Formation of bilayer bernal graphene: layer-by-layer epitaxy via chemical vapor deposition

Programmed synthesis of freestanding graphene nanomembrane arrays

Freestanding graphene membranes are unique materials. The combination of atomically thin dimensions, remarkable mechanical robustness, and chemical stability make porous and non-porous graphene membranes attractive for water purification and various sensing applications. Nanopores in graphene and other 2D materials have been identified as promising devices for next-generation DNA sequencing based on readout of either transverse DNA base-gated current or through-pore ion current. While several ground breaking studies of graphene-based nanopores for DNA analysis have been reported, all methods to date require a physical transfer of the graphene from its source of production onto an aperture support. The transfer process is slow and often leads to tears in the graphene that render many devices useless for nanopore measurements. In this work, we report a novel scalable approach for site-directed fabrication of pinhole-free graphene nanomembranes. Our approach yields high quality few-layer graphene nanomembranes produced in less than a day using a few steps that do not involve transfer. We highlight the functionality of these graphene devices by measuring DNA translocation through electron-beam fabricated nanopores in such membranes.

Graphene growth under Knudsen molecular flow on a confined catalytic metal coil

We have established a simple method for drastically improving the productivity of chemical vapor deposition in large-area graphene synthesis using a roll-stacked Ni coil as a catalyst. Our systematic investigation of the effects of a confined catalytic geometry has shown that the gas flow through interfacial gaps within the stack follows non-continuum fluid dynamics when the size of the gap decreases sufficiently, which enhances the dissolution of the carbon sources into the catalyst during synthesis. Quantitative criteria for graphene growth in the confined geometry are established through the introduction of the Knudsen number, Kn, which is the ratio of the mean-free-path of the gas molecules to the size of the gap. The criteria provided in this article for the synthesis of graphene in the confined geometry are expected to provide the foundations for the efficient mass production of large-area graphene. We also show that the evolution of the catalytic Ni surface in a stacked system results in larger grains in the (111) plane, and consequently in reproducible, uniform, and high-quality multi-layered graphene.

Controlled removal of monolayers for bilayer graphene preparation and visualization

Selective oxidation of monolayer graphene allows the visualization and preparation of bilayer graphene.

Solid-state graphene formation via a nickel carbide intermediate phase

Direct formation of graphene with a controlled number of graphitic layers on dielectric surfaces is achieved with an in-depth understanding of the solid-state growth mechanism.

Large scale bi-layer graphene by suppression of nucleation from a solid precursor

Nucleation was controlled and suppressed by two-way carrier gas insertion and continuous bilayer graphene was synthesized from a botanical derivative, camphor.

Direct synthesis of graphene from adsorbed organic solvent molecules over copper

We show the direct synthesis of graphene from adsorbed organic solvent molecules over copper by annealing in hydrogen.

Breakdown of self-limiting growth on oxidized copper substrates: a facile method for large-size high-quality bi- and trilayer graphene synthesis

Copper nanoparticles induced by oxidation can be utilized to tune the dispersion and size of bi- and trilayer graphene grains.

Equilibrium chemical vapor deposition growth of Bernal-stacked bilayer graphene

Using ethanol as the carbon source, self-limiting growth of AB-stacked bilayer graphene (BLG) has been achieved on Cu via an equilibrium chemical vapor deposition (CVD) process. We found that during this alcohol catalytic CVD (ACCVD) a source-gas pressure range exists to break the self-limitation of monolayer graphene on Cu, and at a certain equilibrium state it prefers to form uniform BLG with a high surface coverage of ∼94% and AB-stacking ratio of nearly 100%. More importantly, once the BLG is completed, this growth shows a self-limiting manner, and an extended ethanol flow time does not result in additional layers. We investigate the mechanism of this equilibrium BLG growth using isotopically labeled (13)C-ethanol and selective surface aryl functionalization, and results reveal that during the equilibrium ACCVD process a continuous substitution of graphene flakes occurs to the as-formed graphene and the BLG growth follows a layer-by-layer epitaxy mechanism. These phenomena are significantly in contrast to those observed for previously reported BLG growth using methane as precursor.

