Graphene-nickel interfaces: a review

Spin filtering by proximity effects at hybridized interfaces in spin-valves with 2D graphene barriers

We report on spin transport in state-of-the-art epitaxial monolayer graphene based 2D-magnetic tunnel junctions (2D-MTJs). In our measurements, supported by ab-initio calculations, the strength of interaction between ferromagnetic electrodes and graphene monolayers is shown to fundamentally control the resulting spin signal. In particular, by switching the graphene/ferromagnet interaction, spin transport reveals magneto-resistance signal MR > 80% in junctions with low resistance × area products. Descriptions based only on a simple K-point filtering picture (i.e. MR increase with the number of layers) are not sufficient to predict the behavior of our devices. We emphasize that hybridization effects need to be taken into account to fully grasp the spin properties (such as spin dependent density of states) when 2D materials are used as ultimately thin interfaces. While this is only a first demonstration, we thus introduce the fruitful potential of spin manipulation by proximity effect at the hybridized 2D material / ferromagnet interface for 2D-MTJs.

On-site growth method of 3D structured multi-layered graphene on silicon nanowires

An experimental method is described in which a orderly 3D array of graphene sheets is grown to conform to the shape of an underlying nanowire (NW) substrate that remains on-site. The procedure uses a sacrificial nickel catalyst-based CVD growth process that is capable of producing graphene onto an insulating SiO₂ substrate. Nano-imprint silicon NWs serve both as the scaffolding for the catalyst and as the final underlying substrate. The graphene is polycrystalline and multi-layered as expected from this nickel catalyzed growth method. This presents a novel and quick method that can be used to produce conductive graphene sheets in precise shapes and configurations seen in complex device applications but which are difficult to produce with current transfer methods. The geometry of the nanostructured substrate itself contributes to the on-site growth method by making it difficult for the graphene to wash off during wet etching. The SiNWs used in this research have increased surface area and a light trapping effect that, in combination with the graphene, can be used in future sensor and photovoltaic device applications.

Trends in the Change in Graphene Conductivity upon Gas Adsorption: The Relevance of Orbital Distortion

The experimental ability to alter graphene (G) conductivity by adsorption of a single gas molecule is promoting the development of ultra-high-sensitivity gas detectors and could ultimately provide a novel playground for future nanoelectronics devices. At present, the underpinning effect is broadly attributed to a variation of G carrier concentration, caused by an adsorption-induced Fermi-level shift. By means of first-principle Kubo-Greenwood calculations, here we demonstrate that adsorbate-induced orbital distortion could also lead to small but finite G conductivity changes, even in the absence of Fermi-level shifts. This mechanism enables a sound physical interpretation of the observed variable sensitivity of G devices to different chemical moieties, and it can be strongly enhanced by using a suitable Ni substrate, thereby opening new pathways for the optimal design of operational nanoscale detectors.

Graphene/Half-Metallic Heusler Alloy: A Novel Heterostructure toward High-Performance Graphene Spintronic Devices

Graphene-based vertical spin valves (SVs) are expected to offer a large magnetoresistance effect without impairing the electrical conductivity, which can pave the way for the next generation of high-speed and low-power-consumption storage and memory technologies. However, the graphene-based vertical SV has failed to prove its competence due to the lack of a graphene/ferromagnet heterostructure, which can provide highly efficient spin transport. Herein, the synthesis and spin-dependent electronic properties of a novel heterostructure consisting of single-layer graphene (SLG) and a half-metallic Co₂ Fe(Ge_0.5 Ga_0.5 ) (CFGG) Heusler alloy ferromagnet are reported. The growth of high-quality SLG with complete coverage by ultrahigh-vacuum chemical vapor deposition on a magnetron-sputtered single-crystalline CFGG thin film is demonstrated. The quasi-free-standing nature of SLG and robust magnetism of CFGG at the SLG/CFGG interface are revealed through depth-resolved X-ray magnetic circular dichroism spectroscopy. Density functional theory (DFT) calculation results indicate that the inherent electronic properties of SLG and CFGG such as the linear Dirac band and half-metallic band structure are preserved in the vicinity of the interface. These exciting findings suggest that the SLG/CFGG heterostructure possesses distinctive advantages over other reported graphene/ferromagnet heterostructures, for realizing effective transport of highly spin-polarized electrons in graphene-based vertical SV and other advanced spintronic devices.

