Quality Graphene Research Articles

Effect of hydrogen on chemical vapor deposition growth of graphene on Au substrates

Chemical vapor deposition (CVD) on substrates with low C solubility such as Cu and Au is promising to grow monolayer graphene selectively in a large scale. Hydrogen is often added to control the domain size of graphene on Cu, while Au does not require H2 since Ar is inert against oxidation. The effect of H2 should be revealed to improve the quality of graphene on Au. Here we report the effect of H2 on the CVD growth of graphene on Au substrates using in situ radiation-mode optical microscopy. The in situ observation and ex situ Raman spectroscopy revealed that whether H2 was supplied or not strongly affected the growth rate, thermal radiation contrast, and compressive strain of graphene on Au. We attributed these features to the surface reconstruction of Au(001) depending on H2 supply. Our results are essential to achieve the graphene growth with high quality on Au for future applications.

An improved process for the graphene preparation via redox potential control

Abstract

One-pot preparation of reduced graphene oxide from liquid nitrogen tetroxide via redox potential control

Reduced graphene oxide (RGO) was prepared by redox method with liquid nitrogen tetroxide (N2O4) as oxidant source and sodium borohydride (NaBH4) as reducing agent, which could provide an effective way for the treatment and reuse of liquid propellant oxidants. After N2O4 was pretreated with 60 wt% hydrogen peroxide (H2O2), fluorocarbon surfactants (FC-4330), a nonionic polymerized yellow viscous liquid that can significantly reduce the surface tension of the solvent at very low concentrations, was added to the mixture of N2O4 and natural graphite. In order to simplify the synthetic route of graphene, we introduced the redox potential (ORP), a comprehensive index to measure the degree of redox of the reaction system, by which we successfully fused the oxidation stage and reduction stage in the same reaction vessel, in other words, the one-pot method for preparation of RGO has been successfully realized. This method can also be used to accumulate graphene products and determine the yields directly without the necessity to purify and deal with numerous intermediate products. Furthermore, the redox potential of the reaction system was continuously optimized and precisely regulated by automatic sample injection to create the best redox atmosphere for the preparation of RGO, so as to achieve the purpose of improving the yield and quality of graphene. Finally, the optimal process conditions for the preparation of RGO by one-pot method were obtained, that is, the optimal initial ORP range was [1700, 1750] mV at the oxidation stage and [−1200, −1180] mV at the reduction stage. The graphene products can eventually be exfoliated easily between layers with uniform fold distribution, more concentrated particle size distribution and smaller microcrystal size, and simultaneously the stacking number of layers can be stabilized at 1 or 2 layers.

Study of vertical graphene growth on silver substrate based on design of experiment

Due to the different catalytic activity between individual metallic materials, it is always necessary to find optimal conditions for the growth of vertical graphene for the new substrate. In this case, the silver substrate was used as the catalytic layer. The growth of graphene was accomplished by chemical vapour deposition supported by plasma discharge. To find the optimal parameters, a 69-round design of experiment was used with the following input factors: the temperature (500–800 °C), the pressure (200–1200 mTorr), the total flow (30–100 sccm), the methane content in the mixture and hydrogen pre-treatment. Based on the observation by electron microscope, the samples with vertical graphene were selected, which were further characterized. To determine the quality (D, G and 2D peaks intensity) of graphene and number of layers, Raman microscopy was used. The analysis of crystalline structure of graphene was carried out by means of a transmission electron microscope. The size of the deposit was determined by a combination of electron and ion microscopy. The analysis of the data obtained from the design of experiment identified the key parameters of vertical graphene deposition on the silver substrate. Besides, the impact of the individual parameters on the total thickness of the deposited graphene was evaluated.

In Situ Strain Tuning in hBN-Encapsulated Graphene Electronic Devices

Using a simple setup to bend a flexible substrate, we demonstrate deterministic and reproducible in situ strain tuning of graphene electronic devices. Central to this method is the full hBN encapsulation of graphene, which preserves the exceptional quality of pristine graphene for transport experiments. In addition, the on-substrate approach allows one to exploit strain effects in the full range of possible sample geometries and at the same time guarantees that changes in the gate capacitance remain negligible during the deformation process. We use Raman spectroscopy to spatially map the strain magnitude in devices with two different geometries and demonstrate the possibility to engineer a strain gradient, which is relevant for accessing the valley degree of freedom with pseudomagnetic fields. Comparing the transport characteristics of a suspended device with those of an on-substrate device, we demonstrate that our new approach does not suffer from the ambiguities encountered in suspended devices.

