Single-layer Graphene Films Research Articles

Growth Kinetics of Graphene on Cu(111) Foils from Methane, Ethyne, Ethylene, and Ethane

AbstractChemical vapor deposition of carbon precursors on Cu‐based substrates at temperatures exceeding 1000 °C is currently a typical route for the scalable synthesis of large‐area high‐quality single‐layer graphene (SLG) films. Using molecules with higher activities than CH4 may afford lower growth temperatures that might yield fold‐ and wrinkle‐free graphene. The kinetics of growth of graphene using hydrocarbons other than CH4 are of interest to the scientific and industrial communities. We measured the growth rates of graphene islands on Cu(111) foils by using C2H2, C2H4, C2H6 and CH4, respectively (each mixed with H2). From such kinetics data we obtain the activation enthalpy (ΔH≠) of graphene growth as shown in parentheses (C2H2 (0.93±0.09 eV); C2H4 (2.05±0.19 eV); C2H6 (2.50±0.11 eV); CH4 (4.59±0.26 eV)); C2Hy (y=2, 4, 6) show similar growth behavior but CH4 is different. Computational fluid dynamics and density functional theory simulations suggest that C2Hy differs from CH4 due to different values of adsorption energy and the lifetime of relevant carbon precursors on the Cu(111) surface. Combining experimental and simulation results, we find that the rate determining step (RDS) is the dissociation of the first C−H bond of CH4 molecules in the gas phase, while the RDS using C2Hy is the first dehydrogenation of adsorbed C2Hy that happens with assistance of H atoms adsorbed on the Cu(111) surface. By using C2H2 as the carbon precursor, high‐quality single‐crystal adlayer‐free SLG films are achieved on Cu(111) foils at 900 °C.

Growth Kinetics of Graphene on Cu(111) Foils from Methane, Ethyne, Ethylene, and Ethane.

Chemical vapor deposition of carbon precursors on Cu-based substrates at temperatures exceeding 1000 °C is currently a typical route for the scalable synthesis of large-area high-quality single-layer graphene (SLG) films. Using molecules with higher activities than CH4 may afford lower growth temperatures that might yield fold- and wrinkle-free graphene. The kinetics of growth of graphene using hydrocarbons other than CH4 are of interest to the scientific and industrial communities. We measured the growth rates of graphene islands on Cu(111) foils by using C2H2, C2H4, C2H6 and CH4, respectively (each mixed with H2). From such kinetics data we obtain the activation enthalpy (ΔH≠) of graphene growth as shown in parentheses (C2H2 (0.93±0.09 eV); C2H4 (2.05±0.19 eV); C2H6 (2.50±0.11 eV); CH4 (4.59±0.26 eV)); C2Hy (y=2, 4, 6) show similar growth behavior but CH4 is different. Computational fluid dynamics and density functional theory simulations suggest that C2Hy differs from CH4 due to different values of adsorption energy and the lifetime of relevant carbon precursors on the Cu(111) surface. Combining experimental and simulation results, we find that the rate determining step (RDS) is the dissociation of the first C-H bond of CH4 molecules in the gas phase, while the RDS using C2Hy is the first dehydrogenation of adsorbed C2Hy that happens with assistance of H atoms adsorbed on the Cu(111) surface. By using C2H2 as the carbon precursor, high-quality single-crystal adlayer-free SLG films are achieved on Cu(111) foils at 900 °C.

