Effects of polycrystalline cu substrate on graphene growth by chemical vapor deposition

Controlled Chemical Synthesis in CVD Graphene

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

Substrate independent approach for synthesis of graphene platelet networks

Graphene platelet networks (GPNs) comprised of randomly oriented graphene flakes two to three atomic layers thick are synthesized using a novel plasma-based approach. The approach uses a substrate capable of withstanding synthesis temperatures around 800 °C, but is fully independent of the substrate material. The synthesis occurs directly on the substrate surface without the necessity of any additional steps. GPNs were synthesized on various substrate materials including silicon (Si), thermally oxidized Si (SiO₂), molybdenum (Mo), nickel (Ni) and copper (Cu), nickel-chromium (NiCr) alloy and alumina ceramics (Al₂O₃). The mismatch between the atomic structures of sp² honeycomb carbon networks and the substrate material is fully eliminated shortly after the synthesis initiation, namely when about 100 nm thick deposits are formed on the substrate. GPN structures synthesized on a substrate at a temperature of about 800 °C are significantly more porous in comparison to the much denser packed amorphous carbon deposits synthesized at lower temperatures. The method proposed here can potentially revolutionize the area of electrochemical energy storage by offering a single-step direct approach for the manufacture of graphene-based electrodes for non-Faradaic supercapacitors. Mass production can be achieved using this method if a roll-to-roll system is utilized.

Large-area, high-quality monolayer graphene from polystyrene at atmospheric pressure

Graphene films have been attracting great interest owing to their unique physical properties. In this paper, we develop an efficient method to prepare large-area monolayer graphene (97.5% coverage) by atmospheric pressure chemical vapor deposition on Cu foils using polystyrene in a short time (3 min). Raman spectroscopy, transmission electron microscopy and scanning electron microscopy are employed to confirm the thickness and uniformity of the graphene films. Graphene films on glass substrates show high optical transmittance and electrical conductivity. Magnetic transport studies demonstrate that the as-grown monolayer graphene exhibits a high carrier mobility of 3395 cm² V^-1 s^-1 at 25 K. On the basis of the analysis, it is concluded that our method is a simple, safe and versatile approach for the synthesis of monolayer graphene.

Nucleation and growth of single layer graphene on electrodeposited Cu by cold wall chemical vapor deposition

The nucleation density and average size of graphene crystallites grown using cold wall chemical vapor deposition (CVD) on 4 μm thick Cu films electrodeposited on W substrates can be tuned by varying growth parameters. Growth at a fixed substrate temperature of 1000 °C and total pressure of 700 Torr using Ar, H₂ and CH₄ mixtures enabled the contribution of total flow rate, CH₄:H₂ ratio and dilution of the CH₄/H₂ mixture by Ar to be identified. The largest variation in nucleation density was obtained by varying the CH₄:H₂ ratio. The observed morphological changes are analogous to those that would be expected if the deposition rate were varied at fixed substrate temperature for physical deposition using thermal evaporation. The graphene crystallite boundary morphology progresses from irregular/jagged through convex hexagonal to regular hexagonal as the effective C deposition rate decreases. This observation suggests that edge diffusion of C atoms along the crystallite boundaries, in addition to H₂ etching, may contribute to shape evolution of the graphene crystallites. These results demonstrate that graphene grown using cold wall CVD follows a nucleation and growth mechanism similar to hot wall CVD. As a consequence, the vast knowledge base relevant to hot wall CVD may be exploited for graphene synthesis by the industrially preferable cold wall method.

Role of limited hydrogen and flow interval on the growth of single crystal to continuous graphene by low-pressure chemical vapor deposition

A method for defect-free large crystallite graphene growth remains unknown despite much research effort. In this work, we discuss the role of flow duration of H₂ gas for the production of graphene as per requirement and production at a minimum flow rate considering the safety issue of hydrogen utilization. The copper substrate used for growth was treated for different time intervals (0 to 35 min) in H₂ flow prior to growth. Structural and chemical changes occurring in the copper substrate surface were probed by grazing incidence x-ray diffraction and x-ray photoelectron spectroscopy. The results were correlated with the Raman spectroscopy data, which can quantify the quality of graphene. With increasing H₂ flow interval, secondary nucleation sites were observed and growth favored few-layer graphene structures. The surface-adsorbed oxygen molecules and its conversion to an OH terminated surface with increasing hydrogen flow interval was found to be a key factor in enhancing nucleation density. The Stranski-Krastanov type of nucleation was observed for samples grown with different time intervals of H₂ treatment, except 5 min of H₂ flow prior to growth for which the Volmer-Weber type of growth favored monolayer graphene crystallite growth.

