Real-time microscopy of graphene growth on epitaxial metal films: role of template thickness and strain

Epitaxial transition metal films have recently been introduced as substrates for the scalable synthesis of transferable graphene. Here, real-time microscopy is used to study graphene growth on epitaxial Ru films on sapphire. At high temperatures, high-quality graphene grows in macroscopic (>100 μm) domains to full surface coverage. Graphene nucleation and growth characteristics on thin (100 nm) Ru films are consistent with a pure surface chemical vapor deposition process, without detectable contributions from C segregation. Experiments on thicker (1 μm) films show a systematic suppression of the C uptake into the metal to levels substantially below those expected from bulk C solubility data, consistent with a strain-induced reduction of the C solubility due to gas bubbles acting as stressors in the epitaxial Ru films. The results identify two powerful approaches--i) limiting the template thickness and ii) tuning the interstitial C solubility via strain--for controlling graphene growth on metals with high C solubility, such as Ru, Pt, Rh, Co, and Ni.     

Epitaxial graphene on ruthenium

Graphene has been used to explore the fascinating electronic properties of ideal two-dimensional carbon, and shows great promise for quantum device architectures. The primary method for isolating graphene, micromechanical cleavage of graphite, is difficult to scale up for applications. Epitaxial growth is an attractive alternative, but achieving large graphene domains with uniform thickness remains a challenge, and substrate bonding may strongly affect the electronic properties of epitaxial graphene layers. Here, we show that epitaxy on Ru(0001) produces arrays of macroscopic single-crystalline graphene domains in a controlled, layer-by-layer fashion. Whereas the first graphene layer indeed interacts strongly with the metal substrate, the second layer is almost completely detached, shows weak electronic coupling to the metal, and hence retains the inherent electronic structure of graphene. Our findings demonstrate a route towards rational graphene synthesis on transition-metal templates for applications in electronics, sensing or catalysis.     

Controllable Growth of Graphene on Liquid Surfaces

Controllable fabrication of graphene is necessary for its practical application. Chemical vapor deposition (CVD) approaches based on solid metal substrates with morphology‐rich surfaces, such as copper (Cu) and nickel (Ni), suffer from the drawbacks of inhomogeneous nucleation and uncontrollable carbon precipitation. Liquid substrates offer a quasiatomically smooth surface, which enables the growth of uniform graphene layers. The fast surface diffusion rates also lead to unique growth and etching kinetics for achieving graphene grains with novel morphologies. The rheological surface endows the graphene grains with self‐adjusted rotation, alignment, and movement that are driven by specific interactions. The intermediary‐free transfer or the direct growth of graphene on insulated substrates is demonstrated using liquid metals. Here, the controllable growth process of graphene on a liquid surface to promote the development of attractive liquid CVD strategies is in focus. The exciting progress in controlled growth, etching, self‐assembly, and delivery of graphene on a liquid surface is presented and discussed in depth. In addition, prospects and further developments in these exciting fields of graphene growth on a liquid surface are discussed.     

Control of thickness uniformity and grain size in graphene films for transparent conductive electrodes

Large-scale and transferable graphene films grown on metal substrates by chemical vapor deposition (CVD) still hold great promise for future nanotechnology. To realize the promise, one of the key issues is to further improve the quality of graphene, e.g., uniform thickness, large grain size, and low defects. Here we grow graphene films on Cu foils by CVD at ambient pressure, and study the graphene nucleation and growth processes under different concentrations of carbon precursor. On the basis of the results, we develop a two-step ambient pressure CVD process to synthesize continuous single-layer graphene films with large grain size (up to hundreds of square micrometers). Scanning electron microscopy and Raman spectroscopy characterizations confirm the film thickness and uniformity. The transferred graphene films on cover glass slips show high electrical conductivity and high optical transmittance that make them suitable as transparent conductive electrodes. The growth mechanism of CVD graphene on Cu is also discussed, and a growth model has been proposed. Our results provide important guidance toward the synthesis of high quality uniform graphene films, and could offer a great driving force for graphene based applications.     

Growth Mechanism of Metal Clusters on a Graphene/Ru(0001) Template

Using first‐principles calculations combined with scanning tunneling microscopy experiments, we investigated the adsorption configurations, electronic structures and the corresponding growth mechanism of several transition metal (TM) atoms (Pt, Ru, Ir, Ti, Pd, Au, Ag, and Cu) on a graphene/Ru(0001) moiré template (G/Ru(0001)) at low coverage. We find that Pt, Ru, Ir, and Ti selectively adsorb on the fcc region of G/Ru(0001) and form ordered dispersed metal nanoclusters. This behavior is due to the unoccupied d orbital of the TM atoms and the strong sp³ hybridization of carbon atoms in the fcc region of G/Ru(0001). Pd, Au, Ag, and Cu form nonselective structures because of the fully occupied d orbital. This mechanism can be extended to metals on a graphene/Rh(111) template. By using Pt as an example, we provide a layer by layer growth path for Pt nanoclusters in the fcc region of the G/Ru(0001). The simulations of growth mechanism agree well with the experimental observations. Moreover, they also provide guidance for the selection of suitable metal atoms to form ordered dispersed metal nanoclusters on similar templates.     

