Few-layer Graphene Research Articles

Preparation and optical-electronic properties of thin films comprising P3HT and few-layer graphene nanosheets

The present work investigates the production of thin films from nanocomposites obtained by solution mixing a synthesized P3HT with different proportions of few-layered graphene (FLG) in chloroform and its influence on crystallography and light absorption in the visible region. Commercial FLG produced through exfoliation in the liquid phase assisted by a non-anionic surfactant was employed, being a material not yet tested for the production of nanocomposites, let alone to be applied in photovoltaic cells. The deposition of thin films was evaluated at low (1000 rpm) and high (2500 rpm) rotation speeds in a dynamic and static deposition process. In addition, the annealing heat treatment in the optimized thin films was evaluated. The results showed that the P3HT was synthesized with suitable molar mass, regioregularity, and thermal resistance as characterized by the analyses of gel permeation chromatography (GPC), 1H-nuclear magnetic resonance (1H NMR), and thermogravimetric analysis (TGA). The thin films produced showed that the FLG enlarged and strengthened the P3HT absorption, increasing the absorption region by up to 34 % in addition to improving the resolution of the absorption peaks favoring the π-π* transition, which is important for the photovoltaic effect. By the X-ray diffraction analysis, it can be suggested that incorporating the FLG improves the charge mobility as there was an increase in the crystallite size without changing the interplanar distance. Based on these results, it can be suggested that P3HT/FLG nanocomposites are promising for application in both the active and hole transport layers of solar cells.

3D graphene sponge biomass-derived with high surface area applied as adsorbent for nitrophenols

The present work used wood sawdust as a carbon precursor to produce 3D graphene sponge material, a carbon material with an added value, successfully used as an adsorbent for removing o-nitrophenol and p-nitrophenol from aqueous effluents. The resulting material’s structural, morphological, textural, and chemical characteristics (WS-Graphene) were evaluated. Textural analysis revealed a high surface area (∼3000 m2g−1) and high total pore volume (1.38 cm3 g−1). Raman, infrared and X-ray diffraction techniques evidenced the presence of a few layers of graphene, with some fraction of defects. Transmission electron microscopy confirmed the presence of a 3D-packed structure for the graphene. These WS-Graphene characteristics favored its application as an adsorbent for removing o-nitrophenol and p-nitrophenol pollutants. The adsorption process of both nitrophenols was extremely rapid and reached the equilibrium time in less than 5 min when the concentration of both adsorbates was fixed at 700 mg L−1. Furthermore, the maximum adsorption capacity (Qmax) values were 1842 mg g−1 (o-nitrophenol) and 879.6 mg g−1 (p-nitrophenol) at 30 °C. These results confirm that the WS-Graphene adsorbent acts as a 3D sponge. Langmuir’s isothermal was the best-fitted model for both nitrophenols in aqueous solution. The o-nitrophenol adsorption onto WS-Graphene might follow pseudo-second-order as well as general-order, while the adsorption of p-nitrophenol might follow only the general-order model. Thermodynamic studies revealed a spontaneous and endothermic adsorption process. The WS-Graphene adsorbent presents a high percentage (∼97 %) of removing the mixture of pollutants in effluents, showing an excellent adsorption capacity and making it a potential material to remove industrial wastewater.

Graphene-Encapsulated Bifunctional Catalysts with High Activity and Durability for Zn-Air Battery.

Carbon-based electrocatalysts with both high activity and high stability are desirable for use in Zn-air batteries. However, the carbon corrosion reaction (CCR) is a critical obstacle in rechargeable Zn-air batteries. In this study, a cost-effective carbon-based novel material is reported with a high catalytic effect and good durability for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), prepared via a simple graphitization process. In situ growth of graphene is utilized in a 3D-metal-coordinated hydrogel by introducing a catalytic lattice of transition metal alloys. Due to the direct growth of few-layer graphene on the metal alloy decorated 3d-carbon network, greatly reduced CCR is observed in a repetitive OER test. As a result, an efficient bifunctional electrocatalytic performance is achieved with a low ΔE value of 0.63V and good electrochemical durability for 83 h at a current density of 10mA cm-2 in an alkaline media. Moreover, graphene-encapsulated transition metal alloys on the nitrogen-doped carbon supporter exhibit an excellent catalytic effect and good durability in a Zn-air battery system. This study suggests a straightforward way to overcome the CCR of carbon-based materials for an electrochemical catalyst with wide application in energy conversion and energy storage devices.

