Copper-Vapor-Assisted Rapid Synthesis of Large AB-Stacked Bilayer Graphene Domains on Cu-Ni Alloy

Two-dimensional material assisted-growth strategy: new insights and opportunities

The exploration and synthesis of novel materials are integral to scientific and technological progress. Since the prediction and synthesis of two-dimensional (2D) materials, it is expected to play an important role in the application of industrialization and the information age, resulting from its excellent physical and chemical properties. Currently, researchers have effectively utilized a range of material synthesis techniques, including mechanical exfoliation, redox reactions, chemical vapor deposition, and chemical vapor transport, to fabricate two-dimensional materials. However, despite their rapid development, the widespread industrial application of 2D materials faces challenges due to demanding synthesis requirements and high costs. To address these challenges, assisted growth techniques such as salt-assisted, gas-assisted, organic-assisted, and template-assisted growth have emerged as promising approaches. Herein, this study gives a summary of important developments in recent years in the assisted growth synthesis of 2D materials. Additionally, it highlights the current difficulties and possible benefits of the assisted-growth approach for 2D materials. It also highlights novel avenues of development and presents opportunities for new lines of investigation.

A Wrinkling and Etching-Assisted Regrowth Strategy for Large-Area Bilayer Graphene Preparation on Cu

Bilayer graphene is a contender of interest for functional electronic applications because of its variable band gap due to interlayer interactions. Graphene growth on Cu is self-limiting, thus despite the fact that chemical vapor deposition (CVD) has made substantial strides in the production of monolayer and single-crystal graphene on Cu substrates, the direct synthesizing of high-quality, large-area bilayer graphene remains an enormous challenge. In order to tackle this issue, we present a simple technique using typical CVD graphene growth followed by a repetitive wrinkling-etching-regrowth procedure. The key element of our approach is the rapid cooling process that causes high-density wrinkles to form in the monolayer area rather than the bilayer area. Next, wrinkled sites are selectively etched with hydrogen, exposing a significant portion of the active Cu surface, and leaving the remaining bilayer areas, which enhance the nucleation and growth of the second graphene layer. A fully covered graphene with 78 ± 2.8% bilayer coverage and a bilayer transmittance of 95.6% at room temperature can be achieved by modifying the process settings. Bilayer graphene samples are examined using optical microscopy (OM), scanning electron microscopy (SEM), Raman spectroscopy, and an atomic force microscope (AFM) during this process. The outcomes of our research are beneficial in clarifying the growth processes and future commercial applications of bilayer graphene.

Spiral Growth of Adlayer Graphene

The morphology of as-grown graphene in chemical vapor deposition (CVD) experiments is sensitive to the reaction environment. Understanding the mechanism of formation of different graphene morphologies is essential to achieve controlled graphene CVD growth. Here the growth and formation mechanism of adlayer graphene spirals are reported. An adlayer graphene spiral is formed by fast propagation of the tips of spiral arms along the edge of the first graphene layer. The driving force to form spirals is the limited availability of carbon diffusing from the Cu surface through the edge of the first graphene layer. In addition, it is found that graphene onions are formed by overlapping graphene spirals with clockwise and anticlockwise arms. Based on these features, a kinetic Monte Carlo (kMC) method is demonstrated using which all the observed graphene spiral structures are successfully reproduced at the atomic level. This study thus unravels the hither-to unresolved mechanism of graphene onion growth and paves the way to the controllable growth of few-layer graphene by increasing the carbon supply at the edge of the first layer graphene.