Tuning the Reactivity of Graphene by Surface Phase Orientation

Tuning the local reactivity of graphene is a subject of paramount importance. Among the available strategies, the activation/passivation of graphene by copper substrate is very promising because it enables the properties of graphene to be influenced without any transfer procedure, since graphene can be grown directly on copper. Herein, it is demonstrated that the reactivity of graphene towards fluorination is strongly influenced by the face of the surface of the copper substrate. Graphene on the copper foil was probed and grain orientations were identified. The results of the reactivity were evaluated by means of X-ray photo electron and Raman spectroscopy. Graphene on the grains with a surface orientation close to the (111) face is the most reactive, whereas graphene on the grains close to the (110) surface is least reactive. The long-term stability test showed that the decomposition of fluorinated graphene was slowest on the grains with a surface orientation close to the (111) face. The results are consistent with the variation of the mechanical strain of graphene on different faces of copper. In contrast, no clear correlation of the graphene reactivity with doping induced by different facets was found.

In situepitaxial growth of GdF3on NaGdF4:Yb,Er nanoparticles

By electron-beam irradiation of TEM, GdF₃(020) was epitaxially grown on the interface of NaGdF₄(111).

Recrystallization of copper at a solid interface for improved CVD graphene growth

Annealing of Cu in contact with a solid cap was found to relax lattice strain and minimize surface roughness which enhanced graphene growth.

Understanding and Controlling Cu-Catalyzed Graphene Nucleation: The Role of Impurities, Roughness, and Oxygen Scavenging

The mechanism by which Cu catalyst pretreatments control graphene nucleation density in scalable chemical vapor deposition (CVD) is systematically explored. The intrinsic and extrinsic carbon contamination in the Cu foil is identified by time-of-flight secondary ion mass spectrometry as a major factor influencing graphene nucleation and growth. By selectively oxidizing the backside of the Cu foil prior to graphene growth, a drastic reduction of the graphene nucleation density by 6 orders of magnitude can be obtained. This approach decouples surface roughness effects and at the same time allows us to trace the scavenging effect of oxygen on deleterious carbon impurities as it permeates through the Cu bulk. Parallels to well-known processes in Cu metallurgy are discussed. We also put into context the relative effectiveness and underlying mechanisms of the most widely used Cu pretreatments, including wet etching and electropolishing, allowing a rationalization of current literature and determination of the relevant parameter space for graphene growth. Taking into account the wider CVD growth parameter space, guidelines are discussed for high-throughput manufacturing of "electronic-quality" monolayer graphene films with domain size exceeding 1 mm, suitable for emerging industrial applications, such as electronics and photonics.

Templating for hierarchical structure control in carbon materials

Carbon-based materials show a remarkable variety of physical properties. For this reason, they have recently been explored for many advanced applications and emerging technologies. In the absence of actual "chemical" functionalities in these materials, tailoring these physical properties requires control on all levels of the structural hierarchy, from the atomic structure (carbon connectivity, defects, impurities), to the supramolecular level (domain orientations), nanoscopic length scale (domain sizes, porosity), microscopic structure (morphology), and macroscopic aspects (shape, surface chemistry). When preparing carbon materials, all these features can be tailored through the use of hard, soft, or molecular templates. Based on such templating approaches or through their combination, tremendous progress towards hierarchically structured carbon materials has recently been accomplished. Novel carbon nanomaterials such as brick-walled carbon tubes, carbon nanotube forests, coral-like carbon monoliths, or functional carbon nanosheets have become available, some of which exhibit unusual combinations of electronic, mechanical, and chemical properties. This review aims to discuss how the different templating approaches allow the control of structure formation on various length scales, how hierarchical structure formation can be realized, and which challenges remain, such as the detailed control over the carbon connectivity or the surface chemistry.

Synthesis of Different Layers of Graphene on Stainless Steel Using the CVD Method

In this study, different types of graphene, including single-, few-, and multi-layer graphene, were grown on a stainless steel (SS) mesh coated with Cu catalyst by using the chemical vapor deposition (CVD) method. Even though the SS mesh consisted of different types of metals, such as Fe, Ni, and Cr, which can also be used as catalysts, the reason for coating Cu catalyst on the SS surface had been related to the nature of the Cu, which promotes the growth of graphene with high quality and quantity at low temperature and time. The reaction temperature and run time, as the most important parameters of the CVD method, were varied, and thus led to the synthesis of different layers of graphene. Moreover, the presence of single-, few-, and multi-layer graphene was confirmed by employing two techniques, namely transmission electron microscopy (TEM) and Raman spectroscopy. On top of that, electron dispersive X-ray (EDX) was further applied to establish the influence of the CVD parameters on the growth of graphene.