The interplay of intra- and inter-layer interactions in bending rigidity of ultrathin 2D materials

Continuum mechanics break down in bending stiffness calculations of mono- and few-layered two-dimensional (2D) van der Waals crystal sheets, because their layered atomistic structures are uniquely characterized by strong in-plane bonding coupled with weak interlayer interactions. Here, we elucidate how the bending rigidities of pristine mono- and few-layered molybdenum disulfide (MoS2), graphene, and hexagonal boron nitride (hBN) are governed by their structural geometry and intra- and inter-layer bonding interactions. Atomic force microscopy experiments on the self-folded conformations of these 2D materials on flat substrates show that the bending rigidity of MoS2 significantly exceeds those of graphene or hBN of comparable layers, despite its much lower tensile modulus. Even on a per-thickness basis, MoS2 is found to possess similar bending stiffness to hBN and is much stiffer than graphene. Density functional theory calculations suggest that this high bending rigidity of MoS2 is due to its large interlayer thickness and strong interlayer shear, which prevail over its weak in-plane bonding.

Domain-Dependent Surface Adhesion in Twisted Few-Layer Graphene: Platform for Moiré-Assisted Chemistry.

Twisted van der Waals multilayers are widely regarded as a rich platform to access novel electronic phases thanks to the multiple degrees of freedom available for controlling their electronic and chemical properties. Here, we propose that the stacking domains that form naturally due to the relative twist between successive layers act as an additional "knob" for controlling the behavior of these systems and report the emergence and engineering of stacking domain-dependent surface chemistry in twisted few-layer graphene. Using mid-infrared near-field optical microscopy and atomic force microscopy, we observe a selective adhesion of metallic nanoparticles and liquid water at the domains with rhombohedral stacking configurations of minimally twisted double bi- and trilayer graphene. Furthermore, we demonstrate that the manipulation of nanoparticles located at certain stacking domains can locally reconfigure the moiré superlattice in their vicinity at the micrometer scale. Our findings establish a new approach to controlling moiré-assisted chemistry and nanoengineering.

Dynamic Observation of the Coulomb Explosion and Field Evaporation of a Few-Layer Graphene Nanoribbon.

The Coulomb explosion and field evaporation are frequently observed physical phenomena for a metallic tip under an external electric field, which can modify the structures of the tip and have broad applications, such as in the atomic-probe tomography and field ion microscopy. However, the mechanistic comprehending of how they change the structures of the tip and the differences between them are not clear. Here, dynamic observations of Coulomb explosions and field evaporations on the positively biased and charged few-layer graphene (FLG) nanoribbon inside a transmission electron microscope are reported. By combining the atomic-scale molecular dynamic simulations, it is shown that the FLG is split into several sheets under Coulomb explosion. It is also observed to break by emitting the carbon ions/segments under the field evaporation. It is further demonstrated that the split and breaking of FLG can be tuned by the shape of the nanoribbon. FLG ribbons with sharp tips have splitting and breaking occur in sequence. FLG with blunt tips break without a split. These results provide a fundamental understanding of Coulomb explosion and field evaporation in graphene nanomaterials and suggest potential methods to engineer graphene-based nanostructures.

