Large-area High-quality Graphene Research Articles

Transfer-Free CVD Growth of High-Quality Wafer-Scale Graphene at 300 °C for Device Mass Fabrication.

Direct chemical vapor deposition of graphene on semiconductors and insulators provides high feasibility for integration of graphene devices and semiconductor electronics. However, the current methods typically rely on high temperatures (>1000 °C), which can damage the substrates. Here, a growth method for high-quality large-area graphene at 300 °C is introduced. A multizone furnace with gradient temperature control was designed according to a computational fluid dynamics model. The crucial roles of the chamber pressure in the film continuity and hydrogen composition in the graphene defect density at low temperature were revealed. As a result, a uniform graphene film with the Raman ratio ID/IG = 0.08 was obtained. Furthermore, a technique of laminating single-crystal Cu foil as a sacrificial layer on the substrate was proposed to realize transfer-free growth, and a wafer-scale graphene transistor array was demonstrated with good performance consistency, which paves the way for mass fabrication of graphene devices.

Effect of Staged Methane Flow on Graphene Quality of Low-Pressure Chemical Vapor Deposition

The chemical vapor deposition method and the method using copper are suitable for high-quality large-area graphene synthesis. Here, we present methane flow conditions for obtaining high-quality graphene over a large area. The conditions of gases other than the flow rate of methane were fixed, and the graphene synthesized by adjusting the flow rate of methane and the exposure time of methane was verified through the Raman spectrum. When the methane flow rate was 5 sccm, the growth of graphene was island-shaped and made into a multilayer graphene. When the methane flow rate increases to 8 sccm, the Irish growth of graphene disappears and stably grows into a single layer. However, if the flow rate exceeds 9 sccm, Irish growth disappears. However, in order to minimize the area where graphene in the multilayer is synthesized, the methane exposure time was analyzed in units of 10 minutes from 5 minutes to 25 minutes. When analyzing the I[2d]/I[g] value and I[d]/I[g] value of the Raman spectrum, single-layer graphene of a large area could be observed.

Quantum transport in CVD graphene synthesized with liquid carbon precursor

Large-area high-quality graphene enabled by chemical vapor deposition (CVD) can possibly pave the path for advanced flexible electronics and spintronics. CVD-grown method utilizing liquid carbon precursor has recently been demonstrated as an appealing choice for mass graphene production, thanks to its low cost and safe operation. However, the quality of the graphene film has been the major obstacle for the implementation of the liquid-precursor-based CVD method. Here we report the growth of centimeter-scale easily-transferable single-layer graphene (SLG) using acetone as a liquid carbon precursor. The dry-transfer technique was used to prepare the graphene device. The typical mobility of the dry-transferred SLG device is as high as 12 500 cm2 V−1 s−1 at room temperature. Thanks to the high quality of the device, the robust quantum Hall effect can survive up to room temperature. The excellent device quality also enables us to observe the Shubnikov–de Haas oscillation in the low magnetic field regime and systemically study the leading scattering mechanism. We extracted both the transport scattering time τ t and the quantum scattering time τ q over a wide range of carrier density. The ratio of the scattering times suggests that the charged-impurity resided near the surface of the graphene restricted the device performance.

MXene-Graphene Field-Effect Transistor Sensing of Influenza Virus and SARS-CoV-2.

An MXene–graphene field-effect transistor (FET) sensor for both influenza virus and 2019-nCoV sensing was developed and characterized. The developed sensor combines the high chemical sensitivity of MXene and the continuity of large-area high-quality graphene to form an ultra-sensitive virus-sensing transduction material (VSTM). Through polymer linking, we are able to utilize antibody–antigen binding to achieve electrochemical signal transduction when viruses are deposited onto the VSTM surface. The MXene–graphene VSTM was integrated into a microfluidic channel that can directly receive viruses in solution. The developed sensor was tested with various concentrations of antigens from two viruses: inactivated influenza A (H1N1) HA virus ranging from 125 to 250,000 copies/mL and a recombinant 2019-nCoV spike protein ranging from 1 fg/mL to 10 pg/mL. The average response time was about ∼50 ms, which is significantly faster than the existing real-time reverse transcription-polymerase chain reaction method (>3 h). The low limit of detection (125 copies/mL for the influenza virus and 1 fg/mL for the recombinant 2019-nCoV spike protein) has demonstrated the sensitivity of the MXene–graphene VSTM on the FET platform to virus sensing. Especially, the high signal-to-viral load ratio (∼10% change in source-drain current and gate voltage) also demonstrates the ultra-sensitivity of the developed MXene–graphene FET sensor. In addition, the specificity of the sensor was also demonstrated by depositing the inactivated influenza A (H1N1) HA virus and the recombinant 2019-nCoV spike protein onto microfluidic channels with opposite antibodies, producing signal differences that are about 10 times lower. Thus, we have successfully fabricated a relatively low-cost, ultrasensitive, fast-responding, and specific inactivated influenza A (H1N1) and 2019-nCoV sensor with the MXene–graphene VSTM.

