Comprehensive study of graphene grown by chemical vapor deposition

Graphene was grown on Cu foil by chemical vapor deposition with CH₄ as carbon source, and then was transferred onto various substrates for device applications. The structural and optical properties of graphene were investigated, comprehensively. Raman spectra indicate as-grown and transferred graphene films are homogenous monolayer graphene. Optical microscopy and scanning electron microscopy images reveal wrinkle-free and smooth surface of transferred graphene, confirming the high quality of graphene. In addition, the transferred graphene on glass exhibits excellent transmittances in the visible region (89.3 % at ~500 nm). Therefore, the results present the controllable approaches to achieve as-grown and transferred high quality graphene for the fabrication of multiple nanoelectronic devices.     

PMMA在石墨烯转移过程中对其表面准周期褶皱的影响

利用CVD方法在铜基底上制备了大面积石墨烯,将其转移到PMMA表面,利用AFM和STM对转移前后的石墨烯表面进行了研究,结果表明,利用CVD方法制备的石墨烯表面存在由Cu基底表面台阶引起的大面积准周期性条纹状褶皱;当石墨烯转移到PMMA表面后,褶皱数量显著减少,表面杂质颗粒和裂痕减少,表明PMMA与石墨烯间的相互作用能够提高石墨烯的平整度,改善石墨烯的质量。     

Control of thickness uniformity and grain size in graphene films for transparent conductive electrodes

Large-scale and transferable graphene films grown on metal substrates by chemical vapor deposition (CVD) still hold great promise for future nanotechnology. To realize the promise, one of the key issues is to further improve the quality of graphene, e.g., uniform thickness, large grain size, and low defects. Here we grow graphene films on Cu foils by CVD at ambient pressure, and study the graphene nucleation and growth processes under different concentrations of carbon precursor. On the basis of the results, we develop a two-step ambient pressure CVD process to synthesize continuous single-layer graphene films with large grain size (up to hundreds of square micrometers). Scanning electron microscopy and Raman spectroscopy characterizations confirm the film thickness and uniformity. The transferred graphene films on cover glass slips show high electrical conductivity and high optical transmittance that make them suitable as transparent conductive electrodes. The growth mechanism of CVD graphene on Cu is also discussed, and a growth model has been proposed. Our results provide important guidance toward the synthesis of high quality uniform graphene films, and could offer a great driving force for graphene based applications.     

Synthesis of hexagonal graphene on polycrystalline Cu foil from solid camphor by atmospheric pressure chemical vapor deposition

Understanding of graphene nucleation and growth on a metal substrate in chemical vapor deposition (CVD) process is critical to obtain high-quality single crystal graphene. Here, we report synthesis of individual hexagonal graphene and their large cluster on Cu foil using solid camphor as a carbon precursor in the atmospheric pressure CVD (AP-CVD) process. Optical and scanning electron microscopy studies show formation of hexagonal graphene crystals across the grain, grain boundaries and twin boundaries of polycrystalline Cu foil. Electron backscattered diffraction analysis is carried out before and after the growth to identify Cu grain orientation correlating with the graphene formation. The influence of growth conditions and Cu grain structure is explored on individual hexagonal graphene formation in the camphor-based AP-CVD process.     

Fast and simultaneous growth of graphene,intermetallic compounds,and silicate on Cu–Ni alloy foils

