Direct chemical vapor deposition of graphene on dielectric surfaces

A review on SEM imaging of graphene layers

Atomic-thick graphene has stimulated great interests for exploring fundamental science and technological applications due to its promising electronic, mechanical and thermal properties. It is important to gain a deeper understanding of geometrical/structural characteristics of graphene and its properties/performance. Scanning electron microscopy (SEM) is indispensable for characterizing graphene layers. This review details SEM imaging of graphene layer, including the SEM image contrast mechanism of graphene layers, imaging parameter-dependent contrast of graphene layers and the influence of polycrystalline substrates on image contrast. Furthermore, a summary of SEM applications in imaging graphene layers is also provided, including layer-number determinations, study of chemical vapor deposition (CVD)-growth mechanism, and reveal of anti-corrosive failure mechanism of graphene layers. This review will provide a systematic and comprehensive understanding on SEM imaging of graphene layers for graphene community.

Graphene-skinned alumina fiber fabricated through metalloid-catalytic graphene CVD growth on nonmetallic substrate and its mass production

Graphene growth on widely used dielectrics/insulators via chemical vapor deposition (CVD) is a strategy toward transfer-free applications of CVD graphene for the realization of advanced composite materials. Here, we develop graphene-skinned alumina fibers/fabrics (GAFs/GAFFs) through graphene CVD growth on commercial alumina fibers/fabrics (AFs/AFFs). We reveal a vapor-surface-solid growth model on a non-metallic substrate, which is distinct from the well-established vapor-solid model on conventional non-catalytic non-metallic substrates, but bears a closer resemblance to that observed on catalytic metallic substrates. The metalloid-catalytic growth of graphene on AFs/AFFs resulted in reduced growth temperature (~200 °C lower) and accelerated growth rate (~3.4 times faster) compared to that obtained on a representative non-metallic counterpart, quartz fiber. The fabricated GAFF features a wide-range tunable electrical conductivity (1-15000 Ω sq-1), high tensile strength (>1.5 GPa), lightweight, flexibility, and a hierarchical macrostructure. These attributes are inherited from both graphene and AFF, making GAFF promising for various applications including electrical heating and electromagnetic interference shielding. Beyond laboratory level preparation, the stable mass production of large-scale GAFF has been achieved through a home-made roll-to-roll system with capacity of 468-93600 m2/year depending on product specifications, providing foundations for the subsequent industrialization of this material, enabling its widespread adoption in various industries.

Recent Advances in Transfer-Free Synthesis of High-Quality Graphene

High-quality graphene obtained by chemical vapor deposition (CVD) technique holds significant importance in constructing innovative electronic and optoelectronic devices. Direct growth of graphene over target substrates readily eliminates cumbersome transfer processes, offering compatibility with practical application scenarios. Recent years have witnessed growing strategic endeavors in the preparation of transfer-free graphene with favorable quality. Nevertheless, timely review articles on this topic are still scarce. In this contribution, a systematic summary of recent advances in transfer-free synthesis of high-quality graphene on insulating substrates, with a focus on discussing synthetic strategies designed by elevating reaction temperature, confining gas flow, introducing growth promotor and regulating substrate surface is presented.

X-ray dynamical diffraction by quasi-monolayer graphene

We study the processes of dynamical diffraction of the plane X-ray waves on the graphene film/SiC substrate system in the case of the Bragg diffraction geometry. The statistical dynamical theory of X-ray diffraction in imperfect crystals is applied to the case of real quasi-two-dimensional systems. The necessity of the taking into account of the variability of the lattice parameter of multilayer graphene, as well as the influence of thickness on the thermal Debye-Waller factor at the calculation of the complex structural factors and Fourier components of polarizability, is demonstrated. It is shown that the change of the structural characteristics of the 3-layer graphene/substrate system, as well as its strained state, leads to a significant change in the diffraction profiles, which makes it possible to determine the characteristics by the X-ray diffraction method.

Amide salt pyrolysis fabrication of graphene nanosheets with multi-excitation single color emission

A novel and simple approach of using amide salt pyrolysis to produce photoluminescent (multi-excitation and single color emission) graphene nanosheets (GNs) with a thickness of <1 nm and a diameter of about 100-200 nm is described herein. It has characteristics of high water solubility, low toxicity, easy manufacturing, etc., and has potential application prospects in analytical chemistry and biomedicine.

Graphene Film Growth on Silicon Carbide by Hot Filament Chemical Vapor Deposition

The electrical properties of graphene on dielectric substrates, such as silicon carbide (SiC), have received much attention due to their interesting applications. This work presents a method to grow graphene on a 6H-SiC substrate at a pressure of 35 Torr by using the hot filament chemical vapor deposition (HFCVD) technique. The graphene deposition was conducted in an atmosphere of methane and hydrogen at a temperature of 950 °C. The graphene films were analyzed using Raman spectroscopy, scanning electron microscopy, atomic force microscopy, energy dispersive X-ray, and X-ray photoelectron spectroscopy. Raman mapping and AFM measurements indicated that few-layer and multilayer graphene were deposited from the external carbon source depending on the growth parameter conditions. The compositional analysis confirmed the presence of graphene deposition on SiC substrates and the absence of any metal involved in the growth process.