Copper-Containing Carbon Feedstock for Growing Superclean Graphene.

Chemical vapor deposition (CVD) enables the large-scale growth of high-quality graphene film and exhibits considerable potential for the industrial production of graphene. However, CVD-grown graphene film contains surface contamination, which in turn hinders its potential applications, for example, in electrical and optoelectronic devices and in graphene-membrane-based applications. To solve this issue, we demonstrated a modified gas-phase reaction to achieve the large-scale growth of contamination-free graphene film, i.e., superclean graphene, using a metal-containing molecule, copper(II) acetate, Cu(OAc)2, as the carbon source. During high-temperature CVD, the Cu-containing carbon source significantly increased the Cu content in the gas phase, which in turn suppressed the formation of contamination on the graphene surface by ensuring sufficient decomposition of the carbon feedstock. The as-received graphene with a surface cleanness of about 99% showed enhanced optical and electrical properties. This study opens a new avenue for improving graphene quality with respect to surface cleanness and provides new insight into the mechanism of graphene growth through the gas-phase reaction pathway.

Exceptional anodic performance of Sb-doped SnO2 nanoparticles on electrochemically exfoliated graphene for lithium-ion batteries

Metal oxides such as SnO2 are widely studied as anode materials for next-generation lithium-ion batteries because of their high theoretical capacities. However, the lithium storage capability of SnO2 is suppressed by its poor electrical conductivity and serious volume change during the charge and discharge processes. In this work, SnO2 nanoparticles are directly grown on the surface of electrochemically exfoliated graphene, followed by doping Sb into SnO2 nanoparticles. As a high quality of graphene, electrochemically exfoliated graphene possesses high electrical conductivity and strong mechanical strength, which is beneficial to the charge transfer from graphene to SnO2 nanoparticles. Moreover, the doping of Sb into SnO2 nanoparticles inhibits the agglomeration and growth of SnO2 nanoparticles, and further improves the electron conductivity of SnO2 by changing the electronic structure of SnO2. As a result, electrochemically exfoliated graphene-based SnO2 with Sb doping exhibits superior electrochemical performance including high reversible capacity, excellent cycling stability and remarkable rate capability. Modifying the electronic structure of metal oxides by doping with appropriate metals may provide an avenue to realize the demand of high-performance electrodes for secondary batteries.

Transition Metal Salts Additive as Antioxidant or Metal Oxide Decorator during Electrochemical Exfoliation of Graphite?

The preparation of graphene via electrochemical exfoliation in aqueous solution is considered very attractive in terms of cost, scalability and non-toxicity. However, the quality of graphene produced by this method is inferior to the material produced by methods such as mechanical exfoliation due to the functionalisation of carbon by reactive oxygen radical species derived from water oxidation.1 There is a need to alleviate the effect of these radicals in order to capitalize on the use of anodic electrochemical exfoliation in aqueous solution. In this presentation, we will describe the use of a new approach, based on the unexpectedly versatile role of transition metal ions to produce high quality graphene via an anodic electrochemical exfoliation route, using sulfate as the intercalating anion.2 Notably, some of the transition metals (Co2+, Fe3+) acted as antioxidants, preventing surface oxidation of graphene, while others (Ru3+, Mn2+, Ir3+) acted as metal oxide decorators (Figure 1). The graphene obtained by electrochemical exfoliation of graphite in the presence of Co2+ is two orders of magnitude more conducting and defect free than the one obtained in the absence of cobalt ion. This Co-treated graphene found to be an excellent cathode material for Aluminum-based batteries in terms of performance and cycling stability. The use of other transition metal ions (Ru3+, Mn2+, Ir3+) in the electrolysis solution resulted in decoration of graphene with the oxide of that metal in a single stage simultaneous process. This process can also allow for producing graphene functionalised with hybrid metal oxides (G-RuO2-Mn3O4) in a one-pot single step process, which can be used as electrodes for supercapacitors (500 F g-1) and as an efficient electrocatalyst for water splitting. References 1) K. Parvez, Z. S. Wu, R. J. Li, X. J. Liu, R. Graf, X. L. Feng, K. Mullen, J. Am. Chem. Soc. 2014, 136, 6083. 2) A. Ejigu, K. Fujisawa, B. F. Spencer, B. Wang, M. Terrones, I. A. Kinloch, R. A. W. Dryfe, Adv. Funct. Mater. 2018, 28, 1804357. Figure 1

