Large Graphene Research Articles

Architectures of Graphene-Based Field-Effect Transistors for Single-Molecule Experiments

With progress in the extreme miniaturisation of electronic components and the discovery of low-dimensional conductive materials, it is now possible to assemble field-effect transistors (FETs) that can incorporate single-molecule components as a channel or gate1,2. Such types of FETs have been recently used to detect and study various fundamental mechanisms at the single-molecule scale, among which the folding and unfolding of molecules, hybridization mechanisms, charge transport or chemical reactions1–5. In these experiments, devices were typically fabricated using architectures based on individual 1D materials, such as carbon nanotubes (CNTs) and silicon nanowires. The 1D topology facilitates the isolation of individual molecules in the circuit, but present drawbacks in scalability due to challenges in the growth, purification and/or assembling of such 1D materials into FET circuits. Here, we present new top-down approaches for the fabrication of single-molecule FETs, based on 2D graphene architectures. As CNTs, graphene is made of an hexagonal carbon lattice enabling excellent conductivity as well as carbon-based chemistry to anchor individual molecules, yet its 2D topology is more compatible with wafer-scale fabrication processes. First, we report the fabrication of large arrays of FETs based on graphene ribbons with controlled electrical properties. These arrays were built from high-quality large area graphene synthesized by chemical vapor deposition (CVD), followed by patterning steps using photolithography and plasma etching processes. Then, we report the design of two different architectures for single-molecule experiments: nanoconstrictions and nanogaps. Nanoconstrictions were achieved using electron-beam lithography (EBL), allowing to pattern high-resolution features (50nm) in the graphene channel. Nanogaps were obtained using the electroburning technique to open a gap of a few nanometers in the graphene channel6. We will present the design and fabrication process of these architectures, as well as their characterization using high-resolution microscopy (SEM/AFM) and transport measurements. Finally, we will discuss approached for the single-molecule functionalization for these architectures and their application in conductance-based single-molecule measurements.

Dynamic electron transfer for reducing nanofriction of graphene at electrified interfaces

Nanofriction properties of graphene at electrified interfaces are important for the application of graphene as a solid lubricant in graphene-based electric-carrying interfaces. The nanofriction experiments based on conductive atomic force microscopy show that dynamic electron transfer determines nanofriction of graphene at the electrified interfaces. Nanofriction at highly conductive interfaces remain constant because the fast electron transfer decreases the electrical potential difference. However, nanofriction at low conductive interfaces increases because electron blocking increases the electrical potential difference. The nanofriction of graphene correlated negatively with loads at low conductive interfaces because high loads decrease the contact resistance to release blocked electrons. Furthermore, large graphene possesses higher nanofriction than small graphene. Scanning Kelvin force microscopy measurements show that the larger electron capacity of large graphene can maintain the electrical potential difference at interfaces. These studies provide beneficial guidelines for the applications of graphene as a solid lubricant at electrified interfaces.

Fabrication of high-strength PET fibers modified with graphene oxide of varying lateral size

The large lateral-sized graphene oxide (LGO) was successfully broken into medium- and small lateral-sized graphene oxide (MGO and SGO, respectively), using a self-designed tubular ultrasonic instrument and other different techniques such as centrifugation, ultrasonication, and freeze-drying. Nanocomposite fibers composed of polyester (PET) material modified with various lateral-sized graphene oxide (GO) were prepared by twin-rotor continuous extrusion and melt spinning. The effects of high-speed mixing and GO lateral size on the mechanical and thermal properties of the composite samples were investigated. Compared to the conventional twin-screw extruder, a twin-rotor continuous extruder achieves a more uniform distribution of GO in the PET matrix. Furthermore, the use of SGO yields the best dispersion and interfacial compatibility properties. It also promotes crystallization, which confirms that, compared to LGO and MGO, SGO plays a more important role in the heterogeneous nucleation of PET. Among the investigated samples, PET/SGO fibers loaded with 0.1 wt% SGO show the highest tensile strength (890 MPa) and Young's modulus (30 GPa). Therefore, the high-speed mixing of PET with nanometer-sized SGO using a twin-rotor continuous extruder has broad application prospects in the field of GO-reinforced polymers, on an industrial scale.