Fabrication of patterned graphitized carbon wires using low voltage near-field electrospinning, pyrolysis, electrodeposition, and chemical vapor deposition

We herein report a high-resolution nanopatterning method using low voltage electromechanical spinning with a rotating collector to obtain aligned graphitized micro and nanowires for carbon nanomanufacturing. A small wire diameter and a small inter-wire spacing were obtained by controlling the electric field, the spinneret-to-collector distance, the pyrolysis parameters, the linear speed of the spinneret, the rotational speed of the collector. Using a simple scaling analysis, we show how the straightness and the diameter of the wires can be controlled by the electric field and the distance of the spinneret to the collector. A small inter-wire spacing, as predicted by a simple model, was achieved by simultaneously controlling the linear speed of the spinneret and the rotational speed of the collector. Rapid drying of the polymer nanowires enabled the facile fabrication of suspended wires over various structures. Patterned polyacrylonitrile wires were carbonized using standard stabilization and pyrolysis to obtain carbon nanowires. Suspended carbon nanowires with a diameter of <50 nm were obtained. We also established a method for making patterned, highly graphitized structures by using the aforementioned carbon wire structures as a template for chemical vapor deposition of graphite. This patterning technique offers high throughput for nano writing, which outperforms other existing nanopatterning techniques, making it a potential candidate for large-scale carbon nanomanufacturing.

Effects of a graphene substrate on the structure and properties of atomically thin metal sheets

The production and use of atomically thin metal sheets are desirable but challenging. Here, density functional theory calculations indicate that the introduction of graphene as a support can play an unexpected role in the stability and function of Rh monolayer, as a representative of single-layer metal nanosheets. The graphene stabilizes the otherwise unstable Rh monolayer by the substrate interaction that not only impedes the out-of-plane movement of the Rh atoms but also decreases the surface energy. The Rh/graphene bilayer has good mechanical properties, comparable to those of emerging 2D graphene-based materials. The interfacial stress from the substrate interaction causes surface corrugations to form on the bilayer, exhibiting a degree of consistency in the direction and the area. Discrete magnetic units, compatible with the substrate interaction, are present in the corrugated Rh sheet. The visible magnetic anisotropy and spin-splitting of polarized carrier states of the corrugated Rh sheet dominate the spin-dependent transport in the bilayer film, which can be used as a building block for ultrathin electronic/spintronic devices.

Room-temperature graphitization in a solid-phase reaction

Graphitized carbon including graphene has recently become one of the most investigated advanced materials for future device applications, but a prerequisite for broadening its range of applications is to lower its growth temperature. Here we report a great decrease in graphitization temperature using the well-known catalyst Ni. Amorphous carbon films with Ni nanoparticles (NPs) were deposited, using a simple one-step magnetron sputtering method, onto microgrids and a SiO₂/Si substrate for transmission electron microscopy (TEM) and Raman spectroscopy analyses, respectively. The amorphous carbon surroundings and locations between the Ni NPs started to become graphitized during the film deposition even at room temperature (RT) and 50 °C. The graphitization was confirmed by both high-resolution TEM (HR-TEM) and Raman 2D peak analyses. The increase in the relative amount of Ni in the amorphous carbon film led to the partial oxidation of the larger Ni NPs, resulting in less graphitization even at an elevated deposition temperature. Based on the detailed HR-TEM analyses, a decreased oxidation of NPs and enhanced solubility of carbon into Ni NPs were believed to be key for achieving low-temperature graphitization.

The spontaneous graphitization for C films containing Ni NPs was attributed mainly to the increased solubility for metallic Ni NPs, and was enhanced at the deposition temperature of 50 °C.

The synergistic influence of polyethyleneimine-grafted graphene oxide and iodide for the protection of steel in acidizing conditions

Herein, graphene oxide (GO) was chemically functionalized with polyethyleneimine (PEI) in a single step to obtain PEI-GO, which was characterized via FTIR spectroscopy, SEM, and TEM. Additionally, for the first time, PEI-GO was employed for the corrosion mitigation of carbon steel in a solution of 15% HCl. The corrosion performance of the inhibitor was evaluated by utilizing weight loss tests, electrochemical measurements with impedance analysis, electrochemical frequency modulation, and potentiodynamic polarization studies. Thorough surface analysis was performed using 3D profilometry and static water contact angle measurements. PEI-GO was adsorbed on the steel surface and showed mixed-type corrosion inhibition behavior with the prevalence of cathodic characteristics. Additionally, potassium iodide was incorporated in the acid solution as a synergistic agent to enhance the corrosion inhibition behavior of PEI-GO. The obtained results showed that PEI-GO alone provided a high corrosion inhibition efficiency of 88.24% at a temperature of 65 °C and in the presence of KI, it showed an I.E. of 95.77% due to their synergistic effect. These interesting results demonstrate that PEI-GO can act as a potential corrosion inhibitor in acidizing conditions. The DFT-based computational studies showed that the inhibitor functioned in both its neutral and protonated forms.

