Nickel carbide as a source of grain rotation in epitaxial graphene

Formation of Graphene on Gold-Nickel Surface Alloys

Nickel (Ni)-catalyzed growth of a single- or rotated-graphene layer is a well-established process above 800 K. In this report, a Au-catalyzed, low-temperature, and facile route at 500 K for graphene formation is described. The substantially lower temperature is enabled by the presence of a surface alloy of Au atoms embedded within Ni(111), which catalyzes the outward segregation of carbon atoms buried in the Ni bulk at temperatures as low as 400-450 K. The resulting surface-bound carbon in turn coalesces into graphene above 450-500 K. Control experiments on a Ni(111) surface show no evidence of carbon segregation or graphene formation at these temperatures. Graphene is identified by its out-of-plane optical phonon mode at 750 cm^-1 and its longitudinal/transverse optical phonon modes at 1470 cm^-1 while surface carbon is identified by its C-Ni stretch mode at 540 cm^-1, as probed by high-resolution electron energy-loss spectroscopy. Dispersion measurements of the phonon modes confirm the presence of graphene. Graphene formation is observed to be maximum at 0.4 ML Au coverage. The results of these systematic molecular-level investigations open the door to graphene synthesis at the low temperatures required for integration with complementary metal-oxide-semiconductor processes.

Antimicrobial Activity of Graphene-Based Nanocomposites: Synthesis, Characterization, and Their Applications for Human Welfare

Graphene (GN)-related nanomaterials such as graphene oxide, reduced graphene oxide, quantum dots, etc., and their composites have attracted significant interest owing to their efficient antimicrobial properties and thus newer GN-based composites are being readily developed, characterized, and explored for clinical applications by scientists worldwide. The GN offers excellent surface properties, i.e., a large surface area, pH sensitivity, and significant biocompatibility with the biological system. In recent years, GN has found applications in tissue engineering owing to its impressive stiffness, mechanical strength, electrical conductivity, and the ability to innovate in two-dimensional (2D) and three-dimensional (3D) design. It also offers a photothermic effect that potentiates the targeted killing of cells via physicochemical interactions. It is generally synthesized by physical and chemical methods and is characterized by modern and sophisticated analytical techniques such as NMR, Raman spectroscopy, electron microscopy, etc. A lot of reports show the successful conjugation of GN with existing repurposed drugs, which improves their therapeutic efficacy against many microbial infections and also its potential application in drug delivery. Thus, in this review, the antimicrobial potentialities of GN-based nanomaterials, their synthesis, and their toxicities in biological systems are discussed.

Controlling the Balance between Remote, Pinhole, and van der Waals Epitaxy of Heusler Films on Graphene/Sapphire

Remote epitaxy is promising for the synthesis of lattice-mismatched materials, exfoliation of membranes, and reuse of expensive substrates. However, clear experimental evidence of a remote mechanism remains elusive. Alternative mechanisms such as pinhole-seeded epitaxy or van der Waals epitaxy can often explain the resulting films. Here, we show that growth of the Heusler compound GdPtSb on clean graphene/sapphire produces a 30° rotated (R30) superstructure that cannot be explained by pinhole epitaxy. With decreasing temperature, the fraction of this R30 domain increases, compared to the direct epitaxial R0 domain, which can be explained by a competition between remote versus pinhole epitaxy. Careful graphene/substrate annealing and consideration of the relative lattice mismatches are required to obtain epitaxy to the underlying substrate across a series of other Heusler films, including LaPtSb and GdAuGe. The R30 superstructure provides a possible experimental fingerprint of remote epitaxy, since it is inconsistent with the leading alternative mechanisms.