Chemically synthesized graphene as a precursor to Prussian blue-based nanocomposite: A multifunctional material for transparent aqueous K-ion battery or electrochromic device

This work describes the preparation of a novel graphene/Prussian blue nanocomposite thin and transparent film and its application as both electrochromic material and cathode for transparent K-ion battery operating in aqueous electrolyte. Large graphene sheets were chemically synthesized from benzene, at a water/benzene liquid/liquid interface, using solid iron chloride as both oxidizing and catalyst, yielding a transparent film stabilized at the liquid/liquid interface that can be easily transferred to desirable substrates. We took advantage of the different iron species remaining as a side-product of the synthesis to promote the Prussian blue growing, through a heterogeneous electrochemical reaction between those iron species in the film and ferrocyanide ions in the electrolyte aqueous solution. Nanocubes of Prussian blue grow over the graphene and cover the entire substrate, yielding a bluish, homogeneous and transparent graphene/Prussian blue multifunctional nanocomposite, which was characterized by different techniques. Finally, we demonstrate two possibilities of a real application of this nanocomposite thin film, as cathodic material for aqueous K+-ion transparent batteries, presenting capacities up to 141 mAh g−1 at 0.093 A g−1, and as electrode in an electrochromic device paired with WO3 and using an aqueous KCl solution as electrolyte, presenting very stable optical variation through 200 electrochromic cycles.

Graphene based plasma-wave devices for terahertz applications

Unique properties of graphene are combined to enable graphene plasmonic devices that could revolutionize the terahertz (THz) electronic technology. A high value of the carrier mobility allows us to excite resonant plasma waves. The graphene bipolar nature allows for different mechanisms of plasma wave excitation. Graphene bilayer and multilayer structures make possible improved THz device configurations. The ability of graphene to form a high quality heterostructure with h-BN, black phosphorus, and other materials systems supports advanced heterostructure devices comprised of the best properties of graphene and other emerging materials. In particular, using black phosphorus compounds for cooling electron–hole plasma in graphene could dramatically improve the conditions for THz lasing. High optical phonon energy allows for reaching higher plasma frequencies that are supported by high sheet carrier densities in graphene. Recent improvements in graphene technology combined with a better understanding of the device physics of graphene THz plasmonics and graphene plasmonic device designs hold promise to make graphene THz plasmonic technology one of the key graphene applications. Commercialization of plasmonic graphene technology is facing the same challenges as other graphene applications, which have difficulties in producing uniform large graphene layers, bilayers, and heterostructures of high quality and making good low resistance stable Ohmic contacts. The time projection for large scale graphene electronic device applications now extends into the 2030s. However, emerging graphene mass production technologies might bring commercial applications of the graphene plasmonic terahertz technology closer.

Effects of precursors’ purity on graphene quality: Synthesis and thermoelectric effect

A chemical vapor deposition method has been proven to produce large scale monolayer graphene. However, it is often reported that such graphene contains a varying amount of defects. In this work, methane precursors of different purities [99.99% (4-9G), 99.90% (3-9G), and 98.90% (2-9G)] were used. It is shown that the introduction of defects occurs during graphene growth. It has been attributed to the presence of trace oxygen molecules in the gas precursors. By controlling the amount of oxygen present, one is able to tune the defect density in graphene at will. It is purported that the oxygen reacts with methane to yield methanol and formaldehyde. The latter is oxidized to formic acid. As the graphene network expands, the alcohol and formic acid are incorporated as C—O and O—C=O functional groups. In turn, the graphene experiences an overall global tensile strain due to local bond distortion induced by the electronegative oxygen containing groups. Furthermore, the presence of N2 molecules impedes the proper coalescing of carbon-containing molecules for the formation of the sp2-rich carbon network. Electrical measurement conducted suggests that a high purity precursor (4-9G) induces the least amount of defects, which confers a high Seebeck coefficient (105.1 µV/K) and a low sheet resistance (58.3 Ω). In the case of a larger volume of oxygen in the precursor, electrical performance decreases generally.