Optical data of graphene grown by chemical vapor deposition on copper foil using spectroscopic ellipsometry

Hexagonal, carbon-based structures were prepared on bare and treated copper foil substrates by Chemical Vapor Deposition at 1075 ^ᵒC using a graphite box for limiting copper reactivity with hydrocarbon gases during growth reactions. Spectroscopic ellipsometry with a digital camera was used to collect the amplitude ratio and the phase of structures various spots. The amplitude ratio of hexagonal structures on copper substrates was collected based on substrate treatment type and thickness as well as reaction zone conditions and methane flow rates. Matched amplitude ratio was obtained only for 75 µm thick acetic acid copper foil substrates inside and outside the graphite box at a low methane flow rate of 0.2 sccm. Moreover, the amplitude ratio of 75 µm bare copper foil and 75 µm electro-polished copper foil substrates at methane flow rates of 0.2 - 0.3 sccm were unmatched. Ellipsometric parameters data use potential is assessment of graphene layer thickness on different surfaces. Data shows dependence of the amplitude ratio on the surface roughness of copper foil, gas concentrations and reaction zone conditions.

Fragmented graphene synthesized on a dielectric substrate for THz applications

Fragmented multi-layered graphene films were directly synthesized via chemical vapor deposition (CVD) on dielectric substrates with a pre-deposited copper catalyst. We demonstrate that the thickness of the sacrificial copper film, process temperature, and growth time essentially influence the integrity, quality, and disorder of the synthesized graphene. Atomic force microscopy and Kelvin probe force microscopy measurements revealed the presence of nano-agglomerates and charge puddles. The potential gradients measured over the sample surface confirmed that the deposited graphene film possessed a multilayered structure, which was modelled as an ensemble of randomly oriented conductive prolate ellipsoids. THz time domain spectroscopy measurements gave theacconductivity of the graphene flakes and homogenized graphitic films as being around 1200 S cm^-1and 1000 S cm^-1, respectively. Our approach offers a scalable fabrication of graphene structures composed of graphene flakes, which have effective conductivity sufficient for a wide variety of THz applications.

Copper acetate-facilitated transfer-free growth of high-quality graphene for hydrovoltaic generators

Direct synthesis of high-quality graphene on dielectric substrates without a transfer process is of vital importance for a variety of applications. Current strategies for boosting high-quality graphene growth, such as remote metal catalyzation, are limited by poor performance with respect to the release of metal catalysts and hence suffer from a problem with metal residues. Herein, we report an effective approach that utilizes a metal-containing species, copper acetate, to continuously supply copper clusters in a gaseous form to aid transfer-free growth of graphene over a wafer scale. The thus-derived graphene films were found to show reduced multilayer density and improved electrical performance and exhibited a carrier mobility of 8500 cm2 V-1 s-1. Furthermore, droplet-based hydrovoltaic electricity generator devices based on directly grown graphene were found to exhibit robust voltage output and long cyclic stability, in stark contrast to their counterparts based on transferred graphene, demonstrating the potential for emerging energy harvesting applications. The work presented here offers a promising solution to organize the metal catalytic booster toward transfer-free synthesis of high-quality graphene and enable smart energy generation.

Anomalies at the Dirac Point in Graphene and Its Hole-Doped Compositions

We study the properties of the Dirac states in SiC-graphene and its hole-doped compositions employing angle-resolved photoemission spectroscopy and density functional theory. The symmetry-selective measurements for the Dirac bands reveal their linearly dispersive behavior across the Dirac point which was termed as the anomalous region in earlier studies. No gap is observed even after boron substitution that reduced the carrier concentration significantly from 3.7×10^{13}  cm^{-2} in SiC-graphene to 0.8×10^{13}  cm^{-2} (5% doping). The anomalies at the Dirac point are attributed to the spectral width arising from the lifetime and momentum broadening in the experiments. The substitution of boron at the graphitic sites leads to a band renormalization and a shift of the Dirac point towards the Fermi level. The internal symmetries appear to be preserved in SiC-graphene even after significant boron substitutions. These results suggest that SiC-graphene is a good platform to realize exotic science as well as advanced technology where the carrier properties like concentration, mobility, etc., can be tuned keeping the Dirac fermionic properties protected.

Mechanisms for Graphene Growth on SiO2 Using Plasma-Enhanced Chemical Vapor Deposition: A Density Functional Theory Study

Plasma-enhanced chemical vapor deposition (PE-CVD) of graphene layers on dielectric substrates is one of the most important processes for the incorporation of graphene in semiconductor devices. Graphene is moving rapidly from the laboratory to practical implementation; therefore, devices may take advantage of the unique properties of such nanomaterial. Conventional approaches rely on pattern transfers after growing graphene on transition metals, which can cause nonuniformities, poor adherence, or other defects. Direct growth of graphene layers on the substrates of interest, mostly dielectrics, is the most logical approach, although it is not free from challenges and obstacles such as obtaining a specific yield of graphene layers with desired properties or accurate control of the growing number of layers. In this work, we use density-functional theory (DFT) coupled with ab initio molecular dynamics (AIMD) to investigate the initial stages of graphene growth on silicon oxide. We select C₂H₂ as the PE-CVD precursor due to its large carbon contribution. On the basis of our simulation results for various surface models and precursor doses, we accurately describe the early stages of graphene growth, from the formation of carbon dimer rows to the critical length required to undergo dynamical folding that results in the formation of low-order polygonal shapes. The differences in bonding with the functionalization of the silicon oxide also mark the nature of the growing carbon layers as well as shed light of potential flaws in the adherence to the substrate. Finally, our dynamical matrix calculations and the obtained infrared (IR) spectra and vibrational characteristics provide accurate recipes to trace experimentally the growth mechanisms described and the corresponding identification of possible stacking faults or defects in the emerging graphene layers.

Carbon-dot doped, transfer-free, low-temperature, high mobility graphene using microwave plasma CVD

Microwave plasma chemical vapor deposition is a well-known method for low-temperature, large-area direct graphene growth on any insulating substrate without any catalysts.