A graphene bilayer was grown on copper–nickel alloy foils (30 at-% Ni: 70 at-% Cu designated as a 30Ni–70Cu) via an inductively coupled plasma–chemical vapor deposition chamber, and was characterized. The first layer fully covered the foil, while there was partial coverage of the second layer. At the same time, the alloy catalyst produced a compound of magnesium silicate in some regions and of copper sulfide in other regions on which a graphene monolayer simultaneously grew without any discontinuity or boundaries of the 1st graphene monolayer between simultaneous growth and graphene-only growth regions. Compared with Cu foils, the alloy foils led to faster growth of the graphene film in graphene-only growth regions, while maintaining the same quality, homogeneity, and thickness uniformity as a monolayer graphene grown on Cu. Raman spectroscopy and scattering demonstrated that the 2D and D bands of the Raman spectra were in the same position for the monolayer graphene on 30Ni–70Cu regardless of the grown regions and for the graphene on the Cu with a full width at half maximum of ∼38 cm⁻¹ ranging from 30 to 55 cm⁻¹ of 2D, and without a D band in the spectra of the graphene monolayer and bilayer. Thus the resulting graphene growth is affected primarily by the Cu catalyst, partly by the compounds grown simultaneously with the graphene monolayer on the foil surface via thermal reactions of the impurities dissolved in the alloy matrix, and partly by the Ni. The quality of the graphene is dependent on the major composition of Cu catalyst in the alloy foils. On the other hands, the alloying element of Ni governs the growth kinetics unless the alloy foils is covered with the intermetallic compounds and silicate.     

Graphene films with large domain size by a two-step chemical vapor deposition process

The fundamental properties of graphene are making it an attractive material for a wide variety of applications. Various techniques have been developed to produce graphene and recently we discovered the synthesis of large area graphene by chemical vapor deposition (CVD) of methane on Cu foils. We also showed that graphene growth on Cu is a surface-mediated process and the films were polycrystalline with domains having an area of tens of square micrometers. In this paper, we report on the effect of growth parameters such as temperature, and methane flow rate and partial pressure on the growth rate, domain size, and surface coverage of graphene as determined by Raman spectroscopy, and transmission and scanning electron microscopy. On the basis of the results, we developed a two-step CVD process to synthesize graphene films with domains having an area of hundreds of square micrometers. Scanning electron microscopy and Raman spectroscopy clearly show an increase in domain size by changing the growth parameters. Transmission electron microscopy further shows that the domains are crystallographically rotated with respect to each other with a range of angles from about 13 to nearly 30°. Electrical transport measurements performed on back-gated FETs show that overall films with larger domains tend to have higher carrier mobility up to about 16,000 cm(2) V(-1) s(-1) at room temperature.     

Shear modulus of monolayer graphene prepared by chemical vapor deposition

We report shear modulus (G) and internal friction (Q(-1)) measurements of large-area monolayer graphene films grown by chemical vapor deposition on copper foil and transferred onto high-Q silicon mechanical oscillators. The shear modulus, extracted from a resonance frequency shift at 0.4 K where the apparatus is most sensitive, averages 280 GPa. This is five times larger than those of the multilayered graphene-based films measured previously. The internal friction is unmeasurable within the sensitivity of our experiment and thus bounded above by Q(-1) ≤ 3 × 10(-5), which is orders-of-magnitude smaller than that of multilayered graphene-based films. Neither annealing nor interface modification has a measurable effect on G or Q(-1). Our results on G are consistent with recent theoretical evaluations and simulations carried out in this work, showing that the shear restoring force transitions from interlayer to intralayer interactions as the film thickness approaches one monolayer.     

Long-range ordered single-crystal graphene on high-quality heteroepitaxial Ni thin films grown on MgO(111)

Large-area single crystal monolayer graphene is synthesized on Ni(111) thin films, which have flat terraces and no grain boundaries. The flat single-crystal Ni films are heteroepitaxially grown on MgO(111) substrates using a buffer layer technique. Low-energy electron diffraction and various spectroscopic methods reveal the long-range single crystallinity and uniform monolayer thickness of the graphene. When transferred onto an insulating wafer, continuous millimeter-scale single domain graphene is obtained.     