Herein, graphene oxide (GO) was chemically functionalized with polyethyleneimine (PEI) in a single step to obtain PEI-GO, which was characterized via FTIR spectroscopy, SEM, and TEM.

Modeling of One-Side Surface Modifications of Graphene

We model, with the use of the force field method, the dependence of mechanical conformations of graphene sheets, located on flat substrates, on the density of unilateral (one-side) attachment of hydrogen, fluorine or chlorine atoms to them. It is shown that a chemically-modified graphene sheet can take four main forms on a flat substrate: the form of a flat sheet located parallel to the surface of the substrate, the form of convex sheet partially detached from the substrate with bent edges adjacent to the substrate, and the form of a single and double roll on the substrate. On the surface of crystalline graphite, the flat form of the sheet is lowest in energy for hydrogenation density p < 0.21 , fluorination density p < 0.20 , and chlorination density p < 0.16 . For higher attachment densities, the flat form of the graphene sheet becomes unstable. The surface of crystalline nickel has higher adsorption energy for graphene monolayer and the flat form of a chemically modified sheet on such a substrate is lowest in energy for hydrogenation density p < 0.47 , fluorination density p < 0.30 and chlorination density p < 0.21 .

Functionalized Hybridization of 2D Nanomaterials

The discovery of graphene and subsequent verification of its unique properties have aroused great research interest to exploit diversified graphene-analogous 2D nanomaterials with fascinating physicochemical properties. Through either physical or chemical doping, linkage, adsorption, and hybridization with other functional species into or onto them, more novel/improved properties are readily created to extend/expand their functionalities and further achieve great performance. Here, various functionalized hybridizations by using different types of 2D nanomaterials are overviewed systematically with emphasis on their interaction formats (e.g., in-plane or inter plane), synergistic properties, and enhanced applications. As the most intensely investigated 2D materials in the post-graphene era, transition metal dichalcogenide nanosheets are comprehensively investigated through their element doping, physical/chemical functionalization, and nanohybridization. Meanwhile, representative hybrids with more types of nanosheets are also presented to understand their unique surface structures and address the special requirements for better applications. More excitingly, the van der Waals heterostructures of diverse 2D materials are specifically summarized to add more functionality or flexibility into 2D material systems. Finally, the current research status and faced challenges are discussed properly and several perspectives are elaborately given to accelerate the rational fabrication of varied and talented 2D hybrids.

Synthesis of edge-rich vertical multilayer graphene nanotube arrays towards high-performance supercapacitors

In this work, we demonstrate the synthesis of edge-rich vertical multilayer graphene nanotube arrays and edge density-dependent capacitance in a supercapacitor application. We employ Ni-Au multi-block vertical nanotubes fabricated by anodic aluminum oxide template-assisted electrodeposition as a designer substrate for multilayer graphene growth. This edge generation of graphene relies on the distinct carbon solubility of Au and Ni under chemical vapor deposition. Therefore the graphene edge density is tailorable by controlling the total number of bimetallic interfaces of alternating electrodeposited Ni and Au blocks. In supercapacitor applications, we found that the capacitance heavily correlates to the graphene edge densities. Multilayer graphene nanotubes with 18 bimetallic interfaces exhibit 8.4 times higher capacitance than those without interfaces. This experimental evaluation shows great promise to significantly enhance the supercapacitor capacitance by creating high-density edges on multilayer graphene.

Faraday-like Screening by Two-Dimensional Nanomaterials: A Scale-Dependent Tunable Effect

The modulation of electric fields by mono- or few-layer two-dimensional (2D) nanomaterials embodies a major challenge through vast technological areas, including 2D nanoscale electronics, ultrathin cable shielding, and nanostructured battery and supercapacitor electrodes. By a quantum-mechanical analysis of Faraday-like electrostatic screening due to diverse 2D nanolayers we demonstrate that electric field screening is triggered by charge response nonlocality. The effective screening factor is not only influenced by average polarizability but further exhibits nontrivial scalings with respect to surface distance: while ideal 2D metallic systems cause complete Faraday-cage screening, semimetallic graphene yields a finite, roughly scale-independent field reduction factor. Conversely, screening by finite-gap MoS₂ appears most effective in the vicinity of the surfaces, gradually vanishing in the long-distance limit because of the intrinsic finiteness of the charge-response length scale. The variability of screening effects and of their scaling laws with respect to accessible physical parameters opens novel pathways for experimental modulation of electric fields, ionic interactions, and adsorption of charged or polar moieties.

