Growing graphene on polycrystalline copper foils by ultra-high vacuum chemical vapor deposition

We show that monolayer graphene can be grown isothermally on polycrystalline copper foils via ultra-high vacuum chemical vapor deposition (UHV-CVD), using acetylene as a carbon precursor. The growth is self-limiting, yielding monolayer graphene with a quality comparable to that of graphene grown by atmospheric- or low-pressure chemical vapor deposition. Copper sublimation, a typical concern for UHV-CVD, is shown to be suppressed by growing graphene domains. Further, the roughness of the copper surface after growth is similar to that of copper foils after growth processes at higher pressures. A dependency of the growth kinetics on the surface orientation of the copper grains is observed and a growth model including all stages of growth is presented and discussed. Similar to observations at higher growth pressures, the graphene domains possess sigmoidal growth, however the overall growth behavior is more complicated with two subsequent growth modes. The role of hydrogen is investigated and shows that, contrary to reports for higher growth pressures, dissolved hydrogen in the copper foil plays an essential role for graphene growth whereas ambient hydrogen does not have a noticeable influence.     

A simple method to synthesize graphene at 633 K by dechlorination of hexachlorobenzene on Cu foils

A modified chemical vapor deposition method to synthesize graphene at 360 °C is described. Hexachlorobenzene (HCB) was used as carbon source, and copper foils were used as not only the substrates for graphene deposition but also the catalyst to HCB dechlorination. The possible growth mechanism was investigated using X-ray photoelectron spectroscopy. Enhancement of HCB dechlorination by copper played a key role in synthesis of graphene at such a low temperature.     

Molecular beam epitaxial growth of graphene using cracked ethylene — Advantage over ethanol in growth

We studied the growth of graphene by molecular beam epitaxy (MBE) using ethylene and evaluated the MBE-grown graphene. The flow rate dependence of the carbon deposition rate in the ethylene MBE indicated a gradual decrease of the deposition rate with increasing flow rate. This is an improvement from that deposition rate with ethanol, which abruptly decreases because of an etching effect. This feature of ethylene enhances the condition for the growth of the high-quality graphene. Raman spectroscopy measurement and atomic force microscopy observation show that the quality of graphene grown using ethylene is improved from that grown using ethanol from the viewpoints of a domain size and crystallinity. These results demonstrate the advantages of ethylene over ethanol in the MBE growth of graphene.     

Layer-by-layer synthesis of large-area graphene films by thermal cracker enhanced gas source molecular beam epitaxy

A thermal cracker enhanced gas source molecular beam epitaxy system was used to synthesize large-area graphene. Hydrocarbon gas molecules were broken by thermal cracker at very high temperature of 1200 °C and then impinged on a nickel substrate. High-quality, large-area graphene films were achieved at 800 °C, and this was confirmed by both Raman spectroscopy and transmission electron microscopy. A rapid cooling rate was not required for few-layer graphene growth in this method, and a high-percentage of single layer and bilayer graphene films was grown by controlling the growth time. The results suggest that in this method, carbon atoms migrate on the nickel surface and bond with each other to form graphene. Few-layer graphene is formed by subsequent growth of carbon layers on top of existing graphene layers. This is completely different from graphene formation through carbon dissolving in nickel and then precipitating from the nickel during rapid substrate cooling in the chemical vapor deposition method.     

Bulk growth of mono- to few-layer graphene on nickel particles by chemical vapor deposition from methane

A method for the bulk growth of mono- to few-layer graphene on nickel particles by chemical vapor deposition from methane at atmospheric pressure is described. A graphene yield of about 2.5% of the weight of nickel particles used was achieved in a growth time of 5 min. Scanning and transmission electron microscopy, Raman spectroscopy, thermogravimetry, and electrical conductivity measurements reveal the high quality of the graphene obtained. Suspended graphene can be prepared during this process, bridging the gaps between nearby nickel grains. After the growth of graphene the nickel particles can be effectively removed by a modest FeCl₃/HCl etching treatment without degradation of the quality of the graphene sheets.     

Synthesis of three-dimensional carbon nanotube/graphene hybrid materials by a two-step chemical vapor deposition process

A three-dimensional carbon nanotube (CNT)/graphene hybrid material was synthesized by a two-step chemical vapor deposition (CVD) process. Due to the separated CVD processes for graphene and CNTs, the structures of the hybrid materials could be easily controlled. It is revealed that graphene film was tightly connected with one end of the CNT arrays, forming “jellyfish” structures. Moreover, our results indicate that the presence of graphene influenced the precipitation and growth rate of CNTs. The precipitation of CNTs was postponed due to the existence of graphene. However, the average growth rate of CNTs in the graphene region for the whole process was faster than that in the region without graphene.     

Large scale metal-free synthesis of graphene on sapphire and transfer-free device fabrication

Metal catalyst-free growth of large scale single layer graphene film on a sapphire substrate by a chemical vapor deposition (CVD) process at 950 °C is demonstrated. A top-gated graphene field effect transistor (FET) device is successfully fabricated without any transfer process. The detailed growth process is investigated by the atomic force microscopy (AFM) studies.     