Production and Mechanical Characterization of Graphene Micro-Ribbons

Patterning of graphene into micro- and nano-ribbons allows for tunability in emerging fields such as flexible electronic and optoelectronic devices, and is gaining interest for the production of more efficient reinforcement for composite materials. In this work we fabricate micro-ribbons from graphene synthesized via chemical vapor deposition (CVD) by combining ultraviolet (UV) photolithography and dry etching oxygen plasma treatments. We used Raman spectral imaging to confirm the effectiveness of the patterning procedure, which is suitable for large-area patterning of graphene on wafer-scale, and confirms that the quality of graphene remains unaltered. The produced micro-ribbons were finally transferred and embedded into a polymeric matrix and the mechanical response was investigated by in-situ mechanical investigation combining Raman spectroscopy and tensile/compressive tests.

The Effects of Hydrogen Flowrate during Pre-Annealing on Graphene Growth by Chemical Vapor Deposition Using Methanol as a Liquid Carbon Precursor

Studying an influence of several parameters on Chemical Vapor Deposition (CVD) used for graphene synthesis is crucial to optimizing the graphene quality to be Compatible with advanced devices. The effect of different hydrogen (H2) flow-rates (0, 50, 100, 150, 200, 250, and 300 sccm) during the pre-annealing process on CVD grown graphene have been reported. This study revealed that hydrogen flow rates during annealing changed the surface roughness/smoothness of the copper substrates. For high hydrogen flow rates, the smoothing effect was increased. Furthermore, the annealed graphene samples emerged a deferent number of layers because of morphological surface changes. According to Raman D- to G-band intensity ratios (ID/IG), the graphene quality was influenced by the annealing hydrogen flowrate. The visible light transmittance values of the grown graphene samples confirmed a few number of layers (mono to seven-layer). Mostly, the samples which annealed under moderate hydrogen flow rates showed less defects intensities and higher crystallite sizes.

The influence of Graphene quality on performance of a Si/Graphene nanocomposite anode

ABSTRACTSi/Graphene nanoparticles represent attractive alternative anode materials for Lithium-ion batteries. Graphene nanosheets with different properties, including surface area, defect distance, and charge-transfer resistance, were fabricated and characterised in Si/Graphene nanocomposites formed by static-electric self-assembly then by an in-situ reduction process. Graphene nanosheets that exhibited the highest surface area, the shortest defect distance, and the lowest charge-transfer resistance demonstrated the best overall electrochemical performance, with a high initial discharge capacity of 2692 mAh g−1, good cycling performance of 1135 mAh g−1, at the 200th cycle at the current rate of 0.5 C. This work shows the preferable graphene quality for Si/Graphene nanocomposite anode and provides insights into the design of graphene nanocomposite electrodes, regardless of the graphene synthesis method.

Oxidation of the polycrystalline copper–graphene nanocomposite

Since the experimental discovery of graphene, research has progressed at an astonishing speed. This 2D honeycomb has already exhibited revolutionary electronic properties and claimed the record for strength. However, getting graphene out of the lab requires improved characterization, especially of imperfections. Hence, this paper explores one of the most important defects in a copper–graphene nanocomposite fabricated by chemical vapor deposition (CVD), which is the method most capable of producing large sheets of high-quality graphene. Through electron microscopies, optical microscopy, and Raman spectroscopy, this work details (1) the room-temperature oxidation of the copper substrate coated with graphene and (2) the heterogeneities of the graphene due to deposition and oxidation. The results indicate that this oxidation can be confined by grain boundaries, forms in the copper substrate as Cu2O, and minimally affects the graphene quality. This investigation also suggests that CVD can produce a variable number of high-quality, graphene layers.