Asymmetric growth of bilayer graphene on copper enclosures using low-pressure chemical vapor deposition

In this work, we investigated the growth mechanisms of bilayer graphene on the outside surface of Cu enclosures at low pressures. We observed that the asymmetric growth environment of a Cu enclosure can yield a much higher (up to 100%) bilayer coverage on the outside surface as compared to the bilayer growth on a flat Cu foil, where both sides are exposed to the same growth environment. By simultaneously examining the graphene films grown on both the outside and inside surfaces of the Cu enclosure, we find that carbon can diffuse from the inside surface to the outside via exposed copper regions on the inside surface. The kinetics of this process are examined by coupling the asymmetric growth between the two surfaces through a carbon diffusion model. Finally, using these results, we show that the coverage of bilayer graphene can be tuned simply by changing the thickness of the Cu foil, further confirming our model of carbon delivery through the Cu foil.

Precisely aligned graphene grown on hexagonal boron nitride by catalyst free chemical vapor deposition

To grow precisely aligned graphene on h-BN without metal catalyst is extremely important, which allows for intriguing physical properties and devices of graphene/h-BN hetero-structure to be studied in a controllable manner. In this report, such hetero-structures were fabricated and investigated by atomic resolution scanning probe microscopy. Moiré patterns are observed and the sensitivity of moiré interferometry proves that the graphene grains can align precisely with the underlying h-BN lattice within an error of less than 0.05°. The occurrence of moiré pattern clearly indicates that the graphene locks into h-BN via van der Waals epitaxy with its interfacial stress greatly released. It is worthy to note that the edges of the graphene grains are primarily oriented along the armchair direction. The field effect mobility in such graphene flakes exceeds 20,000 cm²·V⁻¹·s⁻¹ at ambient condition. This work opens the door of atomic engineering of graphene on h-BN, and sheds light on fundamental research as well as electronic applications based on graphene/h-BN hetero-structure.

Epitaxial growth of asymmetrically-doped bilayer graphene for photocurrent generation

An asymmetrically doped bilayer graphene is grown by modulation-doped chemical vapor deposition, which consists of one intrinsic layer and one nitrogen-doped layer according to AB stacking. The asymmetrically doped bilayer crystalline profile is found to extend the identical registry as adjacent pristine bilayer region, thus forming single-crystalline bilayer graphene p-n junctions. Efficient photocurrent with responsivity as high as 0.2 mA/W is generated at the bilayer p-n junctions via a hot carrier-assisted mechanism.

Layer-stacking growth and electrical transport of hierarchical graphene architectures

Hierarchical graphene architectures (HGAs) that grow by stacking of layers are produced on a liquid copper surface using chemical vapor deposition. The stacking mode--for example hexagonal-hexagonal-hexagonal or hexagonal-snowflake-dendritic--can be simply controlled. Measurements of the electrical properties of HGAs indicate that hierarchical stacking of graphene may be a simple and effective way of tailoring their properties without degrading them.

Direct synthesis of van der Waals solids

The stacking of two-dimensional layered materials, such as semiconducting transition metal dichalcogenides (TMDs), insulating hexagonal boron nitride (hBN), and semimetallic graphene, has been theorized to produce tunable electronic and optoelectronic properties. Here we demonstrate the direct growth of MoS2, WSe2, and hBN on epitaxial graphene to form large-area van der Waals heterostructures. We reveal that the properties of the underlying graphene dictate properties of the heterostructures, where strain, wrinkling, and defects on the surface of graphene act as nucleation centers for lateral growth of the overlayer. Additionally, we show that the direct synthesis of TMDs on epitaxial graphene exhibits atomically sharp interfaces. Finally, we demonstrate that direct growth of MoS2 on epitaxial graphene can lead to a 10(3) improvement in photoresponse compared to MoS2 alone.