Surface Monocrystallization of Copper Foil for Fast Growth of Large Single-Crystal Graphene under Free Molecular Flow

Wafer-sized single-crystalline Cu (100) surface can be readily achieved on stacked polycrystalline Cu foils via simple oxygen chemisorption-induced reconstruction, enabling fast growth of large-scale millimeter-sized single-crystalline graphene arrays under molecular flow. The maximum growth rate can reach 300 μm min^-1 , several orders of magnitude higher than previously reported values for millimeter-sized single-crystalline graphene growth on Cu foils.

Nondestructive optical visualisation of graphene domains and boundaries

The domain boundaries of polycrystalline graphene produced by chemical vapor deposition (CVD) adversely influence the graphene transporting properties. The existing domain visualisation methods for large area graphene always cause detrimental damage or contamination. Here we report a nondestructive method for spatial visualisation of the domains and boundaries of large area continuous graphene grown on Cu foils (Gr/Cu) by CVD. Using a rationally modified optical microscope, we can directly observe novel star-like bright line sets of Gr/Cu in an enhanced dark field mode. Each set of the bright lines is identified as the ridges of one Cu surface pyramid which arises beneath one enlarging graphene domain due to slower evaporation of graphene-covered Cu than that of graphene-free Cu. This one to one correspondence thereby enables nondestructive visualisation. This method offers an advantageous pathway for monitoring the spatial distribution of the graphene domains and boundaries. We have further discovered for the first time various types of star-like ridge structures which are governed by the underlying Cu crystallographic orientations. This gives rise to a new phenomenon for research on the complex 2D material-metal interfacing.

Mechanical Stability of Flexible Graphene-Based Displays

The mechanical behavior of a prototype touch panel display, which consists of two layers of CVD graphene embedded into PET films, is investigated in tension and under contact-stress dynamic loading. In both cases, laser Raman spectroscopy was employed to assess the stress transfer efficiency of the embedded graphene layers. The tensile behavior was found to be governed by the "island-like" microstructure of the CVD graphene, and the stress transfer efficiency was dependent on the size of graphene "islands" but also on the yielding behavior of PET at relatively high strains. Finally, the fatigue tests, which simulate real operation conditions, showed that the maximum temperature gradient developed at the point of "finger" contact after 80 000 cycles does not exceed the glass transition temperature of the PET matrix. The effect of these results on future product development and the design of new graphene-based displays are discussed.

Isotropic Growth of Graphene toward Smoothing Stitching

The quality of graphene grown via chemical vapor deposition still has very great disparity with its theoretical property due to the inevitable formation of grain boundaries. The design of single-crystal substrate with an anisotropic twofold symmetry for the unidirectional alignment of graphene seeds would be a promising way for eliminating the grain boundaries at the wafer scale. However, such a delicate process will be easily terminated by the obstruction of defects or impurities. Here we investigated the isotropic growth behavior of graphene single crystals via melting the growth substrate to obtain an amorphous isotropic surface, which will not offer any specific grain orientation induction or preponderant growth rate toward a certain direction in the graphene growth process. The as-obtained graphene grains are isotropically round with mixed edges that exhibit high activity. The orientation of adjacent grains can be easily self-adjusted to smoothly match each other over a liquid catalyst with facile atom delocalization due to the low rotation steric hindrance of the isotropic grains, thus achieving the smoothing stitching of the adjacent graphene. Therefore, the adverse effects of grain boundaries will be eliminated and the excellent transport performance of graphene will be more guaranteed. What is more, such an isotropic growth mode can be extended to other types of layered nanomaterials such as hexagonal boron nitride and transition metal chalcogenides for obtaining large-size intrinsic film with low defect.

Large-area single-crystal graphene grown on a recrystallized Cu(111) surface by using a hole-pocket method

We describe an efficient chemical vapor deposition (CVD) method for synthesizing graphene with a single crystal orientation on the whole surface of a copper (Cu) foil. We specifically synthesized graphene on the inner surface of a folded Cu foil, on which small holes were made for regulating the permeation and adsorption of the gases used for the synthesis. We compared the results of this method, which we call a "hole-pocket" method, with previously developed methods involving traditional synthesis on an open Cu foil and a Cu "pita-pocket". From these comparisons, we found the orientation of recrystallized Cu to depend on the shape of the Cu foil. Our hole-pocket method did not require treatment of the Cu surface with a complicated process to reduce the density of nucleation seeds in order to synthesize large hexagonal graphene grains, nor did it require the use of a single-crystalline substrate because methane passing through holes on the upper side of the hole-pocket slowly decomposed into carbon atoms and the control of the evaporation of Cu inside the foil pocket helped induce a transformation of the Cu domains to Cu(111). The current hole-pocket method resulted in domains that were both large, ranging from 2-5 mm in size, and oriented in the same manner. By extending the synthesis time, we were able to obtain continuous large-area films of single-crystalline orientation on the whole surface with dimensions of several centimeters.

