Few-layer Graphene Research Articles

Observation of thickness-independent ultrafast relaxation times in MPCVD few-layer graphene

Graphene presents unique optoelectronic properties, making it attractive for the development of a wide range of new and advanced technological applications such as high-speed photodetectors and all-optical modulators. The study and control of the generated carriers in graphene, namely their ultrafast relaxation dynamics, are of great importance for these applications. Here, we report the ultrafast relaxation times of photogenerated carriers in few-layer graphene grown by microwave plasma chemical vapour deposition. Graphene samples with 3, 5 and 6 layers were studied by degenerate femtosecond optical pump-probe spectroscopy. We observed a fast relaxation constant on the order of 120−180fs and a slow relaxation constant below 1 ps, associated with carrier-carrier scattering and carrier-phonon scattering processes, respectively. These results suggest that small variations in the number of graphene layers do not affect the dynamics.

Liquid-phase shear exfoliation of graphite and its application as corrosion protection coating on aluminum substrate

Although aluminum (Al) or its alloys are moderately protected by a native oxide layer, the presence of chloride ions in aqueous media in direct contact causes pitting attacks and accelerates corrosion. Using graphene as a coating protector can obstruct corrosion due to its numerous chemical and physical properties. This study investigates corrosion protection of Al using graphene nanosheets (GNSs) produced by liquid phase shear exfoliation method (LPE). To do so, few layer-thick graphene nanosheet colloidal solutions were produced by a scalable eco-friendlier lower cost approach. The GNSs were thoroughly characterized using complementary characterization techniques to determine its quality. Spectroscopic and microscopic analyses indicate the synthesis of few-layers graphene (≤ 5 layers) with a high quality. Once approved, the colloidal solution of GNSs was spread over Al substrate by spray coating method, widely used in industrial manufacturing. Raman and near infrared spectroscopies, X-ray diffraction, microscopy techniques including scanning electron, transmission electron and atomic force microscopies were used to investigate morphological and structural behavior of coated GNSs films either on glass or Al substrates. Density functional theory (DFT) calculations revealed the interaction between the two materials is of the physisorption type, with almost no charge transfer and almost no effect of the number of graphene layers. Electrochemical measurements enabled to investigate the corrosion resistance of GNSs films and its stability over time in a 0.5 M sodium chloride (NaCl) solution, similar to marine surroundings. It was found that introducing a GNSs film can indeed reduce the corrosion rate and preserve Al substrate for an extended period.

Experimental comparison of the effects of the two designed probes for exfoliation of graphene by sonication.

In the present research, comparative simulation and experimental investigation were carried out for two different probes to improve the efficiency and quality of few-layer graphene utilizing liquid-phase exfoliation. Two differently designed shaped probes are fabricated for this study, i.e., a simple probe and a stepped probe. The acoustic pressure distribution in the vessel is simulated for both probes at different output powers. In the experiment, both probes exfoliate graphite powder in a mixture of deionized water and ethanol. Different sonication times and output power were studied for different pulse modes. The graphene layers were characterized using Ultraviolet-visible spectroscopy (UV‒Vis), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), and Raman Microscope. The simulation shows that the total displacement on the tip of the stepped probe is 5.7% greater than that of the simple probe. Also, the pressure difference produced by the stepped probe is 9.15 × 106 pa compared to the pressure difference of the simple probe which is 8.23 × 106 pa. Consequently, the stepped probe was more effective in exfoliating graphene. The experimental results show that the absorbance peak in the stepped probe is approximately 32% greater than the absorbance peak in the simple probe with the same output power. Also, the graphene with better quality is produced with the stepped probe compared to the simple probe, which verifies the simulation findings. Additionally, the optimum output power, pulse duration, and sonication time to produce graphene with less time and energy consumption are obtained. The best graphene flakes were obtained via sonication with a pulse duration of 0.7 s, an output power of 282 W, and a sonication time of 45 min using a stepped probe. UV-Vis., SEM and TEM analysis show increases in quantity and quality of the exfoliated flakes. These results are approved by the peak ratio of I2D/IG around 1 in Raman spectra revealed the production of the few-layer graphene by the simple probe and the bilayer graphene by the stepped probe. So, this method is suitable for the production of graphene flakes from graphite in suitable quantity and quality.