Ultrafast Growth of Large Area Graphene on Si Wafer by a Single Pulse Current

Graphene has many excellent optical, electrical and mechanical properties due to its unique two-dimensional structure. High-efficiency preparation of large area graphene film is the key to achieve its industrial applications. In this paper, an ultrafast quenching method was firstly carried out to flow a single pulse current through the surface of a Si wafer with a size of 10 mm × 10 mm for growing fully covered graphene film. The wafer surface was firstly coated with a 5-nm-thick carbon layer and then a 25-nm-thick nickel layer by magnetron sputtering. The optimum quenching conditions are a pulse current of 10 A and a pulse width of 2 s. The thus-prepared few-layered graphene film was proved to cover the substrate fully, showing a high conductivity. Our method is simple and highly efficient and does not need any high-power equipment. It is not limited by the size of the heating facility due to its self-heating feature, providing the potential to scale up the size of the substrates easily. Furthermore, this method can be applied to a variety of dielectric substrates, such as glass and quartz.

Graphene adlayer growth between nonepitaxial graphene and the Ni(111) substrate: a theoretical study

Understanding the fundamentals of chemical vapor deposition bilayer graphene growth is crucial for its synthesis. By employing density functional theory calculations and classical molecular dynamics simulations, we have investigated the evolution of carbon structures and the kinetics of the adlayer graphene nucleation between the graphene top layer (GTL) and the Ni(111) substrate. Compared to the epitaxial GTL, the weaker interaction between the nonepitaxial GTL and the Ni(111) substrate makes the nucleation of the adlayer more favorable. Furthermore, the defects involving in the adlayer graphene are easier to be healed by adopting the nonepitaxial GTL. Our results agree well with the experimental observation and demonstrate that the adlayer graphene with a high quality can be grown underneath the nonepitaxial GTL on Ni-like substrates.

Topochemical pyrolytic synthesis of quasi-Mxene hybrids via ionic liquid-iron phthalocyanine as a self-template

A novel quasi-Mxene structure hybrid is first realized through topochemical pyrolysis with the assistance of an ionic liquid.

One Second Formation of Large Area Graphene on a Conical Tip Surface via Direct Transformation of Surface Carbide

Graphene functionalized nanotips are expected to possess promising potential for various applications based on the outstanding electrical and mechanical properties of graphene. However, current methods, usually requiring a high growth temperature and identical crystal surface to match graphene lattice, are suitable for graphene formation on a flat surface. It remains a big challenge to grow graphene on a nanosized convex surface and fabricate functionalized nanotips with high quality graphene at the apex. In this work, a novel ultrafast annealing method is developed for growing large area graphene on Ni nanotips within 1-2 s. Few layered or multiple layered graphene is presented on the apex or sidewall of the conical tip surface. Direct experimental evidences support that thus-produced graphene is formed via the direct conversion of nickel carbide at the outer surface under the instantaneous high temperature, which is different from the conventional segregation mechanism. This newly developed ultrafast method provides a new route to produce graphene efficiently and economically, promising for both convex surfaces and flat substrates. Moreover, the graphene functionalized nanotips exhibit a great potential for nanoelectrical measurements and conductive scanning probe microscopy (SPM) applications.

Atomic Layer Deposition of Nickel Carbide from a Nickel Amidinate Precursor and Hydrogen Plasma

A new atomic layer deposition (ALD) process for depositing nickel carbide (Ni₃C _x) thin films is reported, using bis( N, N'-di- tert-butylacetamidinato)nickel(II) and H₂ plasma. The process shows a good layer-by-layer film growth behavior with a saturated film growth rate of 0.039 nm/cycle for a fairly wide process temperature window from 75 to 250 °C. Comprehensive material characterizations are performed on the Ni₃C _x films deposited at 95 °C with various H₂ plasma pulse lengths from 5 to 12 s, and no appreciable difference is found with the change of the plasma pulse length. The deposited Ni₃C _x films are fairly pure, smooth, and conductive, and the x in the nominal formula of Ni₃C _x is approximately 0.7. The ALD Ni₃C _x films are polycrystalline with a rhombohedral Ni₃C crystal structure, and the films are free of nanocrystalline graphite or amorphous carbon. Last, we demonstrate that, by using this ALD process, highly uniform Ni₃C _x films can be conformally deposited into deep narrow trenches with an aspect ratio as high as 20:1, which thereby highlights the broad and promising applicability of this process for conformal Ni₃C _x film coatings on complex high-aspect-ratio 3D architectures in general.