Monatomic chemical-vapor-deposited graphene membranes bridge a half-millimeter-scale gap

One of the main concerns in nanotechnology is the utilization of nanomaterials in macroscopic applications without losing their extreme properties. In an effort to bridge the gap between the nano- and macroscales, we propose a clever fabrication method, the inverted floating method (IFM), for preparing freestanding chemical-vapor-deposited (CVD) graphene membranes. These freestanding membranes were then successfully suspended over a gap a half-millimeter in diameter. To understand the working principle of IFM, high-speed photography and white light interferometry were used to characterize and analyze the deformation behaviors of the freestanding graphene membranes in contact with a liquid during fabrication. Some nanoscale configurations in the macroscopic graphene membranes were able to be characterized by simple optical microscopy. The proposed IFM is a powerful approach to investigating the macroscopic structures of CVD graphene and enables the exploitation of freestanding CVD graphene for device applications.

The edge- and basal-plane-specific electrochemistry of a single-layer graphene sheet

Graphene has a unique atom-thick two-dimensional structure and excellent properties, making it attractive for a variety of electrochemical applications, including electrosynthesis, electrochemical sensors or electrocatalysis, and energy conversion and storage. However, the electrochemistry of single-layer graphene has not yet been well understood, possibly due to the technical difficulties in handling individual graphene sheet. Here, we report the electrochemical behavior at single-layer graphene-based electrodes, comparing the basal plane of graphene to its edge. The graphene edge showed 4 orders of magnitude higher specific capacitance, much faster electron transfer rate and stronger electrocatalytic activity than those of graphene basal plane. A convergent diffusion effect was observed at the sub-nanometer thick graphene edge-electrode to accelerate the electrochemical reactions. Coupling with the high conductivity of a high-quality graphene basal plane, graphene edge is an ideal electrode for electrocatalysis and for the storage of capacitive charges.

Large hexagonal bi- and trilayer graphene single crystals with varied interlayer rotations

Bi- and trilayer graphene have attracted intensive interest due to their rich electronic and optical properties, which are dependent on interlayer rotations. However, the synthesis of high-quality large-size bi- and trilayer graphene single crystals still remains a challenge. Here, the synthesis of 100 μm pyramid-like hexagonal bi- and trilayer graphene single-crystal domains on Cu foils using chemical vapor deposition is reported. The as-produced graphene domains show almost exclusively either 0° or 30° interlayer rotations. Raman spectroscopy, transmission electron microscopy, and Fourier-transformed infrared spectroscopy were used to demonstrate that bilayer graphene domains with 0° interlayer stacking angles were Bernal stacked. Based on first-principle calculations, it is proposed that rotations originate from the graphene nucleation at the Cu step, which explains the origin of the interlayer rotations and agrees well with the experimental observations.

Van der Waals epitaxy and characterization of hexagonal boron nitride nanosheets on graphene

Graphene is highly sensitive to environmental influences, and thus, it is worthwhile to deposit protective layers on graphene without impairing its excellent properties. Hexagonal boron nitride (h-BN), a well-known dielectric material, may afford the necessary protection. In this research, we demonstrated the van der Waals epitaxy of h-BN nanosheets on mechanically exfoliated graphene by chemical vapor deposition, using borazine as the precursor to h-BN. The h-BN nanosheets had a triangular morphology on a narrow graphene belt but a polygonal morphology on a larger graphene film. The h-BN nanosheets on graphene were highly crystalline, except for various in-plane lattice orientations. Interestingly, the h-BN nanosheets preferred to grow on graphene than on SiO2/Si under the chosen experimental conditions, and this selective growth spoke of potential promise for application to the preparation of graphene/h-BN superlattice structures fabricated on SiO2/Si.

Fabrication of single-crystal few-layer graphene domains on copper by modified low-pressure chemical vapor deposition

Few-layer graphene domains are fabricated by modified LPCVD on Cu and the growth mechanism is schematically shown in the figure.