Electroluminescence from a phthalocyanine monolayer encapsulated in a van der Waals tunnel diode

Monolayers of free base phthalocyanine (H2Pc) are grown on monolayer and few-layer exfoliated flakes of hexagonal boron nitride (hBN) which are subsequently integrated into a van der Waals tunnel diode. This heterostructure consists of two thin hBN flakes between which the H2Pc monolayer is sandwiched and also incorporates upper and lower few-layer graphene contacts. When a voltage is applied between the contacts, a tunnel current flows and the embedded molecules can be excited resulting in the emission of photons with wavelengths which are close to the peaks observed in photoluminescence. We also observe electroluminescence at voltages where the energy gained by a tunnelling electron is lower than the energy of the emitted photon implying a multi-electron excitation pathway which we attribute to the formation of an intermediate triplet state. Our results provide insights into the differences in excitation and relaxation of molecules in supramolecular monolayers and bulk crystals and we discuss how the alignment of the energy levels of the molecules and contact layers determine the emission process.

Bithiophene as a Sulfur-Based Promotor for the Synthesis of Carbon Nanotubes and Carbon-Carbon Composites.

We assess bithiophene (C8H6S2) as a novel sulfur-based promotor for the growth of single-walled carbon nanotubes (SWCNTs) in the aerosol (floating catalyst) CVD method. Technologically suitable equilibrium vapor pressure and an excess of hydrocarbon residuals formed under its decomposition make bithiophene an attractive promoter for the production of carbon nanotubes in general and specifically for ferrocene-based SWCNT growth. Indeed, we detect a moderate enhancement in the carbon nanotube yield and a decrease in the equivalent sheet resistance of the films at a low bithiophene content, indicating the improvement of the product properties. Moreover, the relatively high concentrations and low temperature stability of bithiophene result in non-catalytical decomposition, leading to the formation of pyrolytic carbon deposits; the deposits appear as few-layer graphene structures. Thus, bithiophene pyrolysis opens a route for the cheap production of hierarchical composite thin films comprising carbon nanotubes and few-layer graphene, which might be of practical use for hierarchical adsorbents, protective membranes, or electrocatalysis.

Influence of graphite geography on the yield of mechanically exfoliated few-layer graphene

Some of graphene's exciting properties that inspired countless studies over the last two decades are exclusive to defect-free, few-layer graphene (FLG, ≤4 layers) produced by mechanical exfoliation. Despite graphite exfoliation having been thoroughly studied, the influence of the graphite source on the successful exfoliation of FLG remains unexplored. In this work, we examine the physicochemical properties and exfoliation performance of various graphite types (e.g., natural crystalline flake, natural vein, and synthetic) from around the globe (e.g., Alaska, Tanzania, China, etc.). We first established a normalization method for the facile comparison of FLG (Φ) yields between separately exfoliated graphites. Using this approach, we find that the relative yield of FLG ranges from 0% to 22% across graphite types, with regular natural crystalline flakes (NCF-R) performing the best overall (ΦNCF-R = 16% ± 5%). FLG yield was also found to depend on source geography, evidenced by ΦNCF-R ranging from 8% to 22% for NCF-R sourced from Alabama and Canada, respectively. We employed machine learning and Pearson correlation analysis to determine the graphite characteristics that govern FLG yield. Graphite source-dependent properties such as graphite surface area and mineral impurities, including aluminum and calcium, were found to be critically important. Our findings highlight the importance of the graphite source when trying to maximize the yield of mechanically exfoliated FLG and represent an important step toward producing higher quantities of defect-free FLG for graphene-based research.

One-Step Plasma-Assisted Electrochemical Synthesis of Nanocomposites of Few-Layer Graphene Structures with Manganese Oxides as Electrocatalysts for Oxygen Reduction Reaction

Using the method of plasma-assisted electrochemical exfoliation of graphite, a nanocomposite, which consists of few-layer graphene structures with surface decorated with manganese oxides nanoparticles, is synthesized in one-step process. It is found that this material exhibits a high electrocatalytic activity towards the oxygen reduction reaction due to the presence of manganese in the +2 and +3 oxidation states, and also carbonyl (quinone) functional groups on the surface of graphene structures.

