Hexagonal Graphene Domains Research Articles

Relationship between graphene nucleation density and epitaxial growth orientation on Cu(111) surfaces

Crystal domain alignment is required to produce large, single-crystal, two-dimensional materials which are synthesized by epitaxial growth, but it has been unclear what is needed to achieve the alignment. It is presumed to be possible when the symmetries of the two-dimensional material and substrate are consistent. However, experiments show that although graphene and the Cu(111) surface have the same symmetry, graphene domains grown on Cu(111) are not always well aligned. We developed a physical model and used first principles calculations to explain these observations. It is found that small hexagonal graphene domains are always misorientated on the Cu(111) substrate because the vertexes of the graphene domains have preferred binding sites. These misorientation angles vary with the domain size, with near-perfect alignment of the domains on the substrate only being achieved when they become hundreds of nanometers in size. Our results indicate that low nucleation density of graphene on Cu(111) surfaces is necessary to achieve epitaxial alignment of graphene domains.

Influence of growth kinetics on graphene domains shape under atmospheric pressure chemical vapor deposition

In this work, the role of gas kinetics in the growth of lobed graphene domains by atmospheric pressure chemical vapor deposition (AP-CVD) is elucidated by sandwiching Cu foil between Si/SiO2 wafers. Two different growths were carried out: (1) A Cu foil was placed at the center of a quartz tube in AP-CVD for graphene growth and (2) another Cu foil was sandwiched between Si/SiO2 wafers to alter the nucleation growth kinetics of graphene domains to mimic those in low-pressure chemical vapor deposition (LP-CVD). From the scanning electron microscopy (SEM) images, the graphene domains of the sandwiched Cu foil displayed mostly four-lobed, parallel-sided domains which are usually obtained under LP-CVD as compared to Cu foil without sandwiching which showed typical hexagonal graphene domains of AP-CVD. The Raman spectroscopy confirmed that the domains are single-layer graphene. An electron backscatter diffraction (EBSD) showed that the Cu foil is predominantly (001). The results of this study agree with the theoretical predictions of growth kinetics in graphene synthesis by CVD and showed that it is possible to obtain single-layer graphene domains which are usually obtained under LP-CVD by restricting the gas flux through the boundary layer.Graphic abstract

Millimeter sized graphene domains through in situ oxidation/reduction treatment of the copper substrate

In chemical vapor deposition of graphene, low nucleation density and large domain sizes are desirable. In the particular case of growth on copper, throughout recent years, several works have reported the oxidation of the substrate through various means, which leads to a reduced nucleation density. In this work, an in situ oxidation and reduction treatment is explored, establishing its connection with the removal of carbon impurities from the copper bulk, as well as clarifying the role of these impurities as graphene nucleation sites. The extent of the contribution of these impurities to the growth of graphene is inferred by experiments using carbon-13 methane feed. The effect of the oxidation and reduction treatment on the substrate’s surface roughness is also addressed. Lastly, the synthesis recipe based on this treatment is supplemented with a growth stage splitting, an increased argon flowrate and a short etching stage, resulting in millimeter sized hexagonal graphene domains (diameters 2–3 mm) with improved electrical transport properties. Thus, the deposition process presented here stands out on account of its streamlined nature, which makes it an attractive candidate for industrial scale-up.

The Role of Hydrogen on the Growth of Graphene Nanostructure Using a Two-Step Method.

Chemical vapor deposition (CVD) is widely applied in synthesizing high quality graphene, whose size, shape and structure are strongly impacted by the hydrogen concentration and, however, its role is not fully understood. In the traditional CVD, the concentration of the hydrogen keeps the constant in whole synthesis process and subsequently the nucleation and growth process are carried out simultaneously, therefore, its roles are usually confused and indistinguishable. In this report, the role of hydrogen on the growth of graphene nanostructure was creatively studied by introducing a two-step method which divided the nucleation and growth process for the first time. In the first step, the hexagonal graphene domain grown with the same conditions was used as precursor to eliminate the impact of the nucleation. In the second step, the role of hydrogen on the growth of graphene nanostructure was investigated by controlling the hydrogen concentration. The evolution behavior of the graphene nanostructure with the hydrogen concentration was systematically investigated. Two roles of the hydrogen, namely growth and etching modes, are clearly disclosed and then a possible mechanism was proposed. The results shown here may provide valuable guidance to understand the graphene growth mechanism and further advance the synthesis of unique graphene nanostructure.