Crossover from ballistic to diffusive thermal transport in suspended graphene membranes

We report heat transport measurements on suspended single-layer graphene disks with radius of 150–1600 nm using a high-vacuum scanning thermal microscope. The results of this study revealed a radius-dependent thermal contact resistance between tip and graphene, with values between 1.15 and 1.52 × 108 KW−1. The observed scaling of thermal resistance with radius is interpreted in terms of ballistic phonon transport in suspended graphene discs with radius smaller than 775 nm. In larger suspended graphene discs (radius >775 nm), the thermal resistance increases with radius, which is attributed to in-plane heat transport being limited by phonon–phonon resistive scattering processes, which resulted in a transition from ballistic to diffusive thermal transport. In addition, by simultaneously mapping topography and steady-state heat flux signals between a self-heated scanning probe sensor and graphene with 17 nm thermal spatial resolution, we demonstrated that the surface quality of the suspended graphene and its connectivity with the Si/SiO2 substrate play a determining role in thermal transport. Our approach allows the investigation of heat transport in suspended graphene at sub-micrometre length scales and overcomes major limitations of conventional experimental methods usually caused by extrinsic thermal contact resistances, assumptions on the value of the graphene’s optical absorbance and limited thermal spatial resolution.

Graphene Post-Processing

Graphene is a two-dimensional wonder material, used for electronic and medical applications. To date, various synthesis method of graphene layers is proposed, such as: SiC epitaxial growth, Chemical Vapor Deposition (CVD) and mechanical exfoliation. Graphene single layer can be rapidly produced by CVD. Although the transfer process on different substrates has been researched, a post-processing step after graphene growth for understanding influence of radical of carbon unreacted in graphene properties has not been studied. Deposition of graphene on metal transition substrate involves many reactions, and is not clear what happens with the unreacted methane. The scanning electron microscopy is the best method to visualize this unreacted species as a thin film which covers the graphene layer. The post-processing of graphene after CVD is a crucial step for the performance of graphene-based devices. The graphene film is characterized morphologically and structurally before and after the post-processing step, with the scope of removing the unreacted film and investigating the influence of this step on the graphene properties. To identify the specific vibration of graphene layer before and after post-processing step, Raman spectroscopy has been used. Here, we investigate the quality of CVD graphene before and after removing unreacted hydrocarbon, to better understand the importance of post-processing process for device applications, before the graphene transfer.

Tunable solution-processable anodic exfoliated graphene

Electrochemical exfoliation of graphite is an emerging approach to provide large scale, low cost, efficient and reliable production of high quality, low defect and solution-processable graphene. The use of a sulphuric acid electrolyte allows rapid and efficient anodic graphite exfoliation. However, the highly oxidative process in this electrolyte promotes hydroxyl radicals. These degrade the carbon lattice structurally, hence compromising the quality of graphene produced. Here, we report a simple and effective way to overcome this challenge; the use of an additive to scavenge hydroxyl radicals. The addition of a small volume percentage of ethylene glycol acts to scavenge hydroxyl radicals. Furthermore, it is readily adsorbed onto the intercalated graphite surface, and therefore also serves as an oxidative prevention layer. As a result, the damage on the graphitic structure is greatly minimized, resulting in a low defect and high quality few layers graphene, with high yield (>80%) and a low degree of oxidation with a C/O ratio of 16.9. The graphene solution-processability is readily tuned and improved with stable dispersions up to 5mg/mL in dimethylformamide. This new strategy of a simple and low cost dual purpose additive in scavenging hydroxyl radicals and providing an oxidative prevention layer to efficiently tune the graphene quality and solution-processability; holds promise in industrial scale mass production of high quality graphene.

Double-layer graphene optical modulators based on Fano resonance in all-dielectric metasurfaces

In this paper, two double-layer graphene optical modulators based on all-dielectric metasurfaces are proposed. The double-layer graphene modulators remove the requirement of doped silicon back gates which would cause excessive loss and limit the modulation speed. The first structure is based on the electromagnetically induced transparency phenomenon, and the second one is a polarization-independent modulator that is based on the Fano resonance. The structures are simulated and analyzed using the finite element method. According to the simulation results, large modulation depth of about 95% at the wavelength of 1.55 μm can be obtained. The required voltages of 4.95 V are obtained for both of the modulators. By assuming the device of 50 × 50 unit cells, the 3 dB bandwidth of the first (second) structure is calculated as 630 MHz (482 MHz). It is estimated that by improving the quality of graphene, the 3 dB bandwidth of more than 2 GHz could be attained. The achieved modulation performances are much better than the previously reported free-space modulators with the same device area. The proposed high-performance optical modulators are promising for free-space optics technology especially free-space optical communication networks.