Universal segregation growth approach to wafer-size graphene from non-noble metals

Graphene has been attracting wide interests owing to its excellent electronic, thermal, and mechanical performances. Despite the availability of several production techniques, it is still a great challenge to achieve wafer-size graphene with acceptable uniformity and low cost, which would determine the future of graphene electronics. Here we report a universal segregation growth technique for batch production of high-quality wafer-scale graphene from non-noble metal films. Without any extraneous carbon sources, 4 in. graphene wafers have been obtained from Ni, Co, Cu-Ni alloy, and so forth via thermal annealing with over 82% being 1-3 layers and excellent reproducibility. We demonstrate the first example of monolayer and bilayer graphene wafers using Cu-Ni alloy by combining the distinct segregation behaviors of Cu and Ni. Together with the easy detachment from growth substrates, we believe this facile segregation technique will offer a great driving force for graphene research.     

Single-step formation of a graphene-metal hybrid transparent and electrically conductive film

We report here a rapid (10 s of heating) graphene growth method that can be carried out on any desired substrate, including an insulator, thus negating the need for the transfer from the metal substrate. This technique is based on metal-induced crystallization of amorphous carbon (a-C) to graphene, and involves an ultra-thin metal layer that is less than 10 nm in thickness. Rapid annealing of a bilayer of a-C and metal deposited on the surface leads to the formation of graphene film, and to subsequent breaking-up of the thin metal layer underneath the film, thus resulting in the formation of a graphene–metal hybrid film which is both transparent and electrically conducting. Based on Raman studies, we have also systematically compared ultra-thin metal-induced crystallization behavior with a case of conventional thick metal. Based on the present investigation, it was observed that the dominant growth mechanism in ultra-thin metal-induced crystallization is nucleation controlled.     

Conductance through multilayer graphene films

The ballistic conductance through junctions between multilayer graphene films and several different metals is studied using ab initio calculations within the local density approximation. The system consists of films of up to four graphene layers (Bernal stacking) between metallic electrodes, assuming reasonable metal-graphene epitaxial relationships. For some metals, the conductance decays exponentially with increasing number of layers, while for others the conductance saturates with film thickness. This difference in asymptotic behavior stems from the crystal momentum (mis)match between the bulk Fermi-level states in the electrode and those in the film. In contrast, for sufficiently thin films the bonding between the metal and the adjacent graphene layer dominates, giving a metal dependence for graphene similar to that seen experimentally for single-wall carbon nanotubes. Among the metals considered here, we find Pd to be the best for electrodes to films with up to 4 graphene layers.     

Hexagonal single crystal domains of few-layer graphene on copper foils

Hexagonal-shaped single crystal domains of few layer graphene (FLG) are synthesized on copper foils using atmospheric pressure chemical vapor deposition with a high methane flow. Scanning electron microscopy reveals that the graphene domains have a hexagonal shape and are randomly orientated on the copper foil. However, the sites of graphene nucleation exhibit some correlation by forming linear rows. Transmission electron microscopy is used to examine the folded edges of individual domains and reveals they are few-layer graphene consisting of approximately 5-10 layers in the central region and thinning out toward the edges of the domain. Selected area electron diffraction of individual isolated domains reveals they are single crystals with AB Bernal stacking and free from the intrinsic rotational stacking faults that are associated with turbostratic graphite. We study the time-dependent growth dynamics of the domains and show that the final continuous FLG film is polycrystalline, consisting of randomly connected single crystal domains.     

Self-assembly-induced formation of high-density silicon oxide memristor nanostructures on graphene and metal electrodes

We report the direct formation of ordered memristor nanostructures on metal and graphene electrodes by a block copolymer self-assembly process. Optimized surface functionalization provides stacking structures of Si-containing block copolymer thin films to generate uniform memristor device structures. Both the silicon oxide film and nanodot memristors, which were formed by the plasma oxidation of the self-assembled block copolymer thin films, presented unipolar switching behaviors with appropriate set and reset voltages for resistive memory applications. This approach offers a very convenient pathway to fabricate ultrahigh-density resistive memory devices without relying on high-cost lithography and pattern-transfer processes.     