Spectroscopic Study of Strain, Oxidation, and Formation of Few-Layer Graphene at Steel Surfaces Tribologically Tested against MoS2 Films.

MoS2 not only has unique optoelectronic properties realizing photonic and semiconductor applications but also serves as a promising solid lubricant in tribological three-body contacts due to its advantageous friction and wear behavior. Its functionality is defined by elementary processes including strain, oxidation processes, and material mixing. However, these mechanisms were not elucidated for MoS2 having transferred from the MoS2 film synthesized at the main body to a steel counter body during tribological ball-on-disk tests. Using spatially and spectrally high-resolved Raman spectroscopy, we study the compressive and tensile strain within the MoS2 transfer material and analyze the oxidation of molybdenum, sulfur, and iron. In addition, we elaborate on the impact of transition metals modifying the MoS2 films on the strain distribution and oxidation processes. Decreasing intensities of the MoS2 Raman lines are accompanied with enhanced intensities of sulphur and molybdenum oxide Raman signatures which are particularly agglomerated at the edges of the tribological track. The formation of tribochemical oxides, including Mo4O11 in the Magnéli-phase, depends weakly on the type of modifying element, and an oxidation of a modification element itself is not detected. We also identified a tribologically induced formation of disordered few-layer graphene at counter-body surface areas which experienced weak thermo-mechanical tribological load. Our results characterize structural and chemical features of the MoS2 transfer material, thus predicting material failure at the microscopic level.

Mechanical properties of freestanding few-layer graphene/boron nitride/polymer heterostacks investigated with local and non-local techniques.

van der Waals two-dimensional materials and heterostructures combined with polymer films continue to attract research attention to elucidate their functionality and potential applications. This study presents the fabrication and mechanical testing of 2D material heterostacks, consisting of few-layer boron nitride and graphene heterostructures synthesized via chemical vapor deposition, capped with a polymethyl methacrylate layer and suspended across ∼200 μm wide trenches using a combined wet-dry transfer method. The mechanical characterization of the heterostacks was performed using two independent approaches: (a) non-local testing with a custom-built tensile testing platform and (b) local load-displacement testing employing atomic force microscopy probes, complemented by finite element simulations. Both approaches provided new results, which are in good agreement with each other. Overall, our findings offer new insights into a combined load capacity in complex multi-material two-dimensional systems, and can contribute to advancing micro and nano-scale device designs and implementations.

Scalable Li-Ion Battery with Metal/Metal Oxide Sulfur Cathode and Lithiated Silicon Oxide/Carbon Anode.

A Li-ion battery combines a cathode benefitting from Sn and MnO2 with high sulfur content, and a lithiated anode including fumed silica, few layer graphene (FLG) and amorphous carbon. This battery is considered a scalable version of the system based on lithium-sulfur (Li-S) conversion, since it exploits at the anode the Li-ion electrochemistry instead of Li-metal stripping/deposition. Sn and MnO2 are used as cathode additives to improve the electrochemical process, increase sulfur utilization, while mitigating the polysulfides loss typical of Li-S devices. The cathode demonstrates in half-cell a maximum capacity of ~1170 mAh gS -1, rate performance extended over 1 C, and retention of 250 cycles. The anode undergoes Li-(de)alloying with silicon, Li-(de)insertion into amorphous carbon, and Li-(de)intercalation through FLG, with capacity of 500 mAh g-1 in half-cell, completely retained over 400 cycles. The full-cells are assembled by combining a sulfur cathode with active material loading up to 3 mg cm-2 and lithiated version of the anode, achieved either using an electrochemical pathway or a chemical one. The cells deliver at C/5 initial capacity higher than 1000 mAh gS -1, retained for over ~40 % upon 400 cycles. The battery is considered a promising energy storage system for possible scaling-up in pouch or cylindrical cells.