Graphene Growth Directly on SiO2/Si by Hot Filament Chemical Vapor Deposition

We report the first direct synthesis of graphene on SiO₂/Si by hot-filament chemical vapor deposition. Graphene deposition was conducted at low pressures (35 Torr) with a mixture of methane/hydrogen and a substrate temperature of 970 °C followed by spontaneous cooling to room temperature. A thin copper-strip was deposited in the middle of the SiO₂/Si substrate as catalytic material. Raman spectroscopy mapping and atomic force microscopy measurements indicate the growth of few-layers of graphene over the entire SiO₂/Si substrate, far beyond the thin copper-strip, while X-ray photoelectron spectroscopy and energy-dispersive X-ray spectroscopy showed negligible amounts of copper next to the initially deposited strip. The scale of the graphene nanocrystal was estimated by Raman spectroscopy and scanning electron microscopy.

Directional copper dewetting to grow graphene ribbon arrays

The V-groove confines the anisotropic dewetting of Cu film to form ribbons. The influence mechanism of film thickness and annealing procedure on the confined dewetting, structural and morphological evolution has been investigated. Thus, the synthesized graphene ribbons by CVD have uniform width, regular edges and good crystallinity, and deliver obvious room-temperature PL emission.

Layer exchange synthesis of multilayer graphene

Low-temperature synthesis of multilayer graphene (MLG) on arbitrary substrates is the key to incorporating MLG-based functional thin films, including transparent electrodes, low-resistance wiring, heat spreaders, and battery anodes in advanced electronic devices. This paper reviews the synthesis of MLG via the layer exchange (LE) phenomenon between carbon and metal from its mechanism to the possibility of device applications. The mechanism of LE is completely different from that of conventional MLG precipitation methods using metals, and the resulting MLG exhibits unique features. Modulation of metal species and growth conditions enables synthesis of high-quality MLG over a wide range of growth temperatures (350 °C-1000 °C) and MLG thicknesses (5-500 nm). Device applications are discussed based on the high electrical conductivity (2700 S cm^-1) of MLG and anode operation in Li-ion batteries. Finally, we discuss the future challenges of LE for MLG and its application to flexible devices.

Surface Electron-Hole Rich Species Active in the Electrocatalytic Water Oxidation

Iridium and ruthenium and their oxides/hydroxides are the best candidates for the oxygen evolution reaction under harsh acidic conditions owing to the low overpotentials observed for Ru- and Ir-based anodes and the high corrosion resistance of Ir-oxides. Herein, by means of cutting edge operando surface and bulk sensitive X-ray spectroscopy techniques, specifically designed electrode nanofabrication and ab initio DFT calculations, we were able to reveal the electronic structure of the active IrO_x centers (i.e., oxidation state) during electrocatalytic oxidation of water in the surface and bulk of high-performance Ir-based catalysts. We found the oxygen evolution reaction is controlled by the formation of empty Ir 5d states in the surface ascribed to the formation of formally Ir^V species leading to the appearance of electron-deficient oxygen species bound to single iridium atoms (μ₁-O and μ₁-OH) that are responsible for water activation and oxidation. Oxygen bound to three iridium centers (μ₃-O) remains the dominant species in the bulk but do not participate directly in the electrocatalytic reaction, suggesting bulk oxidation is limited. In addition a high coverage of a μ₁-OO (peroxo) species during the OER is excluded. Moreover, we provide the first photoelectron spectroscopic evidence in bulk electrolyte that the higher surface-to-bulk ratio in thinner electrodes enhances the material usage involving the precipitation of a significant part of the electrode surface and near-surface active species.

Adhesion-Enhanced Vertically Oriented Graphene on Titanium-Covered Quartz Glass toward High-Stability Light-Dimming-Related Applications

Improving the adhesion property of graphene directly grown on an insulating substrate is essential for promoting the reliability and durability of the related applications. However, effective approaches have rarely been reported, especially for vertically oriented graphene (VG) films grown on insulating templates. To tackle this, we have developed a facile synthetic strategy by introducing an ultrathin (10 nm-thick) titanium (Ti) film on a quartz glass substrate as the adhesion layer, for plasma-enhanced chemical vapor deposition (PECVD) growth of VG films. This synthetic process induces the formation of Ti, oxygen (O), carbon (C)-containing adhesion layer (Ti (O, C)), offering improved interfacial adhesion due to the formation of chemical bonds among Ti and C atoms. Dramatically improved surface and interface stabilities have been achieved, with regard to its counterpart without a Ti adhesion layer. Moreover, we have also realized precise controls of the transparent/conductive property, surface roughness, and hydrophobicity, etc., by varying the VG film growth time. We have also demonstrated the very intriguing application potentials of the hybrids in light-dimming related fields, that is, electro-heating defogging lenses and neutral density filters toward medical endoscope defogging and camera photography.

Large-Size Suspended Mono-Layer Graphene Film Transfer Based on the Inverted Floating Method

Suspended graphene can perfectly present the excellent material properties of graphene, which has a good application prospect in graphene sensors. The existing suspended graphene pressure sensor has several problems that need to be solved, one of which is the fabrication of a suspended sample. It is still very difficult to obtain large-size suspended graphene films with a high integrity that are defect-free. Based on the simulation and analysis of the kinetic process of the traditional suspended graphene release process, a novel setup for large-size suspended graphene release was designed based on the inverted floating method (IFM). The success rate of the single-layer suspended graphene with a diameter of 200 μm transferred on a stainless-steel substrate was close to 50%, which is greatly improved compared with the traditional impregnation method. The effects of the defects and burrs around the substrate cavity on the stress concentration of graphene transfer explain why the transfer success rate of large-size suspended graphene is not high. This research lays the foundation for providing large-size suspended graphene films in the area of graphene high-precision sensors.

