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.     

Anisotropic Strain Relaxation of Graphene by Corrugation on Copper Crystal Surfaces

Corrugation is a ubiquitous phenomenon for graphene grown on metal substrates by chemical vapor deposition, which greatly affects the electrical, mechanical, and chemical properties. Recent years have witnessed great progress in controlled growth of large graphene single crystals; however, the issue of surface roughness is far from being addressed. Here, the corrugation at the interface of copper (Cu) and graphene, including Cu step bunches (CuSB) and graphene wrinkles, are investigated and ascribed to the anisotropic strain relaxation. It is found that the corrugation is strongly dependent on Cu crystallographic orientations, specifically, the packed density and anisotropic atomic configuration. Dense Cu step bunches are prone to form on loose packed faces due to the instability of surface dynamics. On an anisotropic Cu crystal surface, Cu step bunches and graphene wrinkles are formed in two perpendicular directions to release the anisotropic interfacial stress, as revealed by morphology imaging and vibrational analysis. Cu(111) is a suitable crystal face for growth of ultraflat graphene with roughness as low as 0.20 nm. It is believed the findings will contribute to clarifying the interplay between graphene and Cu crystal faces, and reducing surface roughness of graphene by engineering the crystallographic orientation of Cu substrates.     

Large Single-Crystal Cu Foils with High-Index Facets by Strain-Engineered Anomalous Grain Growth

The rich and complex arrangements of metal atoms in high-index metal facets afford appealing physical and chemical properties, which attracts extensive research interest in material science for the applications in catalysis and surface chemistry. However, it is still a challenge to prepare large-area high-index single crystals in a controllable and cost-efficient manner. Herein, entire commercially available decimeter-sized polycrystalline Cu foils are successfully transformed into single crystals with a series of high-index facets, relying on a strain-engineered anomalous grain growth technique. The introduction of a moderate thermal-contact stress upon the Cu foil during the annealing leads to the formation of high-index grains dominated by the thermal strain of the Cu foils, rather than the (111) surface driven by the surface energy. Besides, the designed static gradient of the temperature enables the as-formed high-index grain seed to expand throughout the entire Cu foil. The as-received high-index Cu foils can serve as the templates for producing high-index single-crystal Cu-based alloys. This work provides an appealing material basis for the epitaxial growth of 2D materials, and the applications that require the unique surface structures of high-index metal foils and their alloys.     

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。     

Large physisorption strain in chemical vapor deposition of graphene on copper substrates

Graphene single layers grown by chemical vapor deposition on single crystal Cu substrates are subject to nonuniform physisorption strains that depend on the orientation of the Cu surface. The strains are revealed in Raman spectra and quantitatively interpreted by molecular dynamics (MD) simulations. An average compressive strain on the order of 0.5% is determined in graphene on Cu(111). In graphene on Cu (100), MD simulations interpret the observed highly nonuniform strains.     

Epitaxial Growth of 6 in. Single‐Crystalline Graphene on a Cu/Ni (111) Film at 750 °C via Chemical Vapor Deposition

The future electronic application of graphene highly relies on the production of large‐area high‐quality single‐crystal graphene. However, the growth of single‐crystal graphene on different substrates via either single nucleation or seamless stitching is carried out at a temperature of 1000 °C or higher. The usage of this high temperature generates a variety of problems, including complexity of operation, higher contamination, metal evaporation, and wrinkles owing to the mismatch of thermal expansion coefficients between the substrate and graphene. Here, a new approach for the fabrication of ultraflat single‐crystal graphene using Cu/Ni (111)/sapphire wafers at lower temperature is reported. It is found that the temperature of epitaxial growth of graphene using Cu/Ni (111) can be reduced to 750 °C, much lower than that of earlier reports on catalytic surfaces. Devices made of graphene grown at 750 °C have a carrier mobility up to ≈9700 cm² V^?1 s^?1 at room temperature. This work shines light on a way toward a much lower temperature growth of high‐quality graphene in single crystallinity, which could benefit future electronic applications.     