Discontinuity and misorientation of graphene grown on nickel foil: Effect of the substrate crystallographic orientation

Graphene growth by chemical vapor deposition on low cost metal foils is a promising approach for the production of large-scale graphene. However, the precise control of the uniformity of synthesized mono- and multilayer graphene requires elucidation of the factors affecting deposition and growth. In this study, we investigate the influence of the crystallographic orientation of nickel on multilayer graphene growth using electron-backscatter diffraction, Raman and energy dispersive X-ray spectroscopies, as well as scanning electron and atomic force microscopies. We correlated the discontinuities of the graphene sheets on polycrystalline nickel foils with crystallographic orientations of nickel grains. In addition, we observed indications of misoriented (twisted) multilayer graphene on particular grain orientations. We demonstrate that the Raman signature from these misoriented multilayer graphene areas is highly similar to that previously reported for twisted bilayer graphene. Using microscopy methods, we demonstrated dramatic morphological changes in the nickel substrate induced by graphene growth.     

Promoter-assisted chemical vapor deposition of graphene

The synthesis of graphene by chemical vapor deposition (CVD) is a promising approach for producing graphene for novel applications. Especially graphene synthesis on Copper substrates has resulted in high quality, large area graphene growth. This method, however, exhibit limitations in achievable graphene quality due to the low catalytic activity of the growth substrate and occurring catalyst deactivation at high graphene coverage. We here study the effect of adding a material to promote graphene growth on Cu. Catalytic materials such as Nickel and Molybdenum were found to affect the graphene quality and growth rate positively. The origin for this enhancement is a decrease of the energy barrier of catalytic methane decomposition through a process of distributed catalysis. This process can also help overcome the issue of catalyst deactivation and increase film continuity. These findings not only provide aroute for improving the CVD synthesis of graphene but also answer fundamental questions about graphene growth.     

The effect of downstream plasma treatments on graphene surfaces

This paper reports on the effects of growth, transfer and annealing procedures on graphene grown by chemical vapour deposition. A combination of Raman spectroscopy, electrical measurements, atomic force microscopy, and X-ray photoemission spectroscopy allowed for the study of inherent characteristics and electronic structure of graphene films. Contributions from contaminants and surface inhomogeneities such as ripples were also examined. A new cleaning and reconstruction process for graphene, based on plasma treatments and annealing is presented, opening a new pathway for control over the surface chemistry of graphene films. The method has been successfully used on contacted graphene samples, demonstrating its potential for in situ cleaning, passivation and interface engineering of graphene devices.     

Catalytic chemical vapor deposition of methane on graphite to produce graphene structures

Graphene structures, obtained by catalytic chemical vapor deposition of methane on highly oriented pyrolitic graphite (HOPG), were examined using scanning tunneling microscopy. Depending on the Fe catalyst coverage and localization on the substrate steps and terraces, different graphene structures were obtained: curved graphene sheets at the edges of topmost stacked graphene bilayers, laterally grown terraces at the edges of individual graphene layers parallel to the HOPG basal plane and planar graphene islands on the terraces. A growth mechanism is proposed that takes into account the specific features of the spatial distribution of Fe catalytic nanoparticles on the substrate surface, driven by metal film-substrate interaction. The present synthesis approach is promising for the controlled growth and modification of graphene layers, as well as for engineering the edge characteristics of graphene systems at the atomic scales.     

The production of large bilayer hexagonal graphene domains by a two-step growth process of segregation and surface-catalytic chemical vapor deposition

A novel two-step process combining surface catalyzed process with segregation growth was used to prepare single crystal hexagonal bilayer graphene domains on Cu metal substrates by ambient pressure chemical vapor deposition. Carbon atoms are first dissolved into the quasi-melting Cu metal at 1080 °C and then segregated on the Cu surface to form nucleation centers of single-layer graphene during cooling. The graphene crystallites spontaneously act as templates to induce the carbon atoms to form hexagonal bilayer graphene domains. The bilayer graphene domains are size-tunable by controlling the growth conditions. The yield of the bilayer graphene is over 90% and the defect-free domains reach ～100 μm in size, greater than the reported single-layer domains.     

Fabrication of high-quality graphene by hot-filament thermal chemical vapor deposition

To the best of our knowledge, the previously reported graphene fabricated using catalytic chemical vapor deposition techniques contained a high defect density, which will hinder its opto-electronic properties. In this work, the effects of two crucial parameters, namely deposition time and hydrogen flow rate on the growth of graphene using a hot-filament thermal chemical vapor deposition technique were systematically studied. Fabrications were conducted at substrate and filament temperatures of 1000 °C and 1750 °C, respectively. Very low I_D/I_G ratios (≪0.1) were obtained for all the samples, which reflected the formation of high-quality graphene deposited on Cu foils. A quasi-static equilibrium copper vapor inside an alumina tube was found to be an important factor to obtain a low defect density graphene. A growth mechanism was then proposed, where the cuprous oxide (Cu₂O) acted as a nucleation site for graphene growth.     

