Single-crystal Graphene Domains Research Articles

Flexible Devices Based on Soybean-Derived High-Quality N-Doped Graphene

Graphene with exceptional properties has attracted significant attention in many fields. Chemical vapor deposition has been a vital method for synthesizing high-quality graphene with controlled size, thickness, and quality. Intrinsic graphene is a zero bandgap 2D material with weak ambipolar behavior, and the transistors based on such graphene show a low on/off current ratio. It is important to achieve the controllable preparation of graphene with adjustable electrical properties. Doping the graphene with heteroatoms is a standard method to achieve this goal. Here, we demonstrate that high-quality N-doped graphene can be prepared using soybeans as the carbon source. We can control the preparation of high-quality N-doped graphene on Cu catalyst using soybean as the carbon source, including, N-doped single-crystal graphene domains and N-doped monolayer films. Electrical measurements show that the N-doped graphene exhibits an n-type behavior, indicating that doping can effectively modulate graphene’s electrical properties. Based on the high-quality N-doped graphene, we demonstrate its applications in flexible supercapacitors and skin-like electrophysiological monitors, showing high application value in wearable electronic devices.

Surface Engineering of Substrates for Chemical Vapor Deposition Growth of Graphene and Applications in Electronic and Spintronic Devices

Controllable synthesis of large-scale and high-quality graphene film is a basis for development of electronic and spintronic devices. Chemical vapor deposition (CVD) has been considered as a potential method for the industrialized preparation of high-quality graphene. The substrate in CVD is used to support graphene and even help graphene grow. Different substrates may lead to different graphene growth results. Therefore, it is of great importance to select appropriate substrates and growth strategies. In addition, the performance demands of advanced device also urgently require high-quality CVD-derived graphene supports. In this review, we summarize the current advances in surface engineering on substrates for CVD growth of graphene. Surface engineering involves anisotropic disordered, anisotropic ordered, and isotropic homogeneous surfaces of catalytic substrates, and surfaces of insulating substrates. Meanwhile, we describe how to use different surface engineering techniques to control the orientation of graphene domains, grow large-area single-crystal graphene domains, and regulate graphene layers. This information will provide a reference for the effective selection of substrates. Moreover, we review the current progress of graphene in the fields of charge and spin transport, demonstrating that graphene has broad prospects in the next generation of communications and storage. Finally, the opportunities and challenges in the controllable CVD-derived preparation of graphene and its application are discussed.

Growth and carrier transport performance of single-crystalline monolayer graphene over electrodeposited copper film on quartz glass

The synthesis of single crystal graphene domains on cold rolling Cu foil has recently been reported. However, the cold rolling Cu foils have many rolling lines that increase the roughness of the foil. In this work, we developed an electrodeposited method to achieve high-quality Cu film on which the nucleation density of graphene decreased greatly. The Cu film was electrodeposited over chemical plated silver film on quartz glass. The roughness and thickness of the Cu film were positively correlated with the deposition temperature and current density, respectively. High-quality single crystal graphene with low nucleation density was achieved on electrodeposited Cu film. With increasing deposition temperature of Cu film from 20 to 65 °C, the nucleation density of graphene increased from 124 to 448 mm−2. The nucleation density of graphene increased from 124 to 192 mm−2 with increasing current density from 0.03 to 0.07 A/cm−2. Finally, a back-gated graphene field effect transistor (FET) was fabricated with a carrier transport performance of μh = ~4041 cm2V−1s−1 and μe = ~2580 cm2V−1s−1 at room temperature.

Rapid growth of large-area single-crystal graphene film by seamless stitching using resolidified copper foil on a molybdenum substrate

Single-crystal graphene film growth by the seamless stitching of highly oriented single-crystal graphene domains on a resolidified Cu (111) surface.

Sputtering an exterior metal coating on copper enclosure for large-scale growth of single-crystalline graphene

We show the suppression of nucleation density in chemical vapor deposited graphene through the use of a sputtered metal coating on the exterior of a copper catalyst enclosure, resulting in the growth of sub-centimeter scale single crystal graphene domains and complete elimination of multilayer growth. The sputtered coating suppresses nucleation density by acting as both a diffusion barrier and as a sink for excess carbon during the growth, reducing the carbon concentration in the interior of the enclosure. Field effect mobility of hBN-templated devices fabricated from graphene domains grown in this way show room temperature carrier mobilities of 12 000 cm2 V−1 s−1 and an absence of weak localization at low temperature. These results indicate a very low concentration of line and point defects in the grown films, which is further supported by Raman and transmission electron microscopic characterization.

