Synthesis Of High-quality Graphene Films Research Articles

Solar-thermal cold-wall chemical vapor deposition reactor design and characterization for graphene synthesis

Manufacturing processes are often highly energy-intensive, even when the energy is primarily used for direct heating processes. The required energy tends to derive from local utilities, which currently employ a blend of sources ranging from fossil fuels to renewable wind and solar photovoltaics, among others, when the end manufacturing need is thermal energy. Direct solar-thermal capture provides a compelling alternative that utilizes renewable energy to reduce greenhouse gas emissions from industrial processes, but one that has rarely been employed to date. In this study, a 10 kWe custom-built high flux solar simulator (HFSS) that closely approximates the solar spectrum produces a heat flux distribution with an adjustable peak between 1.5 and 4.5 MW/m2. The HFSS system is coupled to a cold-wall chemical vapor deposition (CVD) system that is equipped to automate graphene synthesis while providing safe operation, precise control, and real-time monitoring of process parameters. A numerical heat transfer model of a thin copper substrate is derived and validated to compute the substrate’s temperature profile prior to the synthesis process. The peak substrate temperature is correlated to the HFSS supply current and vacuum pressure, as it serves as a critical design parameter during graphene synthesis. We report the synthesis of high-quality graphene films on copper substrates with an average Raman peak intensity ratio ID/IG of 0.17. Backscattered electron microscopy reveals a characteristic grain size of 120 μm, with an area ratio of 16 when compared to that of low-quality graphene on copper. The reported solar-thermal CVD system demonstrates the ability to produce a high-value product, namely, graphene on copper, directly from a renewable energy resource with process control and automation that enables synthesis under a variety of conditions.

CVD growth of high-quality graphene over Ge (100) by annihilation of thermal pits

The physical and chemical state of the underlying germanium (Ge) substrate is crucial for the CVD synthesis of high-quality graphene. Here, we investigate the main causes responsible for formation of pit-like defects on Ge (100) and (111) surfaces, their elimination, and their influence on the quality of deposited graphene. Our data provides evidence of pit-like defects formation on Ge surfaces during thermal annealing at T ≥ 800 °C. We establish that the pits are due to preferential evaporation of residual Ge suboxides, and that their efficient removal would result in a pit-free surface. Oxide removal on HF, HCl and HBr treated surfaces is investigated using X-ray photoemission spectroscopy (XPS). The results confirm that in contrast to HF and HCl, HBr effectively eliminates GeO2 and Ge suboxides, providing pit-free surfaces, which enable CVD growth of high-quality graphene. Hence, we demonstrate that effective elimination of Ge suboxides enables the synthesis of graphene monolayers with excellent quality (ID/IG ∼ 0.2) as confirmed by Raman spectroscopy measurements. The experimental data provides clear corroboration for the role played by the Ge suboxides in the formation of thermal pits and the control of the graphene quality. Our findings lay the foundation for Ge substrates cleaning and the synthesis of high-quality graphene films for advanced applications.

Large area ultra-thin graphene films for functional photovoltaic devices

Abstract

Tension-controlled single-crystallization of copper foils for roll-to-roll synthesis of high-quality graphene films

It has been known that the crystalline orientation of Cu substrates plays a crucial role in chemical vapor deposition (CVD) synthesis of high-quality graphene. In particular, Cu (1 1 1) surface showing the minimum lattice mismatch with graphene is expected to provide an ideal catalytic reactivity that can minimize the formation of defects, which also induces larger single-crystalline domain sizes of graphene. Usually, the Cu (1 1 1) substrates can be epitaxially grown on single-crystalline inorganic substrates or can be recrystallized by annealing for more than 12 h, which limits the cost and time-effective synthesis of graphene. Here, we demonstrate a new method to optimize the crystalline orientations of vertically suspended Cu foils by tension control during graphene growth, resulting in large-area recrystallization into Cu (1 1 1) surface as the applied tension activates the grain boundary energy of Cu and promotes its abnormal grain growth to single crystals. In addition, we found a clue that the formation of graphene cooperatively assists the recrystallization into Cu (1 1 1) by minimizing the surface energy of Cu. The domain sizes and charge carrier mobility of graphene grown on the single-crystalline Cu (1 1 1) are 5 times and ~50% increased, respectively, in comparison with those of graphene from Cu (1 0 0), indicating that the less lattice mismatch and the lower interaction energy between Cu (1 1 1) and graphene allows the growth of larger single-crystalline graphene with higher charge carrier mobility. Thus, we believe that our finding provides a crucial idea to design a roll-to-roll (R2R) graphene synthesis system where the tension control is inevitably involved, which would be of great importance for the continuous production of high-quality graphene in the future.