Role of Precursor Carbides for Graphene Growth on Ni(111)

Surface X-ray Diffraction was used to study the transformation of a carbon-supersaturated carbidic precursor toward a complete single layer of graphene in the temperature region below 703 K without carbon supply from the gas phase. The excess carbon beyond the 0.45  monolayers of C atoms within a single Ni2C layer is accompanied by sharpened reflections of the |4772| superstructure, along with ring-like diffraction features resulting from non-coincidence rotated Ni2C-type domains. A dynamic Ni2C reordering process, accompanied by slow carbon loss to subsurface regions, is proposed to increase the Ni2C 2D carbide long-range order via ripening toward coherent domains, and to increase the local supersaturation of near-surface dissolved carbon required for spontaneous graphene nucleation and growth. Upon transformation, the intensities of the surface carbide reflections and of specific powder-like diffraction rings vanish. The associated change of the specular X-ray reflectivity allows to quantify a single, fully surface-covering layer of graphene (2 ML C) without diffraction contributions of rotated domains. The simultaneous presence of top-fcc and bridge-top configurations of graphene explains the crystal truncation rod data of the graphene-covered surface. Structure determination of the |4772| precursor surface-carbide using density functional theory is in perfect agreement with the experimentally derived X-ray structure factors.

Catalytic CVD synthesis of boron nitride and carbon nanomaterials - synergies between experiment and theory

Low-dimensional carbon and boron nitride nanomaterials - hexagonal boron nitride, graphene, boron nitride nanotubes and carbon nanotubes - remain at the forefront of advanced materials research. Catalytic chemical vapour deposition has become an invaluable technique for reliably and cost-effectively synthesising these materials. In this review, we will emphasise how a synergy between experimental and theoretical methods has enhanced the understanding and optimisation of this synthetic technique. This review examines recent advances in the application of CVD to synthesising boron nitride and carbon nanomaterials and highlights where, in many cases, molecular simulations and quantum chemistry have provided key insights complementary to experimental investigation. This synergy is particularly prominent in the field of carbon nanotube and graphene CVD synthesis, and we propose here it will be the key to future advances in optimisation of CVD synthesis of boron nitride nanomaterials, boron nitride - carbon composite materials, and other nanomaterials generally.

Structure and Oxidation Behavior of Nickel Nanoparticles Supported by YSZ(111)

Nickel nanoparticles supported by the yttria-stabilized zirconia (111) surface show several preferential epitaxial relationships, as revealed by in situ X-ray diffraction. The two main nanoparticle orientations are found to have their [111] direction parallel to the substrate surface normal and ∼41.3 degrees tilted from this direction. The former orientation is described by a cube-on-cube stacking at the oxide-metal interface and the latter by a so-called coherent tilt strain-relieving mechanism, which is hitherto unreported for nanoparticles in literature. A modified Wulff construction used for the 111-oriented particles results in a value of the adhesion energy ranging from 1.4 to 2.2 Jm², whereby the lower end corresponds to more rounded particles and the upper to relatively flat geometries. Upon oxidation at 10^-3 Pa of molecular oxygen and 673 K, a NiO shell forms epitaxially on the [111]-oriented particles. Only a monolayer of metallic nickel of the top (111) facets oxidizes, whereas the side facets seem to react more severely. An apparent size increase of the remaining metallic Ni core is discussed in relation to a size-dependent oxidation mechanism, whereby smaller nanoparticles react at a faster rate. We argue that such a preferential oxidation mechanism, which inactivates the smallest and most reactive metal nanoparticles, might play a role for the long-term degradation of solid oxide fuel cells.