One-Step Plasma-Assisted Electrochemical Synthesis of Nanocomposites of Few-Layer Graphene Structures with Manganese Oxides as Electrocatalysts for Oxygen Reduction Reaction

One-Step Plasma-Assisted Electrochemical Synthesis of Nanocomposites of Few-Layer Graphene Structures with Manganese Oxides as Electrocatalysts for Oxygen Reduction Reaction

Paradigm metallothermic-sulfidation-carbonization constructing ZIFs-derived TMSs@Graphene/CNx heterostructures for high-capacity and long-life energy storage

Enhancing electrochemical activity and structural stability of transition metal sulfides (TMSs) are critical for improving the capacity output and retention of TMSs-based batteries. Here, we report a new paradigmatic approach for fabricating TMSs/C composites, adopting a metallothermic-sulfidation-carbonization strategy (2TM + CS2 = 2TMS + C) based on zeolitic imidazolate frameworks (ZIFs) to synchronously construct a compact TMSs@Graphene/CNx triple heterostructure. The obtained structure features crystalline TMSs nanoparticles wrapped by few-layer graphene and totally embedded within porous carbonized polyhedral frameworks. All three nanocomponents of TMSs@Graphene/CNx are connected via chemical bonding of S−C and TM−C, forming a chemical cross-linked nanostructure. Such structure design bears intrinsic advantages in improving the volumetric-efficiency for accommodating TMSs and electrical properties, enabling promising electrochemical performance in lithium- and sodium-ion storage. As a representative, the ZnS@Graphene/CNx electrode exhibits a high capacity of 891.5 mAh g−1 and an excellent retention of 80 % after 1000 cycles in lithium-ion batteries. More notably, this general metallothermic-sulfidation-carbonization mechanism can be applicable to all ZIFs, defining a new ZIFs-derived TMSs/C heterostructures.

Ultraslim and highly flexible supercapacitor based on chemical vapor deposited nitrogen-doped bernal graphene for wearable electronics

Nitrogen-doped Bernal graphene has several advantages, including improved electrical conductivity, enhanced stability and better compatibility with electrolytes. This article presents the synthesis of few-layer pristine graphene and nitrogen-doped Bernal graphene (CVDNG) through the Low-pressure chemical vapor deposition approach and its application for flexible supercapacitors. A novel hot lamination-assisted approach is employed to transfer the graphene on the desired substrates, enabling the reusability of copper. Raman mapping is utilized to analyze the characteristic bands and statistical distribution of Bernal stacking in CVDNG. The doping in graphene is confirmed through the X-ray photoelectron spectroscopy suggesting the content of pyrrolic-N (73.38%), pyridinic-N (20.28%) and graphitic-N (6.38%) in the CVDNG. Cyclic voltammetry, galvanostatic charge-discharge and electrochemical impedance spectroscopy are performed by sandwiching the polyvinyl alcohol-sodium perchlorate based hydrogel membrane between two symmetrical supercapacitor electrodes. CVDNG symmetrical supercapacitor device delivers an enhanced areal capacitance of 26.75 mF cm−2 with a high energy density of 2.14 mWh cm−2 with a power density of 72 mW cm−2 at 0.05 mA cm−2. The device shows excellent flexibility by retaining 91.1% of its capacitance at extreme bending (180° angle) conditions and excellent specific capacitance retention (90.6%) after 10,000 charge-discharge cycles.

Thermal conductivity of 2D diamond superstructures in interlayer-bonded twisted bilayer graphene

We report results from a systematic analysis of thermal transport in 2D diamond superstructures in interlayer-bonded twisted bilayer graphene (IB-TBG) based on molecular-dynamics simulations. We find that the introduction of interlayer C–C bonds in these bilayer structures causes an abrupt drop in the thermal conductivity of pristine, non-interlayer-bonded bilayer graphene, while further increase in the interlayer C–C bond density (2D diamond fraction) leads to a monotonic increase in the thermal conductivity of the resulting superstructures with increasing 2D diamond fraction toward the high thermal conductivity of 2D diamond (diamane). We also find that a similar trend is exhibited in the thermal conductivity of interlayer-bonded graphene bilayers with randomly distributed individual interlayer C–C bonds (RD-IBGs) as a function of interlayer C–C bond density, but with the thermal conductivity of the IB-TBG 2D diamond superstructures consistently exceeding that of RD-IBGs at a given interlayer bond density. We analyze the simulation results employing effective medium and percolation theories and explain the predicted thermal conductivity dependence on interlayer bond density on the basis of lattice distortions induced in the bilayer structures as a result of interlayer bonding. Our findings demonstrate that the thermal conductivity of IB-TBG 2D diamond superstructures and RD-IBGs can be precisely tuned by controlling interlayer C–C bond density and have important implications for the thermal management applications of interlayer-bonded few-layer graphene derivatives.