Epitaxy of 2D Materials toward Single Crystals

Two-dimensional (2D) materials exhibit unique electronic, optical, magnetic, mechanical, and thermal properties due to their special crystal structure and thus have promising potential in many fields, such as in electronics and optoelectronics. To realize their real applications, especially in integrated devices, the growth of large-size single crystal is a prerequisite. Up to now, the most feasible way to achieve 2D single crystal growth is the epitaxy: growth of 2D materials of one or more specific orientations with single-crystal substrate. Only when the 2D domains have the same orientation, they can stitch together seamlessly and single-crystal 2D films can be obtained. In this view, four different epitaxy modes of 2D materials on various substrates are presented, including van der Waals epitaxy, edge epitaxy, step-guided epitaxy, and in-plane epitaxy focusing on the growth of graphene, hexagonal boron nitride (h-BN), and transition metal dichalcogenide (TMDC). The lattice symmetry relation and the interaction between 2D materials and the substrate are the key factors determining the epitaxy behaviors and thus are systematically discussed. Finally, the opportunities and challenges about the epitaxy of 2D single crystals in the future are summarized.

Controllable Synthesis of Wafer-Scale Graphene Films: Challenges, Status, and Perspectives

The availability of high-quality, large-scale, and single-crystal wafer-scale graphene films is fundamental for key device applications in the field of electronics, optics, and sensors. Synthesis determines the future: unleashing the full potentials of such emerging materials relies heavily upon their tailored synthesis in a scalable fashion, which is by no means an easy task to date. This review covers the state-of-the-art progress in the synthesis of wafer-scale graphene films by virtue of chemical vapor deposition (CVD), with a focus on main challenges and present status. Particularly, prevailing synthetic strategies are highlighted on a basis of the discussion in the reaction kinetics and gas-phase dynamics during CVD process. Perspectives with respect to key opportunities and promising research directions are proposed to guide the future development of wafer-scale graphene films.

Strategies, Status, and Challenges in Wafer Scale Single Crystalline Two-Dimensional Materials Synthesis

The successful exfoliation of graphene has given a tremendous boost to research on various two-dimensional (2D) materials in the last 15 years. Different from traditional thin films, a 2D material is composed of one to a few atomic layers. While atoms within a layer are chemically bonded, interactions between layers are generally weak van der Waals (vdW) interactions. Due to their particular dimensionality, 2D materials exhibit special electronic, magnetic, mechanical, and thermal properties, not found in their 3D counterparts, and therefore they have great potential in various applications, such as 2D materials-based devices. To fully realize their large-scale practical applications, especially in devices, wafer scale single crystalline (WSSC) 2D materials are indispensable. In this review, we present a detailed overview on strategies toward the synthesis of WSSC 2D materials while highlighting the recent progress on WSSC graphene, hexagonal boron nitride (hBN), and transition metal dichalcogenide (TMDC) synthesis. The challenges that need to be addressed in future studies have also been described. In general, there have been two distinct routes to synthesize WSSC 2D materials: (i) allowing only one nucleus on a wafer scale substrate to be formed and developed into a large single crystal and (ii) seamlessly stitching a large number of unidirectionally aligned 2D islands on a wafer scale substrate, which is generally single crystalline. Currently, the synthesis of WSSC graphene has been realized by both routes, and WSSC hBN and MoS₂ have been synthesized by route (ii). On the other hand, the growth of other WSSC 2D materials and WSSC multilayer 2D materials still remains a big challenge. In the last section, we wrap up this review by summarizing the future challenges and opportunities in the synthesis of various WSSC 2D materials.

Substrate Engineering for CVD Growth of Single Crystal Graphene

Single crystal graphene (SCG) has attracted enormous attention for its unique potential for next-generation high-performance optoelectronics. In the absence of grain boundaries, the exceptional intrinsic properties of graphene are preserved by SCG. Currently, chemical vapor deposition (CVD) has been recognized as an effective method for the large-scale synthesis of graphene films. However, polycrystalline films are usually obtained and the present grain boundaries compromise the carrier mobility, thermal conductivity, optical properties, and mechanical properties. The scalable and controllable synthesis of SCG is challenging. Recently, much attention has been attracted by the engineering of large-size single-crystal substrates for the epitaxial CVD growth of large-area and high-quality SCG films. In this article, a comprehensive and comparative review is provided on the selection and preparation of various single-crystal substrates for CVD growth of SCG under different conditions. The growth mechanisms, current challenges, and future development and perspectives are discussed.