Few-layered graphene from cathodic exfoliation in binary solvents and application as a protective layer for lithium metal anodes

Electrochemical exfoliation is a top-down method recognized for its environmentally friendly and scalable approach for producing solution-processable graphene. Among its variation, cathodic exfoliation stands out for its potential in generating few-layer graphene with minimal defects. In this study, we developed a cathodic exfoliation method for graphite foil using a binary solvent system to produce graphene with a reduced number of layers. By employing a 0.1 M LiClO4 solution in a dimethyl sulfoxide and dimethyl carbonate solvents mixture at a 1:2 vol ratio, we enhanced the intercalation of solvated lithium ions into graphite, successfully exfoliating graphene with 5–6 layers. The exfoliated graphene exhibited a low defect density (ID/IG ∼0.05) and a C/O ratio of 33.2. Furthermore, when used as a coating on a separator, the exfoliated few-layered graphene (FLG) sheets demonstrated notable performance as a protective layer for Li metal anode in Li metal battery technology.

A Lithium‐Oxygen Battery Exploiting Carbon Nanotubes, Graphene and Gold Catalyst

AbstractLithium‐oxygen (Li‐O2) battery is considered a high‐energy alternative to Li‐ion one due its characteristic electrochemical conversion process, with the additional advantage of lower cost and environmental impact. However, this emerging battery still requires an enhancement of stability and lifespan to allow its use as a practical energy storage system. In this work we investigate an electrode material benefitting of multiwalled carbon nanotubes (MWCNTs), few layer graphene (FLG), and gold nano‐powder catalyst to improve the Li‐O2 battery performances in terms of energy efficiency, cycle life and stability. Morphological, structural, and electrochemical tests indicate that the composite electrode can actually boost the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), and enhance the Li‐O2 process reversibility, with a capacity of 1000 mAh g−1 over 70 cycles. On the other hand, the tests reveal the role of the gold in decreasing the polarization and increasing the cell life. Therefore, the results suggest the combination of carbons with various morphologies as a suitable architecture for hosting the Li‐O2 reaction products and allowing their reversible reaction. On the other hand, the results highlight the necessity for a better tuning the noble metal characteristics to further enhance the cell performances.

Hydrogen spillover on Ni@Graphene enables robust and efficient catalytic hydrogenation of aqueous levulinic acid to γ-valerolactone

Hydrogenation of biomass-derived aqueous levulinic acid (LA) to produce γ-valerolactone (GVL) is a promising approach from biomass to sustainable platform chemicals. However, the practical application of this method is limited because aqueous LA is highly corrosive to metal-based hydrogenation catalyst in high-temperature hydrothermal environments. Herein, we encapsulated metallic Ni0 particles with a few layers of graphene using a hydrothermal carbon coating method to obtain Ni@FLG-600 catalyst, which exhibited efficient catalytic activity and excellent cycling stability for the hydrogenation of aqueous LA to GVL. The defect-rich graphene shell prevents the Ni0 from acid corrosion meanwhile selectively permits the passage of small H2 molecules. The resulted active H* on Ni0 spills over to the outer surface of graphene shell and reacts with LA. Remarkably, the amount of H* on graphene shell can be the descriptor of activity. This finding presents a new strategy for the fabrication of acid-resistant catalysts for aqueous biomass hydrogenation.

Continuous Monitoring of Monochloramine in Water, and Its Distinction from Free Chlorine and Dichloramine Using a Functionalized Graphene-Based Array of Chemiresistors.