Controlled Chemical Synthesis in CVD Graphene

AbstractDue to the unique properties of graphene, single layer, bilayer or even few layer graphene peeled off from bulk graphite cannot meet the need of practical applications. Large size graphene with quality comparable to mechanically exfoliated graphene has been synthesized by chemical vapor deposition (CVD). The main development and the key issues in controllable chemical vapor deposition of graphene has been briefly discussed in this chapter. Various strategies for graphene layer number and stacking control, large size single crystal graphene domains on copper, graphene direct growth on dielectric substrates, and doping of graphene have been demonstrated. The methods summarized here will provide guidance on how to synthesize other two-dimensional materials beyond graphene.

Optical-assistant characterization of friction anisotropy properties of single-crystal graphene domains

Recently, large-area high-quality single-crystal graphene domains have been be fabricated using modified chemical vapor deposition (CVD). These hexagonal graphene domains have edges of zigzag chirality and micrometer size, which makes it possible to study the friction anisotropy properties caused by lattice orientations with multi-scale method. In this paper, an optical-assistant characterization method is first put forward. With the micro-scale optical system, the spatial relationship between the hexagonal graphene domain and the probe of the atomic force microscopy (AFM) was determined and changed gradually. Then a series of atomic-scale friction experiments were conducted under ambient conditions utilizing AFM lateral mode. During the process, the probe scanned along various lattice orientations with sample rotation method, which gets ride of the probe's anisotropic effect caused by the cantilever and the tip. And the stick-slip behaviors of the probe during this process were observed and recorded precisely. The scanning experimental results unveiled that the patterns of the stick-slip behaviors varied along different lattice orientation, which is caused by the distribution of graphene surface potential. The comparison between theoretical and experimental results shows that these variations can be regarded as the origin of the friction anisotropy on the graphene domains. The achievements will not only provide an effective method for the identification of lattice orientation, but also lay a more solid experimental base for multiscale research on graphene. The presented work can also provide a standard process for studies on various properties of graphene depended on lattice distribution.

Ultrafast growth of single-crystal graphene assisted by a continuous oxygen supply

Graphene has a range of unique physical properties and could be of use in the development of a variety of electronic, photonic and photovoltaic devices. For most applications, large-area high-quality graphene films are required and chemical vapour deposition (CVD) synthesis of graphene on copper surfaces has been of particular interest due to its simplicity and cost effectiveness. However, the rates of growth for graphene by CVD on copper are less than 0.4 μm s-1, and therefore the synthesis of large, single-crystal graphene domains takes at least a few hours. Here, we show that single-crystal graphene can be grown on copper foils with a growth rate of 60 μm s-1. Our high growth rate is achieved by placing the copper foil above an oxide substrate with a gap of ∼15 μm between them. The oxide substrate provides a continuous supply of oxygen to the surface of the copper catalyst during the CVD growth, which significantly lowers the energy barrier to the decomposition of the carbon feedstock and increases the growth rate. With this approach, we are able to grow single-crystal graphene domains with a lateral size of 0.3 mm in just 5 s.

Electron-diffraction study of graphene-film growth stages during the thermal destruction of 6H-SiC (000 $$\bar 1$$ ) in vacuum

The structure of graphene layers grown by sublimation on a 6H-SiC (000\(\bar 1\)) substrate surface is studied by electron diffraction depending on the sublimation temperature and substrate-surface pretreatment method. It is shown that the use of polishing sublimation etching of the substrate before thermal destruction at a temperature of 1350°C on the substrate surface results in the formation of single-crystal graphene domains with graphene-lattice rotation by 30° with respect to the SiC lattice and a small fraction of amorphous domains. An increase in the temperature to 1500°C leads to the partial formation of a polycrystalline graphene phase with turbostratic structure while retaining the preferred orientation of graphene crystallites as at 1350°C. The use of pregrowth annealing before thermal destruction makes it possible to grow a graphene film with a more ordered and homogeneous structure without inclusions of amorphous and polycrystalline components. The preferred orientation of graphene domains in the film remains unchanged.