Some Like It Flat: Decoupled h-BN Monolayer Substrates for Aligned Graphene Growth

On the path to functional graphene electronics, suitable templates for chemical vapor deposition (CVD) growth of high-mobility graphene are of great interest. Among various substrates, hexagonal boron nitride (h-BN) has established itself as one of the most promising candidates. The nanomesh, a h-BN monolayer grown on the Rh(111) surface where the lattice mismatch of h-BN and rhodium leads to a characteristic corrugation of h-BN, offers an interesting graphene/h-BN interface, different from flat graphene/h-BN systems hitherto studied. In this report, we describe a two-step CVD process for graphene formation on h-BN/Rh(111) at millibar pressures and describe the influence of the surface texture on the CVD process. During a first exposure to the 3-pentanone precursor, carbon atoms are incorporated in the rhodium subsurface, which leads to decoupling of the h-BN layer from the Rh(111) surface. This is reflected in the electronic band structure, where the corrugation-induced splitting of the h-BN bands vanishes. In a second 3-pentanone exposure, a graphene layer is formed on the flat h-BN layer, evidenced by the appearance of the characteristic linear dispersion of its π band. The graphene layer grows incommensurate and highly oriented. The formation of graphene/h-BN on rhodium opens the door to scalable production of well-aligned heterostacks since single-crystalline thin-film Rh substrates are available in large dimensions.

Switchable graphene-substrate coupling through formation/dissolution of an intercalated Ni-carbide layer

Control over the film-substrate interaction is key to the exploitation of graphene’s unique electronic properties. Typically, a buffer layer is irreversibly intercalated “from above” to ensure decoupling. For graphene/Ni(111) we instead tune the film interaction “from below”. By temperature controlling the formation/dissolution of a carbide layer under rotated graphene domains, we reversibly switch graphene’s electronic structure from semi-metallic to metallic. Our results are relevant for the design of controllable graphene/metal interfaces in functional devices.

Simultaneous growth of monolayer graphene on Ni–Cu bimetallic catalyst by atmospheric pressure CVD process

CVD is the most efficient way to produce wafer scale monolayer graphene.

Creating a Nanospace under an h-BN Cover for Adlayer Growth on Nickel(111)

Heterostructures of two-dimensional (2D) atomic crystals have attracted increasing attention, while fabrication of the 2D stacking structures remains a challenge. In this work, we present a route toward formation of 2D heterostructures via confined growth of a 2D adlayer underneath the other 2D overlayer. Taking a hexagonal boron nitride (h-BN) monolayer on Ni(111) as a model system, both epitaxial and nonepitaxial h-BN islands have been identified on the Ni surface. Surface science studies combined with density functional theory calculations reveal that the nonepitaxial h-BN islands interact weakly with the Ni(111) surface, which creates a 2D nanospace underneath the h-BN islands. An additional h-BN or graphene layer can be grown in the space between the nonepitaxial h-BN islands and Ni(111) surface, forming h-BN/h-BN bilayer structures and h-BN/graphene heterostructures. These results suggest that confined growth under 2D covers may provide an effective route to obtain stacks of 2D atomic crystals.

Near-Ambient-Pressure X-ray Photoelectron Spectroscopy Study of Methane-Induced Carbon Deposition on Clean and Copper-Modified Polycrystalline Nickel Materials

In order to simulate solid-oxide fuel cell (SOFC)-related coking mechanisms of Ni, methane-induced surface carbide and carbon growth was studied under close-to-real conditions by synchrotron-based near-ambient-pressure (NAP) X-ray photoelectron spectroscopy (XPS) in the temperature region between 250 and 600 °C. Two complementary polycrystalline Ni samples were used, namely, Ni foam-serving as a model structure for bulk Ni in cermet materials such as Ni/YSZ-and Ni foil. The growth mechanism of graphene/graphite species was found to be closely related to that previously described for ethylene-induced graphene growth on Ni(111). After a sufficiently long "incubation" period of the Ni foam in methane at 0.2 mbar and temperatures around 400 °C, cooling down to ∼250 °C, and keeping the sample at this temperature for 50-60 min, initial formation of a near-surface carbide phase was observed, which exhibited the same spectroscopic fingerprint as the C₂H₄ induced Ni₂C phase on Ni(111). Only in the presence of this carbidic species, subsequent graphene/graphite nucleation and growth was observed. Vice versa, the absence of this species excluded further graphene/graphite formation. At temperatures above 400 °C, decomposition/bulk dissolution of the graphene/graphite phase was observed on the rather "open" surface of the Ni foam. In contrast, Ni foil showed-under otherwise identical conditions-predominant formation of unreactive amorphous carbon, which can only be removed at ≥500 °C by oxidative clean-off. Moreover, the complete suppression of carbide and subsequent graphene/graphite formation by Cu-alloying of the Ni foam and by addition of water to the methane atmosphere was verified.