Highly Efficient Hybrid Ni/Nitrogenated Graphene Electrocatalysts for Hydrogen Evolution Reaction

Two nickel/nitrogenated graphene hybrid electrodes (Ni-NrGO NH3 and Ni-NrGO APTES ) were synthesized, and their catalytic activity with respect to the hydrogen evolution reaction (HER) in alkaline media was analyzed. Incorporation of nitrogen to the carbon structure in graphene oxide (GO) or reduced GO (rGO) flakes in aqueous solutions was carried out based on two different configurations. NrGO NH 3 particles were obtained by a hydrothermal method using ammonium hydroxide as the precursor, and NGO APTES particles were obtained by silanization (APTES functionalization) of GO sheets. Aqueous dispersions containing NrGO NH 3 and NGO APTES particles were added to the traditional nickel Watts plating bath in order to prepare the Ni-NrGO NH 3 and Ni-NrGO APTES catalysts, respectively. Nickel substrates were coated with the hybrid nickel electrodeposits and used as electrodes for hydrogen production. The Ni-NrGO catalysts show a higher activity than the conventional nickel electrodeposited electrodes, particularly the ones containing APTES molecules because they allow obtaining a hydrogen current density 130% higher than conventional Ni-plated electrodes with a Watts bath in the absence of additives. In addition, both catalysts show a low deactivation rate during the ageing treatment, which is a sign of a longer midlife for the catalyst. Cyclic voltammetry and electrochemical impedance spectroscopy measurements were used for examination of the catalytic efficiency of hybrid Ni-NrGO electrodes for HER in KOH solution. High values of exchange current densities, 8.53 × 10-4 and 2.53 × 10-5 mA cm-2 for HER in alkaline solutions on Ni-NrGO NH 3 and Ni-NrGO APTES electrodes, respectively, were obtained.

Carbon Nanotubes and Related Nanomaterials: Critical Advances and Challenges for Synthesis toward Mainstream Commercial Applications

Advances in the synthesis and scalable manufacturing of single-walled carbon nanotubes (SWCNTs) remain critical to realizing many important commercial applications. Here we review recent breakthroughs in the synthesis of SWCNTs and highlight key ongoing research areas and challenges. A few key applications that capitalize on the properties of SWCNTs are also reviewed with respect to the recent synthesis breakthroughs and ways in which synthesis science can enable advances in these applications. While the primary focus of this review is on the science framework of SWCNT growth, we draw connections to mechanisms underlying the synthesis of other 1D and 2D materials such as boron nitride nanotubes and graphene.

Magnetically Recoverable Catalysts: Beyond Magnetic Separation

Here, we discuss several important aspects of magnetically recoverable catalysts which can be realized when magnetic oxide nanoparticles are exposed to catalytic species and catalytic reaction media. In such conditions magnetic oxides can enhance performance of catalytic nanoparticles due to (i) electronic effects, (ii) catalyzing reactions which are beneficial for the final reaction outcome, or (iii) providing a capacity to dilute catalytic metal oxide species, leading to an increase of oxygen vacancies. However, this approach should be used when the magnetic oxides are stable in reaction conditions and do not promote side reactions. Incorporation of another active component, i.e., a graphene derivative, in the magnetically recoverable catalyst constitutes a smart design of a catalytic system due to synergy of its components, further enhancing catalytic properties.

Rigorous and Accurate Contrast Spectroscopy for Ultimate Thickness Determination of Micrometer-Sized Graphene on Gold and Molecular Sensing

The thickness of graphene films can be accurately determined by optical contrast spectroscopy. However, this becomes challenging and complicated when the flake size reduces to the micrometer scale, where the contrast spectrum is sensitively dependent on the polarization and incident angle of light. Here, we report accurate measurement of the optical contrast spectra of micrometer-sized few-layer graphene flakes on Au substrate. Using a high-resolution optical microscopy with a 100× magnification objective, we accurately determined the layer numbers of flakes as small as one micrometer in lateral size. We developed a theoretical model to accurately take into account the appropriate contribution of light incident at various angles and polarizations, which matched the experimental results extremely well. Furthermore, we demonstrate that the optical contrast spectroscopy is highly sensitive to detect the adsorption of submonolayer airborne hydrocarbon molecules, which can reveal whether graphene is contaminated. Though the technique was demonstrated on graphene, it can be readily generalized to many other two-dimensional materials, which opens new avenues for developing miniaturized and ultrasensitive label-free molecular sensors.