Graphene as a transparent conducting and surface field layer in planar Si solar cells

This work presents an experimental and finite difference time domain (FDTD) simulation-based study on the application of graphene as a transparent conducting layer on a planar and untextured crystalline p-n silicon solar cell. A high-quality monolayer graphene with 97% transparency and 350 Ω/□ sheet resistance grown by atmospheric pressure chemical vapor deposition method was transferred onto planar Si cells. An increase in efficiency from 5.38% to 7.85% was observed upon deposition of graphene onto Si cells, which further increases to 8.94% upon SiO2 deposition onto the graphene/Si structure. A large increase in photon conversion efficiency as a result of graphene deposition shows that the electronic interaction and the presence of an electric field at the graphene/Si interface together play an important role in this improvement and additionally lead to a reduction in series resistance due to the conducting nature of graphene.

Controllable atmospheric pressure growth of mono-layer, bi-layer and tri-layer graphene

A coherent three-step growth method has been developed for mono-, bi- and tri-layer graphene with coverage of ∼90% at atmospheric pressure on Cu foil.

Chemical vapour deposition growth of large single crystals of monolayer and bilayer graphene

The growth of large-domain single crystalline graphene with the controllable number of layers is of central importance for large-scale integration of graphene devices. Here we report a new pathway to greatly reduce the graphene nucleation density from ~10(6) to 4 nuclei cm(-2), enabling the growth of giant single crystals of monolayer graphene with a lateral size up to 5 mm and Bernal-stacked bilayer graphene with the lateral size up to 300 μm, both the largest reported to date. The formation of the giant graphene single crystals eliminates the grain boundary scattering to ensure excellent device-to-device uniformity and remarkable electronic properties with the expected quantum Hall effect and the highest carrier mobility up to 16,000 cm(2) V(-1) s(-1). The availability of the ultra large graphene single crystals can allow for high-yield fabrication of integrated graphene devices, paving a pathway to scalable electronic and photonic devices based on graphene materials.

Designed CVD growth of graphene via process engineering

Graphene, the atomic thin carbon film with honeycomb lattice, holds great promise in a wide range of applications, due to its unique band structure and excellent electronic, optical, mechanical, and thermal properties. Scientists are researching this star material because of the development of various emerging preparation techniques, among which chemical vapor deposition (CVD) has received the fastest advances in the past few years. For the CVD growth of graphene, the ultimate goal is to achieve the highest quality in the largest scale and lowest cost with a precise control of layer thickness, stacking order, and crystallinity. To meet this goal, researchers need a comprehensive understanding and effective controlling of the growth process, especially to its elementary steps. In this Account, we focus on our recent progresses toward the controlled surface growth of graphene and its two-dimensional (2D) hybrids via rational designs of CVD elementary processes, namely, process engineering. A typical CVD process consists of four main elementary steps: (A) adsorption and catalytic decomposition of precursor gas, (B) diffusion and dissolution of decomposed carbon species into bulk metal, (C) segregation of dissolved carbon atoms onto the metal surface, and finally, (D) surface nucleation and growth of graphene. Absence or enhancement of each elementary step would lead to significant changes in the whole growth process. Metals with certain carbon solubility, such as nickel and cobalt, involve all four elementary steps in a typical CVD process, thus providing us an ideal system for process engineering. The elementary segregation process can be completely blocked if molybdenum is introduced into the system as an alloy catalyst, yielding perfect monolayer graphene almost independent of growth parameters. On the other hand, the segregation-only process of predissolved solid carbons is also capable of high-quality graphene growth. By using a synergetic Cu-Ni alloy, we are able to further enhance the control to such a segregation technique, especially for the thickness of graphene. By designing a cosegregation process of carbon atoms with other elements, such as nitrogen, doped graphene could be synthesized directly with a tunable doping profile. Copper with negligible carbon solubility provides another platform for process engineering, where both carbon dissolution and segregation steps are negligible in the CVD process. Carbon atoms decomposed from precursors diffuse on the surface and build up the thermodynamically stable honeycomb lattice. As a result, graphene growth on copper is self-limited, and formation of multilayer graphene is generally prohibited. Being able to control this process better, as well as the high quality produced, makes copper-based growth the dominating synthesis procedure in the graphene community. We designed a two-temperature zone system to spatially separate the catalytic decomposition step of carbon precursors and the surface graphitization step for breaking this self-limited growth feature, giving high-quality Bernal stacked bilayer graphene via van der Waals epitaxy. We performed the so-called wrinkle engineering by growing graphene on nanostructured copper foil together with a structure-preserved surface transfer. In such a way, we controlled the wrinkling or folding on graphene and further fabricated graphene nanoribbon arrays by self-masked plasma etching. Moreover, by designing a two-step patching growth process on copper, we succeeded in synthesizing the mosaic graphene, a patchwork of intrinsic and nitrogen-doped graphene connected by single crystalline graphene p-n junctions. By following a general concept of process engineering, our work on the designed CVD growth of graphene and its 2D hybrids provides a unique insight of this research field. It enables the precise growth control of graphene together with the in-depth understanding of CVD growth process, which would further stimulate the pace of graphene applications.