Graphene growth and properties on metal substrates

Graphene-metal interface as one of the interesting graphene-based objects attracts much attention from both application and fundamental science points of view. This paper gives a timely review of the recent experimental works on the growth and the electronic properties of the graphene-metal interfaces. This work makes a link between huge amount of experimental and theoretical data allowing one to understand the influence of the metallic substrate on the electronic properties of a graphene overlayer and how its properties can be modified in a controllable way. The further directions of studies and applications of the graphene-metal interfaces are discussed.

Crystalline Ni3C as both carbon source and catalyst for graphene nucleation: a QM/MD study

Graphene nucleation from crystalline Ni₃C has been investigated using quantum chemical molecular dynamics (QM/MD) simulations based on the self-consistent-charge density-functional tight-binding (SCC-DFTB) method. It was observed that the lattice of Ni₃C was quickly relaxed upon thermal annealing at high temperature, resulting in an amorphous Ni₃C catalyst structure. With the aid of the mobile nickel atoms, inner layer carbon atoms precipitated rapidly out of the surface and then formed polyyne chains and Y-junctions. The frequent sinusoidal-like vibration of the branched carbon configurations led to the formation of nascent graphene precursors. In light of the rapid decomposition of the crystalline Ni₃C, it is proposed that the crystalline Ni₃C is unlikely to be a reaction intermediate in the CVD-growth of graphene at high temperatures. However, results present here indicate that Ni₃C films can be employed as precursors in the synthesis of graphene with exciting possibility.

Pentacene on Ni(111): room-temperature molecular packing and temperature-activated conversion to graphene

We investigate, using scanning tunnelling microscopy, the adsorption of pentacene on Ni(111) at room temperature and the behaviour of these monolayer films with annealing up to 700 °C. We observe the conversion of pentacene into graphene, which begins from as low as 220 °C with the coalescence of pentacene molecules into large planar aggregates. Then, by annealing at 350 °C for 20 minutes, these aggregates expand into irregular domains of graphene tens of nanometers in size. On surfaces where graphene and nickel carbide coexist, pentacene shows preferential adsorption on the nickel carbide phase. The same pentacene to graphene transformation was also achieved on Cu(111), but at a higher activation temperature, producing large graphene domains that exhibit a range of moiré superlattice periodicities.

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.

Graphene-nickel interfaces: a review

Graphene on nickel is a prototypical example of an interface between graphene and a strongly interacting metal, as well as a special case of a lattice matched system. The chemical interaction between graphene and nickel is due to hybridization of the metal d-electrons with the π-orbitals of graphene. This interaction causes a smaller separation between the nickel surface and graphene (0.21 nm) than the typical van der Waals gap-distance between graphitic layers (0.33 nm). Furthermore, the physical properties of graphene are significantly altered. Main differences are the opening of a band gap in the electronic structure and a shifting of the π-band by ∼2 eV below the Fermi-level. Experimental evidence suggests that the ferromagnetic nickel induces a magnetic moment in the carbon. Substrate induced geometric and electronic changes alter the phonon dispersion. As a consequence, monolayer graphene on nickel does not exhibit a Raman spectrum. In addition to reviewing these fundamental physical properties of graphene on Ni(111), we also discuss the formation and thermal stability of graphene and a surface-confined nickel-carbide. The fundamental growth mechanisms of graphene by chemical vapor deposition are also described. Different growth modes depending on the sample temperature have been identified in ultra high vacuum surface science studies. Finally, we give a brief summary for the synthesis of more complex graphene and graphitic structures using nickel as catalyst and point out some potential applications for graphene-nickel interfaces.