Facile synthesis 3D flexible core-shell graphene/glass fiber via chemical vapor deposition

Direct deposition of graphene layers on the flexible glass fiber surface to form the three-dimensional (3D) core-shell structures is offered using a two-heating reactor chemical vapor deposition system. The two-heating reactor is utilized to offer sufficient, well-proportioned floating C atoms and provide a facile way for low-temperature deposition. Graphene layers, which are controlled by changing the growth time, can be grown on the surface of wire-type glass fiber with the diameter from 30 nm to 120 um. The core-shell graphene/glass fiber deposition mechanism is proposed, suggesting that the 3D graphene films can be deposited on any proper wire-type substrates. These results open a facile way for direct and high-efficiency deposition of the transfer-free graphene layers on the low-temperature dielectric wire-type substrates.

PACS

81.05.U-; 81.07.-b; 81.15.Gh     

Multilayered graphene films prepared at moderate temperatures using energetic physical vapour deposition

Carbon films were energetically deposited onto copper and nickel foil using a filtered cathodic vacuum arc deposition system. Raman spectroscopy, scanning electron microscopy, transmission electron microscopy and UV–visible spectroscopy showed that graphene films of uniform thickness with up to 10 layers can be deposited onto copper foil at moderate temperatures of 750 °C. The resulting films, which can be prepared at high deposition rates, were comparable to graphene films grown at 1050 °C using chemical vapour deposition (CVD). This difference in growth temperature is attributed to dynamic annealing which occurs as the film grows from the energetic carbon flux. In the case of nickel substrates, it was found that graphene films can also be prepared at moderate substrate temperatures. However much higher carbon doses were required, indicating that the growth mode differs between substrates as observed in CVD grown graphene. The films deposited onto nickel were also highly non uniform in thickness, indicating that the grain structure of the nickel substrate influenced the growth of graphene layers.     

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 H₂ flow rate is favorable for the growth of high-quality graphene films. The graphene film prepared without the presence of H₂ 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.     

Transfer-free growth of graphene on SiO2 insulator substrate from sputtered carbon and nickel films

Here we demonstrate the growth of transfer-free graphene on SiO₂ insulator substrates from sputtered carbon and metal layers with rapid thermal processing in the same evacuation. It was found that graphene always grows atop the stack and in close contact with the Ni. Raman spectra typical of high quality exfoliated monolayer graphene were obtained for samples under optimised conditions with monolayer surface coverage of up to 40% and overall graphene surface coverage of over 90%. Transfer-free graphene is produced on SiO₂ substrates with the removal of Ni in acid when Ni thickness is below 100 nm, which effectively eliminates the need to transfer graphene from metal to insulator substrates and paves the way to mass production of graphene directly on insulator substrates. The characteristics of Raman spectrum depend on the size of Ni grains, which in turn depend on the thickness of Ni, layer deposition sequence of the stack and RTP temperature. The mechanism of the transfer-free growth process was studied by AFM in combination with Raman. A model is proposed to depict the graphene growth process. Results also suggest a monolayer self-limiting growth for graphene on individual Ni grains.     

Chemical vapor deposition of graphene on large-domain ultra-flat copper

Copper foil is the most commonly used substrate for chemical vapor deposition (CVD) growth of graphene, despite the impact of its surface roughness and polycrystalline structure on the resulting graphene. Here we present a method of preparing thick, ultra-flat copper substrates for growing graphene by CVD. We demonstrate the growth of graphene on these substrates using the common Atmospheric Pressure CVD (APCVD) and Low Pressure CVD (LPCVD) methods. We show that compared to copper foil, graphene grown on these thick ultra-flat copper substrates by APCVD results in 50 times smoother graphene on copper. Furthermore, the thick copper substrates have at least 5 times larger copper domains, compared to conventionally prepared copper foil. The evolution of the surface roughness in each growth method is also presented.     

Growth of graphene on Cu by plasma enhanced chemical vapor deposition

The growth of graphene on Cu substrates by plasma enhanced chemical vapor deposition (PE-CVD) was investigated and its growth mechanism was discussed. At a substrate temperature of 500 °C, formation of graphene was found to precede the growth of carbon nanowalls (CNWs), which are often fabricated by PE-CVD. The growth of graphene was investigated in various conditions, changing the plasma power, gas pressures, and the substrate temperature. The catalytic nature of Cu also affects the growth of monolayer graphene at high substrate temperatures, while the growth at low temperatures and growth of multilayer graphene are dominated mostly by radicals generated in the plasma.     

Two-step growth of graphene with separate controlling nucleation and edge growth directly on SiO2 substrates

Recently, we have developed a catalyst-free and direct growth approach for nanographene on various substrates by a remote-plasma assisted chemical vapor deposition. A two-step growth strategy for separately controlling the nucleation and subsequent edge growth was further developed for growing graphene sheets with adjusted nuclei density and large domain size of 500 nm. The key for tuning the growth mode from nucleation to edge growth is the growth temperature; at a specific growth temperature (∼510–545 °C), only edge growth is available while the nucleation can be largely suppressed. This fine tuning of growth process yields a continuous polycrystalline graphene film with domain size of ∼150 nm. This domain size is controllable in this tunable growth to thus giving more freedom to control the graphene film properties.