Interplay of wrinkles, strain, and lattice parameter in graphene on iridium

Following graphene growth by thermal decomposition of ethylene on Ir(111) at high temperatures we analyzed the strain state and the wrinkle formation kinetics as function of temperature. Using the moiré spot separation in a low energy electron diffraction pattern as a magnifying mechanism for the difference in the lattice parameters between Ir and graphene, we achieved an unrivaled relative precision of ±0.1 pm for the graphene lattice parameter. Our data reveals a characteristic hysteresis of the graphene lattice parameter that is explained by the interplay of reversible wrinkle formation and film strain. We show that graphene on Ir(111) always exhibits residual compressive strain at room temperature. Our results provide important guidelines for strategies to avoid wrinkling.     

Unraveling the Water Impermeability Discrepancy in CVD‐Grown Graphene

Graphene has recently attracted particular interest as a flexible barrier film preventing permeation of gases and moistures. However, it has been proved to be exceptionally challenging to develop large‐scale graphene films with little oxygen and moisture permeation suitable for industrial uses, mainly due to the presence of nanometer‐sized defects of obscure origins. Here, the origins of water permeable routes on graphene‐coated Cu foils are investigated by observing the micrometer‐sized rusts in the underlying Cu substrates, and a site‐selective passivation method of the nanometer‐sized routes is devised. It is revealed that nanometer‐sized holes or cracks are primarily concentrated on graphene wrinkles rather than on other structural imperfections, resulting in severe degradation of its water impermeability. They are found to be predominantly induced by the delamination of graphene bound to Cu as a release of thermal stress during the cooling stage after graphene growth, especially at the intersection of the Cu step edges and wrinkles owing to their higher adhesion energy. Furthermore, the investigated routes are site‐selectively passivated by an electron‐beam‐induced amorphous carbon layer, thus a substantial improvement in water impermeability is achieved. This approach is likely to be extended for offering novel barrier properties in flexible films based on graphene and on other atomic crystals.     

Graphene epitaxy by chemical vapor deposition on SiC

We demonstrate the growth of high quality graphene layers by chemical vapor deposition (CVD) on insulating and conductive SiC substrates. This method provides key advantages over the well-developed epitaxial graphene growth by Si sublimation that has been known for decades. (1) CVD growth is much less sensitive to SiC surface defects resulting in high electron mobilities of ～1800 cm(2)/(V s) and enables the controlled synthesis of a determined number of graphene layers with a defined doping level. The high quality of graphene is evidenced by a unique combination of angle-resolved photoemission spectroscopy, Raman spectroscopy, transport measurements, scanning tunneling microscopy and ellipsometry. Our measurements indicate that CVD grown graphene is under less compressive strain than its epitaxial counterpart and confirms the existence of an electronic energy band gap. These features are essential for future applications of graphene electronics based on wafer scale graphene growth.     

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.     

Improved mechanical properties of magnesium–graphene composites with copper–graphene hybrids

New magnesium nanocomposites reinforced with copper–graphene nanoplatelet hybrid particles have been prepared through the semipowder metallurgy method. Compared with the monolithic Mg, the Mg–1Cu–xGNPs nanocomposites exhibited higher tensile and compressive strength. In tension, nanocomposites revealed substantial enhancement in elastic modulus, 0.2% yield strength, ultimate tensile strength and failure strain (up to +89, +117, +58 and +96% respectively) compared to monolithic Mg. In compression, the nanocomposites showed the greatest improvement in 0.2% yield strength, and the ultimate compressive strength and failure strain (%) (up to +34, +59 and +61% respectively), whilst the compressive elastic modulus first increases and then decreases with an increase in the graphene nanoplatelets (GNPs) contents. The enhanced strength of the composites is likely to result from strengthening mechanisms invoked by the addition of Cu–GNPs hybrids.     