Electrochemical synthesis of hierarchical porous PbSe film and its infrared photodetection properties

Lead selenide (PbSe) has been extensively studied as an infrared detection material capable of operating at room temperature. In this research, we proposed a simple cyclic voltammetry (CV) electrodeposition of hierarchical porous PbSe on single-layer graphene film. Scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy were used to evaluate the physical and surface characteristics of porous PbSe films. The porous structure with large surface to volume ratio will enhance the light absorption ability after effectively sensitized in oxygen atmosphere. Furthermore, oxygen-sensitized processes create electron traps at grain boundaries, extending the lifetime of the hole carrier and improving photoconductive sensitivity. At room temperature, the porous PbSe/graphene photodetector exhibit a responsivity of 12.3 A/W, a detectivity of 5.95 * 108 cm Hz1/2W−1 (@1 kHz) at 1.55 µm, and a responsivity of 15.2 A/W, a detectivity of 1.4 * 109 cm Hz1/2W−1(@1 kHz) at 2.7 µm. This work highlights the low cost and effective electrodeposition strategy for constructing hierarchical porous structure with further application in infrared photodetection.

Tunable terahertz absorption of ion gel-graphene hybrids based on the Salisbury effect.

The gate-tunable absorption properties of graphene make it suitable for terahertz (THz) absorbers. However, the realization of a graphene-based THz absorber faces challenges between the difficulty of patterning graphene for processing and the intrinsically low absorbance of graphene with the high electric field needed to change the conductivity of graphene. This report presents an electrically tunable graphene THz absorber where a single-layer graphene film and a gold reflective layer are separated by a polyimide (PI) dielectric layer to form an easily fabricated three-layer Salisbury screen structure. The carrier density of the graphene layer can be efficiently tuned by a small external electrical gating (-5V-5 V) with the assistance of an ion gel layer. The voltage modulation of the Fermi energy level (EF) of graphene was confirmed by Raman spectra, and the variation of the device absorbance was confirmed using a THz time-domain spectroscopy system (THz-TDS). The measurements show that the EF is adjusted in the range of 0-0.5 eV, and THz absorbance is adjusted in the range of 60%-99%. The absorber performs well under different curvatures, and the peak absorbance is all over 95%. We conducted further analysis of the absorber absorbance by varying the thickness of the PI dielectric layer, aiming to examine the correlation between the resonant frequency of the absorber and the dielectric layer thickness. Our research findings indicate that the proposed absorber holds significant potential for application in diverse fields such as communication, medicine, and sensing.

Flexible field-effect transistors with high-quality and uniform single-layer graphene for high mobility

In this work, a fully flexible graphene field-effect transistor with high carrier mobility is reported. Patterned high-quality and uniform single-layer graphene films are successfully realized by combining the selective growth on a patterned copper foil and the direct transfer method to minimize degradation factors. The selectively grown single-layer graphene is directly transferred to the target substrate through the deposition of poly-para-xylylene (Parylene) C. The quality of the graphene films is confirmed by Raman spectroscopy. The analysis reveals that the use of Parylene C as the substrate, gate dielectric, and encapsulation layer has the advantage of reducing the scattering by the optical phonons and charge puddles. The estimated residual carrier density is 1.72 × 1011 cm−2, and the intrinsic hole and electron carrier mobilities are found to be as high as 10 260 and 10 010 cm2 V−1 s−1, respectively. This study can pave the way for the development and mass production of high-performance and fully flexible graphene electronics.

Sequential Bayesian-optimized graphene synthesis by direct solar-thermal chemical vapor deposition

This work reports the use of a high-flux solar simulator that mimics the solar spectrum and a cold-wall CVD reactor to demonstrate the feasibility of utilizing a renewable energy resource in synthesizing graphene under various conditions. A parametric study of process parameters was conducted using a probabilistic approach. Gaussian process regression serves as a surrogate to establish a prior for Bayesian optimization, and an information acquisition function is employed to identify conditions that yield high-quality products. Backscattered electron images and Raman mapping were used to assess the effects of growth conditions on graphene characteristic sizes, film quality, and uniformity. We report the synthesis of high-quality single-layer graphene (SLG) and AB-stacked bilayer graphene films in a one-step, short-time process with ID/IG\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$I_{D}/I_{G}$$\\end{document} ratios of 0.21 and 0.14, respectively. Electron diffraction analysis shows peak intensities that resemble SLG and AB-bilayer graphene with up to 5 and 20 μ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\\upmu }$$\\end{document}m grain sizes, respectively. The optical transmissivities of SLG and AB-bilayer graphene fall between 0.959–0.977 and 0.929–0.953, whereas the sheet resistances measured by a 4-point probe with 1 mm spacing are 15.5 ± 4.6 and 3.4 ± 1.5 kΩ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\Omega$$\\end{document}/sq, respectively. Further scale-up of the optimized graphene growth area was achieved by flattening the insolation profile, leading to spatial uniformity up to 13 mm in radius. Direct solar capture for CVD synthesis enable a practical and sustainable option for synthesizing graphene films applicable for photonic and electronic applications.