Effects of Graphene Quality on Lithium Storage Performances of Fe3O4/Thermally Reduced Graphene Oxide Hybrid Anodes

AbstractTill now, there are no systematic reports on the effect of the quality of thermally reduced graphene oxide (TRG) on lithium storage properties, as the reported methods to fabricate graphene‐based electrodes are usually not effective enough to reduce graphene oxides to graphene. Herein, graphite oxide is thermally exfoliated and annealed with a high‐temperature graphitization oven at different temperatures (800–2000 °C). A series of Fe3O4/TRG hybrids are synthesized as anode materials for lithium‐ion batteries with TRGs acting as substrates of in‐situ formed Fe3O4 nanoparticles. Both, the electroconductivity of the TRGs and their interfacial interaction with Fe3O4 influence the lithium storage performances of the hybrids. However, the electroconductivity of the TRGs and the formation of interfacial bonds are conflicting. Because the oxygen‐containing groups and defects of TRG are greatly removed leading to enhanced electrical conductivity with the increase of thermal annealing temperature. Hence, the resulting Fe3O4/TRG hybrids show first decreased then increased electrochemical performances with increasing annealed temperatures. In a word, the effect of interfacial interaction is dominant at a relatively low annealing temperature, while the effect of conductivity is dominant at a relatively high annealing temperature. The optimized hybrids exhibit excellent cycling and rate performances. This work should provide useful information for the rational design and construction of high‐performance electrodes for energy storage applications.

Solid-diffusion-facilitated cleaning of copper foil improves the quality of CVD graphene

The quality of CVD-grown graphene is limited by the parallel nucleation of grains from surface impurities which leads to increased grain boundary densities. Currently employed cleaning methods cannot completely remove surface impurities since impurity diffusion from the bulk to the surface occurs during growth. We here introduce a new method to remove impurities not only on the surface but also from the bulk. By employing a solid cap during annealing that acts as a sink for impurities and leads to an enhancement of copper purity throughout the catalyst thickness. The high efficiency of the solid-diffusion-based transport pathway results in a drastic decrease in the surface particle concentration in a relatively short time, as evident in AFM and SIMS characterization of copper foils. Graphene grown on those substrates displays enhanced grain sizes and room-temperature, large-area carrier mobilities in excess of 5000 cm2/Vs which emphasizes the suitability of our approach for future graphene applications.

Gigahertz Field-Effect Transistors with CMOS-Compatible Transfer-Free Graphene.

High-quality graphene grown on metal-free substrates represents a vital milestone that provides an atomic clean interface and a complementary metal-oxide-semiconductor-compatible manufacturing process for electronic applications. We report a scalable approach to fabricate radio frequency field-effect transistors with a graphene channel grown directly on the sapphire substrate using the technique of remote-catalyzed chemical vapor deposition (CVD). A mushroom-shaped AlO x top gate is used to allow the self-aligned drain/source contacts, yielding remarkable increase of device transconductance and reduction of the associated parasitic resistance. The quality of thus-grown graphene is reflected in the high extrinsic cutoff frequency and maximum oscillation frequency of 10.1 and 5.6 GHz for the graphene channel of length 200 nm and width 80 μm, respectively, potentially comparable with those of transferred CVD graphene at the same channel length and holding promise for applications in high-speed wireless communications.

Abrupt changes in the graphene on Ge(001) system at the onset of surface melting

By combining scanning probe microscopy with Raman and x-ray photoelectron spectroscopies, we investigate the evolution of CVD-grown graphene/Ge(001) as a function of the deposition temperature in close proximity to the Ge melting point, highlighting an abrupt change of the graphene's quality, morphology, electronic properties and growth mode at 930 °C. We attribute this discontinuity to the incomplete surface melting of the Ge substrate and show how incomplete melting explains a variety of diverse and long-debated peculiar features of the graphene/Ge(001), including the characteristic nanostructuring of the Ge substrate induced by graphene overgrowth. We find that the quasi-liquid Ge layer formed close to 930 °C is fundamental to obtain high-quality graphene, while a temperature decrease of 10° already results in a wrinkled and defective graphene film.