Effect of copper foil annealing process on large graphene domain growth by solid source-based chemical vapor deposition

Controlling the metal catalyst surface structure is a critical factor to achieve growth of large graphene domains. In this prospect, we explored the annealing process to create an oxide layer and subsequent recrystallization of Cu foil for growth of large graphene domain by the atmospheric pressure chemical vapor deposition (AP-CVD) technique. We revealed the transformation of Cu surface crystallographic structures in every step of annealing process by electron back-scattered diffraction analysis. Initially, electroless polished Cu foils are annealed in Ar and then in H₂ atmosphere to obtain a smoother surface with reduced graphene nucleation sites. The transformation of Cu grain structures at various annealing steps was confirmed, where the gas atmosphere and annealing duration have significant influence. Graphene domains with the size more than 560 µm are obtained on the processed Cu surface using polystyrene as solid precursor. It is obtained that the oxidation and recrystallization process of Cu foil surface significantly influence the nucleation density, which enable growth of larger graphene domain in the developed CVD process.     

A novel method for large area graphene transfer on the polymer optical fiber

We develop two simple methods-the dip coat stamping and lift-off methods-to transfer large area, high quality graphene films onto the top and side faces of the polymer optical fiber. The graphene films can be synthesized using chemical vapor deposition method on large Cu foils. After synthesis, the graphene films are characterized by scanning electron microscopy, atomic force microscopy and Raman spectroscopy. The polymer optical fiber probe with the transferred graphene film can be used as a chemical sensor for the detection of various organic aerosols.     

Near-field microwave scanning probe imaging of conductivity inhomogeneities in CVD graphene

We have performed near-field scanning microwave microscopy (SMM) of graphene grown by chemical vapor deposition. Due to the use of probe-sample capacitive coupling and a relatively high ac frequency of a few GHz, this scanning probe method allows mapping of local conductivity without a dedicated counter electrode, with a spatial resolution of about 50?nm. Here, the coupling was enabled by atomic layer deposition of alumina on top of graphene, which in turn enabled imaging both large-area films, as well as micron-sized islands, with a dynamic range covering a low sheet resistance of a metal film and a high resistance of highly disordered graphene. The structures of graphene grown on Ni films and Cu foils are explored, and the effects of growth conditions are elucidated. We present a simple general scheme for interpretation of the contrast in the SMM images of our graphene samples and other two-dimensional conductors, which is supported by extensive numerical finite-element modeling. We further demonstrate that combination of the SMM and numerical modeling allows quantitative information about the sheet resistance of graphene to be obtained, paving the pathway for characterization of graphene conductivity with a sub-100?nm special resolution.     

Direct formation of wafer scale graphene thin layers on insulating substrates by chemical vapor deposition

Direct formation of high-quality and wafer scale graphene thin layers on insulating gate dielectrics such as SiO(2) is emergent for graphene electronics using Si-wafer compatible fabrication. Here, we report that in a chemical vapor deposition process the carbon species dissociated on Cu surfaces not only result in graphene layers on top of the catalytic Cu thin films but also diffuse through Cu grain boundaries to the interface between Cu and underlying dielectrics. Optimization of the process parameters leads to a continuous and large-area graphene thin layers directly formed on top of the dielectrics. The bottom-gated transistor characteristics for the graphene films have shown quite comparable carrier mobility compared to the top-layer graphene. The proposed method allows us to achieve wafer-sized graphene on versatile insulating substrates without the need of graphene transfer.     

Wafer scale homogeneous bilayer graphene films by chemical vapor deposition

The discovery of electric field induced band gap opening in bilayer graphene opens a new door for making semiconducting graphene without aggressive size scaling or using expensive substrates. However, bilayer graphene samples have been limited to μm(2) size scale thus far, and synthesis of wafer scale bilayer graphene poses a tremendous challenge. Here we report homogeneous bilayer graphene films over at least a 2 in. × 2 in. area, synthesized by chemical vapor deposition on copper foil and subsequently transferred to arbitrary substrates. The bilayer nature of graphene film is verified by Raman spectroscopy, atomic force microscopy, and transmission electron microscopy. Importantly, spatially resolved Raman spectroscopy confirms a bilayer coverage of over 99%. The homogeneity of the film is further supported by electrical transport measurements on dual-gate bilayer graphene transistors, in which a band gap opening is observed in 98% of the devices.     