Chemical vapor deposition growth of large single-crystal bernal-stacked bilayer graphene from ethanol

Using ethanol as a precursor, single-crystal bilayer graphene domains with dimensions up to hundreds of micrometer, one of the largest reported so far, were synthesized on Cu foils by chemical vapor deposition (CVD). Raman spectroscopy analysis revealed that the bilayers are homogeneously AB-stacked, with very low D-band intensity. Selected area electron diffraction analysis also confirmed the bernal stacking order and the large area crystallinity. Surprisingly, decreasing ethanol pressure in CVD shows an unambiguous tread from self-limited single layer to a multi-layer graphene, and there exists a narrow window of ethanol pressure which is preferred for the formation of large domain bilayer graphene. This suggests the dual effect of ethanol in graphene growth and demonstrates that the formation of different layers of graphene can be controlled by carefully tuning single parameter: ethanol pressure.

Growth of large-area aligned pentagonal graphene domains on high-index copper surfaces

Single-crystal graphene domains grown by chemical vapor deposition (CVD) intrinsically tend to have a six-fold symmetry; however, several factors can influence the growth kinetics, which can in turn lead to the formation of graphene with different shapes. Here we report the growth of oriented large-area pentagonal single-crystal graphene domains on Cu foils by CVD. We found that high-index Cu planes contributed selectively to the formation of pentagonal graphene. Our results indicated that lattice steps present on the crystalline surface of the underlying Cu promoted graphene growth in the direction perpendicular to the steps and finally led to the disappearance of one of the edges forming a pentagon. In addition, hydrogen promoted the formation of pentagonal domains. This work provides new insights into the mechanism of graphene growth.

Few layers isolated graphene domains grown on copper foils by microwave surface wave plasma CVD using camphor as a precursor

Few layers isolated graphene domains were grown by microwave surface wave plasma CVD technique using camphor at low temperature. Graphene nucleation centers were suppressed on pre-annealed copper foils by supplying low dissociation energy. Scanning electron microscopy study of time dependent growth reveals that graphene nucleation centers were preciously suppressed, which indicates the possibility of controlled growth of large area single crystal graphene domains by plasma processing. Raman spectroscopy revealed that the graphene domains are few layered which consist of relatively low defects.

Surface Engineering of Copper Foils for Growing Centimeter-Sized Single-Crystalline Graphene.

The controlled growth of high-quality graphene on a large scale is of central importance for applications in electronics and optoelectronics. To minimize the adverse impacts of grain boundaries in large-area polycrystalline graphene, the synthesis of large single crystals of monolayer graphene is one of the key challenges for graphene production. Here, we develop a facile surface-engineering method to grow large single-crystalline monolayer graphene by the passivation of the active sites and the control of graphene nucleation on copper surface using the melamine pretreatment. Centimeter-sized hexagonal single-crystal graphene domains were successfully grown, which exhibit ultrahigh carrier mobilities exceeding 25,000 cm(2) V(-1) s(-1) and quantum Hall effects on SiO2 substrates. The underlying mechanism of melamine pretreatments were systematically investigated through elemental analyses of copper surface in the growth process of large single-crystals. This present work provides a surface design of a catalytic substrate for the controlled growth of large-area graphene single crystals.

Repeated growth-etching-regrowth for large-area defect-free single-crystal graphene by chemical vapor deposition.

Reducing nucleation density and healing structural defects are two challenges for fabricating large-area high-quality single-crystal graphene, which is essential for its electronic and optoelectronic applications. We have developed a method involving chemical vapor deposition (CVD) growth followed by repeated etching-regrowth, to solve both problems at once. Using this method, we can obtain single-crystal graphene domains with a size much larger than that allowed by the nucleation density in the initial growth and efficiently heal structural defects similar to graphitization but at a much lower temperature, both of which are impossible to realize by conventional CVD. Using this method with Pt as a growth substrate, we have grown ∼3 mm defect-free single-crystal graphene domains with a carrier mobility up to 13,000 cm2 V(-1) s(-1) under ambient conditions.