Greatly Enhanced Anticorrosion of Cu by Commensurate Graphene Coating

Metal corrosion is a long‐lasting problem in history and ultrahigh anticorrosion is one ultimate pursuit in the metal‐related industry. Graphene, in principle, can be a revolutionary material for anticorrosion due to its excellent impermeability to any molecule or ion (except for protons). However, in real applications, it is found that the metallic graphene forms an electrochemical circuit with the protected metals to accelerate the corrosion once the corrosive fluids leaks into the interface. Therefore, whether graphene can be used as an excellent anticorrosion material is under intense debate now. Here, graphene‐coated Cu is employed to investigate the facet‐dependent anticorrosion of metals. It is demonstrated that as‐grown graphene can protect Cu(111) surface from oxidation in humid air lasting for more than 2.5 years, in sharp contrast with the accelerated oxidation of graphene‐coated Cu(100) surface. Further atomic‐scale characterization and ab initio calculations reveal that the strong interfacial coupling of the commensurate graphene/Cu(111) prevents H₂O diffusion into the graphene/Cu(111) interface, but the one‐dimensional wrinkles formed in the incommensurate graphene on Cu(100) can facilitate the H₂O diffusion at the interface. This study resolves the contradiction on the anticorrosion capacity of graphene and opens a new opportunity for ultrahigh metal anticorrosion through commensurate graphene coating.     

Weak mismatch epitaxy and structural Feedback in graphene growth on copper foil

Graphene growth by low-pressure chemical vapor deposition on low cost copper foils shows great promise for large scale applications. It is known that the local crystallography of the foil influences the graphene growth rate. Here we find an epitaxial relationship between graphene and copper foil. Interfacial restructuring between graphene and copper drives the formation of (n10) facets on what is otherwise a mostly Cu(100) surface, and the facets in turn influence the graphene orientations from the onset of growth. Angle resolved photoemission shows that the electronic structure of the graphene is decoupled from the copper indicating a weak interaction between them. Despite this, two preferred orientations of graphene are found, ±8° from the Cu[010] direction, creating a non-uniform distribution of graphene grain boundary misorientation angles. Comparison with the model system of graphene growth on single crystal Cu(110) indicates that this orientational alignment is due to mismatch epitaxy. Despite the differences in symmetry the orientation of the graphene is defined by that of the copper. We expect these observations to not only have importance for controlling and understanding the growth process for graphene on copper, but also to have wider implications for the growth of two-dimensional materials on low cost metal substrates.      

Remote catalyzation for direct formation of graphene layers on oxides

Direct deposition of high-quality graphene layers on insulating substrates such as SiO(2) paves the way toward the development of graphene-based high-speed electronics. Here, we describe a novel growth technique that enables the direct deposition of graphene layers on SiO(2) with crystalline quality potentially comparable to graphene grown on Cu foils using chemical vapor deposition (CVD). Rather than using Cu foils as substrates, our approach uses them to provide subliming Cu atoms in the CVD process. The prime feature of the proposed technique is remote catalyzation using floating Cu and H atoms for the decomposition of hydrocarbons. This allows for the direct graphitization of carbon radicals on oxide surfaces, forming isolated low-defect graphene layers without the need for postgrowth etching or evaporation of the metal catalyst. The defect density of the resulting graphene layers can be significantly reduced by tuning growth parameters such as the gas ratios, Cu surface areas, and substrate-to-Cu distance. Under optimized conditions, graphene layers with nondiscernible Raman D peaks can be obtained when predeposited graphite flakes are used as seeds for extended growth.     

Direct evidence of advantage of Cu(111) for graphene synthesis by using Raman mapping and electron backscatter diffraction

The advantageous crystallographic orientation of Cu surface for graphene synthesis by using thermal chemical vapor deposition (CVD) is examined by Raman mapping and electron backscatter diffraction. It is found that Cu(111) predominates over (110) and (100) for single- (SLG) or few-layer graphene (FLG) growth. To confirm this result we attempt the synthesis of graphene on Cu(111) single crystal film surfaces. We confirmed the formation of high quality and high uniformity SLG or FLG over more than 97% of the substrate surface area of 10 mm × 10 mm by Raman mapping.     