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.

Growth of Ni nanoclusters on irradiated graphene: a molecular dynamics study

We studied the soft landing of Ni atoms on a previously damaged graphene sheet by means of molecular dynamics simulations. We found a monotonic decrease of the cluster frequency as a function of its size, but few big clusters comprise an appreciable fraction of the total number of Ni atoms. The aggregation of Ni atoms is also modeled by means of a simple phenomenological model. The results are in clear contrast with the case of hard or energetic landing of metal atoms, where there is a tendency to form mono-disperse metal clusters. This behavior is attributed to the high diffusion of unattached Ni atoms, together with vacancies acting as capture centers. The findings of this work show that a simple study of the energetics of the system is not enough in the soft landing regime, where it is unavoidable to also consider the growth process of metal clusters.

Origin of the moiré superlattice scale lateral force modulation of graphene on a transition metal substrate

The moiré superlattice formed between graphene and a transition metal substrate is capable of tuning the frictional properties of graphene. For instance, a moiré superlattice scale modulation on the lateral force will be experienced by the tip of an atomic force microscope (AFM). However, the origin of this long-range force modulation still needs to be clarified. In this study, density functional theory (DFT) calculations have been carried out to investigate the indentation processes of a one-Ar-atom tip and a 10-atom Ir tip, sliding on graphene/Re(0001) and graphene/Pt(111) moiré superlattices, respectively. The calculation results indicate that the interfacial interaction between graphene and a transition metal substrate determines the morphological corrugation of graphene and the characteristics of the lateral force modulation. Moreover, when the tip-graphene interaction is strong enough, it will influence the evolutions of the adsorption energy Ead and tip sliding trajectory. Thus, the moiré superlattice scale lateral force modulation of graphene on a transition metal substrate originates from the joint effects of the graphene-substrate interfacial interaction and tip-graphene interaction.

Insulator-to-Metallic Spin-Filtering in 2D-Magnetic Tunnel Junctions Based on Hexagonal Boron Nitride

We report on the integration of atomically thin 2D insulating hexagonal boron nitride (h-BN) tunnel barriers into magnetic tunnel junctions (2D-MTJs) by fabricating two illustrative systems (Co/h-BN/Co and Co/h-BN/Fe) and by discussing h-BN potential for metallic spin filtering. The h-BN is directly grown by chemical vapor deposition on prepatterned Co and Fe stripes. Spin-transport measurements reveal tunnel magneto-resistances in these h-BN-based MTJs as high as 12% for Co/h-BN/h-BN/Co and 50% for Co/h-BN/Fe. We analyze the spin polarizations of h-BN/Co and h-BN/Fe interfaces extracted from experimental spin signals in light of spin filtering at hybrid chemisorbed/physisorbed h-BN, with support of ab initio calculations. These experiments illustrate the strong potential of h-BN for MTJs and are expected to ignite further investigations of 2D materials for large signal spin devices.

Tuning ultrafast electron injection dynamics at organic-graphene/metal interfaces

We compare the ultrafast charge transfer dynamics of molecules on epitaxial graphene and bilayer graphene grown on Ni(111) interfaces through first principles calculations and X-ray resonant photoemission spectroscopy. We use 4,4'-bipyridine as a prototypical molecule for these explorations as the energy level alignment of core-excited molecular orbitals allows ultrafast injection of electrons from a substrate to a molecule on a femtosecond timescale. We show that the ultrafast injection of electrons from the substrate to the molecule is ∼4 times slower on weakly coupled bilayer graphene than on epitaxial graphene. Through our experiments and calculations, we can attribute this to a difference in the density of states close to the Fermi level between graphene and bilayer graphene. We therefore show how graphene coupling with the substrate influences charge transfer dynamics between organic molecules and graphene interfaces.

Large local lattice expansion in graphene adlayers grown on copper

Variations of the lattice parameter can significantly change the properties of a material, and, in particular, its electronic behaviour. In the case of graphene, however, variations of the lattice constant with respect to graphite have been limited to less than 2.5% due to its well-established high in-plane stiffness. Here, through systematic electronic and lattice structure studies, we report regions where the lattice constant of graphene monolayers grown on copper by chemical vapour deposition increases up to ~7.5% of its relaxed value. Density functional theory calculations confirm that this expanded phase is energetically metastable and driven by the enhanced interaction between the substrate and the graphene adlayer. We also prove that this phase possesses distinctive chemical and electronic properties. The inherent phase complexity of graphene grown on copper foils revealed in this study may inspire the investigation of possible metastable phases in other seemingly simple heterostructure systems.