Rational Design of Binary Alloys for Catalytic Growth of Graphene via Chemical Vapor Deposition

Chemical vapor deposition is the most promising technique for the mass production of high-quality graphene, in which the metal substrate plays a crucial role in the catalytic decomposition of the carbon source, assisting the attachment of the active carbon species, and regulating the structure of the graphene film. Due to some drawbacks of single metal substrates, alloy substrates have gradually attracted attention owing to their complementarity in the catalytic growth of graphene. In this review, we focus on the rational design of binary alloys, such as Cu/Ni, Ni/Mo, and Cu/Si, to control the layer numbers and growth rate of graphene. By analyzing the elementary steps of graphene growth, general principles are summarized in terms of the catalytic activity, metal–carbon interactions, carbon solubility, and mutual miscibility. Several challenges in this field are also put forward to inspire the novel design of alloy catalysts and the synthesis of graphene films bearing desirable properties.

Isothermal Growth and Stacking Evolution in Highly Uniform Bernal-Stacked Bilayer Graphene

Controlling the stacking order in bilayer graphene (BLG) allows realizing interesting physical properties. In particular, the possibility of tuning the band gap in Bernal-stacked (AB) BLG (AB-BLG) has a great technological importance for electronic and optoelectronic applications. Most of the current methods to produce AB-BLG suffer from inhomogeneous layer thickness and/or coexistence with twisted BLG. Here, we demonstrate a method to synthesize highly pure large-area AB-BLG by chemical vapor deposition using Cu-Ni films. Increasing the reaction time resulted in a gradual increase of the AB stacking, with the BLG eventually free from twist regions for the longer growth times (99.4% of BLG has AB stacking), due to catalyst-assisted continuous BLG reconstruction driven by carbon dissolution-segregation processes. The band gap opening was confirmed by the electrical measurements on field-effect transistors using two different device configurations. The concept of the continuous reconstruction to achieve highly pure AB-BLG offers a way to control the stacking order of catalytically grown two-dimensional materials.

Doping effect in graphene-graphene oxide interlayer

Interlayer coupling in graphene-based van der Waals (vdW) heterostructures plays a key role in determining and modulating their physical properties. Hence, its influence on the optical and electronic properties cannot be overlooked in order to promote various next-generation applications in electronic and opto-electronic devices based on the low-dimensional materials. Herein, the optical and electrical properties of the vertically stacked large area heterostructure of the monolayer graphene transferred onto a monolayer graphene oxide film are investigated. An effective and stable p-doping property of this structure is shown by comparison to that of the graphene device fabricated on a silicon oxide substrate. Through Raman spectroscopy and density functional theory calculations of the charge transport characteristics, it is found that graphene is affected by sustainable p-doping effects induced from underneath graphene oxide even though they have weak interlayer interactions. This finding can facilitate the development of various fascinating graphene-based heterostructures and extend their practical applications in integrated devices with advanced functionalities.