Scanning tunneling microscope observations of non-AB stacking of graphene on Ni films

Microscopic features of graphene segregated on Ni films prior to chemical transfer—including atomic structures of monolayers and bilayers, Moiré patterns due to non-AB stacking, as well as wrinkles and ripples caused by strain effects-have been characterized in detail by high-resolution scanning tunneling microscopy (STM). We found that the stacking geometry of the bilayer graphene usually deviates from the traditional Bernal stacking (or so-called AB stacking), resulting in the formation of a variety of Moiré patterns. The relative rotations inside the bilayer were then qualitatively deduced from the relationship between Moiré patterns and carbon lattices. Moreover, we found that typical defects such as wrinkles and ripples tend to evolve around multi-step boundaries of Ni, thus reflecting strong perturbations from substrate corrugations. These investigations of the morphology and the mechanism of formation of wrinkles and ripples are fundamental topics in graphene research. This work is expected to contribute to the exploration of electronic and transport properties of wrinkles and ripples.      

Layer‐by‐Layer Decoupling of Twisted Graphene Sheets Epitaxially Grown on a Metal Substrate

The electronic properties of graphene can be efficiently altered upon interaction with the underlying substrate resulting in a dramatic change of charge carrier behavior. Here, the evolution of the local electronic properties of epitaxial graphene on a metal upon the controlled formation of multilayers, which are produced by intercalation of atomic carbon in graphene/Ir(111), is investigated. Using scanning tunneling microscopy and Landau‐level spectroscopy, it is shown that for a monolayer and bilayers with small‐angle rotations, Landau levels are fully suppressed, indicating that the metal–graphene interaction is largely confined to the first graphene layer. Bilayers with large twist angles as well as twisted trilayers demonstrate a sequence of pronounced Landau levels characteristic for a free‐standing graphene monolayer pointing toward an effective decoupling of the top layer from the metal substrate. These findings give evidence for the controlled preparation of epitaxial graphene multilayers with a different degree of decoupling, which represent an ideal platform for future electronic and spintronic applications.     

Ridge formation and removal via annealing in exfoliated graphene

It is well known that graphene is a very promising material due to its excellent physical, chemical, and thermal properties. Previously, ridges in graphene on a substrate were found in epitaxial graphene on a SiC substrate. It was found in this study that ridges can be made on a graphene layer via mechanical exfoliation on a sapphire substrate, and that ridges can be created or removed through heating and cooling. Due to the difference of the thermal-expansion coefficients of the substrate and graphene, it can be said that thermal cycling causes compressive strain, which is released by forming ridges. Annealing was carried out in a vacuum chamber within the pressure range of 10(-3)-10(-6) Torr and at 900-1100 degrees C. To analyze the shapes and mechanical properties of the ridges, Raman spectroscopy and AFM measurement were performed. It was found that the ridges can be extended by defect as a nucleation center, and the graphene layer can be folded along the preexisting ridge during heating and cooling.     

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.     

Direct growth of special-shape graphene on different templates by remote catalyzation of Cu nanoparticles

A novel method avoiding the complex transfer process is proposed to directly grow low-defect and few-layer graphene on different insulating substrates(SiO_2, Al_2O_3, etc.) by remote catalyzation of Cu nanoparticles(NPs) using ambient pressure chemical vapor deposition(APCVD). The insulating substrates with special structure are used as templates to grow wrapped graphene sheets with special shapes.Hollow graphene species are obtained by removing the substrates. The prime feature of the proposed method is using Cu NPs as catalyst rather than metal foils. The Cu NPs play an important role in the remote catalyzation during the nucleation of graphene. This method can improve the quality and relatively decrease the growth temperature of the graphene on the insulating substrates, which displays the great potential of APCVD direct growth of graphene on dielectric substrates for electronic and photovoltaic applications.