Monochloramine (MCA) is commonly added to drinking water as a disinfectant to prevent pathogen growth. The generation of MCA at the treatment plant requires tight control over both pH and the ratio of free chlorine (FC) to ammonia to avoid forming undesirable byproducts such as dichloramine (DCA) and trichloramine (TCA), which can impart odor and toxicity to the water. Therefore, continuous monitoring of MCA is essential to ensuring drinking water quality. Currently, standard colorimetric methods to measure MCA rely on the use of reagents and are unsuitable for online monitoring. In addition, other oxidants can interfere with MCA measurement. Here, we present a solid-state, reagent-free MCA sensing method using an array of few-layer graphene (FLG) chemiresistors. The array consists of exfoliated FLG chemiresistors functionalized with specific redox-active molecules that have differential responses to MCA, FC, and DCA over a range of concentrations. Chemometric methods were employed to separate the analytes' responses and to generate multivariate calibration for quantification. A minimum of three sensors are required in the array to maintain full functionality. The array has been demonstrated to quantify MCA in buffered and tap water as a low-cost, reagent-free approach to continuous monitoring.

Highly Tunable Interlayer Coupling and Electronic Structures of Few-Layer Graphene with Pressure.

The interlayer electronic coupling is responsible for the electronic structure evolution from monolayer graphene to graphite and for the moiré potential in twisted bilayer graphene. Here we demonstrate that the interlayer transfer integral (hopping parameter) increases nearly 40% with a quite moderate pressure of ∼3.5 GPa, manifested by the resonance peak shift in the infrared spectra of all 2-10 L graphene. A simple model based on the Morse potential enabled us to establish the relationship between the transfer integral and pressure. Our work provides fundamental insights into the dependence of the electronic coupling on the interlayer distance.

Electrochemical Synthesis of a Composite of Few-Layer Graphene Structures with PdNi-Alloy Nanoparticles and Its Electrocatalytic Activity in the Methanol Oxidation Reaction

Electrochemical Synthesis of a Composite of Few-Layer Graphene Structures with PdNi-Alloy Nanoparticles and Its Electrocatalytic Activity in the Methanol Oxidation Reaction

Temperature dependence of phonon energies and lifetimes in single- and few-layered graphene

Temperature dependence of phonon energies and lifetimes in single- and few-layered graphene

Thermophysical Profile of Industrial Graphene Water-Based Nanofluids.

The exceptional properties of high-grade graphene make it an ideal candidate for thermal dissipation and heat exchange in energy applications and nanofluid development. Here, we present a comprehensive study of few-layer graphene (FLG) nanofluids prepared in an industrial context. FLG nanofluids were synthesized through an ultrasound-assisted mechanical exfoliation process of graphite in water with a green solvent. This method produces FLG of high structural quality and stable nanofluids, as demonstrated by electron microscope, dynamic light scattering and ζeta potential analyses. Thermal conductivity measurements of FLG-based nanofluids were conducted in the temperature range of 283.15 K to 313.15 K, with FLG concentrations ranging from 0.005 to 0.200% in wt. The thermal conductivity of FLG nanofluids is up to 20% higher than water. The modeling of nanofluid thermal conductivity reveals that this enhancement is supported by the influence of the thermal resistance at the FLG interface, and the content, average dimensions and flatness of FLG sheets; this latter varying with the FLG concentration in the nanofluid. Additionally, the density and heat capacity of FLG suspensions were measured and compared with theoretical models, and the rheological behavior of FLG nanofluids was evaluated. This behavior is mainly Newtonian, with a weak 5% viscosity increase.

Exceptional fabrication of dead-weight-free silicon and graphitic heterostructures anodes for the next advancement of Li-ion batteries