Synthesis of large-area graphene films on rolled-up Cu foils by a “breathing” method

Chemical vapor deposition (CVD) is considered to be the most promising method for the synthesis of large-area high-quality graphene films. However, with an increase in reactor size, manufacturing difficulty and significant increases in CVD reactor costs will occur. These effects result in considerable limitations on the size and throughput of graphene films. We report a “breathing” CVD method to effectively utilize the reactor space for producing large-area graphene films with a small reactor. A rectangular Cu foil substrate is rolled into a spiral to increase the substrate loading density. Graphite mat strips are placed between the Cu layers at the two ends of the spiral to avoid Cu layer adhesion at high temperature while leaving enough space inside the Cu spiral for gas exchange. Reactant gases are introduced in a way similar to breathing by repeatedly increasing and decreasing the pressure inside the reactor as well as the Cu spiral, which assists gas exchange. In addition, in order to avoid the collapse of the middle Cu layers due to high temperature softening and gravity, the Cu spiral is placed vertically. With this method, both the size and throughput of graphene films can be an order of magnitude larger than conventional methods, as demonstrated by the synthesis of the large-area high-quality graphene films of 1.1 m × 0.25 m with a 2-in. diameter quartz tube CVD reactor. The transverse length (1.1 m) of the graphene film is 25 times the inner diameter (0.044 m) of the reactor tube. This method can also be used for the synthesis of other large-area 2D films and the production of large-area metal single crystal foils.

Controlled Graphene Synthesis from Solid Carbon Sources

Here the authors present the investigation of controllable growth of large area high quality graphene from different solid carbon sources. A home‐made cold‐wall chamber is used for chemical vapor deposition realization to grow graphene on copper foils. It is supplied with tools for in situ monitoring the temperature along the entire sample. Monolayer graphene films are synthesized using solid poly(methyl methacrylate) (PMMA) and paraffin precursors at a growth temperature of about 850 °C and under low pressure conditions. The area of 1 × 1 cm2 appears to be covered by graphene. Raman spectroscopy and scanning electron microscopy are used to estimate the quality of graphene films fabricated and the graphene layer number.

Growth of Large-Area High-Quality Graphene on Different Types of Copper Foil Preannealed under Positive Pressure H2 Ambience.

Large-area high-quality graphene was synthesized on different types of copper foils preannealed under positive pressure H2 atmosphere between 1 and 2 standard atmospheres. The prepared graphene showed good electrical conductivity, transmittance, and uniformity. The sheet resistance values of the grown monolayer graphene film were all about 500 Ω/□, and the transmittance was as high as 97.24%. The carrier mobility of the monolayer graphene film was around 2000–3000 cm2/(V s). Furthermore, the monolayer coverage could be more than 95.00% controlled by adjusting the process parameters. The properties of the four layer-superposed graphene film nearly reached that of the commercialized indium tin oxide (ITO) glasses, which showed that the prepared graphene could be well applied to the transparent conductive electrode. The obtained graphene film has been used to construct Si/graphene solar cells without an antireflection film, which showed energy conversion efficiency among 4.99–5.62%.