In situ observations of the atomistic mechanisms of Ni catalyzed low temperature graphene growth

The key atomistic mechanisms of graphene formation on Ni for technologically relevant hydrocarbon exposures below 600 °C are directly revealed via complementary in situ scanning tunneling microscopy and X-ray photoelectron spectroscopy. For clean Ni(111) below 500 °C, two different surface carbide (Ni2C) conversion mechanisms are dominant which both yield epitaxial graphene, whereas above 500 °C, graphene predominantly grows directly on Ni(111) via replacement mechanisms leading to embedded epitaxial and/or rotated graphene domains. Upon cooling, additional carbon structures form exclusively underneath rotated graphene domains. The dominant graphene growth mechanism also critically depends on the near-surface carbon concentration and hence is intimately linked to the full history of the catalyst and all possible sources of contamination. The detailed XPS fingerprinting of these processes allows a direct link to high pressure XPS measurements of a wide range of growth conditions, including polycrystalline Ni catalysts and recipes commonly used in industrial reactors for graphene and carbon nanotube CVD. This enables an unambiguous and consistent interpretation of prior literature and an assessment of how the quality/structure of as-grown carbon nanostructures relates to the growth modes.

Nanocomposite catalysts producing durable, super-black carbon nanotube systems: applications in solar thermal harvesting

A novel two-step approach for preparing carbon nanotube (CNT) systems, exhibiting an extraordinary combination of functional properties, is presented. It is based upon nanocomposite films consisting of metal (Me = Ni, Fe, Mo, Sn) nanoparticles embedded into diamond-like carbon (DLC). The main concept behind this approach is that DLC inhibits the growth of Me, resulting in the formation of small nanospheres instead of layers or extended grains. In the second step, DLC:Me substrates were used as catalyst templates for the growth of CNTs by the thermal chemical vapor deposition (T-CVD) process. X-ray photoelectron spectroscopy (XPS) has shown that at the T-CVD temperature of 700 °C DLC is completely graphitized and NiC is formed, making DLC:Ni a very effective catalyst for CNT growth. The catalyst layers and the CNT systems have been characterized with a wide range of analytical techniques such as Auger electron spectroscopy and X-ray photoelectron spectroscopy (AES/XPS), X-ray diffraction, reflectivity and scattering, Raman spectroscopy, scanning electron microscopy, atomic force microscopy, and optical and electrical testing. The produced CNTs are of excellent quality, without needing any further purification, durable, firmly attached to the substrate, and of varying morphology depending on the density of catalyst nanoparticles. The produced CNTs exhibit exceptional properties, such as super-hydrophobic surfaces (contact angle up to 165°) and exceptionally low optical reflection (reflectivity <10(-4)) in the entirety of the visible range. The combination of the functional properties makes these CNT systems promising candidates for solar thermal harvesting, as it is demonstrated by solar simulation experiments.

Yield and shape selection of graphene nanoislands grown on Ni(111)

The catalytic decomposition of hydrocarbons on transition-metal surfaces has attracted increasing interest as a method to prepare high quality graphene layers. Here, we study the optimal reaction path for the preparation of graphene nanoislands of selected shape using controlled decomposition of propene on Ni(111). Scanning tunneling microscopy performed at different stages of the reaction provides insight into the temperature and dose-dependent growth of graphene islands, which precedes the formation of monolayer graphene. The effect of postreaction annealing on the morphology of the islands is studied. By adjusting the initial propene dose, reaction temperature, and postannealing procedure, islands with a triangular or hexagonal shape can be selectively obtained.