Rapid growth of large area graphene on glass from olive oil by laser irradiation

Although homogeneous, high quality graphene can be fabricated on a Cu or Ni sheet using the traditional chemical vapour deposition method at high temperatures (over 1000 °C) under specific atmospheric conditions, their transfer to another substrate is difficult. In this paper a novel method of rapidly (i.e. 3–6 s of laser irradiation) producing a large area (>3 cm2) graphene film from olive oil on a glass surface (pre-coated with a 5–28 nm thick Ni film) with defocused, large area continuous laser irradiation is described. The turbostratic graphene film (6 layers) grown in such a way has shown high electrical conductivity (sheet resistance of around 20 Ω sq−1) and an optical transmittance of 40–50%. With femtosecond laser patterning, 70% optical transparency was demonstrated. Continuous large area graphene was formed at relatively lower temperatures (<250 °C) and without the need for specific atmospheric conditions. The basic process characteristics and mechanisms involved are discussed.

Growth of Single-Layer and Multilayer Graphene on Cu/Ni Alloy Substrates

ConspectusGraphene, a one-atom-thick layer of carbon with a honeycomb lattice, has drawn great attention due to its outstanding properties and its various applications in electronic and photonic devices. Mechanical exfoliation has been used for preparing graphene flakes (from monolayer to multilayer with thick pieces also typically present), but with sizes limited typically to less than millimeters, its usefulness is limited. Chemical vapor deposition (CVD) has been shown to be the most effective technique for the scalable preparation of graphene films with high quality and uniformity. To date, CVD growth of graphene on the most commonly used substrates (Cu and Ni foils) has been demonstrated and intensively studied. However, a survey of the existing literature and earlier work using Cu or Ni substrates for CVD growth indicates that the bilayer and multilayer graphene over a large area, particularly single crystals, have not been obtained.In this Account, we review current progress and development in the CVD growth of graphene and highlight the important challenges that need to be addressed, for example, how to achieve large single crystal graphene films with a controlled number of layers. A single-layer graphene film grown on polycrystalline Cu foil was first reported by our group, and since then various techniques have been devoted to achieving the fast growth of large-area graphene films with high quality. Commercially available Cu/Ni foils, sputtered Cu/Ni thin films, and polycrystalline Cu/Ni foils have been used for the CVD synthesis of bilayer, trilayer, and multilayer graphene. Cu/Ni alloy substrates are particularly interesting due to their greater carbon solubility than pure Cu substrates and this solubility can be finely controlled by changing the alloy composition. These substrates with controlled compositions have shown the potential for the growth of layer-tunable graphene films in addition to providing a much higher growth rate due to their stronger catalytic activity. However, the well-controlled preparation of single crystal graphene with a defined number of layers on Cu/Ni substrates is still challenging.Due to its small lattice mismatch with graphene, a single crystal Cu(111) foil has been shown to be an ideal substrate for the epitaxial growth of graphene. Our group has reported the synthesis of large-size single crystal Cu(111) foils by the contact-free annealing of commercial Cu foils, and single crystal Cu/Ni(111) alloy foils have also been obtained after the heat-treatment of Ni-coated Cu(111) foils. The use of these single crystal foils (especially the Cu/Ni alloy foils) as growth substrates has enabled the fast growth of single crystal single-layer graphene films. By increase of the Ni content, single crystal bilayer, trilayer, and even multilayer graphene films have been synthesized. In addition, we also discuss the wafer-scale growth of single-layer graphene on the single crystalline Cu/Ni(111) thin films.Recent research results on the large-scale preparation of single crystal graphene films with different numbers of layers on various types of Cu/Ni alloy substrates with different compositions are reviewed and discussed in detail. Despite the remarkable progress in this field, further challenges, such as the wafer-scale synthesis of single crystal graphene with a controlled number of layers and a deeper understanding of the growth mechanism of bilayer and multilayer graphene growth on Cu/Ni substrates, still need to be addressed.