Facet-Mediated Growth of High-Quality Monolayer Graphene on Arbitrarily Rough Copper Surfaces

A synthetic approach for high-quality graphene on rough Cu surfaces via chemical vapor deposition is proposed. High-quality graphene is synthesized on rough Cu surfaces by inducing surface faceting of Cu surfaces prior to graphene growth. The electron mobility of synthesized graphene on the rough Cu surfaces is enhanced to 10 335 cm(2) V(-1) s(-1).

Surface Diffusion Directed Growth of Anisotropic Graphene Domains on Different Copper Lattices

Anisotropic graphene domains are of significant interest since the electronic properties of pristine graphene strongly depend on its size, shape, and edge structures. In this work, considering that the growth of graphene domains is governable by the dynamics of the graphene-substrate interface during growth, we investigated the shape and defects of graphene domains grown on copper lattices with different indices by chemical vapor deposition of methane at either low pressure or atmospheric pressure. Computational modeling identified that the crystallographic orientation of copper strongly influences the shape of the graphene at low pressure, yet does not play a critical role at atmospheric pressure. Moreover, the defects that have been previously observed in the center of four-lobed graphene domains grown under low pressure conditions were demonstrated for the first time to be caused by a lattice mismatch between graphene and the copper substrate.

A fast transfer-free synthesis of high-quality monolayer graphene on insulating substrates by a simple rapid thermal treatment

The transfer-free synthesis of high-quality, large-area graphene on a given dielectric substrate, which is highly desirable for device applications, remains a significant challenge. In this paper, we report on a simple rapid thermal treatment (RTT) method for the fast and direct growth of high-quality, large-scale monolayer graphene on a SiO2/Si substrate from solid carbon sources. The stack structure of a solid carbon layer/copper film/SiO2 is adopted in the RTT process. The inserted copper film does not only act as an active catalyst for the carbon precursor but also serves as a "filter" that prevents premature carbon dissolution, and thus, contributes to graphene growth on SiO2/Si. The produced graphene exhibits a high carrier mobility of up to 3000 cm(2) V(-1) s(-1) at room temperature and standard half-integer quantum oscillations. Our work provides a promising simple transfer-free approach using solid carbon sources to obtain high-quality graphene for practical applications.

Large scale atomistic simulation of single-layer graphene growth on Ni(111) surface: molecular dynamics simulation based on a new generation of carbon-metal potential

To explore the mechanism of graphene chemical vapor deposition (CVD) growth on a catalyst surface, a molecular dynamics (MD) simulation of carbon atom self-assembly on a Ni(111) surface based on a well-designed empirical reactive bond order potential was performed. We simulated single layer graphene with recorded size (up to 300 atoms per super-cell) and reasonably good quality by MD trajectories up to 15 ns. Detailed processes of graphene CVD growth, such as carbon atom dissolution and precipitation, formation of carbon chains of various lengths, polygons and small graphene domains were observed during the initial process of the MD simulation. The atomistic processes of typical defect healing, such as the transformation from a pentagon into a hexagon and from a pentagon-heptagon pair (5|7) to two adjacent hexagons (6|6), were revealed as well. The study also showed that higher temperature and longer annealing time are essential to form high quality graphene layers, which is in agreement with experimental reports and previous theoretical results.

Effects of thermally-induced changes of Cu grains on domain structure and electrical performance of CVD-grown graphene

During the chemical vapor deposition (CVD) growth of graphene on Cu foils, evaporation of Cu and changes in the dimensions of Cu grains in directions both parallel and perpendicular to the foils are induced by thermal effects. Such changes in the Cu foil could subsequently change the shape and distribution of individual graphene domains grown on the foil surface, and thus influence the domain structure and electrical properties of the resulting graphene films. Here, a slower cooling rate is used after the CVD process, and the graphene films are found to have an improved electrical performance, which is considered to be associated with the Cu surface evaporation and grain structure changes in the Cu substrate.

Anodic electrogenerated chemiluminescence behavior and the choline biosensing application of blue emitting conjugated polymer dots

The anodic electrochemiluminescence (ECL) behavior of poly(9,9-dioctylfluorenyl-2,7-diyl) (PFO) dots was studied and applied in oxidoreductase-based ECL biosensing using Chox as the model enzyme.