Synaptic transistors and neuromorphic systems based on carbon nano-materials

Carbon-based materials possessing a nanometer size and unique electrical properties perfectly address the two critical issues of transistors, the low power consumption and scalability, and are considered as a promising material in next-generation synaptic devices. In this review, carbon-based synaptic transistors were systematically summarized. In the carbon nanotube section, the synthesis of carbon nanotubes, purification of carbon nanotubes, the effect of architecture on the device performance and related carbon nanotube-based devices for neuromorphic computing were discussed. In the graphene section, the synthesis of graphene and its derivative, as well as graphene-based devices for neuromorphic computing, was systematically studied. Finally, the current challenges for carbon-based synaptic transistors were discussed.

Seeded Growth of Ultrathin Carbon Films Directly onto Silicon Substrates

The production of graphene films is of importance for the large-scale application of graphene-based materials; however, there is still a lack of an efficient and effective approach to synthesize graphene films directly on dielectric substrates. Here, we report the controlled growth of ultrathin carbon films, which have a similar structure to graphene, directly on silicon substrates in a process of seeded chemical vapor deposition (CVD). Crystalline silicon with a thermally grown 300 nm oxide layer was first treated with 3-trimethoxysilyl-1-propanamine (APS), which was used as an anchor point for the covalent deposition of small graphene flakes, obtained from graphite using the Hummers' method. Surface coverage of these flakes on the silicon substrate was estimated by scanning electron microscopy (SEM) to be around only 0.01% of the total area. By treating the covalently deposited graphene as seeds for CVD growth, the coverage was increased to >40% when using ethanol as the carbon source. Examination of the carbon thin films with SEM, X-ray photoelectron spectroscopy, and Raman spectroscopy indicated that they consist of domains of coherent, single-layer graphene produced by the coalescence of the expanding graphene islands. This approach potentially lends itself to the production of high-quality graphene films that may be suitable for device fabrication.

Chemical Vapour Deposition of Graphene for Durable Anticorrosive Coating on Copper

Due to the excellent chemical inertness, graphene can be used as an anti-corrosive coating to protect metal surfaces. Here, we report the growth of graphene by using a chemical vapour deposition (CVD) process with ethanol as a carbon source. Surface and structural characterisations of CVD grown films suggest the formation of double-layer graphene. Electrochemical impedance spectroscopy has been used to study the anticorrosion behaviour of the CVD grown graphene layer. The observed corrosion rate of 8.08 × 10-14 m/s for graphene-coated copper is 24 times lower than the value for pure copper which shows the potential of graphene as the anticorrosive layer. Furthermore, we observed no significant changes in anticorrosive behaviour of the graphene coated copper samples stored in ambient environment for more than one year.

Enhanced Microwave Absorption Bandwidth in Graphene-Encapsulated Iron Nanoparticles with Core-Shell Structure

Graphene-encapsulated iron nanoparticles (Fe(G)) hold great promise as microwave absorbers owing to the combined dielectric loss of the graphene shell and the magnetic loss of the ferromagnetic metal core. Transmission electron microscopy (TEM) revealed transition metal nanoparticles encapsulated by graphene layers. The microwave electromagnetic parameters and reflection loss (R) of the Fe(G) were investigated. Graphene provided Fe(G) with a distinctive dielectric behavior via interfacial polarizations taking place at the interface between the iron cores and the graphene shells. The R of Fe(G)/paraffin composites with different Fe(G) contents and coating thickness was simulated according to the transmit-line theory and the measured complex permittivity and permeability. The Fe(G)/paraffin composites showed an excellent microwave absorption with a minimum calculated R of −58 dB at 11 GHz and a 60 wt% Fe(G) loading. The composites showed a wide bandwidth (the bandwidth of less than −10 dB was about 11 GHz). The R of composites with 1–3 mm coating thickness was measured using the Arch method. The absorption position was in line with the calculated results, suggesting that the graphene-coated iron nanoparticles can generate a suitable electromagnetic match and provide an intense microwave absorption. Excellent Fe(G) microwave absorbers can be obtained by selecting optimum layer numbers and Fe(G) loadings in the composites.

Van der Waals Epitaxy of III-Nitride Semiconductors Based on 2D Materials for Flexible Applications

III-nitride semiconductors have attracted considerable attention in recent years owing to their excellent physical properties and wide applications in solid-state lighting, flat-panel displays, and solar energy and power electronics. Generally, GaN-based devices are heteroepitaxially grown on c-plane sapphire, Si (111), or 6H-SiC substrates. However, it is very difficult to release the GaN-based films from such single-crystalline substrates and transfer them onto other foreign substrates. Consequently, it is difficult to meet the ever-increasing demand for wearable and foldable applications. On the other hand, sp² -bonded two-dimensional (2D) materials, which exhibit hexagonal in-plane lattice arrangements and weakly bonded layers, can be transferred onto flexible substrates with ease. Hence, flexible III-nitride devices can be implemented through such 2D release layers. In this progress report, the recent advances in the different strategies for the growth of III-nitrides based on 2D materials are reviewed, with a focus on van der Waals epitaxy and transfer printing. Various attempts are presented and discussed herein, including the different kinds of 2D materials (graphene, hexagonal boron nitride, and transition metal dichalcogenides) used as release layers. Finally, current challenges and future perspectives regarding the development of flexible III-nitride devices are discussed.