Flexible field-effect transistors with a high on/off current ratio based on large-area single-crystal graphene

Large-area single-crystal graphene (LSG) over half a millimeter were synthesized via chemical vapor deposition, and a flexible ion gel-gated graphene field-effect transistor (FET) was fabricated to explore the electrical properties of the graphene based on the mechanism of electronic double layers. A procedure of cyclical oxidation/hydrogen-annealing processes by chemical vapor deposition (CVD) was proposed and optimized as a new growth method for the large single-crystal graphene, to facilitate the reconstruction of copper foil surface. To form a flexible FET on the polyethylene terephthalate substrate, LSG was utilized as a channel layer, and a spin-coated ion gel film was used as a gate dielectric. The flexible device demonstrates excellent electrical properties and high degree of bendability. When the device was bent by 8.1%, the on/off current ratio exceeded 400 because the deformation of the graphene crystal lattice widened graphene’s bandgap. This work is a fundamental study on the growth mechanism of LSG and the two-dimensional-material-based ion gel-gated transistor for application in flexible electronic devices.

Highly Ordered and Dense Thermally Conductive Graphitic Films from a Graphene Oxide/Reduced Graphene Oxide Mixture

Summary We report a new approach to making highly dense, oriented, and crystalline graphite films from heat-treated and pressed graphene oxide (G-O). By introducing small-diameter reduced graphene oxide (rG-O) flakes into the graphene oxide starting material, we found that after heat treatment at 3,000°C, the sample density and atomic order substantially improved over a film composed, at the outset, only of pure G-O flakes. A subsequent mechanical press increased the density but reduced the atomic order. A second 3,000°C heat treatment restored the graphitic structure with graphitization metrics exceeding even those of the first heat treatment. The optimized graphitic film with an original concentration of 15 wt % reduced G-O in G-O gave well-oriented graphitic films with a density of 2.1 g cm−3, cross-plane thermal conductivity of 5.65 W m−1 K−1, and in-plane thermal conductivity of 2,025 ± 25 W m−1 K−1.

Inducing and controlling magnetism in the honeycomb lattice through a harmonic trapping potential

We study strongly interacting ultracold spin-1/2 fermions in a honeycomb lattice in the presence of a harmonic trap. Tuning the strength of the harmonic trap we show that it is possible to confine the fermions in artificial structures reminiscent of graphene nanoflakes in solid state. The confinement on small structures induces magnetic effects which are absent in a large graphene sheet. Increasing the strength of the harmonic potential we are able to induce different magnetic states, such as a N\'eel-like antiferromagnetic or ferromagnetic states, as well as mixtures of these basic states. The realization of different magnetic patterns is associated with the terminations of the artificial structures, in turn controlled by the confining potential.

Climate-zone-dependent effect mechanism of humic acid and fulvic acid extracted from river sediments on aggregation behavior of graphene oxide

Climate factors could affect the physicochemical properties of dissolved organic matter (DOM) in river sediments, which potentially influence the stability of nanoparticles in natural waters. In this study, we extracted humic acid (HA) and fulvic acid (FA) from river sediments in different climate zones of China. Their effect with different concentrations (0.2 and 1 mg·C·L−1) on the aggregation kinetics of large (589 nm) and small graphene oxide (GO, 200 nm) in NaCl solutions was investigated. Both concentrations of HA/FA significantly inhibited the aggregation of small GO because of the steric forces rendered by DOM. For large GO, the inhibition effect of HA on aggregation was higher than FA because of the higher molecular weight, longer carbon chain length, and more structure complexity of HA. Interestingly, with 0.2 mg·C·L−1 HA and large GO, Makou in Subtropical monsoon climate zone decreased the aggregation rate more significantly due to its larger molecular weight, while, Maqin in the Plateau and mountain (PM) climate zone with smaller molecular weight and greater hydrophobicity showed lower inhibition effect on the aggregation. One mg·C·L−1 FA with high polarity from Tangke in PM climate zone and Panjin from Temperate monsoon climate zone showed more stability ability towards large GO. Derjaguin-landau-verwey-overbeek (DLVO) theory indicated that the interaction energy barrier between GO particles dependent on physicochemical characteristics of DOM and GO size. Understanding the climate-zone-dependent effect of sediment DOM on stability of GO is essential for anticipating its fate in natural systems.