Carbon content and layers number controlling electronic properties of hybridized graphene and boron nitride

Abstract Using the first-principles density functional theory calculation, the most stable structure of hybridized hexagonal boron nitride and graphene domains (h-BNC) was studied, and the influence of carbon atomic ratio and layers number on band gap was investigated. The energy calculation shows that the more h-BN and graphene concentrate in h-BNC, the more stable structure will be. The calculation of band gap shows that band gap of single-layer h-BNC decrease with increasing of carbon atomic ratio. When graphene is in h-BN, the band gap decreases in a rough law; however, when h-BN is in graphene, there is only a general trend of band gap decreasing. In addition, we found that as the number of h-BNC layers increases, the band gap “collapse”. Therefore, the band gap of h-BNC can be adjusted by controlling the carbon content and the number of layers synergistically. The reason for the large drop of the band gap is charge transfer between the h-BNC layers. This research proves the rationality of the structure of h-BNC appearing in previous experiments, and has a referring role in adjusting h-BNC band gap to an appropriate value in the process. It is significant for application of h-BNC in band gap engineering.

Chemical composition and interaction strength of two-dimensional boron‑nitrogen‑carbon heterostructures driven by polycrystalline metallic surfaces

Two-dimensional boron‑nitrogen‑carbon heterostructures with variable composition and interaction strength were obtained by one-step one-precursor synthetic route based on dimethylamine borane pyrolysis by changing the metallic surfaces and the temperature of growth. Hybrid single layers composed mainly of hexagonal boron nitride and graphene domains (h–BNG) with different relative concentration and surface hybridization were grown on polycrystalline metallic foils including Pt and earth abundant metals such as Ni and Fe. An almost free-standing layer of h–BNG grown on Pt was successfully transferred on a SiO2/Si substrate by a simple electrolytic delamination. The atmospheric gas intercalation between h–BNG and Pt of the sample stored in air before the transfer procedure was demonstrated to be effective for a facile h–BNG detachment from the metal during the delamination bubbling process. This facile and flexible synthesis and transfer route opens up the way to applications of hybrid B–N–C monolayer in technology, catalysis and 2D confined chemistry, gas-sensing and hydrogen storage.

In situ chemical probing of hole defects and cracks in graphene at room temperature.

Chemical vapor deposition (CVD) has emerged as the most promising technique towards manufacturing of large area, high quality graphene. Characterization, understanding, and controlling of various structural defects in CVD-grown graphene are essential to realize its true potential for real-world applications. We report a new method for in situ chemical probing of vacancy defects in CVD-grown graphene at room temperature. Our approach is based on a solid-gas phase reaction that occurs selectively in graphene vacancy defect regions such as holes and cracks. Our new probing technique has a unique combination of the following advantages: (1) no exposure to liquids; (2) non-damaging in situ probing; (3) high selectivity, sensitivity, and reliability towards vacancy defects; (4) simplicity and scalability. By focusing on hexagonal graphene domains, we have made the following findings: (1) the nucleation centers of graphene domains are favorable locations of hole defects. (2) The lengthy electron-beam irradiation at very low energy (3.5 keV) could etch the graphene. (3) Graphene cracks often kink at the angle of primarily 150° or 120°. (4) There exist complex graphene cracks such as cracks with clock-hands patterns, and cracks with snowflake-like branched structures. (5) There exist discontinuous cracks in some graphene domains, where hole defects are oriented along a straight or curved line. Such discontinuous cracks may arise from the ductile fracture of graphene. In addition, we have shown that our method is also applicable to chemical probing of vacancy defects such as holes, continuous and discontinuous cracks in CVD-grown monolayer polycrystalline graphene films on copper. Our study also suggests that the copper grain and copper grain boundary play significant roles in formation and distribution of graphene vacancy defects.

Oxide-assisted growth of scalable single-crystalline graphene with seamlessly stitched millimeter-sized domains on commercial copper foils.

Chemical vapor deposition (CVD) is considered as an effective route to obtain large-area and high-quality polycrystalline graphene; however, there are still technological challenges associated with its application to achieve single crystals of graphene. Herein, we present the CVD growth of scalable single-crystalline graphene by seamless stitching millimeter-sized unidirectional aligned hexagonal domains using different types of commercial Cu foils without repeated substrate polishing and H2-annealing processes. Compared with that reported in previous studies, herein, the average size for the hexagonal graphene domains is enlarged by 1–2 orders of magnitude (from tens of micrometers to millimeter). The key factor for growth is the Cu surface monocrystallization achieved by a pre-introduced oxide layer and the sequential Ar annealing. The graphene domains exhibit an average growth rate of >20 μm min−1 and a misorientation possibility of <2%, and seamless stitching at the domain coalescence interfaces is confirmed by atomic force microscopy measurements.

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.