Observation of low energy Raman modes in twisted bilayer graphene

Two new Raman modes below 100 cm(-1) are observed in twisted bilayer graphene grown by chemical vapor deposition. The two modes are observed in a small range of twisting angle at which the intensity of the G Raman peak is strongly enhanced, indicating that these low energy modes and the G Raman mode share the same resonance enhancement mechanism, as a function of twisting angle. The ~94 cm(-1) mode (measured with a 532 nm laser excitation) is assigned to the fundamental layer breathing vibration (ZO' mode) mediated by the twisted bilayer graphene lattice, which lacks long-range translational symmetry. The dependence of this mode's frequency and line width on the rotational angle can be explained by the double resonance Raman process that is different from the previously identified Raman processes activated by twisted bilayer graphene superlattice. The dependence also reveals the strong impact of electronic-band overlaps of the two graphene layers. Another new mode at ~52 cm(-1), not observed previously in the bilayer graphene system, is tentatively attributed to a torsion mode in which the bottom and top graphene layers rotate out-of-phase in the plane.

Thinning of large-area graphene film from multilayer to bilayer with a low-power CO2 laser

Bilayer graphene has attracted a great deal of attention for many electronic and optical applications. Although large-area bilayer graphene can be synthesized by chemical vapor deposition (CVD), multilayer growth often occurs and subsequent processes are required to obtain uniform bilayer films. We report an efficient way of thinning multilayer graphene film by low-power CO2 laser irradiation in vacuum. With a laser power density of ~10(2) W cm(-2), pristine graphene film of 4-5 layers can be thinned to a bilayer free of defects in 30 s. Contrary to previous laser-assisted graphene thinning processes, which reduced graphene layers precisely and locally with a high power density and a small beam diameter, our approach enables high-efficiency thinning of large-area graphene film whilst using a significantly reduced power density and an increased laser beam diameter.

Self-regulating homogenous growth of high-quality graphene on Co-Cu composite substrate for layer control

The composite substrate of Co and Cu was proposed to grow homogenous high quality wafer-size graphene films by an atmosphere pressure CVD method. The composite substrate consists of a moderate-carbon-solubility metal top (Co coating) as a C-dissolving layer and a low-carbon-solubility metal base (Cu foil) as a C-rejecting layer. During the CVD process, the interdiffusion of Co and Cu atoms occurs in the composite. With the dynamic control on Co and Cu alloying process to affect the carbon solubility, active carbon atoms captured by the Co layer were segregated to form spontaneously a high-quality graphene film on the top of Cu-Co substrate. The tunable layer-number of the graphene films can be precisely controlled by adjusting the thickness of the Co layer. High quality single-layered graphene films with a 98% yield were prepared on an 80 nm-Co-coated Cu foil and insensitive to growth temperature and time. More importantly, this type of composite substrate has also been developed to grow AB-stacked bilayers and three-layer graphene with 99% surface coverage and absence of defects. The approach is opening up a new avenue for high-quality graphene production with precise layer control through composite substrate design.

