced adhesion energy was attributed to the improved conformal contacts of monolayer graphene with corrugated SiO2 surfaces.

An Efficient GFET Structure

A graphene field effect transistor, where the active area is made of monolayer large-area graphene, is simulated including a full 2D Poisson equation and a drift-diffusion model with mobilities deduced by a direct numerical solution of the semiclassical Boltzmann equations for charge transport by a suitable discontinuous Galerkin approach. The critical issue in a graphene field effect transistor is the difficulty of fixing the off state which requires an accurate calibration of the gate voltages. In the present paper we propose and simulate a graphene field effect transistor structure which has well-behaved characteristic curves similar to those of conventional (with gap) semiconductor materials. The introduced device has a clear off region and can be the prototype of devices suited for post-silicon nanoscale electron technology. The specific geometry overcomes the problems of triggering the minority charge current and gives a viable way for the design of electron devices based on large area monolayer graphene as substitute of standard semiconductors in the active area. The good field effect transistor behavior of the current versus the gate voltage makes the simulated device very promising and a challenging case for experimentalists.

Adlayer-free large-area single-crystal CVD graphene growth on copper

Chemical vapor deposition (CVD) graphene growth on copper foil has the potential to produce high-quality graphene at the industrial scale to realize its applications in future electronics and optoelectronic devices. During the graphene growth process, various parameters such as catalytic substrate, temperature, flow rate of precursor gases (methane and hydrogen) and presence of oxygen on the Cu substrate, directly affect graphene grain size, thickness and crystallinity. Researchers have been able to achieve large-area single-crystal monolayer graphene on Cu substrate. However, small fractions of bilayer or multilayer (adlayer) regions also appear along with single-crystal monolayer graphene. In recent years, researchers have been studying the effect of various growth parameters on adlayer formation. In this review, we have discussed the role and optimization of these growth parameters to achieve adlayer-free large-area single-crystal graphene on copper foil.

Molecular dynamics simulation of graphene sinking during chemical vapor deposition growth on semi-molten Cu substrate

Copper foil is the most promising catalyst for the synthesis of large-area, high-quality monolayer graphene. Experimentally, it has been found that the Cu substrate is semi-molten at graphene growth temperatures. In this study, based on a self-developed C–Cu empirical potential and density functional theory (DFT) methods, we performed systematic molecular dynamics simulations to explore the stability of graphene nanostructures, i.e., carbon nanoclusters and graphene nanoribbons, on semi-molten Cu substrates. Many atomic details observed in the classical MD simulations agree well with those seen in DFT-MD simulations, confirming the high accuracy of the C–Cu potential. Depending on the size of the graphene island, two different sunken-modes are observed: (i) graphene island sinks into the first layer of the metal substrate and (ii) many metal atoms surround the graphene island. Further study reveals that the sinking graphene leads to the unidirectional alignment and seamless stitching of the graphene islands, which explains the growth of large single-crystal graphene on Cu foil. This study deepens our physical insights into the CVD growth of graphene on semi-molten Cu substrate with multiple experimental mysteries well explained and provides theoretic references for the controlled synthesis of large-area single-crystalline monolayer graphene.

On the production, mechanical and electro-optic properties of large-area monolayer graphene films using Langmuir-Blodgett technique

We managed to produce truly transparent conductive monolayer graphene films using commercially available graphene nano-flakes by utilizing the Langmuir-Blodgett technique. The monolayer films could only be produced under specific solution concentration, sonication time, and evaporation conditions. The described method eliminates the need for intermediate processing steps, such as adding surfactant or annealing during the process. Film structure is characterized using scanning tunneling microscopy and high-resolution electron transmission microscopy. Mechanical characterization shows that the film exhibits a nonlinear stress-strain curve with a strain stiffening behavior in agreement with previous theoretical predictions. Film stiffness ranges between 40 and 80 MPa. The produced monolayer graphene films exhibit full transparency in the wavelength range of 400 nm to 1100 nm, and an excellent electrical conductivity ∼104 S cm−1 which is comparable to the transferred films produced by CVD, and comparable to that of ITO coating. These films have great potential applications in many different fields including transparent electrodes, sensor, optoelectronic devices and photovoltaics.