Room temperature observation of the correlation between atomic and electronic structure of graphene on Cu(110)

We have used atomically-resolved scanning tunneling microscopy and spectroscopy to study the interplay between the atomic and electronic structure of graphene formed on copper via chemical vapor deposition.

High catalytic activity of oriented 2.0.0 copper(I) oxide grown on graphene film

Metal oxide nanoparticles supported on graphene exhibit high catalytic activity for oxidation, reduction and coupling reactions. Here we show that pyrolysis at 900 °C under inert atmosphere of copper(II) nitrate embedded in chitosan films affords 1.1.1 facet-oriented copper nanoplatelets supported on few-layered graphene. Oriented (1.1.1) copper nanoplatelets on graphene undergo spontaneous oxidation to render oriented (2.0.0) copper(I) oxide nanoplatelets on few-layered graphene. These films containing oriented copper(I) oxide exhibit as catalyst turnover numbers that can be three orders of magnitude higher for the Ullmann-type coupling, dehydrogenative coupling of dimethylphenylsilane with n-butanol and C–N cross-coupling than those of analogous unoriented graphene-supported copper(I) oxide nanoplatelets.

Supported metal nanoparticles have been widely used as heterogeneous catalysts. Here, the authors report the synthesis of (1.1.1) copper on few layer graphene which oxidize to orientated (2.0.0) copper(I) oxide nanoplatelets which display high catalytic activity for a number of organic coupling reactions.

Probing Crystallinity of Graphene Samples via the Vibrational Density of States

The purity of graphene samples is of crucial importance for their experimental and practical use. In this regard, the detection of the defects is of direct relevance. Here, we show that structural defects in graphene samples give rise to clear signals in the vibrational density of states (VDOS) at specific peaks at high and low frequencies. These can be used as an independent probe of the defect density. In particular, we consider grain boundaries made of pentagon-heptagon pairs, and show that they lead to a shift of the characteristic vibrational D mode toward higher frequency; this distinguishes these line defects from Stone-Wales point defects, which do not lead to such a shift. Our findings may be instrumental for the detection of structural lattice defects using experimental techniques that can directly measure VDOS, such as inelastic electron tunneling and inelastic neutron spectroscopy.

A wafer-scale graphene and ferroelectric multilayer for flexible and fast-switched modulation applications

Here we report a wafer-scale graphene/P(VDF-TrFE)/graphene multilayer for light-weight, flexible and fast-switched broadband modulation applications. The P(VDF-TrFE) film not only significantly reduces the sheet resistance of graphene throughout heavy doping of ∼0.8 × 10(13) cm(-2) by nonvolatile ferroelectric dipoles, but also acts as an efficient electro-optic (EO) layer. Such multilayered structural integration with remarkable ferroelectric polarization, high transparency (>90%), low sheet resistance (∼302 Ω□(-1)), and excellent mechanic flexibility shows the potential of a flexible modulation application over a broad range of wavelengths. Moreover, the derived device also exhibits strong field-induced EO modulation even under bending and one large Pockels coefficient (∼54.3 pm V(-1)) is obtained. Finally, the graphene and ferroelectric hybrid demonstrates a fast switching time (∼2 μs) and works well below low sheet resistance level over a long time. This work gives insights into the potential of graphene and ferroelectric hybrid structures, enabling future exploration on next-generation high-performance, flexible transparent electronics and photonics.

Roll-to-Roll Green Transfer of CVD Graphene onto Plastic for a Transparent and Flexible Triboelectric Nanogenerator

A novel roll-to-roll, etching-free, clean transfer of CVD-grown graphene from copper to plastic using surface-energy-assisted delamination in hot deionized water is reported. The delamination process is realized by water penetration between the hydrophobic graphene and a hydrophilic native oxide layer on a copper foil.The transferred graphene on plastic is used as a high-output flexible and transparent triboelectric nanogenerator.

Controlled growth of large area multilayer graphene on copper by chemical vapour deposition

The growth of nearly full coverage of multilayer graphene on the surface of a 99.8% purity copper foil has been experimentally studied. It has been shown that the film thickness can be controlled by a single parameter, the growth time, and growth can be extended until nearly full coverage of more than one layer graphene over the copper surface. The results are supported by scanning electron microscopy and Raman analysis together with optical transmittance and sheet resistance measurements. It has been verified that silicon oxide impurity particles within the copper act as catalysts and the seeds of multilayer graphene islands. The linear increase of the average thickness of graphene to the growth time has been attributed to the interplay between the mean distance between the impurities on the surface and the molecular mean free path in the process gas. A qualitative model is proposed to explain the microscopic mechanism of the multilayer growth on copper. These results contribute to the understanding of the chemical vapour deposition growth kinetics towards the objective of large area high quality graphene production with tuneable layer thickness.