Spray-Drying-Assisted Layer-by-Layer Assembly of Alginate, 3-Aminopropyltriethoxysilane, and Magnesium Hydroxide Flame Retardant and Its Catalytic Graphitization in Ethylene-Vinyl Acetate Resin

Alginates (nickel alginate, NiA; copper alginate, CuA; zinc alginate, ZnA) and 3-aminopropyltriethoxysilane (APTES) were alternately deposited on a magnesium hydroxide (MH) surface by the spray-drying-assisted layer-by-layer assembly technique, fabricating some efficient and environmentally benign flame retardants (M-FR, including Ni-FR, Cu-FR, and Zn-FR). The morphology, chemical compositions, and structures of M-FR were investigated. With 50 wt % loading, compared with EVA28+MH, the peak heat release rate, smoke production rate, and CO production rate of EVA28+Ni-FR decreased by 50.78%, 61.76%, and 66.67%, respectively. The metals or metal oxide nanoparticles arising from alginates could catalyze the pyrolysis intermediates of EVA into graphene and amorphous carbon, which could bind the inorganic compounds (the decomposition products of MH and APTES) together and form some more protective barriers. For each M-FR, the flame retardant and smoke suppression efficiency were different, which were caused by the diverse carbonization and graphitization behaviors of three alginates. ZnA generated some ZnO aggregations and could not catalyze the graphitization of intermediates. For CuA, the catalytic graphitization was limited by the tightly binding graphene layer. As for NiA, the configuration of the Ni atom could not provide strong binding of Ni substrate and carbon. The liquid-like Ni nanoparticles could restructure and get out from firm graphene shells, so the catalytic graphitization of NiA was efficient and sustainable. This work displayed the catalytic graphitization mechanism of alginates while exploring a simple and novel strategy for fabricating efficient green flame retardants.

CVD Graphene/Ni Interface Evolution in Sulfuric Electrolyte

Systems comprising single and multilayer graphene deposited on metals and immersed in acid environments have been investigated, with the aim of elucidating the mechanisms involved, for instance, in hydrogen production or metal protection from corrosion. In this work, a relevant system, namely chemical vapor deposited (CVD) multilayer graphene/Ni (MLGr/Ni), is studied when immersed in a diluted sulfuric electrolyte. The MLGr/Ni electrochemical and morphological properties are studied in situ and interpreted in light of the highly oriented pyrolytic graphite (HOPG) electrode behavior, when immersed in the same electrolyte. Following this interpretative framework, the dominant role of the Ni substrate in hydrogen production is clarified.

Bimetallic junction mediated synthesis of multilayer graphene edges towards ultrahigh capacity for lithium ion batteries

In this work, we report on a novel strategy to synthesize high-density graphene edges on a vertically-aligned nanorod array substrate based on multiple segmented Ni-Au units. The growth of graphene layers on Ni and Au was performed by chemical vapor deposition (CVD) leading to the effective generation of edge-rich multilayer graphene due to the distinct carbon solubility. The composite material was applied as an anode in a lithium ion battery (LIB) whose discharging capacity was found to closely depend on the total number of Ni-Au junctions within the vertical nanorods. Graphene deposited on the 19-junction composite Ni-(Au-Ni)₉ exhibited an ultrahigh capacity of 86.3 μAh cm^-2 at 50 μA cm^-2 which was much higher than graphene deposited on 1-junction, 2-junction and pure Ni nanorods. This ultrahigh capacity was mainly ascribed to the generation of high-density graphene edges engineered by the bimetallic junction. The proposed strategy opens new appealing routes to synthesize high-density graphene edges using bimetallic junctions, which is promising for increasing the performance of LIBs and other electrochemical energy systems (supercapacitors, fuel cells, etc.).

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.

Tune the chemical activity of graphene via the transition metal substrate

To achieve the chemical modification of graphene efficiently is desirable and essential to promote the technological applications of graphene. In this study, the density functional theory (DFT) calculations have been carried out to investigate the hydrogenation and fluorination activities of graphene on Ni(111), Re(0001) and Pt(111). The calculation results indicate that the chemical activity of graphene is related to both the characteristics of the graphene–substrate interfacial interaction and the local atomic stacking, namely the chemical activity of graphene is position-dependent. The strong covalently interacting substrates Ni(111) and Re(0001) will remarkably enhance the chemical activity of graphene, while the modulation effects from the weak van der Waals interacting substrate Pt(111) is trivial. Electronic structure studies reveal that the intensive graphene–substrate interfacial interaction can gain in chemical energy to offset the strain energy caused by the C atom sp²–sp³ transition and stabilize the absorbing state.