Measuring optical reflectivity of graphene films using compensated Fabry-Perot interferometry

Graphene optical applications rely on its exceptional optical characteristics, for which accurate measurement of graphene reflectivity is critical. Fabry-Perot (F-P) interferometry is a standard reflectivity measurement method with a simple structure, fast measurement speed, and high system plasticity. However, a strong coupling relationship exists between the coupling coefficient and the reflectivity of graphene in the F-P interferometry measurements of graphene reflectivity. Therefore, a method for measuring the reflectivity of thin graphene films using F-P interferometry with a compensation factor is proposed and experimentally demonstrated. Four alternative graphene film reflectivity solution models are established within four different F-P cavity lengths. The experimental results of the solution model with cavity lengths of 50–100 μm show that the average reflectivity of single-layer, two-layer, five-layer, eight-layer, and ten-layer graphene films are 0.0131%, 0.101%, 0.793%, 1.335%, and 1.978% with the errors of 0.7%, 1.0%, 1.0%, 1.1%, and 1.1%, respectively. Meanwhile, the standard deviation of graphene reflectivity increases with the number of graphene layers. The highly consistent experimental data and comparative results confirmed the accuracy of our method. This method proposed in this paper could be further extended to measure the reflectivity of other 2-D materials.

An ultrathin frequency dual-tunable Salisbury screen absorber based on graphene and strontium titanate in the terahertz region

We present a frequency dual-tunable Salisbury screen absorber (SSA) based on graphene and strontium titanate (STO) in the terahertz region. Different from the classical SSAs, the absorber is composed of an optically thick layer of gold at bottom, a high-permittivity STO spacer and a single-layer nonstructural graphene film on top, which has the advantages of high absorption, lightweight, ultrathin thickness and frequency tuning. Simulation results show that the proposed SSA can obtain a strong Fabry–Perot resonance peak with 99.96% absorbance at 1.55 THz, whose performance is comparable with metamaterial-based perfect absorbers. When changing the STO temperature from 200 to 400 K, the resonance frequency shifts from 1.22 to 1.83 THz. As varying the graphene Fermi energy level from 0 to 1 eV, the resonance frequency shifts from 1.44 to 1.65 THz. Moreover, by changing the thickness of the STO spacer from 1.5 to 5.5μm, the resonance frequency can be passively tuned from 2.96 to 0.90 THz, achieving a larger frequency tuning range under small thickness variations. Meanwhile, this SSA exhibits excellent polarization insensitivity and maintains high absorption over 90% with varied incidence angle from 0° to 58° for both TE and TM waves. Such a design would have great potential applications in the fields of detecting, thermal emitting, and imaging.

Effect of the electrolyte composition on the corrosion resistance of single-layer CVD-graphene

The corrosion resistance of a single-layer graphene film synthetized via chemical vapor deposition (CVD) on a polycrystalline copper substrate is studied. The anticorrosive properties were identified by means of electrochemical tests of potentiodynamic polarization and electrochemical impedance spectroscopy in the following environments: (a) 0.5 M H2SO4, (b) 0.5 M HCl, (c) 3.5wt% NaCl + 0.5 M H2SO4, and (d) Na2S2O3 (10−4 mol / l) + 5% NaCl + (CH3COOH) 0.50%). For the characterization of the graphene and corrosion products, the following techniques were used: Raman spectroscopy, AUGER electron spectroscopy, X-ray energy dispersion probe (EDS), scanning electron microscopy (SEM), and confocal laser scanning microscopy (CLSM). The corrosion mechanism and the degree of degradation of the graphene are discussed as a function of the corrosive electrolyte.