The Hide-and-Seek of Grain Boundaries from Moiré Pattern Fringe of Two-Dimensional Graphene

Grain boundaries (GBs) commonly exist in crystalline materials and affect various properties of materials. The facile identification of GBs is one of the significant requirements for systematical study of polycrystalline materials including recently emerging two-dimensional materials. Previous observations of GBs have been performed by various tools including high resolution transmission electron microscopy. However, a method to easily identify GBs, especially in the case of low-angle GBs, has not yet been well established. In this paper, we choose graphene bilayers with a GB as a model system and investigate the effects of interlayer rotations to the identification of GBs. We provide a critical condition between adjacent moiré fringe spacings, which determines the possibility of GB recognition. In addition, for monolayer graphene with a grain boundary, we demonstrate that low-angle GBs can be distinguished easily by inducing moiré patterns deliberately with an artificial reference overlay.

Electron backscatter diffraction study of hexagonal boron nitride growth on Cu single-crystal substrates

Hexagonal boron nitride (h-BN) is an important material for the development of new 2D heterostructures. To enable this development, the relationship between crystal growth and the substrate orientation must be explored and understood. In this study, we simultaneously grew h-BN on different orientations of Cu substrates to establish the impact of substrate structure on the growth habit of thin h-BN layers. The substrates studied were a polycrystalline Cu foil, Cu(100), Cu(110), and Cu(111). Fourier transform grazing-incidence infrared reflection absorption spectroscopy (FT-IRRAS) was used to identify h-BN on copper substrates. X-ray photoelectron spectroscopy (XPS) was used to determine the effective thickness of the h-BN. Scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) were used to measure the morphology of the films and postgrowth crystal structure of the Cu substrates, respectively. Combining the SEM and EBSD images allowed for the correlation between h-BN film coverage and the crystal structure of Cu. It was found that the growth rate was inversely proportional to the surface free energy of the Cu surface, with Cu(111) having the most h-BN surface coverage. The Cu foil predominately crystallized with a (100) surface orientation, and likewise had a film coverage very close to the Cu(100).

Rapid epitaxy-free graphene synthesis on silicidated polycrystalline platinum

Large-area synthesis of high-quality graphene by chemical vapour deposition on metallic substrates requires polishing or substrate grain enlargement followed by a lengthy growth period. Here we demonstrate a novel substrate processing method for facile synthesis of mm-sized, single-crystal graphene by coating polycrystalline platinum foils with a silicon-containing film. The film reacts with platinum on heating, resulting in the formation of a liquid platinum silicide layer that screens the platinum lattice and fills topographic defects. This reduces the dependence on the surface properties of the catalytic substrate, improving the crystallinity, uniformity and size of graphene domains. At elevated temperatures growth rates of more than an order of magnitude higher (120 μm min(-1)) than typically reported are achieved, allowing savings in costs for consumable materials, energy and time. This generic technique paves the way for using a whole new range of eutectic substrates for the large-area synthesis of 2D materials.

Growth kinetics of white graphene (h-BN) on a planarised Ni foil surface

The morphology of the surface and the grain orientation of metal catalysts have been considered to be two important factors for the growth of white graphene (h-BN) by chemical vapour deposition (CVD). We report a correlation between the growth rate of h-BN and the orientation of the nickel grains. The surface of the nickel (Ni) foil was first polished by electrochemical polishing (ECP) and subsequently annealed in hydrogen at atmospheric pressure to suppress the effect of the surface morphology. Atmospheric annealing with hydrogen reduced the nucleation sites of h-BN, which induced a large crystal size mainly grown from the grain boundary with few other nucleation sites in the Ni foil. A higher growth rate was observed from the Ni grains that had the {110} or {100} orientation due to their higher surface energy.

Design of catalytic substrates for uniform graphene films: from solid-metal to liquid-metal

The controllable synthesis of uniform graphene with a specific layer number is crucial for both fundamental research and emerging applications due to the high sensitivity of the various extraordinary physicochemical properties of graphene to its layer numbers. However, the excessive segregation of extra C, the inactivation of the self-limiting of Cu and the superabundant nucleation at grain boundaries and defect sites render that the controllable synthesis of uniform graphene is still a challenge. By the employment of various solid and liquid metals with quasi-atomically smooth surfaces to avoid defects or grain boundaries, a series of studies have been performed and significant improvements have been achieved in the controllable synthesis of uniform graphene films. In this review, the representative strategies of designing catalytic substrates, including polycrystalline metals, single-crystalline metals, binary metal alloys and liquid metals, are highlighted. The future of the controllable synthesis of uniform graphene is also discussed.