The chemical activity of graphene can be tuned by the transition metal substrate.

Atomic layer deposited oxide films as protective interface layers for integrated graphene transfer

The transfer of chemical vapour deposited graphene from its parent growth catalyst has become a bottleneck for many of its emerging applications. The sacrificial polymer layers that are typically deposited onto graphene for mechanical support during transfer are challenging to remove completely and hence leave graphene and subsequent device interfaces contaminated. Here, we report on the use of atomic layer deposited (ALD) oxide films as protective interface and support layers during graphene transfer. The method avoids any direct contact of the graphene with polymers and through the use of thicker ALD layers (≥100 nm), polymers can be eliminated from the transfer-process altogether. The ALD film can be kept as a functional device layer, facilitating integrated device manufacturing. We demonstrate back-gated field effect devices based on single-layer graphene transferred with a protective Al₂O₃ film onto SiO₂ that show significantly reduced charge trap and residual carrier densities. We critically discuss the advantages and challenges of processing graphene/ALD bilayer structures.

Theory of Finite-Length Grain Boundaries of Controlled Misfit Angle in Two-Dimensional Materials

Grain boundaries in two-dimensional crystals are usually thought to separate distinct crystallites and as such they must either form closed loops or terminate at the boundary of a sample. However, when an atomically thin two-dimensional crystal grows on a substrate of nonzero Gaussian curvature, it can develop finite-length grain boundaries that terminate abruptly within a monocrystalline domain. We show that by properly designing the substrate topography, these grain boundaries can be placed at desired locations and at specified misfit angles, as the thermodynamic ground state of a two-dimensional (2D) system bound to a substrate. Compared against the hypothetical competition of growing defectless 2D materials on a Gaussian-curved substrate with consequential fold development or detachment from the substrate, the nucleation and formation of finite-length grain boundaries can be made energetically favorably given sufficient substrate adhesion on the order of tens of meV/Ų for typical 2D materials. New properties specific to certain grain boundary geometries, including magnetism and metallicity, can thus be engineered into 2D crystals through topographic design of their substrates.

From Growth Surface to Device Interface: Preserving Metallic Fe under Monolayer Hexagonal Boron Nitride

We investigate the interfacial chemistry between Fe catalyst foils and monolayer hexagonal boron nitride (h-BN) following chemical vapor deposition and during subsequent atmospheric exposure, using scanning electron microscopy, X-ray photoemission spectroscopy, and scanning photoelectron microscopy. We show that regions of the Fe surface covered by h-BN remain in a metallic state during exposure to moist air for ∼40 h at room temperature. This protection is attributed to the strong interfacial interaction between h-BN and Fe, which prevents the rapid intercalation of oxidizing species. Local Fe oxidation is observed on bare Fe regions and close to defects in the h-BN film (e.g., domain boundaries, wrinkles, and edges), which over the longer-term provide pathways for slow bulk oxidation of Fe. We further confirm that the interface between h-BN and metallic Fe can be recovered by vacuum annealing at ∼600 °C, although this is accompanied by the creation of defects within the h-BN film. We discuss the importance of these findings in the context of integrated manufacturing and transfer-free device integration of h-BN, particularly for technologically important applications where h-BN has potential as a tunnel barrier such as magnetic tunnel junctions.

Graphene and Graphene Analogs toward Optical, Electronic, Spintronic, Green-Chemical, Energy-Material, Sensing, and Medical Applications

This spotlight discusses intriguing properties and diverse applications of graphene (Gr) and Gr analogs. Gr has brought us two-dimensional (2D) chemistry with its exotic 2D features of density of states. Yet, some of the 2D or 2D-like features can be seen on surfaces and at interfaces of bulk materials. The substrate on Gr and functionalization of Gr (including metal decoration, intercalation, doping, and hybridization) modify the unique 2D features of Gr. Despite abundant literature on physical properties and well-known applications of Gr, spotlight works based on the conceptual understanding of the 2D physical and chemical nature of Gr toward vast-ranging applications are hardly found. Here we focus on applications of Gr, based on conceptual understanding of 2D phenomena toward 2D chemistry. Thus, 2D features, defects, edges, and substrate effects of Gr are discussed first. Then, to pattern Gr electronic circuits, insight into differentiating conducting and nonconducting regions is introduced. By utilizing the unique ballistic electron transport properties and edge spin states of Gr, Gr nanoribbons (GNRs) are exploited for the design of ultrasensitive molecular sensing electronic devices (including molecular fingerprinting) and spintronic devices. The highly stable nature of Gr can be utilized for protection of corrosive metals, moisture-sensitive perovskite solar cells, and highly environment-susceptible topological insulators (TIs). Gr analogs have become new types of 2D materials having novel features such as half-metals, TIs, and nonlinear optical properties. The key insights into the functionalized Gr hybrid materials lead to the applications for not only energy storage and electrochemical catalysis, green chemistry, and electronic/spintronic devices but also biosensing and medical applications. All these topics are discussed here with the focus on conceptual understanding. Further possible applications of Gr, GNRs, and Gr analogs are also addressed in a section on outlook and future challenges.