Epitaxial Graphene Growth on the Step‐Structured Surface of Off‐Axis C‐Face 3C‐SiC(1¯1¯1¯)

Graphene layers grown on the C‐face SiC exhibit quite different structural and electronic properties compared with those grown on the Si‐face SiC. Herein, the growth and structural properties of graphene on the off‐axis C‐face 3C‐SiC() are studied. The as‐grown 4° off‐axis 3C‐SiC() exhibits highly periodic steps with step height of ≈0.75 nm and terrace width of ≈50 nm. After annealing at 1800 °C under 850 mbar argon atmosphere, relatively uniform large graphene domains can be grown. The low‐energy electron microscopy (LEEM) results demonstrate that one monolayer (ML) to four‐ML graphene domains are grown over several micrometers square, which enables us to measure micro low‐energy electron diffraction (μ‐LEED) on the single graphene domain. The μ‐LEED pattern collected on the monolayer domain mainly exhibits four sets of graphene (1 × 1) spots, indicating the presence of graphene grains with different azimuthal orientations in the same graphene sheet. Raman spectra collected on the graphene domains show rather small D peaks, indicating the presence of less defects and higher crystalline quality of the graphene layers grown on the C‐face off‐axis 3C‐SiC().

Molecular dynamics simulation of graphene sinking during chemical vapor deposition growth on semi-molten Cu substrate

Copper foil is the most promising catalyst for the synthesis of large-area, high-quality monolayer graphene. Experimentally, it has been found that the Cu substrate is semi-molten at graphene growth temperatures. In this study, based on a self-developed C–Cu empirical potential and density functional theory (DFT) methods, we performed systematic molecular dynamics simulations to explore the stability of graphene nanostructures, i.e., carbon nanoclusters and graphene nanoribbons, on semi-molten Cu substrates. Many atomic details observed in the classical MD simulations agree well with those seen in DFT-MD simulations, confirming the high accuracy of the C–Cu potential. Depending on the size of the graphene island, two different sunken-modes are observed: (i) graphene island sinks into the first layer of the metal substrate and (ii) many metal atoms surround the graphene island. Further study reveals that the sinking graphene leads to the unidirectional alignment and seamless stitching of the graphene islands, which explains the growth of large single-crystal graphene on Cu foil. This study deepens our physical insights into the CVD growth of graphene on semi-molten Cu substrate with multiple experimental mysteries well explained and provides theoretic references for the controlled synthesis of large-area single-crystalline monolayer graphene.

ADVANCES IN EXPERIMENTAL TECHNOLOGY BY THE EUROGAM DETECTOR SYSTEM

For the high resolution gamma ray detector array EUROGAM new data acquisition solutions are being developed. Large volume high purity Ge detectors, with new shapes for increased efficiency, have been proposed and prototypes are now being tested. High resolution analog and digital electronics will be integrated within the VXIbus system to be used as the front end instrumentation bus. Also, distributed processing solutions are adopted in order to cope with the high data rates.