Highly uniform growth of monolayer graphene by chemical vapor deposition on Cu–Ag alloy catalysts

One of the major challenges for the practical application of graphene is the large scale synthesis of uniform films with high quality at lower temperature. Here, we demonstrate the use of Ag-plated Cu substrates in the synthesis of high-quality graphene films via chemical vapor deposition (CVD) of methane gas at temperatures as low as 900 °C. Various experimental analyses show that the plated Ag diffuses into Cu to form a uniform Cu-Ag alloy that suppresses the formation of multilayer nucleation and decreases the activation energy of precursor formation, leading to a lower synthesis temperature with enhanced monolayer coverage. In addition, we also observed an unusual Ag-assisted abnormal grain growth of Cu into the cube texture with larger grain sizes and reduced grain boundaries, which is believed to provide the homogeneous environment needed for uniform graphene growth.

Process Optimization for Synthesis of High-Quality Graphene Films by Low-Pressure Chemical Vapor Deposition

Low-pressure chemical vapor deposition (LPCVD) is a simple and useful method for the large-area synthesis of graphene films. Here, we have investigated how to adjust and optimize process conditions for the synthesis of single-layer graphene films by LPCVD. Through our experimental procedure, uniform and high-quality graphene films could be grown on Cu foil at 1000 °C for 20 min with an H2 flow rate of 20 sccm, CH4 flow rate of 40 sccm, total pressure of 1.7 Torr, and a fast cooling rate (>10 °C/s). In a Raman spectrum measured from synthesized graphene film, we found that the full width at half-maximum (FWHM) of a symmetric 2D peak centered at 2682.5 cm-1 was 34 cm-1 and the 2D-to-G intensity ratio was 1.35.

Process Optimization for Synthesis of High-Quality Graphene Films by Low-Pressure Chemical Vapor Deposition

Process Optimization for Synthesis of High-Quality Graphene Films by Low-Pressure Chemical Vapor Deposition

Synthesis of high-quality graphene films on nickel foils by rapid thermal chemical vapor deposition

We report the synthesis of high-quality graphene films on Ni foils using a cold-wall reactor by rapid thermal chemical vapor deposition (CVD). The graphene films were produced by shortening the growth time to 10 s, suggesting that a direct growth mechanism may play a larger role rather than a precipitation mechanism. A lower H 2 flow rate is favorable for the growth of high-quality graphene films. The graphene film prepared without the presence of H 2 has a sheet resistance as low as ∼367 ohm/sq coupled with 97.3% optical transmittance at 550 nm wavelength, which is much better than for those grown by hot-wall CVD systems. These data suggest that the structural and electrical characteristics of these graphene films are comparable to those prepared by CVD on Cu.

Production of Extended Single-Layer Graphene

Graphene has attracted an enormous amount of interest recently because of its unique electronic, optical, mechanical, and other properties. We report a promising method for producing single-layer graphene fully covering an entire substrate at low temperature. Single-layer graphene sheets have been synthesized on a whole 2 cm ×2 cm nickel (Ni) film deposited on a highly oriented pyrolytic graphite (HOPG) substrate by heating the Ni/HOPG in a vacuum. The carbon atoms forming our graphene are diffused from the graphite substrate through the nickel template. Our results demonstrate how to control the amount of carbon atoms for graphene formation to yield graphene films with a fine controlled thickness and crystal structure. Our method represents a significant step toward the scalable synthesis of high-quality graphene films with predefined thickness and toward realizing the unique properties of graphene films.

Large-Scale Synthesis of Graphene Films by Joule-Heating-Induced Chemical Vapor Deposition

We report large-area synthesis of few-layer graphene films by chemical vapor deposition (CVD) in a cold-wall reactor. The key feature of this method is that the catalytic metal layers on the SiO2/Si substrates are self-heated to high growth temperature (900°C to 1000°C) by high-current Joule heating. Synthesis of high-quality graphene films, whose structural and electrical characteristics are comparable to those grown by hot-wall CVD systems, was confirmed by transmission electron microscopy images, Raman spectra, and current–voltage analysis. Optical transmittance spectra of the graphene films allowed us to estimate the number of graphene layers, which revealed that high-temperature exposure of Ni thin layers to a carbon precursor (CH4) was critical in determining the number of graphene layers. In particular, exposure to CH4 for 20 s produces very thin graphene films with an optical transmittance of 93%, corresponding to an average layer number of three and a sheet resistance of ~600 Ω/square.