Dry Transfer Process of Single-Layer Graphene on Multi-Layer Hexagonal Boron Nitride for High Quality Heterostructure

Carrier mobility of the graphene thin film is one of the most crucial factors to achieve superior device performance. Factors that influence the carrier mobility include film crystallinity, layer number, presence of contaminations, and effect of the substrate. In this work, to obtain high-quality graphene film, single-layer graphene film was formed on a hexagonal boron nitride (hBN) thin-film via mechanical exfoliation technique. Mechanical exfoliation of graphite was done on a silicon substrate covered with poly (methyl methacrylate)/ poly (acrylic acid). Three adjacent thin films, each having approximately 20 - 30 μm length and 3 - 4 μm width making an average area of 87.5 μm2 for each film. From the measured Raman spectrum, the 2D-to-G peak intensity ratio was 1.7, confirming that exfoliated graphene is a single layer. The absence of the D peak indicates minimum crystal defects in the film. The graphene layers were subsequently dry-transferred onto mechanically exfoliated multi-layer hBN thin film. The area of the exfoliated hBN is around 6,000 μm2. The quality of the transfer process was evaluated through inspection under an optical microscope. All three single-layer graphene thin films were successfully placed exactly on the hBN without any presence of bubble and wrinkle. The process was described very detailed. The formed single-layer graphene on hBN layer is expected to have superior electrical characteristics and can be utilized for the fabrication of high-performance nano device.

Folding and Fracture of Single-Crystal Graphene Grown on a Cu(111) Foil.

A single-crystal graphene film grown on a Cu(111) foil by chemical vapor deposition (CVD) has ribbon-like fold structures. These graphene folds are highly oriented and essentially parallel to each other. Cu surface steps underneath the graphene are along the <110>and <211>directions, leading to the formation of the arrays of folds. The folds in the single-layer graphene (SLG) are not continuous but break up into alternating patterns. A "joint" (an AB-stacked bilayer graphene) region connects two neighboring alternating regions, and the breaks are always along zigzag or armchair directions. Folds formed in bilayer or few-layer graphene are continuous with no breaks. Molecular dynamics simulations show that SLG suffers a significantly higher compressive stress compared to bilayer graphene when both are under the same compression, thus leading to the rupture of SLG in these fold regions. The fracture strength of a CVD-grown single-crystal SLG film is simulated to be about 70GPa. This study greatly deepens the understanding of the mechanics of CVD-grown single-crystal graphene and such folds, and sheds light on the fabrication of various graphene origami/kirigami structures by substrate engineering. Such oriented folds can be used in a variety of further studies.

Effect of single atomic layer graphene film on the thermal stability and hydrogen permeation of Pd-coated Nb composite membrane

Pd coated Nb-base composite membranes are preferable in the fields of hydrogen permeation. However, the rapid reduction of hydrogen permeability caused by high-temperature interfacial diffusion of Pd and Nb atoms hinders their large-scale application. In this paper, a single atomic layer graphene film was used for improving the thermal stability of a hydrogen-permeable composite membrane comprising a Pd coating on the Nb substrate. First, the graphene film was transferred onto the surface of the “clean” niobium substrate. Then a thin palladium coating was deposited on it by magnetron sputtering to form the niobium/graphene/palladium (Nb/Gr/Pd) composite membrane. The interfacial stability was evaluated in the temperature range of 673–973 K under vacuum, and the hydrogen permeation behavior was studied by gas-driven permeation method at 573–823 K. The results show that the single atomic layer graphene film can effectively compress the interdiffusion of Pd coating and Nb substrate and achieve a good hydrogen permeability below 823 K. However, it would be broken due to the micro-deformation of Nb substrate, the high mobility of Pd atoms, and the grain growth at a higher temperature. Therefore, it is concluded that the single atomic layer graphene film is unsuitable as an intermediate hindering layer for Nb-based hydrogen-permeable membranes.