Growth of Single-Layer and Multilayer Graphene on Cu/Ni Alloy Substrates

ConspectusGraphene, a one-atom-thick layer of carbon with a honeycomb lattice, has drawn great attention due to its outstanding properties and its various applications in electronic and photonic devices. Mechanical exfoliation has been used for preparing graphene flakes (from monolayer to multilayer with thick pieces also typically present), but with sizes limited typically to less than millimeters, its usefulness is limited. Chemical vapor deposition (CVD) has been shown to be the most effective technique for the scalable preparation of graphene films with high quality and uniformity. To date, CVD growth of graphene on the most commonly used substrates (Cu and Ni foils) has been demonstrated and intensively studied. However, a survey of the existing literature and earlier work using Cu or Ni substrates for CVD growth indicates that the bilayer and multilayer graphene over a large area, particularly single crystals, have not been obtained.In this Account, we review current progress and development in the CVD growth of graphene and highlight the important challenges that need to be addressed, for example, how to achieve large single crystal graphene films with a controlled number of layers. A single-layer graphene film grown on polycrystalline Cu foil was first reported by our group, and since then various techniques have been devoted to achieving the fast growth of large-area graphene films with high quality. Commercially available Cu/Ni foils, sputtered Cu/Ni thin films, and polycrystalline Cu/Ni foils have been used for the CVD synthesis of bilayer, trilayer, and multilayer graphene. Cu/Ni alloy substrates are particularly interesting due to their greater carbon solubility than pure Cu substrates and this solubility can be finely controlled by changing the alloy composition. These substrates with controlled compositions have shown the potential for the growth of layer-tunable graphene films in addition to providing a much higher growth rate due to their stronger catalytic activity. However, the well-controlled preparation of single crystal graphene with a defined number of layers on Cu/Ni substrates is still challenging.Due to its small lattice mismatch with graphene, a single crystal Cu(111) foil has been shown to be an ideal substrate for the epitaxial growth of graphene. Our group has reported the synthesis of large-size single crystal Cu(111) foils by the contact-free annealing of commercial Cu foils, and single crystal Cu/Ni(111) alloy foils have also been obtained after the heat-treatment of Ni-coated Cu(111) foils. The use of these single crystal foils (especially the Cu/Ni alloy foils) as growth substrates has enabled the fast growth of single crystal single-layer graphene films. By increase of the Ni content, single crystal bilayer, trilayer, and even multilayer graphene films have been synthesized. In addition, we also discuss the wafer-scale growth of single-layer graphene on the single crystalline Cu/Ni(111) thin films.Recent research results on the large-scale preparation of single crystal graphene films with different numbers of layers on various types of Cu/Ni alloy substrates with different compositions are reviewed and discussed in detail. Despite the remarkable progress in this field, further challenges, such as the wafer-scale synthesis of single crystal graphene with a controlled number of layers and a deeper understanding of the growth mechanism of bilayer and multilayer graphene growth on Cu/Ni substrates, still need to be addressed.

Interlayer epitaxy of wafer-scale high-quality uniform AB-stacked bilayer graphene films on liquid Pt3Si/solid Pt

Large-area high-quality AB-stacked bilayer graphene films are highly desired for the applications in electronics, photonics and spintronics. However, the existing growth methods can only produce discontinuous bilayer graphene with variable stacking orders because of the non-uniform surface and strong potential field of the solid substrates used. Here we report the growth of wafer-scale continuous uniform AB-stacked bilayer graphene films on a liquid Pt₃Si/solid Pt substrate by chemical vapor deposition. The films show quality, mechanical and electrical properties comparable to the mechanically exfoliated samples. Growth mechanism studies show that the second layer is grown underneath the first layer by precipitation of carbon atoms from the solid Pt, and the small energy requirements for the movements of graphene nucleus on the liquid Pt₃Si enables the interlayer epitaxy to form energy-favorable AB stacking. This interlayer epitaxy also allows the growth of ABA-stacked trilayer graphene and is applicable to other liquid/solid substrates.

Specific stacking sequence of graphene can enable observation of unusual properties however it has been difficult to obtain this over wider areas. Here, the authors report wafer-scale growth of 100% AB-stacked bilayer graphene films by CVD on liquid Pt₃Si/solid Pt substrates showing high quality and improved mechanical properties comparable to the mechanically exfoliated flakes.

A review of theoretical study of graphene chemical vapor deposition synthesis on metals: nucleation, growth, and the role of hydrogen and oxygen

Graphene has attracted intense research interest due to its extraordinary properties and great application potential. Various methods have been proposed for the synthesis of graphene, among which chemical vapor deposition has drawn a great deal of attention for synthesizing large-area and high-quality graphene. Theoretical understanding of the synthesis mechanism is crucial for optimizing the experimental design for desired graphene production. In this review, we discuss the three fundamental steps of graphene synthesis in details, i.e. (1) decomposition of carbon feedstocks and formation of various active carbon species, (2) nucleation, and (3) attachment and extension. We provide a complete scenario of graphene synthesis on metal surfaces at atomistic level by means of density functional theory, molecular dynamics (MD), Monte Carlo (MC) and their combination and interface with other simulation methods such as quantum mechanical molecular dynamics, density functional tight binding molecular dynamics, and combination of MD and MC. We also address the latest investigation of the influences of the hydrogen and oxygen on the synthesis and the quality of the synthesized graphene.