Chemical Vapor Deposition of Bernal‐Stacked Graphene on a Cu Surface by Breaking the Carbon Solubility Symmetry in Cu Foils

The synthesis of Bernal‐stacked multilayer graphene over large areas is intensively investigated due to the value of this material's tunable electronic structure, which makes it promising for use in a wide range of optoelectronic applications. Multilayer graphene is typically formed via chemical vapor deposition onto a metal catalyst, such as Ni, a Cu–Ni alloy, or a Cu pocket. These methods, however, require sophisticated control over the process parameters, which limits the process reproducibility and reliability. Here, a new synthetic method for the facile growth of large‐area Bernal‐stacked multilayer graphene with precise layer control is proposed. A thin Ni film is deposited onto the back side of a Cu foil to induce controlled diffusion of carbon atoms through bulk Cu from the back to the front. The resulting multilayer graphene exhibits a 97% uniformity and a sheet resistance of 50 Ω sq^?1 with a 90% transmittance after doping. The growth mechanism is elucidated and a generalized kinetic model is developed to describe Bernal‐stacked multilayer graphene growth by the carbon atoms diffused through bulk Cu.     

Different growth behaviors of ambient pressure chemical vapor deposition graphene on Ni(111) and Ni films: A scanning tunneling microscopy study

Graphene growth on the same metal substrate with different crystal morphologies, such as single crystalline and polycrystalline, may involve different mechanisms. We deal with this issue by preparing graphene on single crystal Ni(111) and on ∼300 nm thick Ni films on SiO₂ using an ambient pressure chemical vapor deposition (APCVD) method, and analyze the different growth behaviors for different growth parameters by atomically-resolved scanning tunneling microscopy (STM) and complementary macroscopic analysis methods. Interestingly, we obtained monolayer graphene on Ni(111), and multilayer graphene on Ni films under the same growth conditions. Based on the experimental results, it is proposed that the graphene growth on Ni(111) is strongly templated by the Ni(111) lattice due to the strong Ni-C interactions, leading to monolayer graphene growth. Multilayer graphene flakes formed on polycrystalline Ni films are usually stacked with deviations from the Bernal stacking type and show small rotations among the carbon layers. Considering the different substrate features, the inevitable grain boundaries on polycrystalline Ni films are considered to serve as the growth fronts for bilayer and even multilayer graphene.      

Patterned Growth of Graphene over Epitaxial Catalyst

Rectangle‐ and triangle‐shaped microscale graphene films are grown on epitaxial Co films deposited on single‐crystal MgO substrates with (001) and (111) planes, respectively. A thin film of Co or Ni metal is epitaxially deposited on a MgO substrate by sputtering while heating the substrate. Thermal decomposition of polystyrene over this epitaxial metal film in vacuum gives rectangular or triangular pit structures whose orientation and shape are strongly dependent on the crystallographic orientation of the MgO substrate. Raman mapping measurements indicate preferential formation of few‐layer graphene films inside these pits. The rectangular graphene films are transferred onto a SiO₂/Si substrate while maintaining the original shape and field‐effect transistors are fabricated using the transferred films. These findings on the formation of rectangular/triangular graphene give new insights on the formation mechanism of graphene and can be applied for more advanced/controlled graphene growth.     