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.

Light and complex 3D MoS2/graphene heterostructures as efficient catalysts for the hydrogen evolution reaction

Multi-component 3D porous structures are highly promising hierarchical materials for numerous applications. Herein we show that atomic-layer deposition (ALD) of MoS₂ on graphene foams with variable pore size is a promising methodology to prepare complex 3D heterostructures to be used as electrocatalysts for the hydrogen evolution reaction (HER). The effect of MoS₂ crystallinity is studied and a trade-off between the high density of defects naturally presented in amorphous MoS₂ coatings and the highly crystalline phase obtained after annealing at 800 °C is established. Specifically, an optimal annealing at 500 °C is shown to yield improved catalytic performance with an overpotential of 180 mV, a low Tafel slope of 47 mV dec^-1, and a high exchange current of 17 μA cm^-2. The ALD deposition is highly conformal, and thus advantageous when coating 3D porous structures with small pore sizes, as required for real-world applications. This approach is enabled by conformal thin film deposition on porous structures with controlled crystallinity by tuning the annealing temperature. The results presented here therefore serve as an effective and general platform for the design of chemically and structurally tunable, binder-free, complex, lightweight, and highly efficient 3D porous heterostructures to be used for catalysis, energy storage, composite materials, sensors, water treatment, and more.

Direct Conversion of CO2 to Multi-Layer Graphene using Cu-Pd Alloys

A straightforward one-step process was developed, in which CO₂ gas is directly converted into multi-layer graphene via atmospheric pressure chemical vapor deposition (APCVD). A bimetallic alloy film based on Cu and Pd was employed as the catalyst and substrate. In this study, we found that the quantity of Cu required for the CO₂ conversion process is high (>82 at %). The findings gained in this study serve as a foundation for further studies of metallic alloys for the thermo-reduction of CO₂ to graphene under CVD conditions.

Direct Synthesis of Large-Area Graphene on Insulating Substrates at Low Temperature using Microwave Plasma CVD

With a combination of outstanding properties and a wide spectrum of applications, graphene has emerged as a significant nanomaterial. However, to realize its full potential for practical applications, a number of obstacles have to be overcome, such as low-temperature, transfer-free growth on desired substrates. In most of the reports, direct graphene growth is confined to either a small area or high sheet resistance. Here, an attempt has been made to grow large-area graphene directly on insulating substrates, such as quartz and glass, using magnetron-generated microwave plasma chemical vapor deposition at a substrate temperature of 300 °C with a sheet resistance of 1.3k Ω/□ and transmittance of 80%. Graphene is characterized using Raman microscopy, atomic force microscopy, scanning electron microscopy, optical imaging, UV-vis spectroscopy, and X-ray photoelectron spectroscopy. Four-probe resistivity and Hall effect measurements were performed to investigate electronic properties. Key to this report is the use of 0.3 sccm CO₂ during growth to put a control over vertical graphene growth, generally forming carbon walls, and 15-20 min of O₃ treatment on as-synthesized graphene to improve sheet carrier mobility and transmittance. This report can be helpful in growing large-area graphene directly on insulating transparent substrates at low temperatures with advanced electronic properties for applications in transparent conducting electrodes and optoelectronics.

Lift-Off Assisted Patterning of Few Layers Graphene

Graphene and 2D materials have been exploited in a growing number of applications and the quality of the deposited layer has been found to be a critical issue for the functionality of the developed devices. Particularly, Chemical Vapor Deposition (CVD) of high quality graphene should be preserved without defects also in the subsequent processes of transferring and patterning. In this work, a lift-off assisted patterning process of Few Layer Graphene (FLG) has been developed to obtain a significant simplification of the whole transferring method and a conformal growth on micrometre size features. The process is based on the lift-off of the catalyst seed layer prior to the FLG deposition. Starting from a SiO₂ finished Silicon substrate, a photolithographic step has been carried out to define the micro patterns, then an evaporation of Pt thin film on Al₂O₃ adhesion layer has been performed. Subsequently, the Pt/Al₂O₃ lift-off step has been attained using a dimethyl sulfoxide (DMSO) bath. The FLG was grown directly on the patterned Pt seed layer by Chemical Vapor Deposition (CVD). Raman spectroscopy was applied on the patterned area in order to investigate the quality of the obtained graphene. Following the novel lift-off assisted patterning technique a minimization of the de-wetting phenomenon for temperatures up to 1000 °C was achieved and micropatterns, down to 10 µm, were easily covered with a high quality FLG.

Ultrasensitive Optical Detection of Water Pressure in Microfluidics Using Smart Reduced Graphene Oxide Glass

Despite recent progresses in the field of microfluidics, the effect of liquid pressure on the detection accuracy has been rarely studied. Here, we perform a quantitative analysis of such effect, by utilizing the sensitive optical responses of graphene to the refractive index (RI) change of its surrounding environment. We utilize a reflection coupling configuration by combining the total internal reflection (TIR) and ultrasonic waves. The high-performance graphene is processed on common glasses by using the solution-processable oxidation-reduction method. We find that the RI change of water caused by a pressure as small as 500 Pa generated by the liquid level change in the microfluidics can be measured directly. The detection accuracy and response time limits are approximately 280 Pa and 100 ns, respectively. The Maxwell's boundary conditions, Fresnel's law, and Pascal's law are used in theoretical analyses. This work highlights the importance of liquid pressure in microfluidics and provides guidance in designing and accurate detection of microfluidic devices.