Free radical reactions in two dimensions: a case study on photochlorination of graphene

Graphene, a two-dimensional giant-molecule of sp(2) -bonded carbon atoms, provides a perfect platform for studying free radical reaction chemistry in two-dimensions, which holds promise to control the chemical functionality of graphene. Free-radical photochlorination of graphene is used as an example to investigate the thickness, stacking order, and single- and double-side dependent reactivity in graphene. Anomalously low reactivity is observed in the photochlorination of AB-stacked bilayer graphene in comparison with that of few-layer graphene. Double-sided chlorination of graphene shows higher reactivity and chlorine coverage than single-sided reaction. It is also experimentally and theoretically demonstrated that chlorine free radicals at low coverage prefer to form a stable charge-transfer complex with graphene, which highly enhances graphene's conductivity and simultaneously generates a pseudo-bandgap through noninvasive doping. Moreover, the initial accumulation of chlorine radicals is considered as the rate-determining step of photochlorination of graphene.

Photo-induced free radical modification of graphene

Graphene has stimulated enormous interest due to its intriguing structure and fascinating properties. The extremely high carrier mobility, mechanical flexibility, and optical transparency as well as the versatility for band structure engineering make graphene a promising candidate for next-generation carbon-based electronic devices. Graphene chemistry, the covalent functionalization of graphene as a 2D giant molecule, offers a promising direction to controllably tailor its properties through the introduction of various chemical decorations. One of the great challenges for graphene functionalization originates from its strong chemical stability, thus highly reactive chemical species are needed as the reactants. In recent years, novel photochemical approaches have been developed to achieve efficient graphene modification and bandgap modulation, following a general concept of "Photochemical Bandgap Engineering of Graphene". In this article, such kinds of photochemical graphene engineering are demonstrated, together with a brief discussion on the future directions, challenges, and opportunities in this fascinating research area.

Rapid identification of stacking orientation in isotopically labeled chemical-vapor grown bilayer graphene by Raman spectroscopy

The growth of large-area bilayer graphene has been of technological importance for graphene electronics. The successful application of graphene bilayers critically relies on the precise control of the stacking orientation, which determines both electronic and vibrational properties of the bilayer system. Toward this goal, an effective characterization method is critically needed to allow researchers to easily distinguish the bilayer stacking orientation (i.e., AB stacked or turbostratic). In this work, we developed such a method to provide facile identification of the stacking orientation by isotope labeling. Raman spectroscopy of these isotopically labeled bilayer samples shows a clear signature associated with AB stacking between layers, enabling rapid differentiation between turbostratic and AB-stacked bilayer regions. Using this method, we were able to characterize the stacking orientation in bilayer graphene grown through Low Pressure Chemical Vapor Deposition (LPCVD) with enclosed Cu foils, achieving almost 70% AB-stacked bilayer graphene. Furthermore, by combining surface sensitive fluorination with such hybrid (12)C/(13)C bilayer samples, we are able to identify that the second layer grows underneath the first-grown layer, which is similar to a recently reported observation.

Single- and double-sided chemical functionalization of bilayer graphene

An experimental study on the interaction between the top and bottom layer of a chemically functionalized graphene bilayer by mild oxygen plasma is reported. Structural, chemical, and electrical properties are monitored using Raman spectroscopy, transport measurements, conductive atomic force microscopy and X-ray photoelectron spectroscopy. Single- and double-sided chemical functionalization are found to give very different results: single-sided modified bilayers show relatively high mobility (200-600 cm(2) V(-1) s(-1) at room temperature) and a stable structure with a limited amount of defects, even after long plasma treatment (>60 s). This is attributed to preferential modification and limited coverage of the top layer during plasma exposure, while the bottom layer remains almost unperturbed. This could eventually lead to decoupling between top and bottom layers. Double-sided chemical functionalization leads to a structure containing a high concentration of defects, very similar to graphene oxide. This opens the possibility to use plasma treatment not only for etching and patterning of graphene, but also to make heterostructures (through single-sided modification of bilayers) for sensors and transistors and new graphene-derivatives materials (through double-sided modification).