Copper vapor-assisted growth of hexagonal graphene domains on silica islands

Silica (SiO2) islands with a dendritic structure were prepared on polycrystalline copper foil, using silane (SiH4) as a precursor, by annealing at high temperature. Assisted by copper vapor from bare sections of the foil, single-layer hexagonal graphene domains were grown directly on the SiO2 islands by chemical vapor deposition. Scanning electron microscopy, atomic force microscopy, Raman spectra, and X-ray photoelectron spectroscopy confirm that hexagonal graphene domains, each measuring several microns, were synthesized on the silica islands.

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.

Double hexagonal graphene ring synthesized using a growth-etching method.

Precisely controlling the layer number, stacking order, edge configuration, shape and structure of graphene is extremely challenging but highly desirable in scientific research. In this report, a new concept named the growth-etching method has been explored to synthesize a graphene ring using the chemical vapor deposition process. The graphene ring is a hexagonal structure, which contains a hexagonal exterior edge and a hexagonal hole in the centre region. The most important concept introduced here is that the oxide nanoparticle derived from annealing is found to play a dual role. Firstly, it acts as a nucleation site to grow the hexagonal graphene domain and then it works as a defect for etching to form a hole. The evolution process of the graphene ring with the etching time was carefully studied. In addition, a double hexagonal graphene ring was successfully synthesized for the first time by repeating the growth-etching process, which not only confirms the validity and repeatability of the method developed here but may also be further extended to grow unique graphene nanostructures with three, four, or even tens of graphene rings. Finally, a schematic model was drawn to illustrate how the double hexagonal graphene ring is generated and propagated. The results shown here may provide valuable guidance for the design and growth of unique nanostructures of graphene and other two-dimensional materials.

Fourier transform analysis of hexagonal domain for transparent conductive graphene.

In this paper, we applied the Fourier Transformation as a notion to calculate the orientation of hexagonal graphene domains on Cu substrate. We developed that a hexagon function to describe the diffraction pattern of hexagonal graphene. Hexagonal graphene domains grown on Cu (111) has an average value of orientation surrounding 3° in the frequency domain. For transparent conducting electrode applications, optical and electrical properties of large-area graphene film (2cm(2)) was measured. The results demonstrate that graphene grown on Cu (111) was greater than graphene grown on polycrystalline Cu.

(Invited) Synthesis, Characterization, and Applications of Single- and Double-Layer Graphene Grown on Epitaxial Metal Films

To develop graphene’s excellent and unique properties for future electronic devices, the synthesis of large area, high-quality graphene sheets with a controlled number of layers and well-defined structure is important. Here, we present our work to grow high-quality graphene with controlled orientation. Our method is based on heteroepitaxial CVD using highly crystalline metal catalyst deposited on single crystalline substrates, such as sapphire and MgO [1-6]. We investigated the CVD growth of single-layer graphene on Cu(111) and Cu(100) and found that the orientation of the graphene is strongly influenced by the underlying Cu lattice [2-4]. Large hexagonal graphene domains with a lateral size of 50-100 μm are aligned on the Cu(111) surface [5]. On the other hand, the orientation of graphene on Cu(100) has two main domain orientations, reflecting the symmetry mismatch between graphene and Cu(100) lattice. The carrier transport across the neighboring domains merged with the same orientation was investigated. We found a higher carrier mobility for devices inside one domain (mean value is 7,200 cm2/Vs at 280 K) than those with a domain interface in the channel (2,000 cm2/Vs), suggesting that the domain interface acts as a scattering site even when they merged with the same orientation [6]. Moreover, we have realized the selective growth of double-layer graphene on the heteroepitaxial metal films, which is expected to be applicable to semiconductor devices. The growth mechanism and the stacking order of the double-layer graphene will be discussed. Finally, our recent progress on the synthesis of two-dimensional heterostructures, such as graphene-MoS2 and graphene-NbS2hybrid structures, will be also presented [7,8]. [1] H. Ago et al., ACS Nano, 4, 7407 (2010). [2] B. Hu et al., Carbon 50, 57 (2012). [3] C. M. Orofeo et al., Carbon, 50, 2189 (2012). [4] Y. Ogawa et al., J. Phys. Chem. Lett., 3, 219 (2012). [5] H. Ago et al., Appl. Phys. Exp., 6, 75101 (2013). [6] Y. Ogawa et al., Nanoscale, 6, 7288 (2014). [7] W. Ge et al., Nanoscale, 5, 5773 (2013). [8] H. Ago et al., submitted.

Electromagnetic induction heating for single crystal graphene growth: morphology control by rapid heating and quenching.