Exploring and suppressing the kink effect of black phosphorus field-effect transistors operating in the saturation regime

With continuous device scaling, avalanche breakdown in two-dimensional (2D) transistors severely degrades device output characteristics and overall reliability. It is highly desirable to understand the origin of such electrical breakdown for exploring high-performance 2D transistors. Here, we report an anomalous increase in the drain currents of black phosphorus (BP)-based transistors operating in the saturation regime. Through the comprehensive investigation of various channel thicknesses, channel lengths and operating temperatures, such novel behavior is attributed to the kink effect originating from impact ionization and the related potential shift inside the channel, which is further confirmed by device numerical simulations. Furthermore, nitrogen plasma treatment is carried out to eliminate the current anomalous increase and suppress the kink effect with improved saturation current. This work not only sheds light on the understanding of carrier transport within BP transistors, but also could open up a new avenue for achieving high-performance and reliable electronic devices based on 2D materials.

Seed-Assisted Synthesis of Graphene Films on Insulating Substrate

Synthesizing graphene at a large-scale and of high quality on insulating substrate is a prerequisite for graphene applications in electronic devices. Typically, graphene is synthesized and then transferred to the proper substrate for subsequent device preparation. However, the complicated and skilled transfer process involves some issues such as wrinkles, residual contamination and breakage of graphene films, which will greatly degrade its performance. Direct synthesis of graphene on insulating substrates without a transfer process is highly desirable for device preparation. Here, we report a simple, transfer-free method to synthesize graphene directly on insulating substrates (SiO₂/Si, quartz) by using a Cu layer, graphene oxide and Poly (vinyl alcohol) as the catalyst, seeds and carbon sources, respectively. Atomic force microscope (AFM), scanning electronic microscope (SEM) and Raman spectroscopy are used to characterize the interface of insulating substrate and graphene. The graphene films directly grown on quartz glass can attain a high transmittance of 92.8% and a low sheet resistance of 620 Ω/square. The growth mechanism is also revealed. This approach provides a highly efficient method for the direct production of graphene on insulating substrates.

Direct CVD Growth of Graphene on Traditional Glass: Methods and Mechanisms

Chemical vapor deposition (CVD) on catalytic metal surfaces is considered to be the most effective way to obtain large-area, high-quality graphene films. For practical applications, a transfer process from metal catalysts to target substrates (e.g., poly(ethylene terephthalate) (PET), glass, and SiO₂ /Si) is unavoidable and severely degrades the quality of graphene. In particular, the direct growth of graphene on glass can avoid the tedious transfer process and endow traditional glass with prominent electrical and thermal conductivities. Such a combination of graphene and glass creates a new type of glass, the so-called "super graphene glass," which has attracted great interest from the viewpoints of both fundamental research and daily-life applications. In the last few years, great progress has been achieved in pursuit of this goal. Here, these growth methods as well as the specific growth mechanisms of graphene on glass surfaces are summarized. The typical techniques developed include direct thermal CVD growth, molten-bed CVD growth, metal-catalyst-assisted growth, and plasma-enhanced growth. Emphasis is placed on the strategy of growth corresponding to the different natures of glass substrates. A comprehensive understanding of graphene growth on nonmetal glass substrates and the latest status of "super graphene glass" production are provided.

Tunable Perfect THz Absorber Based on a Stretchable Ultrathin Carbon-Polymer Bilayer

By exploring the Salisbury screen approach, we propose and demonstrate a thin film absorber of terahertz (THz) radiation. The absorber is comprised of a less than 100 nm thick layer of pyrolytic carbon deposited on a stretchable polydimethylsiloxane (PDMS) film followed by the metal film. We demonstrate that being overall less than 200 microns thick, such a sandwich structure absorbs resonantly up to 99.9%of the incident THz radiation, and that the absorption resonance is determined by the polymer thickness, which can be adjusted by stretching.

Role of the Cu substrate in the growth of ultra-flat crack-free highly-crystalline single-layer graphene

Producing ultra-flat crack-free single-layer high-quality graphene over large areas has remained the key challenge to fully exploit graphene's potential into next-generation technological applications. In this regard, we show that epitaxial Cu(111) film represents the most promising catalyst for the chemical vapor deposition (CVD) of graphene with superior planarity and physical integrity. We first compare the most widely used Cu catalysts (foils, polycrystalline films and epitaxial films) in order to benchmark the roughness of the Cu surface which serves as a template for graphene growth. We then discuss the correlation between the formation of cracks and wrinkles in as-grown graphene and the surface morphology of these various Cu catalysts. In particular, Cu grain boundary grooves, inherently present in polycrystalline substrates, are found to contribute to the formation of cracks. Finally, we focused on tuning the CVD protocol in order to successfully grow highly crystalline graphene made of millimeter-size domains on every type of catalyst while mitigating Cu surface roughening. Putting into context the challenges and opportunities associated with the most widely used Cu catalysts provides valuable guidelines for high-throughput manufacturing of graphene suitable for emerging industrial applications.

Flatlands in the Holy Land: The Evolution of Layered Materials Research in Israel

The experimental identification of fullerenes in 1985, carbon nanotubes in 1991, inorganic nanotubes in 1992, and graphene in 2004 are cornerstone events that have marked the beginning of the layered nanostructures era of materials science. Nowadays, the synthesis of such low-dimensional systems is a routine practice allowing the controlled fabrication of 0-, 1-, and 2D layered structures of diverse chemical compositions. These systems possess unique physical properties that stem from their structural anisotropy characterized by strong intralayer covalent bonding and weaker interlayer dispersive interactions. This, in turn, results in promising functionality that attracts the attention of scientists from many disciplines including chemists, physicists, material scientists, engineers, as well as life scientists that are interested in both their basic and applied science aspects. Here, a short review of the contribution of the Israeli scientific community to this effort over the past 3 decades, is provided.