Growth of adlayer graphene on Cu studied by carbon isotope labeling

The growth of bilayer and multilayer graphene on copper foils was studied by isotopic labeling of the methane precursor. Isotope-labeled graphene films were characterized by micro-Raman mapping and time-of-flight secondary ion mass spectrometry. Our investigation shows that during growth at high temperature, the adlayers formed simultaneously and beneath the top, continuous layer of graphene and the Cu substrate. Additionally, the adlayers share the same nucleation center and all adlayers nucleating in one place have the same edge termination. These results suggest that adlayer growth proceeds by catalytic decomposition of methane (or CH(x), x < 4) trapped in a "nano-chemical vapor deposition" chamber between the first layer and the substrate. On the basis of these results, submillimeter bilayer graphene was synthesized by applying a much lower growth rate.

CVD growth of large area smooth-edged graphene nanomesh by nanosphere lithography

Current etching routes to process large graphene sheets into nanoscale graphene so as to open up a bandgap tend to produce structures with rough and disordered edges. This leads to detrimental electron scattering and reduces carrier mobility. In this work, we present a novel yet simple direct-growth strategy to yield graphene nanomesh (GNM) on a patterned Cu foil via nanosphere lithography. Raman spectroscopy and TEM characterizations show that the as-grown GNM has significantly smoother edges than post-growth etched GNM. More importantly, the transistors based on as-grown GNM with neck widths of 65-75 nm have a near 3-fold higher mobility than those derived from etched GNM with the similar neck widths.

Imaging layer number and stacking order through formulating Raman fingerprints obtained from hexagonal single crystals of few layer graphene

Quantitative mapping of layer number and stacking order for CVD-grown graphene layers is realized by formulating Raman fingerprints obtained on two stepwise stacked graphene single-crystal domains with AB Bernal and turbostratic stacking (with ~30°interlayer rotation), respectively. The integrated peak area ratio of the G band to the Si band, A(G)/A(Si), is proven to be a good fingerprint for layer number determination, while the area ratio of the 2D and G bands, A(2D)/A(G), is shown to differentiate effectively between the two different stacking orders. The two fingerprints are well formulated and resolve, quantitatively, the layer number and stacking type of various graphene domains that used to rely on tedious transmission electron microscopy for structural analysis. The approach is also noticeable in easy discrimination of the turbostratic graphene region (~30° rotation), the structure of which resembles the well known high-mobility graphene R30/R2(±) fault pairs found on the vacuum-annealed C-face SiC and suggests an electron mobility reaching 14,700 cm(3) V(-1) s(-1). The methodology may shed light on monitoring and control of high-quality graphene growth, and thereby facilitate future mass production of potential high-speed graphene applications.

Interfacial properties of bilayer and trilayer graphene on metal substrates

One popular approach to prepare graphene is to grow them on transition metal substrates via chemical vapor deposition. By using the density functional theory with dispersion correction, we systematically investigate for the first time the interfacial properties of bilayer (BLG) and trilayer graphene (TLG) on metal substrates. Three categories of interfacial structures are revealed. The adsorption of B(T)LG on Al, Ag, Cu, Au, and Pt substrates is a weak physisorption, but a band gap can be opened. The adsorption of B(T)LG on Ti, Ni, and Co substrates is a strong chemisorption, and a stacking-insensitive band gap is opened for the two uncontacted layers of TLG. The adsorption of B(T)LG on Pd substrate is a weaker chemisorption, with a band gap opened for the uncontacted layers. This fundamental study also helps for B(T)LG device study due to inevitable graphene/metal contact.

Synthesis of transfer-free graphene on an insulating substrate using a solid phase reaction

We demonstrate a solid phase reaction approach to synthesise transfer-free graphene on an insulating substrate by controlling the C diffusion process. Metal assisted crystallization by annealing of a C thin film was carried out to synthesise transfer-free graphene, in the presence of a top metal oxide and metal layer. Without the metal oxide layer, a large amount of C atoms diffused to the top of the metal surface and hence the formation of only small graphene domains was observed on the underneath of the metal layer. Introducing the metal oxide layer at the top surface, C diffusion was reduced and consequently the thin C film was crystallized to form large area graphene at the metal-insulating substrate interface. The metal oxide or metal catalyst layer was removed after graphene formation and transfer-free graphene was obtained directly on the base substrate. This finding shows that the thin metal oxide layer is critical to synthesise graphene with better quality and continuous domain structures.