Enhanced dynamic performance of twisted and coiled soft actuators using graphene coating

Twisted and coiled actuators (TCAs) composed of fishing lines or sewing threads are one of the most promising artificial muscles because of their high specific power, large stroke, and low fabrication cost. Since the TCA can be thermally actuated, the thermal properties of the actuator determine its actuation performance. Here, graphene materials with high thermal conductivity and low thermal mass were used as coating materials on nylon TCA surfaces to enhance the dynamic responses of the TCA. Large-area monolayer graphene and porous graphene foam were wrapped on the TCA surface and provided a reduction of the actuation cycle times by 10.1 and 17.8%, respectively. Furthermore, high-quality multi-layer graphene flakes were spray-coated on the surface of the TCA, showing a dramatic decrease in the cycle time by 30.9%. The enhanced heat transfer due to the graphene flake coating also increased the maximum strain of the TCA over 3 times under a duty cycle of a pulsed current input. This demonstrates an alternative way to improve the dynamic performance of the TCA using a simple coating method.

Criteria for the growth of large-area adlayer-free monolayer graphene films by chemical vapor deposition

Homogeneity is important to material applications for good performance of individual devices, for making AB-stacked bilayer graphene in a layer-by-layer stacking order, and from the point of view of industrial production. Among many properties to be controlled, for the case of graphene, the thickness (or layer number) uniformity is the prerequisite. Chemical vapor deposition (CVD) of C precursors on Cu substrates is the most popular method to produce large-area graphene films. To date, precise control on the number of graphene layers as well as the uniformity over a large area is still very challenging. In this work, with a further understanding of the factors affecting adlayer growth, the synthesis of large-area adlayer-free monolayer graphene (MLG) films was achieved up to tens of squared centimeters in area by just using untreated Cu foil and a normal CVD process. We found that keeping equal C precursor concentration on the two sides of the Cu substrate is a criterion in addition to other factors such as the ratio of H:C and the substrate surface morphology for the growth of adlayer-free MLG. This finding is not only of great significance for the industrial production of large-area adlayer-free MLG films but also instructive for the synthesis of homogeneous few-layer graphene.

Homogeneous Large-Area Quasi-Free-Standing Monolayer and Bilayer Graphene on SiC

In this study, we first show that the argon flow during epitaxial graphene growth is an important parameter to control the quality of the buffer and the graphene layer. Atomic force microscopy (AFM) and low-energy electron diffraction (LEED) measurements reveal that the decomposition of the SiC substrate strongly depends on the Ar mass flow rate while pressure and temperature are kept constant. Our data are interpreted by a model based on the competition of the SiC decomposition rate, controlled by the Ar flow, with a uniform graphene buffer layer formation under the equilibrium process at the SiC surface. The proper choice of a set of growth parameters allows the growth of defect-free, ultra-smooth and coherent graphene-free buffer layer and bilayer-free monolayer graphene sheets which can be transformed into large-area high-quality quasi-freestanding monolayer and bilayer graphene (QFMLG and QFBLG) by hydrogen intercalation. AFM, scanning tunneling microscopy (STM), Raman spectroscopy and electronic transport measurements underline the excellent homogeneity of the resulting quasi-freestanding layers. Electronic transport measurements in four-point probe configuration reveal a homogeneous low resistance anisotropy on both {\mu}m- and mm scales.

Flexible and transparent supercapacitor based on ultrathin Au/graphene composite electrodes

Large-area monolayer graphene by chemical vapor deposition as electrode in electronic devices always suffers from shortages of low conductivity and high contact resistance. In this work, we demonstrate a composite electrode by depositing a continuous and homogeneous ultrathin 7-nm Au film on monolayer graphene with low average sheet resistance of 24.6 Ω/□ and high transmittance of 74.6% at 520 nm. The excellent properties of the composite electrode are attributed to the suppressed Au Volmer-Weber growth mode of the ultrathin Au film by the organized hexagonal carbon lattices of graphene. Rigid supercapacitor based on the composite electrode exhibits a capacitance of 81.5 μF/cm2 at the scan rate of 0.1 V/s, which is 17 times higher than the device based on monolayer graphene. Flexible supercapacitor based on the composite electrode can be bending at curvature radius of 2 mm and with slight performance decrease over 1800 bending cycles.