Synthesis of large area, homogeneous, single layer graphene films by annealing amorphous carbon on Co and Ni

The synthesis of large area, homogenous, single layer graphene on cobalt (Co) and nickel (Ni) is reported. The process involves vacuum annealing of sputtered amorphous carbon (a-C) deposited on Co/sapphire or Ni/sapphire substrates. The improved crystallinity of the metal film, assisted by the sapphire substrate, proves to be the key to the quality of as-grown graphene film. The crystallinity of the Co and Ni metal films was improved by sputtering the metal at elevated temperature as was verified by X-ray diffraction (XRD). After sputtering of a-C and annealing, large area, single layer graphene that occupies almost the entire area of the substrate was produced. With this method, 100 mm²-area single layer graphene can be synthesized and is limited only by the substrate and vacuum chamber size. The homogeneity of the graphene film is not dependent on the cooling rate, in contrast to syntheses using polycrystalline metal films and conventional chemical vapor deposition (CVD) growth. Our facile method of producing single layer graphene on Co and Ni metal films should lead to large scale graphene-based applications.     

Topology evolution of graphene in chemical vapor deposition, a combined theoretical/experimental approach toward shape control of graphene domains

Morphology control of thin film relies on understanding multiple ongoing processes during deposition and growth. To reveal the shape evolution of graphene domains on copper surfaces in chemical vapor deposition (CVD), a combinative study is performed on the CVD growth of graphene on copper surfaces. To identify the factors that influence the adsorption and diffusion of carbon atoms and further determine the domain shape, simulations based on kinetic Monte Carlo techniques are carried out. The results reveal the dependence of the graphene domain shapes on the crystalline orientation of the underlying copper substrate surfaces.     

In situ observations of catalyst dynamics during surface-bound carbon nanotube nucleation

We present atomic-scale, video-rate environmental transmission electron microscopy and in situ time-resolved X-ray photoelectron spectroscopy of surface-bound catalytic chemical vapor deposition of single-walled carbon nanotubes and nanofibers. We observe that transition metal catalyst nanoparticles on SiOx support show crystalline lattice fringe contrast and high deformability before and during nanotube formation. A single-walled carbon nanotube nucleates by lift-off of a carbon cap. Cap stabilization and nanotube growth involve the dynamic reshaping of the catalyst nanocrystal itself. For a carbon nanofiber, the graphene layer stacking is determined by the successive elongation and contraction of the catalyst nanoparticle at its tip.     

High‐Quality Monolayer Graphene Synthesis on Pd Foils via the Suppression of Multilayer Growth at Grain Boundaries

The segregation of carbon from metals in which carbon is highly soluble, such as Ni (≈1.1 atom% at 1000 °C), is a typical method for graphene growth; this method differs from the surface‐catalyzed growth of graphene that occurs on other metals such as Cu (<0.04 atom%). It has not been established whether strictly monolayer graphene could be synthesized through the traditional chemical vapor deposition route on metals where carbon is highly soluble, such as Pd (≈3.5 atom%). In this work, this issue is investigated by suppressing the grain boundary segregation using a pretreatment comprising the annealing of the Pd foils; this method was motivated by the fact that the typical thick growths at the grain boundaries revealed that the grain boundary functions as the main segregation channel in polycrystalline metals. To evaluate the high crystallinity of the as‐grown graphene, detailed atomic‐scale characterization with scanning tunneling microscopy is performed.     

Stable Low‐Current Electrodeposition of α‐MnO2 on Superaligned Electrospun Carbon Nanofibers for High‐Performance Energy Storage

Metal oxide/carbonaceous nanomaterials are promising candidates for energy‐storage applications. However, inhomogeneous mass and charge transfer across the electrode/electrolyte interface due to unstable metal oxide/carbonaceous nanomaterial synthesis limit their performance in supercapacitors. Here, it is shown that the above problems can be mitigated through stable low‐current electrodeposition of MnO₂ on superaligned electrospun carbon nanofibers (ECNFs). The key to this approach is coupling a self‐designed four steel poles collector for aligned ECNFs and a constant low‐current (40 µA) electrodeposition technique to form a uniform Na⁺‐induced α‐MnO₂ film which proceeds by a time‐dependent growth mechanism involving cluster‐“kebab” structures and ending with a compact, uniform MnO₂ film for high‐performance energy storage.     