Substrate Facet Effect on the Growth of Monolayer MoS2 on Au Foils

MoS2 on polycrystalline metal substrates emerges as an intriguing growth system compared to that on insulating substrates due to its direct application as an electrocatalyst in hydrogen evolution. However, the growth is still indistinct with regard to the effects of the inevitably evolved facets. Herein, we demonstrate for the first time that the crystallography of Au foil substrates can mediate a strong effect on the growth of monolayer MoS2, where large-domain single-crystal MoS2 triangles are more preferentially evolved on Au(100) and Au(110) facets than on Au(111) at relative high growth temperatures (>680 °C). Intriguingly, this substrate effect can be weakened at a low growth temperature (∼530 °C), reflected with uniform distributions of domain size and nucleation density among the different facets. The preferential nucleation and growth on some specific Au facets are explained from the facet-dependent binding energy of MoS2 according to density functional theory calculations. In brief, this work should shed light on the effect of substrate crystallography on the synthesis of monolayer MoS2, thus paving the way for achieving batch-produced, large-domain or domain size-tunable growth through an appropriate selection of the growth substrate.

The Influence of Cu Lattices on the Structure and Electrical Properties of Graphene Domains during Low-Pressure Chemical Vapor Deposition

The influence of various Cu lattices on the texturing of graphene domains during low-pressure chemical vapor deposition was investigated in a large area. The results show that the sizes and shapes of graphene domains grown on Cu(111) substrates match well with those of the underlying Cu(111) domains, which seem to be quasi-single-crystalline. In contrast, on other Cu substrates such as (100) and more intermediate domains, graphene islands with poly-domains (ca. 85 %) are significantly nucleated, eventually merging into polycrystalline graphene. Within the overall channel-length range, graphene from a Cu foil shows a higher resistance compared to graphene from a Cu(111) domain, with the extracted average channel resistances being 34.51 Ω μm(-1) for Cu(111) and 66.17 Ω μm(-1) for the Cu foil.

Grain Structures and Boundaries on Microcrystalline Copper Covered with an Octadecanethiol Monolayer Revealed by Sum Frequency Generation Microscopy

An octadecanethiol (ODT) self-assembled monolayer on microcrystalline copper was investigated by sum frequency generation (SFG) imaging microscopy. The crystal grain and grain boundaries of the copper surface were mapped in the SFG image based on the strong brightness contrast of the SFG signal across the boundary. Local SFG spectra reveal significant difference with each other as well as the average SFG spectra, indicating the heterogeneity of the copper surface resulting from copper grains with distinct crystallographic facets and orientations. It is demonstrated that the SFG signal of crystalline domain areas contains azimuthal anisotropy with respect to the plane of incidence. In addition, the statistical orientation analyses of amplitude ratio of CH3-sym/CH3-asym and corresponding contour maps imply that the orientation of ODT molecules is affected by the underlying copper.

Direct observation of graphene growth and associated copper substrate dynamics by in situ scanning electron microscopy

This work highlights the importance of in situ experiments for an improved understanding of graphene growth on copper via metal-catalyzed chemical vapor deposition (CVD). Graphene growth inside the chamber of a modified environmental scanning electron microscope under relevant low-pressure CVD conditions allows visualizing structural dynamics of the active catalyst simultaneously with graphene nucleation and growth in an unparalleled way. It enables the observation of a complete CVD process from substrate annealing through graphene nucleation and growth and, finally, substrate cooling in real time and nanometer-scale resolution without the need of sample transfer. A strong dependence of surface dynamics such as sublimation and surface premelting on grain orientation is demonstrated, and the influence of substrate dynamics on graphene nucleation and growth is presented. Insights on the growth mechanism are provided by a simultaneous observation of the growth front propagation and nucleation rate. Furthermore, the role of trace amounts of oxygen during growth is discussed and related to graphene-induced surface reconstructions during cooling. Above all, this work demonstrates the potential of the method for in situ studies of surface dynamics on active metal catalysts.

Comparing graphene growth on Cu(111) versus oxidized Cu(111)

The epitaxial growth of graphene on catalytically active metallic surfaces via chemical vapor deposition (CVD) is known to be one of the most reliable routes toward high-quality large-area graphene. This CVD-grown graphene is generally coupled to its metallic support resulting in a modification of its intrinsic properties. Growth on oxides is a promising alternative that might lead to a decoupled graphene layer. Here, we compare graphene on a pure metallic to graphene on an oxidized copper surface in both cases grown by a single step CVD process under similar conditions. Remarkably, the growth on copper oxide, a high-k dielectric material, preserves the intrinsic properties of graphene; it is not doped and a linear dispersion is observed close to the Fermi energy. Density functional theory calculations give additional insight into the reaction processes and help explaining the catalytic activity of the copper oxide surface.