The Efficiency Study of Graphene Synthesis on Copper Substrate via Chemical Vapor Deposition Method with Methanol Precursor.

Few-layer graphene was successfully synthesized on copper foil via chemical vapor deposition with methanol as a carbon source. This was confirmed by optical microscopy observation, Raman spectra measurement, I2D/IG ratio calculation, and 2D-FWHM value comparisons. Monolayer graphene was also found in similar standard procedures, but it required higher growth temperature and longer time periods. The cost-efficient growth conditions for few-layer graphene are thoroughly discussed via TEM observation and AFM measurement. In addition, it has been confirmed that the growth period can be shortened by increasing growth temperature. With the H2 gas flow rate fixed at 15 sccm, few-layer graphene was synthesized at the lower growth temperature of 700 °C in 30 min, and at 900 °C growth temperature in only 5 min. Successful growth was also achieved without adding hydrogen gas flow; this is probably because H2 can be induced from the decomposition of methanol. Through further defects study of few-layer graphene via TEM observation and AFM measurement, we tried to find possible ways for efficiency and quality management in graphene synthesis in industrial applications. Lastly, we investigated graphene formation after pre-treatment with different gas compositions, and found that gas selection is a crucial factor for a successful synthesis.

Electronic Properties of FLG/InP Schottky Contacts

Graphene (Gr) is of great interest in the development of new electronic, photonic, and composite materials. The physical properties of Gr can vary depending on the number of layers, and this unique property makes it a potential material for different electronic applications. In this study, few-layer graphene (FLG) film was spin-coated onto the InP semiconductor surface and the FLG/n-InP Schottky contact was produced. The properties and quality of the FLG nano-film were determined by using Raman spectroscopy. Parameters such as ideality factor, barrier height, and series resistance of Schottky contacts were calculated using current-voltage (I-V) curves. With the Gaussian distribution, the mean ideality factor of the Gr/InP contacts was found to be =1,47, and the mean barrier height values were found to be =0.68 eV. The standard deviation values were calculated as σ=0.32 for the ideality factor and σ=0.06 eV for the barrier height. In addition, the series resistance values were calculated from the Cheung functions and were found to be in agreement with the literature. Finally, the current conduction mechanisms of the Gr/n-InP structure were revealed by examining the logarithmic I-V characteristics.

Mobile Trions in Electrically Tunable 2D Hybrid Perovskites.

2D hybrid perovskites are currently in the spotlight of material research for light-harvesting and -emitting applications. It remains extremely challenging, however, to externally control their optical response due to the difficulties of introducing electrical doping. Here, an approach of interfacing ultrathin sheets of perovskites with few-layer graphene and hexagonal boron nitride into gate-tunable, hybrid heterostructures, is demonstrated. It allows for bipolar, continuous tuning of light emission and absorption in 2D perovskites by electrically injecting carriers to densities as high as 1012 cm-2 . This reveals the emergence of both negatively and positively charged excitons, or trions, with binding energies up to 46meV, among the highest measured for 2D systems. Trions are shown to dominate light emission and propagate with mobilities reaching 200cm2 V-1 s-1 at elevated temperatures. The findings introduce the physics of interacting mixtures of optical and electrical excitations to the broad family of 2D inorganic-organic nanostructures. The presented strategy to electrically control the optical response of 2D perovskites highlights it as a promising material platform toward electrically modulated light-emitters, externally guided charged exciton currents, and exciton transistors based on layered, hybridsemiconductors.