A facile method to observe graphene growth on copper foil

A novel scanning electron microscope (SEM) method is presented for high contrast identification of each layer of pyramidal graphene domains grown on copper. We obtained SEM images by combining the advantages of the high resolution property of the secondary electron signal and the elemental sensitivity of the backscattering electron signal. Through this method, we investigated the difference in the growth mechanisms of mono-layer and few-layer graphene. Due to different lattice mismatches, both the surface adsorption process and the epitaxial growth process existed under the atmospheric growth conditions. Moreover, the copper oxidation process can be easily discovered. It is obvious from the SEM images that the graphene greatly delayed the oxidation process of the copper surface. Finally, the nucleation and growth speed of graphene domains was found to depend on the linear array distribution of surface ledges and terraces of annealed rolled copper foil. This result explains the linear rows of graphene during the growth process and accords with theoretical results.

Graphene: an emerging electronic material

Graphene, a single layer of carbon atoms in a honeycomb lattice, offers a number of fundamentally superior qualities that make it a promising material for a wide range of applications, particularly in electronic devices. Its unique form factor and exceptional physical properties have the potential to enable an entirely new generation of technologies beyond the limits of conventional materials. The extraordinarily high carrier mobility and saturation velocity can enable a fast switching speed for radio-frequency analog circuits. Unadulterated graphene is a semi-metal, incapable of a true off-state, which typically precludes its applications in digital logic electronics without bandgap engineering. The versatility of graphene-based devices goes beyond conventional transistor circuits and includes flexible and transparent electronics, optoelectronics, sensors, electromechanical systems, and energy technologies. Many challenges remain before this relatively new material becomes commercially viable, but laboratory prototypes have already shown the numerous advantages and novel functionality that graphene provides.

Direct optical characterization of graphene growth and domains on growth substrates

We detailed a facile detection technique to optically characterize graphene growth and domains directly on growth substrates through a simple thermal annealing process. It was found that thermal annealing transformed the naked Cu to Cu oxides while keeping graphene and graphene-covered Cu intact. This increases the interference color contrast between Cu oxides and Cu, thus making graphene easily visible under an optical microscope. By using this simple method, we studied the factors that affect graphene nucleation and growth and achieved graphene domains with the domain size as large as ~100 μm. The concept of chemically making graphene visible is universal, as demonstrated by the fact that a solution process based on selective H₂O₂ oxidation has been developed to achieve the similar results in a shorter time. These techniques should be valuable for studies towards elucidating the parameters that control the grains, boundaries, structures and properties of graphene.

High-yield chemical vapor deposition growth of high-quality large-area AB-stacked bilayer graphene

Bernal-stacked (AB-stacked) bilayer graphene is of significant interest for functional electronic and photonic devices due to the feasibility to continuously tune its band gap with a vertical electric field. Mechanical exfoliation can be used to produce AB-stacked bilayer graphene flakes but typically with the sizes limited to a few micrometers. Chemical vapor deposition (CVD) has been recently explored for the synthesis of bilayer graphene but usually with limited coverage and a mixture of AB- and randomly stacked structures. Herein we report a rational approach to produce large-area high-quality AB-stacked bilayer graphene. We show that the self-limiting effect of graphene growth on Cu foil can be broken by using a high H(2)/CH(4) ratio in a low-pressure CVD process to enable the continued growth of bilayer graphene. A high-temperature and low-pressure nucleation step is found to be critical for the formation of bilayer graphene nuclei with high AB stacking ratio. A rational design of a two-step CVD process is developed for the growth of bilayer graphene with high AB stacking ratio (up to 90%) and high coverage (up to 99%). The electrical transport studies demonstrate that devices made of the as-grown bilayer graphene exhibit typical characteristics of AB-stacked bilayer graphene with the highest carrier mobility exceeding 4000 cm(2)/V · s at room temperature, comparable to that of the exfoliated bilayer graphene.