Physical vapor deposition of metal nanoparticles on chemically modified graphene: observations on metal-graphene interactions

The growth of metallic nanoparticles formed on chemically modified graphene (CMG) by physical vapor deposition is investigated. Fine control over the size (down to ～1.5 nm for Au) and coverage (up to 5 × 10(4) μm(-2) for Au) of nanoparticles can be achieved. Analysis of the particle size distributions gives evidence for Au nanocluster diffusion at room temperature, while particle size statistics differ clearly between metal deposited on single- and multilayer regions. The morphology of the nanoparticles varies markedly for different metals (Ag, Au, Fe, Pd, Pt, Ti), from a uniform thin film for Ti to a droplet-like growth for Ag. A simple model explains these morphologies, based only on consideration of 1) the different energy barriers to surface diffusion of metal adatoms on graphene, and 2) the ratio of the bulk cohesive energy of the metal to the metal-graphene binding energy. Understanding these interactions is important for controlling nanoparticle and thin-film growth on graphene, and for understanding the resultant charge transfer between metal and graphene.     

Perforated Carbon Nanotube Film Assisted Growth of Uniform Monolayer MoS2 (Small 23/2023)

Scaling up the chemical vapor deposition (CVD) of monolayer transition metal dichalcogenides (TMDCs) is in high demand for practical applications. However, for CVD-grown TMDCs on a large scale, there are many existing factors that result in their poor uniformity. In particular, gas flow, which usually leads to inhomogeneous distributions of precursor concentrations, has yet to be well controlled. In this work, the growth of uniform monolayer MoS₂ on a large scale by the delicate control of gas flows of precursors, which is realized by vertically aligning a well-designed perforated carbon nanotube (p-CNT) film face-to-face with the substrate in a horizontal tube furnace, is achieved. The p-CNT film releases gaseous Mo precursor from the solid part and allows S vapor to pass through the hollow part, resulting in uniform distributions of both gas flow rate and precursor concentrations near the substrate. Simulation results further verify that the well-designed p-CNT film guarantees a steady gas flow and a uniform spatial distribution of precursors. Consequently, the as-grown monolayer MoS₂ shows quite good uniformity in geometry, density, structure, and electrical properties. This work provides a universal pathway for the synthesis of large-scale uniform monolayer TMDCs, and will advance their applications in high-performance electronic devices.     

Catalyst-free growth of nanographene films on various substrates

We have developed a new method to grow uniform graphene films directly on various substrates, such as insulators, semiconductors, and even metals, without using any catalyst. The growth was carried out using a remote plasma enhancement chemical vapor deposition (r-PECVD) system at relatively low temperatures, enabling the deposition of graphene films up to 4-inch wafer scale. Scanning tunneling microscopy (STM) confirmed that the films are made up of nanocrystalline graphene particles of tens of nanometers in lateral size. The growth mechanism for the nanographene is analogous to that for diamond grown by PECVD methods, in spite of sp2 carbon atoms being formed in the case of graphene rather than sp3 carbon atoms as in diamond. This growth approach is simple, low-cost, and scalable, and might have potential applications in fields such as thin film resistors, gas sensors, electrode materials, and transparent conductive films.     

Ultrathin Metal Crystals: Growth on Supported Graphene Surfaces and Applications

Controlled nucleation and growth of metal clusters in metal deposition processes is a long‐standing issue for thin‐film‐based electronic devices. When metal atoms are deposited on solid surfaces, unintended defects sites always lead to a heterogeneous nucleation, resulting in a spatially nonuniform nucleation with irregular growth rates for individual nuclei, resulting in a rough film that requires a thicker film to be deposited to reach the percolation threshold. In the present study, it is shown that substrate‐supported graphene promotes the lateral 2D growth of metal atoms on the graphene. Transmission electron microscopy reveals that 2D metallic single crystals are grown epitaxially on supported graphene surfaces while a pristine graphene layer hardly yields any metal nucleation. A surface energy barrier calculation based on density functional theory predicts a suppression of diffusion of metal atoms on electronically perturbed graphene (supported graphene). 2D single Au crystals grown on supported graphene surfaces exhibit unusual near‐infrared plasmonic resonance, and the unique 2D growth of metal crystals and self‐healing nature of graphene lead to the formation of ultrathin, semitransparent, and biodegradable metallic thin films that could be utilized in various biomedical applications.     

Graphene synthesis by cold implantation of carbon recoil atoms

A new method of introducing carbon into catalytic metal films for graphene synthesis is proposed. The method is based on the phenomenon of carbon recoil atoms from a layer of methane molecules that are adsorbed on a metal film being incorporated into this film under the action of bombardment with inert gas ions. To increase the thickness of adsorbed methane layer, the substrate is cooled down to ?190°C. The proposed method has been implemented on a polycrystalline nickel film. After the final annealing, Raman spectroscopy showed the presence of numerous fragments of multilayer graphene on the film surface.