Epitaxial Growth of Uniform Single-Layer and Bilayer Graphene with Assistance of Nitrogen Plasma.

Graphene was reported as the first-discovered two-dimensional material, and the thermal decomposition of SiC is a feasible route to prepare graphene films. However, it is difficult to obtain a uniform single-layer graphene avoiding the coexistence of multilayer graphene islands or bare substrate holes, which give rise to the degradation of device performance and becomes an obstacle for the further applications. Here, with the assistance of nitrogen plasma, we successfully obtained high-quality single-layer and bilayer graphene with large-scale and uniform surface via annealing 4H-SiC(0001) wafers. The highly flat surface and ordered terraces of the samples were characterized using in situ scanning tunneling microscopy. The Dirac bands in single-layer and bilayer graphene were measured using angle-resolved photoemission spectroscopy. X-ray photoelectron spectroscopy combined with Raman spectroscopy were used to determine the composition of the samples and to ensure no intercalation or chemical reaction of nitrogen with graphene. Our work has provided an efficient way to obtain the uniform single-layer and bilayer graphene films grown on a semiconductive substrate, which would be an ideal platform for fabricating two-dimensional devices based on graphene.

Single nucleotide discrimination with sub-two nanometer monolayer graphene pore

The effective sensing area is the key point of solid-state nanopore single-molecule detection, which is determined by the diameter of the hole and the thickness of the membrane. High spatial resolution can be achieved by using single-atom thickness two-dimensional materials, such as single-layer graphene and molybdenum disulfide. Helium ion beam microscopy provides an effective technique for manufacturing small-sized nanopores. Here, we show that the precise fabrication of sub-2 nanometer nanopores has been successfully achieved on a single-layer graphene film, which provides the best spatial resolution in room-temperature ionic liquid-assisted single-molecule detection. Many graphene nanopores of different sizes have been fabricated, with a minimum pore diameter of 1.4 nm. The application of these graphene nanopores manufactured by HIM shows that single-layer graphene nanopores can distinguish different homopolymer single-stranded DNA lengths and types, and can even identify four single nucleotides.

Investigating the Impact of Growth Time and Methane Flow on Graphene Synthesis using Nickel Foil

Graphene has been one of the most investigated nanomaterials in recent years due to its one atom thickness nature and its extraordinary properties such as high conductivity and excellent transmittance in visible range, bendability and high carrier mobility. Although there are many techniques to grow graphene, chemical vapor deposition technique is one of the best approach to synthesize homogeneous and commercial scale graphene in thin film form. Many parameters can affect the quality and homogeneity of the graphene film that is grown with this technique. In this study, two growth parameters which are methane flow rate and growth time were investigated to find out their effect on the quality of the graphene film, which is grown on nickel foil. Graphene film grown with different flow rates of methane (from 20 to 50 sccm) and growth time (50 to 20 mins.) were examined. We found that single layer graphene film could only be grown under 20 sccm methane flow in 20 mins. growth time as evidenced via Raman spectroscopy measurements. Furthermore, the single layer graphene film was found to be in high homogeneity as confirmed by Raman mapping. On the other hand, multi-layer graphene film was obtained by increasing both methane flow and growth time.

The role of etching on growth of adlayer graphene by chemical vapor deposition

Understanding the growth mechanism of adlayer graphene is key to preparing a large-area uniform and continuous bilayer or multilayer graphene film, which is essential for electronic and photonic device applications. We synthesized high-quality single-layer graphene (SLG) film with numerous adlayer grains on liquid copper by controllable chemical vapor deposition (CVD). By adjusting parameters related to hydrogen and methane flow, we confirmed a competitive relationship between the etching and the growth of the adlayer, and they can reach dynamic equilibrium during the CVD process. Through the reasonable design of growth time, more details about adlayer growth were revealed. The etched defect is used as the center of carbon injection to grow adlayer under the first layer. For the first time, this research proposes and proves the growth mechanism of etching controlled adlayer growth, which opens a new door for the controllable synthesis of bilayer or multilayer graphene.