The magnetic proximity effect and electrical field tunable valley degeneracy in MoS2/EuS van der Waals heterojunctions

We report the magnetic proximity effect (MPE) and valley non-degeneracy in monolayer MoS₂ and magnetic semiconductor EuS thin film heterojunctions studied by density functional theory (DFT) with the vdW-DF2 correlations. Magnetic moments are observed in MoS₂ due to the MPE when forming chemical or van der Waals (vdW) adsorption states with EuS. Spin-orbit coupling (SOC) leads to observable valley non-degeneracy of MoS₂ at the K (K') points in the Brillouin zone. The valley Zeeman splitting energy E_z can reach 5.1 meV and 37.3 meV for the vdW and chemical adsorption states, corresponding to a magnetic exchange field (MEF) of 22 T and 160 T respectively. By applying a gate voltage across the MoS₂/EuS interface, it is found that E_z can be tuned from 1.8 meV to 8.2 meV and from 24.5 meV to 53.8 meV for vdW and chemical adsorption states respectively. The strong MPE, large and tunable valley degeneracy in 2D material and ferromagnetic semiconductor/insulator vdW heterojunctions demonstrate their promising potential for novel optoelectronic and valleytronic device applications.

Raman Spectroscopy of Lattice-Matched Graphene on Strongly Interacting Metal Surfaces

Regardless of the widely accepted opinion that there is no Raman signal from single-layer graphene when it is strongly bonded to a metal surface, we present Raman spectra of a graphene monolayer on Ni(111) and Co(0001) substrates. The high binding energy of carbon to these surfaces allows formation of lattice-matched (1 × 1) structures where graphene is significantly stretched. This is reflected in a record-breaking shift of the Raman G band by more than 100 cm^-1 relative to the case of freestanding graphene. Using electron diffraction and photoemission spectroscopy, we explore the aforementioned systems together with polycrystalline graphene on Co and analyze possible intercalation of oxygen at ambient conditions. The results obtained are fully supported by Raman spectroscopy. Performing a theoretical investigation of the phonon dispersions of freestanding graphene and stretched graphene on the strongly interacting Co surface, we explain the main features of the Raman spectra. Our results create a reliable platform for application of Raman spectroscopy in diagnostics of chemisorbed graphene and related materials.

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.

Ammonia Generation via a Graphene-Coated Nickel Catalyst

A novel graphene-coated Ni electrode was developed in this investigation to improve corrosion resistance while unexpectedly enhancing the ammonia generation rate in the electrochemically induced urea to ammonia (eU2A) process, which is an electrochemical onsite ammonia generation method. The development of the electrode is crucial for the eU2A reactions since in the ammonia generation process, the concentration of ammonia is inevitably high on the surface of the electrode, leading to severe corrosion of the electrode and the loss of generated ammonia as well. In this paper, the graphene was derived from raw coal by using the chemical vapor deposition method and self-lifted onto a Ni electrode to form a protective layer for corrosion prevention. Transmission electron microscopy showed the synthesized graphene had few-layers and Raman spectroscopy indicated that the coating of graphene was stable during the eU2A reaction. As a result, the ammonia corrosion of the Ni electrode was dramatically reduced by ~20 times with the graphene coating method. More importantly, a higher ammonia generation rate (~2 times) was achieved using the graphene-coated Ni working electrode compared to a bare Ni electrode in the eU2A process.

Microscopic Evidence for Strong Interaction between Pd and Graphene Oxide that Results in Metal-Decoration-Induced Reduction of Graphene Oxide

Graphene oxide (GO) is reduced spontaneously when palladium nanoparticles are decorated on the surface. The oxygen functional groups at the GO surface near the nanoparticles are absorbed to the palladium to produce a palladium oxide interlayer. Palladium therefore grows on the GO with preferred orientations, resulting in unique microstructural and electrical properties.