The direct observation of single crystal graphene growth and its shape evolution is of fundamental importance to the understanding of graphene growth physicochemical mechanisms and the achievement of wafer-scale single crystalline graphene. Here we demonstrate the controlled formation of single crystal graphene with varying shapes and directly observe the shape evolution of single crystal graphene by developing a localized-heating and rapid-quenching chemical vapor deposition (CVD) system based on electromagnetic induction heating. Importantly, rational control of circular, hexagonal and dendritic single crystalline graphene domains can be readily obtained for the first time by changing the growth condition. Systematic studies suggest that the graphene nucleation only occurs during the initial stage, while the domain density is independent of the growth temperatures due to the surface-limiting effect. In addition, the direct observation of graphene domain shape evolution is employed for the identification of competing growth mechanisms including diffusion-limited, attachment-limited and detachment-limited processes. Our study not only provides a novel method for morphology-controlled graphene synthesis, but also offers fundamental insights into the kinetics of single crystal graphene growth.

Controlled van der Waals epitaxy of monolayer MoS2 triangular domains on graphene.

Multilayered heterostructures of two-dimensional materials have recently attracted increased interest because of their unique electronic and optical properties. Here, we present chemical vapor deposition (CVD) growth of triangular crystals of monolayer MoS2 on single-crystalline hexagonal graphene domains which are also grown by CVD. We found that MoS2 grows selectively on the graphene domains rather than on the bare supporting SiO2 surface. Reflecting the heteroepitaxy of the growth process, the MoS2 domains grown on graphene present two preferred equivalent orientations. The interaction between the MoS2 and the graphene induced an upshift of the Raman G and 2D bands of the graphene, while significant photoluminescence quenching was observed for the monolayer MoS2. Furthermore, photoinduced current modulation along with an optical memory effect was demonstrated for the MoS2-graphene heterostructure. Our work highlights that heterostructures synthesized by CVD offer an effective interlayer van der Waals interaction which can be developed for large-area multilayer electronic and photonic devices.

Formation of Graphene Grain Boundaries on Cu(100) Surface and a Route Towards Their Elimination in Chemical Vapor Deposition Growth

Grain boundaries (GBs) in graphene prepared by chemical vapor deposition (CVD) greatly degrade the electrical and mechanical properties of graphene and thus hinder the applications of graphene in electronic devices. The seamless stitching of graphene flakes can avoid GBs, wherein the identical orientation of graphene domain is required. In this letter, the graphene orientation on one of the most used catalyst surface — Cu(100) surface, is explored by density functional theory (DFT) calculations. Our calculation demonstrates that a zigzag edged hexagonal graphene domain on a Cu(100) surface has two equivalent energetically preferred orientations, which are 30 degree away from each other. Therefore, the fusion of graphene domains on Cu(100) surface during CVD growth will inevitably lead to densely distributed GBs in the synthesized graphene. Aiming to solve this problem, a simple route, that applies external strain to break the symmetry of the Cu(100) surface, was proposed and proved efficient.

Nanoscale structure and texture of highly anisotropic pyrocarbons revisited with transmission electron microscopy, image processing, neutron diffraction and atomistic modeling

We present a comparative study of the structure and texture at the nanoscale of two well-known high-textured laminar pyrocarbons (PyCs), the rough laminar (RL) and regenerative laminar (ReL) PyCs. Structure is assessed with diffraction data in the reciprocal space (coherence lengths La and Lc), and in the real space, using a pair distribution function analysis, to finely describe the in-plane features. Texture is characterized from high resolution transmission electron microscopy (HRTEM) images, by analyses of the 002 fringe properties (length and tortuosity) and of new descriptors based on the spatial variations of local orientations, allowing the measurement of the average misorientation angle between crystallites. However, the structure and texture of the RL PyC are recovered when the ReL PyC is heat-treated at 1500°C. In addition, atomistic models are generated from HRTEM images and thoroughly validated against structural and textural indicators. The latter, essentially containing three-coordinated atoms arranged in hexagonal rings, show that the materials differ from the way these rings are clustered together in large hexagonal graphene domains and connected by grain boundaries (pentagon/heptagon pairs), interlayer crosslinks (screw dislocations) and hydrogen-saturated edges.

Effect of Cu substrate roughness on growth of graphene domains at atmospheric pressure

In this paper, the morphologies of copper surface such as roughness and grain boundary were found to have crucial roles in the formation of nucleation seeds of graphene domains. We reduced the number density of graphene domains via polishing the Cu substrate in a chemical mechanical method. Scanning electron microscope and optical microscope images showed that it was preferential to form nucleation seeds along the Cu grain boundary and scratched area of the polished Cu substrate. We demonstrated that a very flat surface morphology of Cu substrate was beneficial for growing hexagonal single-crystal graphene domain which could enhance the homogeneity and electronic transport properties of graphene films.

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.