The Way towards Ultrafast Growth of Single-Crystal Graphene on Copper

The exceptional properties of graphene make it a promising candidate in the development of next-generation electronic, optoelectronic, photonic and photovoltaic devices. A holy grail in graphene research is the synthesis of large-sized single-crystal graphene, in which the absence of grain boundaries guarantees its excellent intrinsic properties and high performance in the devices. Nowadays, most attention has been drawn to the suppression of nucleation density by using low feeding gas during the growth process to allow only one nucleus to grow with enough space. However, because the nucleation is a random event and new nuclei are likely to form in the very long growth process, it is difficult to achieve industrial-level wafer-scale or beyond (e.g. 30 cm in diameter) single-crystal graphene. Another possible way to obtain large single-crystal graphene is to realize ultrafast growth, where once a nucleus forms, it grows up so quickly before new nuclei form. Therefore ultrafast growth provides a new direction for the synthesis of large single-crystal graphene, and is also of great significance to realize large-scale production of graphene films (fast growth is more time-efficient and cost-effective), which is likely to accelerate various graphene applications in industry.

Edge control of graphene domains grown on hexagonal boron nitride

The edge structure of graphene has a significant influence on its electronic properties. However, control over the edge structure of graphene domains on insulating substrates is still challenging. Here we demonstrate edge control of graphene domains on hexagonal boron nitride (h-BN) by modifying the ratio of working-gases. Edge directions were determined with the help of both moiré patterns and atomic-resolution images obtained via atomic force microscopy measurements. It is believed that the variation of graphene edges is mainly attributed to different growth rates of armchair and zigzag edges. This work demonstrates a potential approach to fabricate smooth-edge graphene ribbons on h-BN.

Stranski-Krastanov and Volmer-Weber CVD Growth Regimes To Control the Stacking Order in Bilayer Graphene

Aside from unusual properties of monolayer graphene, bilayer has been shown to have even more interesting physics, in particular allowing bandgap opening with dual gating for proper interlayer symmetry. Such properties, promising for device applications, ignited significant interest in understanding and controlling the growth of bilayer graphene. Here we systematically investigate a broad set of flow rates and relative gas ratio of CH₄ to H₂ in atmospheric pressure chemical vapor deposition of multilayered graphene. Two very different growth windows are identified. For relatively high CH₄ to H₂ ratios, graphene growth is relatively rapid with an initial first full layer forming in seconds upon which new graphene flakes nucleate then grow on top of the first layer. The stacking of these flakes versus the initial graphene layer is mostly turbostratic. This growth mode can be likened to Stranski-Krastanov growth. With relatively low CH₄ to H₂ ratios, growth rates are reduced due to a lower carbon supply rate. In addition bi-, tri-, and few-layer flakes form directly over the Cu substrate as individual islands. Etching studies show that in this growth mode subsequent layers form beneath the first layer presumably through carbon radical intercalation. This growth mode is similar to that found with Volmer-Weber growth and was shown to produce highly oriented AB-stacked materials. These systematic studies provide new insight into bilayer graphene formation and define the synthetic range where gapped bilayer graphene can be reliably produced.

Wafer-Scale Single-Crystalline AB-Stacked Bilayer Graphene

Single-crystalline artificial AB-stacked bilayer graphene is formed by aligned transfer of two single-crystalline monolayers on a wafer-scale. The obtained bilayer has a well-defined interface and is electronically equivalent to exfoliated or direct-grown AB-stacked bilayers.