Annealing free, clean graphene transfer using alternative polymer scaffolds

We examine the transfer of graphene grown by chemical vapor deposition (CVD) with polymer scaffolds of poly(methyl methacrylate) (PMMA), poly(lactic acid) (PLA), poly(phthalaldehyde) (PPA), and poly(bisphenol A carbonate) (PC). We find that optimally reactive PC scaffolds provide the cleanest graphene transfers without any annealing, after extensive comparison with optical microscopy, x-ray photoelectron spectroscopy, atomic force microscopy, and scanning tunneling microscopy. Comparatively, films transferred with PLA, PPA, PMMA/PC, and PMMA have a two-fold higher roughness and a five-fold higher chemical doping. Using PC scaffolds, we demonstrate the clean transfer of CVD multilayer graphene, fluorinated graphene, and hexagonal boron nitride. Our annealing free, PC transfers enable the use of atomically-clean nanomaterials in biomolecule encapsulation and flexible electronic applications.

Insights into carbon nanotube and graphene formation mechanisms from molecular simulations: a review

The discovery of carbon nanotubes (CNTs) and graphene over the last two decades has heralded a new era in physics, chemistry and nanotechnology. During this time, intense efforts have been made towards understanding the atomic-scale mechanisms by which these remarkable nanostructures grow. Molecular simulations have made significant contributions in this regard; indeed, they are responsible for many of the key discoveries and advancements towards this goal. Here we review molecular simulations of CNT and graphene growth, and in doing so we highlight the many invaluable insights gained from molecular simulations into these complex nanoscale self-assembly processes. This review highlights an often-overlooked aspect of CNT and graphene formation-that the two processes, although seldom discussed in the same terms, are in fact remarkably similar. Both can be viewed as a 0D → 1D → 2D transformation, which converts carbon atoms (0D) to polyyne chains (1D) to a complete sp(2)-carbon network (2D). The difference in the final structure (CNT or graphene) is determined only by the curvature of the catalyst and the strength of the carbon-metal interaction. We conclude our review by summarizing the present shortcomings of CNT/graphene growth simulations, and future challenges to this important area.

'Bubble-free' electrochemical delamination of CVD graphene films

The production of large amounts of hydrogen bubbles, typical of electrochemical delamination methods based on the electrolysis of water, results in mechanical damage to graphene during the delamination, transfer, and drying steps. Here a novel 'bubble-free' delamination method is introduced which exploits the electrochemical dissolution of native copper oxide at a potential lower than that required for the formation of hydrogen bubbles, enabling the production of defect-free graphene stack.

Stripe distributions of graphene-coated Cu foils and their effects on the reduction of graphene wrinkles

The wrinkle distribution of graphene domain was obtained as trenches after hydrogen etching. Parallel stripes on graphene domains are always perpendicular to these trenches, suggesting the suppressed wrinkle formation along the stripes' direction.

The role of substrate purity and its crystallographic orientation in the defect density of chemical vapor deposition grown monolayer graphene

Here, we are reporting about the role of the copper substrate purity and its crystallographic orientation in the quality of the graphene grown using a low pressure chemical vapor deposition technique.

Toward 300 mm wafer-scalable high-performance polycrystalline chemical vapor deposited graphene transistors

The largest applications of high-performance graphene will likely be realized when combined with ubiquitous Si very large scale integrated (VLSI) technology, affording a new portfolio of "back end of the line" devices including graphene radio frequency transistors, heat and transparent conductors, interconnects, mechanical actuators, sensors, and optical devices. To this end, we investigate the scalable growth of polycrystalline graphene through chemical vapor deposition (CVD) and its integration with Si VLSI technology. The large-area Raman mapping on CVD polycrystalline graphene on 150 and 300 mm wafers reveals >95% monolayer uniformity with negligible defects. About 26,000 graphene field-effect transistors were realized, and statistical evaluation indicates a device yield of ∼ 74% is achieved, 20% higher than previous reports. About 18% of devices show mobility of >3000 cm(2)/(V s), more than 3 times higher than prior results obtained over the same range from CVD polycrystalline graphene. The peak mobility observed here is ∼ 40% higher than the peak mobility values reported for single-crystalline graphene, a major advancement for polycrystalline graphene that can be readily manufactured. Intrinsic graphene features such as soft current saturation and three-region output characteristics at high field have also been observed on wafer-scale CVD graphene on which frequency doubler and amplifiers are demonstrated as well. Our growth and transport results on scalable CVD graphene have enabled 300 mm synthesis instrumentation that is now commercially available.