Overlook of current chemical vapor deposition-grown large single-crystal graphene domains

Exceptional progress has been made with chemical vapor deposition (CVD) of graphene in the past few years. Not only has good monolayer growth of graphene been achieved, but large-area synthesis of graphene sheets has been successful too. However, the polycrystalline nature of CVD graphene is hampering further progress as graphene property degrades due to presence of grain boundaries. This review will cover factors that affect nucleation of graphene and how other scientists sought to obtain large graphene domains. In addition, the limitation of the current research trend will be touched upon as well.

Wrinkle-dependent hydrogen etching of chemical vapor deposition-grown graphene domains

Hexagonal single-crystal graphene domains were grown on copper (Cu) foil via chemical vapor deposition and were etched with hydrogen at 950°C from 7 to 60min at atmospheric pressure. Numerous trenches were observed on the initial graphene domains after etching, and the trench patterns were closely associated with the Cu crystal orientation. No trenches were found if the etching process was conducted before cooling down. Thus, the etching trenches were bound up with the wrinkles formed during the cooling down process. Then, the process of etching on the wrinkles was examined. This simple hydrogen etching technology proved that wrinkles and point defects existed even in hexagonal single-crystal graphene domains. This method could be a convenient way to detect the distribution and morphology of wrinkles in graphene.

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.

Edge-controlled growth and kinetics of single-crystal graphene domains by chemical vapor deposition

The controlled growth of large-area, high-quality, single-crystal graphene is highly desired for applications in electronics and optoelectronics; however, the production of this material remains challenging because the atomistic mechanism that governs graphene growth is not well understood. The edges of graphene, which are the sites at which carbon accumulates in the two-dimensional honeycomb lattice, influence many properties, including the electronic properties and chemical reactivity of graphene, and they are expected to significantly influence its growth. We demonstrate the growth of single-crystal graphene domains with controlled edges that range from zigzag to armchair orientations via growth-etching-regrowth in a chemical vapor deposition process. We have observed that both the growth and the etching rates of a single-crystal graphene domain increase linearly with the slanted angle of its edges from 0° to ∼19° and that the rates for an armchair edge are faster than those for a zigzag edge. Such edge-structure-dependent growth/etching kinetics of graphene can be well explained at the atomic level based on the concentrations of the kinks on various edges and allow the evolution and control of the edge and morphology in single-crystal graphene following the classical kinetic Wulff construction theory. Using these findings, we propose several strategies for the fabrication of wafer-sized, high-quality, single-crystal graphene.

Synthesis of Millimeter-Size Hexagon-Shaped Graphene Single Crystals on Resolidified Copper

We present a facile method to grow millimeter-size, hexagon-shaped, monolayer, single-crystal graphene domains on commercial metal foils. After a brief in situ treatment, namely, melting and subsequent resolidification of copper at atmospheric pressure, a smooth surface is obtained, resulting in the low nucleation density necessary for the growth of large-size single-crystal graphene domains. Comparison with other pretreatment methods reveals the importance of copper surface morphology and the critical role of the melting-resolidification pretreatment. The effect of important growth process parameters is also studied to determine their roles in achieving low nucleation density. Insight into the growth mechanism has thus been gained. Raman spectroscopy and selected area electron diffraction confirm that the synthesized millimeter-size graphene domains are high-quality monolayer single crystals with zigzag edge terminations.

Hexagonal Graphene Onion Rings

Precise spatial control of materials is the key capability of engineering their optical, electronic, and mechanical properties. However, growth of graphene on Cu was revealed to be seed-induced two-dimensional (2D) growth, limiting the synthesis of complex graphene spatial structures. In this research, we report the growth of onion ring like three-dimensional (3D) graphene structures, which are comprised of concentric one-dimensional hexagonal graphene ribbon rings grown under 2D single-crystal monolayer graphene domains. The ring formation arises from the hydrogenation-induced edge nucleation and 3D growth of a new graphene layer on the edge and under the previous one, as supported by first principles calculations. This work reveals a new graphene-nucleation mechanism and could also offer impetus for the design of new 3D spatial structures of graphene or other 2D layered materials. Additionally, in this research, two special features of this new 3D graphene structure were demonstrated, including nanoribbon fabrication and potential use in lithium storage upon scaling.