Silicon (Si)-graphite and graphite (without Si) anodes for Li-ion batteries are developed at ambient conditions through the direct irradiation of CO2 laser, resulting in avoiding the use of binders, conductive carbon additives, and organic and water-based solvents. Furfuryl alcohol (FA) is mixed with Si-graphite and graphite, prepared viscous slurry through the heating, cast on the copper foil, and dried. Irradiation of CO2 laser at the electrode of Si-graphite-FA generates in-situ hetero-structures of Si particles, SiC/Si-SiC nanowires made of spontaneously carbon-coated/oxidized nano-layer (∼ 3nm), and graphitic carbon containing few layers of graphene. All the laser-fabricated electrode material is electrochemically active (∼ 0% dead weight) in the Si-graphite and graphite anodes since the FA converts to the graphitic carbon containing few layers of graphene; thus, the fabrication method could benefit the next industrial development. The discharge capacity of the Si-graphite electrode with total material loading is recorded at 502.2 mAh/g (∼ 1.9 mAh cm−2) and capacity retention of ∼ 80 % over the 50 cycles vs. Li/Li+. Also, it shows good discharge capacity when fabricated with NMC622 in full-cell format. Therefore, this performance is highly stable without any binder and conductive carbon, especially for Si-based battery, which fails rapidly due to substantial volume changes (∼ 300 %). Moreover, the performance is highly stable for graphite anode with a discharge capacity retention of 91 % over the 160 cycles as considered 100 % (∼ 345 mAh/g) after the formation cycle.

Hybrid graphene nanoflakes for electrochemical sensing with multianalyte detection capability

This work illustrates the production of highly electrochemically active graphene by liquid phase exfoliation technique for ultrasensitive electrochemical sensors. An airless high-pressure spray technique was designed to exfoliate bulk natural graphite into nm-scaled flakes (referred as HP50). Transmission electron microscopy, Raman and X-ray photoelectron spectroscopy studies reveal that the HP50 sample has a mixed structure composed of amorphous carbon and a few-layer graphene fraction with high amount of edge plane defects. The HP50 flakes are drop cast on glassy carbon electrodes to test the simultaneous and selective electrochemical detection of hydrogen peroxide, ascorbic acid, and dopamine. In cyclic voltammetry measurements, hydrogen peroxide reduces at −0.2 V vs Ag/AgCl (1 M), while ascorbic acid and dopamine oxidize at 0.1 V vs Ag/AgCl (1 M) and 0.3 V vs Ag/AgCl (1 M), respectively. Linear sweep voltammetry demonstrates high sensitivity (164, 739, and 3357 μA mM−1 cm−2) and low limit of detection (15, 1.7, and 2.1 μM) for the three analytes, respectively. Interference studies confirm a high sensitivity and selectivity towards the different chemical species. The HP50-based multianalyte sensors are also tested against different environmental and commercial samples, illustrating their viability in practical applications.

Excimer-ultraviolet-lamp-assisted selective etching of single-layer graphene and its application in edge-contact devices

Since the discovery of graphene and its remarkable properties, researchers have actively explored advanced graphene-patterning technologies. While the etching process is pivotal in shaping graphene channels, existing etching techniques have limitations such as low speed, high cost, residue contamination, and rough edges. Therefore, the development of facile and efficient etching methods is necessary. This study entailed the development of a novel technique for patterning graphene through dry etching, utilizing selective photochemical reactions precisely targeted at single-layer graphene (SLG) surfaces. This process is facilitated by an excimer ultraviolet lamp emitting light at a wavelength of 172 nm. The effectiveness of this technique in selectively removing SLG over large areas, leaving the few-layer graphene intact and clean, was confirmed by various spectroscopic analyses. Furthermore, we explored the application of this technique to device fabrication, revealing its potential to enhance the electrical properties of SLG-based devices. One-dimensional (1D) edge contacts fabricated using this method not only exhibited enhanced electrical transport characteristics compared to two-dimensional contact devices but also demonstrated enhanced efficiency in fabricating conventional 1D-contacted devices. This study addresses the demand for advanced technologies suitable for next-generation graphene devices, providing a promising and versatile graphene-patterning approach with broad applicability and high efficiency.