Highly Conductive Nitrogen-Doped Graphene Grown on Glass toward Electrochromic Applications

The direct synthesis of low sheet resistance graphene on glass can promote the applications of such intriguing hybrid materials in transparent electronics and energy-related fields. Chemical doping is efficient for tailoring the carrier concentration and the electronic properties of graphene that previously derived from metal substrates. Herein, we report the direct synthesis of 5 in. uniform nitrogen-doped (N-doped) graphene on the quartz glass through a designed low-pressure chemical vapor deposition (LPCVD) route. Ethanol and methylamine were selected respectively as precursor and dopant for acquiring predominantly graphitic-N-doped graphene. We reveal that by a precise control of growth temperature and thus the doping level the sheet resistance of graphene on glass can be as low as one-half that of nondoped graphene, accompanied by relative high crystal quality and transparency. Significantly, we demonstrate that this scalable, 5 in. uniform N-doped graphene glass can serve as excellent electrode materials for fabricating high performance electrochromic smart windows, featured with a much simplified device structure. This work should pave ways for the direct synthesis and application of the new type graphene-based hybrid material.

Bridging the Gap between Reality and Ideal in Chemical Vapor Deposition Growth of Graphene

Graphene, in its ideal form, is a two-dimensional (2D) material consisting of a single layer of carbon atoms arranged in a hexagonal lattice. The richness in morphological, physical, mechanical, and optical properties of ideal graphene has stimulated enormous scientific and industrial interest, since its first exfoliation in 2004. In turn, the production of graphene in a reliable, controllable, and scalable manner has become significantly important to bring us closer to practical applications of graphene. To this end, chemical vapor deposition (CVD) offers tantalizing opportunities for the synthesis of large-area, uniform, and high-quality graphene films. However, quite different from the ideal 2D structure of graphene, in reality, the currently available CVD-grown graphene films are still suffering from intrinsic defective grain boundaries, surface contaminations, and wrinkles, together with low growth rate and the requirement of inevitable transfer. Clearly, a gap still exits between the reality of CVD-derived graphene, especially in industrial production, and ideal graphene with outstanding properties. This Review will emphasize the recent advances and strategies in CVD production of graphene for settling these issues to bridge the giant gap. We begin with brief background information about the synthesis of nanoscale carbon allotropes, followed by the discussion of fundamental growth mechanism and kinetics of CVD growth of graphene. We then discuss the strategies for perfecting the quality of CVD-derived graphene with regard to domain size, cleanness, flatness, growth rate, scalability, and direct growth of graphene on functional substrate. Finally, a perspective on future development in the research relevant to scalable growth of high-quality graphene is presented.

High-Contrast SEM Imaging of Supported Few-Layer Graphene for Differentiating Distinct Layers and Resolving Fine Features: There is Plenty of Room at the Bottom

For supported graphene, reliable differentiation and clear visualization of distinct graphene layers and fine features such as wrinkles are essential for revealing the structure-property relationships for graphene and graphene-based devices. Scanning electron microscopy (SEM) has been frequently used for this purpose where high-quality image contrast is critical. However, it is surprising that the effect of key imaging parameters on the image contrast has been seriously undermined by the graphene community. Here, superior image contrast of secondary electron (SE) images for few-layer graphene supported on SiC and SiO₂ /Si is realized through simultaneously tuning two key parameters-acceleration voltage (V_acc ) and working distance (WD). The overlooked role of WD in characterizing graphene is highlighted and clearly demonstrated. A unified model of V_acc and WD dependence of three types of SE collected by the standard side-attached Everhart-Thornley (E-T) SE detector is conceptually developed for mechanistically understanding the improved mass thickness contrast for supported few-layer graphene. The findings reported here will have important implications for effective characterizations of atomically thick 2D materials and devices.

Direct Growth of Highly Stable Patterned Graphene on Dielectric Insulators using a Surface-Adhered Solid Carbon Source

A novel method is described for the direct growth of patterned graphene on dielectric substrates by chemical vapor deposition (CVD) in the presence of Cu vapor and using a solid aromatic carbon source, 1,2,3,4-tetraphenylnapthalene (TPN), as the precursor. The UV/O₃ treatment of the TPN film both crosslinks TPN and results in a strong interaction between the substrate and the TPN that prevents complete sublimation of the carbon source from the substrate during CVD. Substrate-adhered crosslinked TPN is successfully converted to graphene on the substrate without any organic contamination. The graphene synthesized by this method shows excellent mechanical and chemical stability. This process also enables the simultaneous patterning of graphene materials, which can thus be used as transparent electrodes for electronic devices. The proposed method for the synthesis directly on substrates of patterned graphene is expected to have wide applications in organic and soft hybrid electronics.

Direct growth of graphene on rigid and flexible substrates: progress, applications, and challenges

Graphene has recently been attracting considerable interest because of its exceptional conductivity, mechanical strength, thermal stability, etc. Graphene-based devices exhibit high potential for applications in electronics, optoelectronics, and energy harvesting. In this paper, we review various growth strategies including metal-catalyzed transfer-free growth and direct-growth of graphene on flexible and rigid insulating substrates which are "major issues" for avoiding the complicated transfer processes that cause graphene defects, residues, tears and performance degradation in graphene-based functional devices. Recent advances in practical applications based on "direct-grown graphene" are discussed. Finally, several important directions, challenges and perspectives in the commercialization of 'direct growth of graphene' are also discussed and addressed.