Recovery of the Dirac states of graphene by intercalating two-dimensional traditional semiconductors

The epitaxial growth of graphene on transition-metal substrates has proved to be an efficient method to synthesize high-quality large-area graphene. However, due to the interaction between graphene and the transition-metal substrate, the electronic structure of the as-fabricated graphene is distorted. Here, using density functional theory calculations, we investigated the effect of intercalating two-dimensional (2D) silicon and III–V materials, such as double-layer honeycomb AlAs, into the graphene-metal interface. We found that the intercalation of these 2D materials significantly reduces the interaction between graphene and the transition-metal substrate. The Dirac state is largely restored. The doping level of graphene induced by 2D intercalated material and the metal substrate is proportional to the work function difference between graphene and 2D materials/metal. This work provides a way for the formation of freestanding graphene and further fabrication of graphene-based devices.

Influence of surface energy and elastic strain energy on the graphene growth in chemical vapor deposition

Polycrystalline metal substrates such as copper (Cu) have been intensively used to grow graphene in chemical vapor deposition (CVD) technique. It has been observed that crystal orientations affect the quality of graphene produced to some degree. The existence of crystal orientations caused graphene to grow randomly on top of Cu which also resulted on the formation of polycrystalline graphene. Despite this, the influence of crystal orientations on the quality of graphene produced are not yet fully understood. There are two possible factors that could affect graphene growth from crystal orientation point of view; surface energy and elastic strain energy. The understanding towards these both factors might beneficial to control the quality of graphene. This paper thus aims to highlight the influence of surface energy and elastic strain energy on the graphene growth in CVD. Substrate used were single crystal Cu with (111), (110) and (100) orientations. The graphene was grown inside a closed reaction chamber with the presence of argon (Ar), hydrogen (H2) and methane (CH4) gases with partial pressure ratio of 0.6: 0.2: 0.2 at 1 atm, 1000 ˚C in 30 minutes. The quality of the as-grown graphene was identified using Raman spectroscopy. The Raman spectra show the existence of graphene peak for all the Cu substrates. The calculation of ID/IG ratio revealed that the Cu (100) possessed the lowest amount of defects compared to Cu(110) and Cu(111). While I2D/IG ratio fluctuated between 0.22 to 0.34 suggested that the crystal orientation does not control the thickness of graphene layer at these reaction conditions. The usage of higher CH4 partial pressure produced a thicker graphene. It is assumed that the thickness of the graphene exceeded the critical thickness thus elastic strain energy becomes the dominant factor in controlling graphene growth. Larger lattice mismatch causes major defects on graphene and this result has been shown in graphene grown on top of Cu(111). These findings thus would give a new insight to tailor the high-quality large-area graphene growth in CVD.

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.

Fast synthesis of high-quality large-area graphene by laser CVD

Few layer graphene (FLG) has attracted tremendous interest in recent years because of its unique properties. However, how to synthesize graphene quickly and control the layer number is an important issue. Herein, high-quality and layer-controlled graphene was obtained on polycrystalline nickel foil by laser chemical vapor deposition (LCVD) within a few minute, which is a cold-wall CVD type based on irradiation of a continuous wave laser with Super-Gaussian distribution spot. The number of graphene layers and quality was controlled by controlling the growth time, laser power and cooling rate. The growth mechanism of the FLG has also been discussed.

Direct formation of wafer-scale single-layer graphene films on the rough surface substrate by PECVD

The technical advance of plasma enhanced chemical vapor deposition (PECVD) exhibits the potential to grow large-area high-quality graphene films at relatively low growth temperature, which is beneficial to the fabrication of graphene-based electronic devices/sensors and transparent electrode. However, it remains a challenge to overcome the degradation of graphene quality during growth by PECVD, due to the continuous bombardment of plasma ions on the catalyst surface. Herein, the combined techniques of PECVD and the growth of graphene underneath the catalyst layer were proposed. As a result, transfer-free single-layer graphene films with 2.5 inch in diameter on quartz substrate can be obtained with the growth temperature of 700 °C, which is 250 °C lower than that for graphene synthesis using thermal CVD. The graphene films prepared by our method show the ability to form on the rough surfaces with millimeter-scale grooves and have minimal surface contamination, compared to that of conventionally transferred CVD graphene.

Low-Temperature and Rapid Growth of Large Single-Crystalline Graphene with Ethane.