High Efficient Seawater Desalination Based on Parallel Nanopore Systems

Graphene is one of the most attractive two-dimensional materials that can be used for efficient desalination due to its ideal physical properties and high performance in ion selectivity and salt rejection. Here, in this paper, molecular dynamics simulations were applied to investigate the possibility of using a parallel nanopore system to pump ions so that the ions of both cation and anion species in the middle compartment could be evacuated at an extremely rapid rate. By building hexagonal parallel single-layer graphene films with spacing of 3.0 nm and changing the pore numbers and surface charge densities of the nanopores, the efficiency of desalination could be well controlled. It is found that the ion concentration decreases exponentially with time. The more the number of nanopore is, the stronger the surface charge density of nanopore is, the evacuation of ions in the middle compartment is more obvious, offering a new means for controlling the desalination efficiency. The simulations performed here provide theoretical insights for designing and fabricating high efficient and less energy consumption graphene desalination devices in the future.

Fabrication of Reduced Graphene Oxide Thin Films on Corona Treated Silicon Substrates

Graphene is a material with excellent properties. However, the manufacture of a single-layer graphene film with a large area is challenging. It requires modern and expensive equipment. In this study, we suggested another approach to improve these problems. The raw material graphite was used to prepare graphene oxide solutions. The resulting graphene oxide solutions were then dispersed onto silicon substrates first treated with corona plasma, which increases the adhesion between materials and substrates. In the next step, these covered substrates were deoxygenated by heating at very high temperatures for a short time to form reduced graphene oxide, which has graphene-like properties. The reduced graphene oxide films formed had a uniform structure over a large area of 2 × 2 cm2 with an oxygen content of about 4%, a film roughness of less than 5 nm, and a sheet resistance of 4-5 ohm/square.

Direct electrosynthesis of 52% concentrated CO on silver\u2019s twin boundary

The gaseous product concentration in direct electrochemical CO2 reduction is usually hurdled by the electrode’s Faradaic efficiency, current density, and inevitable mixing with the unreacted CO2. A concentrated gaseous product with high purity will greatly lower the barrier for large-scale CO2 fixation and follow-up industrial usage. Here, we developed a pneumatic trough setup to collect the CO2 reduction product from a precisely engineered nanotwinned electrocatalyst, without using ion-exchange membrane. The silver catalyst’s twin boundary density can be tuned from 0.3 to 1.5 × 104 cm−1. With the lengthy and winding twin boundaries, this catalyst exhibits a Faradaic efficiency up to 92% at −1.0 V and a turnover frequency of 127 s−1 in converting CO2 to CO. Through a tandem electrochemical-CVD system, we successfully produced CO with a volume percentage of up to 52%, and further transformed it into single layer graphene film.

Preparation and Test of NH3 Gas Sensor Based on Single-Layer Graphene Film.

The ammonia sensing properties of single-layer graphene synthesized by chemical vapor deposition (CVD) were studied. The Au interdigitated electrode (IDE) was prepared by microelectromechanical systems (MEMS) technology, and then, the single-layer graphene was transferred to the IDE by wet transfer technology. Raman spectroscopy was used to monitor the quality of graphene films transferred to SiO2/Si substrates. Moreover, the theory of graphene’s adsorption of gases is explained. The results show that gas sensing characteristics such as response/recovery time and response are related to the target gas, gas concentration, test temperature, and so on. In the stability test, the difference between the maximum resistance and the minimum resistance of the device is 1 ohm without ammonia, the change is less than 1% of its initial resistance, and the repeatability is up to 98.58%. Therefore, the sensor prepared with high quality single-layer graphene has good repeatability and stability for ammonia detection.