Layer-by-Layer Decoupling of Twisted Graphene Sheets Epitaxially Grown on a Metal Substrate

The electronic properties of graphene can be efficiently altered upon interaction with the underlying substrate resulting in a dramatic change of charge carrier behavior. Here, the evolution of the local electronic properties of epitaxial graphene on a metal upon the controlled formation of multilayers, which are produced by intercalation of atomic carbon in graphene/Ir(111), is investigated. Using scanning tunneling microscopy and Landau-level spectroscopy, it is shown that for a monolayer and bilayers with small-angle rotations, Landau levels are fully suppressed, indicating that the metal-graphene interaction is largely confined to the first graphene layer. Bilayers with large twist angles as well as twisted trilayers demonstrate a sequence of pronounced Landau levels characteristic for a free-standing graphene monolayer pointing toward an effective decoupling of the top layer from the metal substrate. These findings give evidence for the controlled preparation of epitaxial graphene multilayers with a different degree of decoupling, which represent an ideal platform for future electronic and spintronic applications.

Direct Growth of Graphene on Silicon by Metal-Free Chemical Vapor Deposition

The metal-free synthesis of graphene on single-crystal silicon substrates, the most common commercial semiconductor, is of paramount significance for many technological applications. In this work, we report the growth of graphene directly on an upside-down placed, single-crystal silicon substrate using metal-free, ambient-pressure chemical vapor deposition. By controlling the growth temperature, in-plane propagation, edge-propagation, and core-propagation, the process of graphene growth on silicon can be identified. This process produces atomically flat monolayer or bilayer graphene domains, concave bilayer graphene domains, and bulging few-layer graphene domains. This work would be a significant step toward the synthesis of large-area and layer-controlled, high-quality graphene on single-crystal silicon substrates.

A novel intracellular pH-responsive formulation for FTY720 based on PEGylated graphene oxide nano-sheets

OBJECTIVE

This study was performed to investigate a novel pH-responsive nanocarrier based on modified nano graphene oxide (nGO) to promote the acid-triggered intracellular release of a poorly soluble drug, FTY720.

METHODS

To synthesize a drug conjugated to modified nGO, first the polyethylene glycol (PEG) was conjugated to nGO, then the produced PEG-nGO was functionalized with the anticancer drug, FTY720, through amide bonding. It was characterized by the scanning electron microscopy (SEM), the atomic force microscopy (AFM), the Fourier transform infrared (FTIR) spectroscopy and the UV-vis spectroscopy. In vitro drug release of the FTY720-conjugated PEG-nGO was evaluated at pH 7.4 and 4.6 PBS at 37 °C. Furthermore, the antineoplastic action of unloaded and drug-loaded carrier against the human breast adenocarcinoma cell line MCF7 was explored using MTT and BrdU assays.

RESULTS

Characterization methods indicated successful drug deposition on the surface of nGO. In vitro, drug release results revealed a significantly faster release of FTY720 from PEG-nGO at acidic pH, compared with physiological pH. The proliferation assays proved that the unloaded nGO had no significant cytotoxicity against MCF7 cells, while free FTY720- and FTY720-loaded PEG-nGO had an approximately equal cytotoxic effect on the MCF7 cells. It was found that the extended release characteristic of FTY720 was well fitted to Korsmeyer-Peppas model and the release profile of FTY720 from PEG-nGO is diffusion controlled.

CONCLUSION

PEGylated GO can act as a pH-responsive drug carrier to improve the efficacy of anticancer drug delivery.

Preparation of Graphene Sheets by Electrochemical Exfoliation of Graphite in Confined Space and Their Application in Transparent Conductive Films

A novel electrochemical exfoliation mode was established to prepare graphene sheets efficiently with potential applications in transparent conductive films. The graphite electrode was coated with paraffin to keep the electrochemical exfoliation in confined space in the presence of concentrated sodium hydroxide as the electrolyte, yielding ∼100% low-defect (the D band to G band intensity ratio, I_D/I_G = 0.26) graphene sheets. Furthermore, ozone was first detected with ozone test strips, and the effect of ozone on the exfoliation of graphite foil and the microstructure of the as-prepared graphene sheets was investigated. Findings indicate that upon applying a low voltage (3 V) on the graphite foil partially coated with paraffin wax that the coating can prevent the insufficiently intercalated graphite sheets from prematurely peeling off from the graphite electrode thereby affording few-layer (<5 layers) holey graphene sheets in a yield of as much as 60%. Besides, the ozone generated during the electrochemical exfoliation process plays a crucial role in the exfoliation of graphite, and the amount of defect in the as-prepared graphene sheets is dependent on electrolytic potential and electrode distance. Moreover, the graphene-based transparent conductive films prepared by simple modified vacuum filtration exhibit an excellent transparency and a low sheet resistance after being treated with NH₄NO₃ and annealing (∼1.21 kΩ/□ at ∼72.4% transmittance).

Review on mechanism of directly fabricating wafer-scale graphene on dielectric substrates by chemical vapor deposition

To date, chemical vapor deposition on transition metal catalysts is a potential way to achieve low cost, high quality and uniform wafer-scale graphene. However, the removal and transfer process of the annoying catalytic metals underneath can bring large amounts of uncertain factors causing the performance deterioration of graphene, such as the pollution of surface polymeric residues, unmentioned doping and structural damages. Thus, to develop a technique of directly fabricating graphene on dielectric substrates is quite meaningful. In this review, we will present specific methods of catalyst- or transfer-free techniques for graphene growth and discuss the diversity of growth mechanisms.