Substrate Developments for the Chemical Vapor Deposition Synthesis of Graphene

AbstractSince the isolation of graphene and numerous demonstrations of its unique properties, the expectations for this material to be implemented in many future commercial applications have been enormous. However, to date, challenges still remain. One of the key challenges is the fabrication of graphene in a manner that satisfies processing requirements. While transfer of graphene can be used, this tends to damage or contaminate it, which degrades its performance. Hence, there is an important drive to grow graphene directly over a number of technologically important materials, viz., different substrate materials, so as to avoid the need for transfer. One of the more successful approaches to synthesis graphene is chemical vapor deposition (CVD), which is well established. Historically, transition metal substrates are used due to their catalytic properties. However, in recent years this has developed to include many nonmetal substrate systems. Moreover, both solid and molten substrate forms have also been demonstrated. In addition, the current trend to progress flexible devices has spurred interest in graphene growth directly over flexible materials surfaces. All these aspects are presented in this review which presents the developments in available substrates for graphene fabrication by CVD, with a focus primarily on large area graphene.

3D printing geopolymer nanocomposites: Graphene oxide size effects on a reactive matrix

In previous study, we reported that the addition of graphene oxide (GO) into geopolymer can significantly modified its rheology properties and make it 3D printable. However, the high concentration of GO employment limits the enhancement of mechanical properties. Improving GO reinforcement efficiency with lower GO concentration without sacrificing 3D printability is still a challenge. More importantly, although it has been proved that large graphene oxide (LGO) can reinforce various matrix materials including polymers and ceramics better than that of small graphene oxide (SGO), it is still unclear whether such conclusion can be directly applied to reactive matrix system. We investigate the effects of GO size on mechanical properties of geopolymer with low GO concentration, both at material level and printed structure level. Surprisingly, it is found that the SGO can enhance the mechanical performance of geopolymer more effectively than LGO at material level, which is resulted from the fact that the chemical reaction between geopolymer particles could be severally blocked by LGO. In addition, we also found that the efficiency for the reinforcement at structure level is higher for LGO. Therefore, the size of nano reinforcement should be optimized both for the reinforcement at material and structure level.

The History of the German Association for Crystal Growth

The History of the German Association for Crystal Growth

Two-dimensional spintronic circuit architectures on large scale graphene

Solid state electronics based on utilizing the electron spin degree of freedom for storing and processing information can pave the way for next-generation spin-based computing. However, the realization of spin communication between multiple devices in complex spin circuit geometries, essential for practical applications, is still lacking. Here, we demonstrate the spin current propagation in two-dimensional (2D) circuit architectures consisting of multiple devices and configurations using a large area CVD graphene on SiO2/Si substrate at room temperature. Taking advantage of the significant spin transport distance reaching 34 μm in commercially available wafer-scale graphene grown on Cu foil, we demonstrate that the spin current can be effectively communicated between the magnetic memory elements in graphene channels within 2D circuits of Y-junction and hexa-arm architectures. We further show that by designing graphene channels and ferromagnetic elements at different geometrical angles, the symmetric and antisymmetric components of the Hanle spin precession signal can be remarkably controlled. These findings lay the foundation for the design of complex 2D spintronic circuits, which can be integrated into efficient electronics based on the transport of pure spin currents.

Growth, charge and thermal transport of flowered graphene

We report on the structural and transport properties of the smallest dislocation loop in graphene, known as a flower defect. First, by means of advanced experimental imaging techniques, we deduce how flower defects are formed during recrystallization of chemical vapor deposited graphene. We propose that the flower defects arise from a bulge type mechanism in which the flower domains are the grains left over by dynamic recrystallization. Next, in order to evaluate the use of such defects as possible building blocks for all-graphene electronics, we combine multiscale modeling tools to investigate the structure and the electron and phonon transport properties of large monolayer graphene samples with a random distribution of flower defects. For large enough flower densities, we find that electron transport is strongly suppressed while, surprisingly, hole transport remains almost unaffected. These results suggest possible applications of flowered graphene for electron energy filtering. For the same defect densities, phonon transport is reduced by orders of magnitude as elastic scattering by defects becomes dominant. Heat transport by flexural phonons, key in graphene, is largely suppressed even for very low concentrations.