Elimination of step bunching in the growth of large-area monolayer and multilayer graphene on off-axis 3C[sbnd]SiC (111)

Multilayer graphene has exhibited distinct electronic properties such as the tunable bandgap for optoelectronic applications. Among all graphene growth techniques, thermal decomposition of SiC is regarded as a promising method for production of device-quality graphene. However, it is still very challenging to grow uniform graphene over a large-area, especially multilayer graphene. One of the main obstacles is the occurrence of step bunching on the SiC surface, which significantly influences the formation process and the uniformity of the multilayer graphene. In this work, we have systematically studied the growth of monolayer and multilayer graphene on off-axis 3CSiC(111). Taking advantage of the synergistic effect of periodic SiC step edges as graphene nucleation sites and the unique thermal decomposition energy of 3CSiC steps, we demonstrate that the step bunching can be fully eliminated during graphene growth and large-area monolayer, bilayer, and four-layer graphene can be controllably obtained on high-quality off-axis 3CSiC(111) surface. The low energy electron microscopy results demonstrate that a uniform four-layer graphene has been grown over areas of tens of square micrometers, which opens the possibility to tune the bandgap for optoelectronic devices. Furthermore, a model for graphene growth along with the step bunching elimination is proposed.

Folding Large Graphene-on-Polymer Films Yields Laminated Composites with Enhanced Mechanical Performance.

A folding technique is reported to incorporate large-area monolayer graphene films in polymer composites for mechanical reinforcement. Compared with the classic stacking method, the folding strategy results in further stiffening, strengthening, and toughening of the composite. By using a water-air-interface-facilitated procedure, an A5-size 400 nm thin polycarbonate (PC) film is folded in half 10 times to a ≈0.4 mm thick material (1024 layers). A large PC/graphene film is also folded by the same process, resulting in a composite with graphene distributed uniformly. A three-point bending test is performed to study the mechanical performance of the composites. With a low volume fraction of graphene (0.085%), the Young's modulus, strength, and toughness modulus are enhanced in the folded composite by an average of 73.5%, 73.2%, and 59.1%, respectively, versus the pristine stacked polymer films, or 40.2%, 38.5%, and 37.3% versus the folded polymer film, proving a remarkable mechanical reinforcement from the combined folding and reinforcement of graphene. These results are rationalized with combined theoretical and computational analyses, which also allow the synergistic behavior between the reinforcement and folding to be quantified. The folding approach could be extended/applied to other 2D nanomaterials to design and make macroscale laminated composites with enhanced mechanical properties.

Highly Oriented Monolayer Graphene Grown on a Cu/Ni(111) Alloy Foil.

Fast-growth of single crystal monolayer graphene by CVD using methane and hydrogen has been achieved on "homemade" single crystal Cu/Ni(111) alloy foils over large area. Full coverage was achieved in 5 min or less for a particular range of composition (1.3 at.% to 8.6 at.% Ni), as compared to 60 min for a pure Cu(111) foil under identical growth conditions. These are the bulk atomic percentages of Ni, as a superstructure at the surface of these foils with stoichiometry Cu6Ni1 (for 1.3 to 7.8 bulk at.% Ni in the Cu/Ni(111) foil) was discovered by low energy electron diffraction (LEED). Complete large area monolayer graphene films are either single crystal or close to single crystal, and include folded regions that are essentially parallel and that were likely wrinkles that "fell over" to bind to the surface; these folds are separated by large, wrinkle-free regions. The folds occur due to the buildup of interfacial compressive stress (and its release) during cooling of the foils from 1075 °C to room temperature. The fold heights measured by atomic force microscopy (AFM) and scanning tunneling microscopy (STM) prove them to all be 3 layers thick, and scanning electron microscopy (SEM) imaging shows them to be around 10 to 300 nm wide and separated by roughly 20 μm. These folds are always essentially perpendicular to the steps in this Cu/Ni(111) substrate. Joining of well-aligned graphene islands (in growths that were terminated prior to full film coverage) was investigated with high magnification SEM and aberration-corrected high-resolution transmission electron microscopy (TEM) as well as AFM, STM, and optical microscopy. These methods show that many of the "join regions" have folds, and these arise from interfacial adhesion mechanics (they are due to the buildup of compressive stress during cool-down, but these folds are different than for the continuous graphene films-they occur due to "weak links" in terms of the interface mechanics). Such Cu/Ni(111) alloy foils are promising substrates for the large-scale synthesis of single-crystal graphene film.