Characterization of nanocrystalline Ni–Cu thin films electrodeposited onto ITO coated glass substrates: effect of pretreatment current density

In this work, Ni–Cu films were grown onto indium tin oxide coated glass substrates without and with galvanostatic pretreatment process at different current densities. In all cases, Ni–Cu films were electrodeposited at a constant deposition potential of ?1,800 mV versus saturated calomel electrode. After that, the surface morphology and structural properties of electrodeposited Ni–Cu films in dependence of pretreatment current density were studied. X-ray diffraction analysis showed that all films have face-centered cubic structure and consist of segregated two Ni-rich and Cu-rich phases regardless of pretreatment current density. The compositional analysis carried out by energy dispersive X-ray spectroscopy revealed that all films contain almost 90 wt% Ni and 10 wt% Cu. The average crystallite size decreased with decreasing pretreatment current density towards more negative values without inducing significant changes in the composition of the films. It was found that the preferred orientation of all films is in the [111] direction regardless of pretreatment current density. The effect of galvanostatic pretreatment process on the surface morphology investigated using a scanning electron microscopy and an atomic force microcopy were also discussed by means of obtained results.     

Synthesis of isotopically-labeled graphite films by cold-wall chemical vapor deposition and electronic properties of graphene obtained from such films

We report the synthesis of isotopically-labeled graphite films on nickel substrates by using cold-wall chemical vapor deposition (CVD). During the synthesis, carbon from ¹²C- and ¹³C-methane was deposited on, and dissolved in, a nickel foil at high temperature, and a uniform graphite film was segregated from the nickel surface by cooling the sample to room temperature. Scanning and transmission electron microscopy, micro-Raman spectroscopy, and X-ray diffraction prove the presence of a graphite film. Monolayer graphene films obtained from such isotopically-labeled graphite films by mechanical methods have electron mobility values greater than 5000 cm²·V⁻¹·s⁻¹ at low temperatures. Furthermore, such films exhibit the half-integer quantum Hall effect over a wide temperature range from 2 K to 200 K, implying that the graphite grown by this cold-wall CVD approach has a quality as high as highly oriented pyrolytic graphite (HOPG). The results from transport measurements indicate that ¹³C-labeling does not significantly affect the electrical transport properties of graphene.     

Adlayer‐Free Large‐Area Single Crystal Graphene Grown on a Cu(111) Foil

To date, thousands of publications have reported chemical vapor deposition growth of “single layer” graphene, but none of them has described truly single layer graphene over large area because a fraction of the area has adlayers. It is found that the amount of subsurface carbon (leading to additional nuclei) in Cu foils directly correlates with the extent of adlayer growth. Annealing in hydrogen gas atmosphere depletes the subsurface carbon in the Cu foil. Adlayer‐free single crystal and polycrystalline single layer graphene films are grown on Cu(111) and polycrystalline Cu foils containing no subsurface carbon, respectively. This single crystal graphene contains parallel, centimeter‐long ≈100 nm wide “folds,” separated by 20 to 50 µm, while folds (and wrinkles) are distributed quasi‐randomly in the polycrystalline graphene film. High‐performance field‐effect transistors are readily fabricated in the large regions between adjacent parallel folds in the adlayer‐free single crystal graphene film.     

Electrical characterization of graphene synthesized by chemical vapor deposition using Ni substrate

In this work, the electrical characterization of graphene films grown by chemical vapor deposition (CVD) on a Ni thin film is carried out and a simple relation between the gate-dependent electrical transport and the thickness of the films is presented. Arrays of two-terminal devices with an average graphene film thickness of 6.9 nm were obtained using standard fabrication techniques. A simple two-band model is used to describe the graphene films, with a band overlap parameter E(0) = 17 meV determined by the dependence of conductivity on temperature. Statistical electrical measurement data are presented for 126 devices, with an extracted average background conductivity σ = 0.91 mS, average carrier mobility μ = 1300 cm(2) V(-1) s(-1) and residual resistivity ρ = 1.65 kΩ. The ratio of mobility to conductivity is calculated to be inversely proportional to the graphene film thickness and this calculation is statistically verified for the ensemble of 126 devices. This result is a new method of graphene film thickness determination and is useful for films which cannot have their thickness measured by AFM or optical interference, but which are electrically contacted and gated. This general approach provides a framework for comparing graphene devices made using different fabrication methods and graphene growth techniques, even without prior knowledge of their uniformity or thickness.