Future applications of graphene rely highly on the production of large-area high-quality graphene, especially large single-crystalline graphene, due to the reduction of defects caused by grain boundaries. However, current large single-crystalline graphene growing methodologies are suffering from low growth rate and as a result, industrial graphene production is always confronted by high energy consumption, which is primarily caused by high growth temperature and long growth time. Herein, a new growth condition achieved via ethane being the carbon feedstock to achieve low-temperature yet rapid growth of large single-crystalline graphene is reported. Ethane condition gives a growth rate about four times faster than methane, achieving about 420 µm min-1 for the growth of sub-centimeter graphene single crystals at temperature about 1000 °C. In addition, the temperature threshold to obtain graphene using ethane can be reduced to 750 °C, lower than the general growth temperature threshold (about 1000 °C) with methane on copper foil. Meanwhile ethane always keeps higher graphene growth rate than methane under the same growth temperature. This study demonstrates that ethane is indeed a potential carbon source for efficient growth of large single-crystalline graphene, thus paves the way for graphene in high-end electronical and optoelectronical applications.

Versatile Water-Based Transfer of Large-Area Graphene Films onto Flexible Substrates

ABSTRACTNext-generation electronic devices are expected to demonstrate greater utility, efficiency and durability. Meanwhile, plastics such as polyethylene terephthalate (PET), polyethylene naphthalate (PEN) and variety of poly(para-xylylene) polymers enable transformational advantages to device shape, flexibility, weight, transparency and recyclability. Exhibiting a combination of outstanding mechanical, electrical, optical, and chemical properties of graphene with the plastic substrates could propose ideal material for the future flexible electronics. Chemical vapor deposition (CVD) allows cost-effective fabrication of a high-quality large-area graphene films, however, the critical issue is clean and noninvasive transfer of the films onto a desired substrate. The water-based delamination of CVD grown graphene on Cu can be considered as a “green” transfer process utilizing only hot deionized water. We investigated a method requiring only two essential steps: coating of 6-inch monolayer CVD graphene with transparent and flexible polymer, and Cu delamination in hot water. Proposed method is inexpensive, reproducible, environmentally friendly, waste-free and suitable for large-scale, high quality graphene. The transfer process demonstrated films with enhanced charge carrier mobility, high uniformity, free of mechanical defects, and sheet resistance as low as ∼50 Ω/sq with 96.5 % transparency at 550 nm wavelength.

Observation of the unexpected morphology of graphene wrinkle on copper substrate

Graphene, a two-dimensional material, has a wide range of unique properties and could be used in the development of varieties of mechanic, electronic and photonic devices, therefore methods to synthesis large-area high-quality graphene films are urgently required. Chemical vapor deposition (CVD) has been of particular interest recently due to its simplicity and low cost. However, because of the mismatch of thermal expansion coefficients, high densities of wrinkles are commonly observed. Despite their prevalence and potential impact on large-scale graphene properties, relatively little is known about their structural morphology and formation mechanism. In this article, morphologies of graphene obtained by CVD are experimentally investigated by an atomic force microscope (AFM) and results show that the profiles of wrinkles are much larger than they should be. By using theoretical methods and molecular dynamics simulations (MD), we find internal molecules created during CVD process which supply additional pressure is the main mechanism.

Source identification and method for drastic reduction of Fe contamination on wet transferred graphene

Graphene is a 2D material with extraordinary electronic and mechanical properties and has a wide range of applications. Among existing synthesis methods, chemical vapor deposition (CVD) onto Cu foil is the most prominent because it allows large area high quality graphene to be prepared at relatively low costs and then subsequently wet transferred to arbitrary substrates. Typically, PMMA and FeCl3 are used as a polymeric support layer and a metal etchant respectively during wet transfer. Unfortunately, this method leaves graphene susceptible to contamination, degrading transferred graphene's extraordinary properties. In this paper, contamination in the form of black particulates is identified to be Fe wrapped in graphene and a model of how it originates was developed. Then, a procedure to reduce this contamination during wet transfer was developed and its effectiveness was systematically studied. Transfers were characterized using Raman spectroscopy and optical microscopy with image segmentation. Experimental data obtained indicates that this simple and economic method is very effective in reducing Fe contamination on transferred CVD graphene, offering a way to improve the wet transfer process.