The excellent anti-wear and friction-reducing performance of magnesium silicate hydroxide/graphite nanocomposite (MSH-C) as a lubricant additive

Abstract In this paper, MSH-C and MSH-GO composites were prepared by doping MSH with micron-sized few-layer graphene oxide (∼10 um) and nanoscale graphite (∼300 nm), respectively, during the hydrothermal synthesis process. Octadecyltrimethoxysilane (ODTMS) was used as a surfactant to disperse these two types of additives into the fully formulated oil. Tribological tests demonstrated that MSH-C exhibits much more superior tribological properties when compared to MSH-GO. The findings reveal that the oil sample containing 0.5 wt% MSH-C can effectively reduce wear volume (by approximately 26.4%) as compared to fully formulated oil and that the friction coefficient is decreased to ∼0.02. SEM, Raman, FIB-TEM, and TOF-SIMS characterizations and molecular dynamics simulation were used to investigate the tribological mechanism. A tribo-film composed of two layers of different compositions was formed on the worn surface of MSH-C, indicating the synergistic effect of MSH and graphite.

Beyond lithium-ion batteries: Are effective electrodes possible for alkaline and other alkali elements? Exploring ion intercalation in surface-modified few-layer graphene and examining layer quantity and stages

In the pursuit of reliable energy storage solutions, the significance of engineering electrodes cannot be overstated. Previous research has explored the use of surface modifiers (SMs), such as single-side fluorinated graphene, to enhance the thermodynamic stability of ion intercalation when applied atop few-layer graphene (FLG). As we seek alternatives to lithium-ion batteries (LIBs), earth-abundant elements like sodium and potassium have emerged as promising candidates. However, a comprehensive investigation into staging intercalation has been lacking thus far. By delving into staging assemblies, we have uncovered a previously unknown intercalation site that offers the most energetically favorable binding. Here, we study the first three elements in both alkali (Li, Na, K) and alkaline (Be, Mg, Ca) earth metals. Furthermore, the precise mechanism underlying this intercalation system has remained elusive in prior studies. In our work, we employed density functional theory calculations with advanced hybrid functionals to determine the electrical properties at various stages of intercalation. This approach has been proven to yield more accurate and reliable electrical information. Through the analysis of projecting density of states and Mulliken population, we have gained valuable insights into the intricate interactions among the SM, ions, and FLG as the ions progressively insert into the structures. Notably, we expanded our investigation beyond lithium and explored the effectiveness of the SM on ions with varying radii and valence, encompassing six alkali and alkaline earth metals. Additionally, we discovered that the number of graphene layers significantly influences the binding energy. Our findings present groundbreaking concepts for material design, offering diverse and economically viable alternatives to LIBs. Furthermore, they serve as a valuable reference for fine-tuning electrical properties through staging intercalation and the application of SMs.

A High-Performance, Low Defected, and Binder-Free Graphene-Based Supercapacitor Obtained via Synergistic Electrochemical Exfoliation and Electrophoretic Deposition Process.

An integrated electrochemical exfoliation and electrophoretic deposition (EPD) method is developed to achieve a high-performance graphene supercapacitor. The electrochemical delamination of graphite sheet has obtained a low-defected few-layer graphene adorned with oxygen-containing functional groups. Then, the EPD process produced a binder-free electrode to alleviate the graphene restacking problem. The electrode prepared using a deposition voltage of 5 V exhibits the highest specific capacitance of 145.95 F/g at 0.5 A/g from three-electrode measurement. Moreover, this EPD-prepared electrode also demonstrates superior electrochemical properties compared to electrodes fabricated using PVDF binder. In the real symmetrical cell, the EPD-prepared electrode also shows excellent performance with a high rate capability of 82.31 % (from 0.5 A/g to 10 A/g), high cycling stability of 95.00 % (at 5 A/g) after 10,000 cycles, and rapid frequency response with short relaxation time ( ) of 9.73 ms. These results indicate that this integration method is beneficial to construct a high performance binder-free supercapacitor electrode consisting of low-defected graphene materials, low electrode resistance, and less agglomeration of graphene sheets by utilizing an environmentally friendly process.