Nanoelectromechanical Temperature Sensor Based on Piezoresistive Properties of Suspended Graphene Film

The substrate impurities scattering will lead to unstable temperature-sensitive behavior and poor linearity in graphene temperature sensors. And this can be weakened by suspending the graphene structure. Herein, we report a graphene temperature sensing structure, with suspended graphene membranes fabricated on the cavity and non-cavity SiO2/Si substrate, using monolayer, few-layer, and multilayer graphene. The results show that the sensor provides direct electrical readout from temperature to resistance transduction by the nano piezoresistive effect in graphene. And the cavity structure can weaken the substrate impurity scattering and thermal resistance effect, which results in better sensitivity and wide-range temperature sensing. In addition, monolayer graphene is almost no temperature sensitivity. And the few-layer graphene temperature sensitivity, lower than that of the multilayer graphene cavity structure (3.50%/°C), is 1.07%/°C. This work demonstrates that piezoresistive in suspended graphene membranes can effectively enhance the sensitivity and widen the temperature sensor range in NEMS temperature sensors.

Facile synthesis of nitrogen-doped graphene, and its advanced electrochemical activity toward efficient lithium ion storage

Nitrogen-doped graphene (N-Graphene) has been extensively researched as the anode material for lithium-ion batteries, as the nitrogen doping provides massive active sites and improves the electrical conductivity and the ion diffusion kinetics, thus, significantly enhancing the lithium storage performance with reversible capacity and cycling stability. However, the high-rate performance and cyclability of the N-Graphene-based anodes are still to be achieved for fast-charging applications. Here, a new N-Graphene was successfully fabricated by annealing the commercial few-layer graphene with dicyandiamide. Along with the plenty of defects in the pristine graphene sheets, the doping of nitrogen effectively reduces the resistance and increases pore volume and the diffusion coefficient. Consequently, the N-Graphene anode exhibits outstanding lithium storage performance with superior high-rate performance and remarkable long-term capability (up to 10,000 cycles at 15 A g[Formula: see text] with a reversible capacity of 133 mAh g[Formula: see text]). Further kinetic analysis reveals that this excellent electrochemical behavior during the rapid discharge/charge operations can be attributed to the enhanced diffusion-controlled and surface capacitive storages. Our process may lead to an alternative way for producing competitive N-Graphene anode materials for efficient lithium ion storage.

Evolution of particle size and morphology in plasma synthesis of few-layer graphene and soot

This paper examines the evolution of particle size and morphology of plasma-synthesized carbonaceous nanoparticles for various reactants, reactant concentrations, reactor temperatures, and positions within the reactor. Aerosol particles were characterized at various positions downstream from the plasma zone by spatially-resolved thermophoretic sampling and transmission electron microscopy, Raman spectroscopy, and by in situ time-resolved laser-induced incandescence. Depending on the carbon-bearing reactant (ethanol or toluene) and its concentration, either pure few-layer graphene (FLG) or soot-like particles, or a mixture of both, were generated. The initial carbon nucleation has been found to commence less than 12.4 cm downstream from the plasma nozzle. In the case of FLG formation, particles show an increasing level of crumpling with increasing distance downstream from the plasma zone. The process was numerically simulated with a 1D plug-flow reactor model employing a joint population balance model for graphitic and amorphous-like particles (FLG and soot-like, respectively). The simulation includes modified versions of inception, hydrogen-abstraction-carbon-addition (HACA), and polycyclic-aromatic hydrocarbon (PAH) adsorption models adapted from a pre-existing soot model. The simulations showed that the ratio of HACA/PAH adsorption determined by post-plasma C2H2 is the main factor that affects the soot-to-FLG ratio. This is also consistent with experimental observations as reducing the rate at which carbon precursor is supplied to the reactor leads to less C2H2 in the post-plasma zone, which results in less PAH adsorption, and consequently suppressed soot formation in favor of FLG synthesis.