Consequence of oxidation method on graphene oxide produced with different size graphite precursors

Exfoliation method is a most cost effective method for producing chemically derived-graphene (CDG) yet electronic performance of CDG strongly depends on the lateral and vertical dimensions of graphene especially in regards to the development of macroscopic scale films. This work demonstrates the variation in functionality of Graphene oxide (GO) prepared via Marcano-Tour’s and modified Hummer’s method using two different sized graphite precursors. Reduced GO papers were fabricated using vacuum filtration and examined by Hall probe and Raman spectroscopy to ensure their quality. A correlation between oxidation-controlled GO size with their overall electrical conductivity is explored by Fourier Transform Infrared spectroscopy (FTIR), Thermo Gravimetric Analysis (TGA) and X-ray Diffraction (XRD) techniques. This study reveals that the modified Hummer’s method suitably preserves basal planes of GO produced from larger GO flakes with less inter-sheet resistance and illustrates the importance of comparing synthetic methods for producing large area high quality graphene related nanostructures.

Direct transfer of wafer-scale graphene films

Flexible electronics serve as the ubiquitous platform for the next-generation life science, environmental monitoring, display, and energy conversion applications. Outstanding multi-functional mechanical, thermal, electrical, and chemical properties of graphene combined with transparency and flexibility solidifies it as ideal for these applications. Although chemical vapor deposition (CVD) enables cost-effective fabrication of high-quality large-area graphene films, one critical bottleneck is an efficient and reproducible transfer of graphene to flexible substrates. We explore and describe a direct transfer method of 6-inch monolayer CVD graphene onto transparent and flexible substrate based on direct vapor phase deposition of conformal parylene on as-grown graphene/copper (Cu) film. The method is straightforward, scalable, cost-effective and reproducible. The transferred film showed high uniformity, lack of mechanical defects and sheet resistance for doped graphene as low as 18 Ω/sq and 96.5% transparency at 550 nm while withstanding high strain. To underline that the introduced technique is capable of delivering graphene films for next-generation flexible applications we demonstrate a wearable capacitive controller, a heater, and a self-powered triboelectric sensor.

Toward fast growth of large area high quality graphene using a cold-wall CVD reactor

In this work we provide a detailed analysis on graphene synthesis by Chemical Vapor Deposition (CVD) using a cold wall CVD reactor to achieve fast production of large area high quality graphene.

Intercalation and its mechanism of high quality large area graphene on metal substrate

Graphene, a two-dimensional material with honeycomb lattice, has attracted great attention from the communities of fundamental research and industry, due to novel phenomena such as quantum Hall effect at room temperature, Berry phase, and Klein tunneling, and excellent properties including extremely high carrier mobility, high Young's modulus, high thermal conductivity and high flexibility. Some key issues hinder graphene from being used in electronics, including how to integrate it with Si, since Si based technology is widely used in modern microelectronics, and how to place high-quality large area graphene on semiconducting or insulating substrates. A well-known method of generating large-area and high-quality graphene is to epitaxially grow it on a single crystal metal substrate. However, due to the strong interaction between graphene and metal substrate, the intrinsic electronic structure is greatly changed and the conducting substrate also prevents it from being directly used in electronics. Recently, we have developed a technique, which intercalates silicon between epitaxial graphene and metal substrate such as Ru (0001) and Ir (111). Experimental results from Raman, angle-resolved photoemission spectroscopy, and scanning tunneling spectroscopy confirm that the intercalation layer decouples the interaction between graphene and metal substrate, which results in the recovery of its intrinsic band structure. Furthermore, we can use this technique to intercalate thick Si beyond one layer and intercalate Si between graphene and metal film, which indicates the possibility of integrating both graphene and Si device and vast potential applications in industry by reducing its cost. Besides Si, many other metal elements including Hf, Pb, Pt, Pd, Ni, Co, Au, In, and Ce can also be intercalated between graphene and metal substrate, implying the universality of this technique. Considering the versatility of these elements, we can expect this intercalation technique to have wide applications in tuning graphene properties. We also investigate the intercalation mechanism in detail experimentally and theoretically, and find that the intercalation process is composed of four steps:creation of defects, migration of heteroatoms, self-repairing of graphene, and growth of intercalation layers. The intercalation of versatile elements with different structures by this technique provides a new route to the construction of graphene heterostructures, espectially van der Waals heterostructure such as graphene/silicene and graphene/hafnene, and also opens the way for placing graphene on insulating substrate for electronic applications if the intercalation layer can be oxidized by further oxygen intercalation.