High-quality Few-layer Graphene Research Articles

Dual frequency ultrasonic liquid phase exfoliation method for the production of few layer graphene in green solvents

In this work, we implement a dual frequency (24 kHz and 1174 kHz) ultrasonic assisted liquid phase exfoliation (ULPE) technique in deionized water (DIW) and other eco-friendly solvents, to produce a variety of high-quality few-layer graphene (FLG) solutions under controlled ultrasonication conditions. The resulting FLG dispersions of variable sizes (∼0.2–1.5 μm2) confirmed by characterisation techniques comprising UV–Vis spectroscopy, Raman spectroscopy and high-resolution transmission electron microscopy (HR-TEM). For the first time we demonstrate that high yield of FLG flakes with minimal defects, stable for 6 + months in a solution (stability ∼ 70 %), can be obtained in less than 1-hour of treatment in either water/ethanol (DIW:EtOH) or water/isopropyl alcohol (DIW:IPA) eco-friendly mixtures.We also scrutinized the underlying mechanisms of cavitation using high-speed imaging synchronized with acoustic pressure measurements. The addition of ethanol or IPA to deionized water is proposed to play a central role in exfoliation as it regulates the extend of the cavitation zone, the intensity of the ultrasonic field and, thus, the cavitation effectiveness. Our study revealed that lateral sizes of the obtained FLG depend on the choice of exfoliating media and the diameter of a sonotrode used. This variability offers flexibility in producing FLG of different sizes, applicable in a wide spectrum of size-specific applications.

Bubble-Mediated Production of Few-Layer Graphene via Vapor-Liquid Reaction between Carbon Dioxide and Magnesium Melt.

It is urgent to develop novel technologies to convert carbon dioxide to graphene. In this work, a bubble-mediated approach via a chemical reaction between carbon dioxide gas and magnesium melt to fabricate a few-layer graphene was illustrated. The morphology and defects of graphene can be regulated by manipulating the melt temperature. The preparation of graphene at 720 °C exhibited an excellent quality of surface and graphitization degree. The high-quality few-layer graphene can be grown under the combined effect of carbon dioxide bubbles and in-situ grown MgO. This preparation method possesses the advantages of high efficiency, low cost, and environmental protection, which may provide a new strategy for the recovery and reuse of greenhouse gases.

Laser-Induced Graphene Formation on Polyimide Using UV to Mid-Infrared Laser Radiation

Our study presents laser-assisted methods to produce conductive graphene layers on the polymer surface. Specimens were treated using two different lasers at ambient and nitrogen atmospheres. A solid-state picosecond laser generating 355 nm, 532 nm, or 1064 nm wavelengths and a CO2 laser generating mid-infrared 10.6 µm wavelength radiation operating in a pulsed regime were used in experiments. Sheet resistance measurements and microscopic analysis of treated sample surfaces were made. The chemical structure of laser-treated surfaces was investigated using Raman spectroscopy, and it showed the formation of high-quality few-layer graphene structures on the PI surface. The intensity ratios I(2D)/I(G) and I(D)/I(G) of samples treated with 1064 nm wavelength in nitrogen atmosphere were 0.81 and 0.46, respectively. After laser treatment, a conductive laser-induced graphene layer with a sheet resistance as low as 5 Ω was formed. Further, copper layers with a thickness of 3–10 µm were deposited on laser-formed graphene using a galvanic plating. The techniques of forming a conductive graphene layer on a polymer surface have a great perspective in many fields, especially in advanced electronic applications to fabricate copper tracks on 3D materials.

Enhancing charge extraction in inverted perovskite solar cells contacts via ultrathin graphene:fullerene composite interlayers.

Improving the perovskite/electron-transporting layer (ETL) interface is a crucial task to boost the performance of perovskite solar cells (PSCs). This is utterly fundamental in an inverted (p-i-n) configuration using fullerene-based ETLs. Here, we propose a scalable strategy to improve fullerene-based ETLs by incorporating high-quality few-layer graphene flakes (GFs), industrially produced through wet-jet milling exfoliation of graphite, into phenyl-C61-butyric acid methyl ester (PCBM). Our new composite ETL (GF:PCBM) can be processed into an ultrathin (∼10 nm), pinhole-free film atop the perovskite. We find that the presence of GFs in the PCBM matrix reduces defect-mediated recombination, while creating preferential paths for the extraction of electrons towards the current collector. The use of our GF-based composite ETL resulted in a significant enhancement in the open circuit voltage and fill factor of triple cation-based inverted PSCs, boosting the power conversion efficiency from ∼19% up to 20.8% upon the incorporation of GFs into the ETL.

Improvement of electromagnetic interference properties of 3D few-layer graphene composite by means of freeze-drying

In this paper, we present a facile, green, and scalable approach for preparing high-quality few-layer graphene (FLG) nanoplatelets through a jet cavitation process. The electromagnetic interference (EMI) performance of FLG of three different lateral sizes is investigated. The midsized FLG (M-FLG) exhibits the best EMI performance. The jet-exfoliation technology used herein preserves the structure of FLG with few structural defects. The ID/IG ratio of M-FLG is as low as 0.092. Furthermore, we introduce 20 wt% M-FLG into paraffin wax to obtain a bulk material for conducting shielding measurements. The shielding effectiveness (SE) in the X-band is 19.28 dB, which exceeds that of unexfoliated graphite. Finally, we use freeze-drying technology to inhibit stacking of the graphene suspension during drying. The SE of the freeze-dried M-FLG (M-FLG-fd) increased considerably to 39 dB. The results indicate that jet cavitation combined with freeze-drying substantially improved the dielectric properties and absorption capabilities of FLG.

Rapid fabrication of high-quality few-layer graphene through gel-phase electrochemical exfoliation of graphite for high-energy-density ionogel-based micro-supercapacitors

Electrochemical exfoliation has been identified as a promising top-down method for obtaining high-quality graphene from graphite. However, conventional anodic exfoliation introduces undesirable oxidation and defects in resultant graphene while cathodic exfoliation eliminates that issue but is time and energy intensive. Herein we report a versatile and rapid fabrication approach for gentle (<1 h ultrasonication) cathodic exfoliation of graphite using only 0.05 M K2SO4 in a polyvinyl alcohol (PVA)-based aqueous gel medium as electrolyte. The few-layer graphene obtained via this route demonstrates high quality (ID/IG ratio of 0.07) and minimal oxidation (3.02 at% oxygen). This gel-exfoliated graphene is found to be particularly suitable for micro-supercapacitors (MSCs) achieving a stable working voltage of 1.2 V in aqueous gel electrolyte, good rate capability, high capacitance of 13.7 F cm−3 at 10 mV s−1 and energy density of 2.74 mW h cm−3. Importantly, gel-exfoliated graphene is also compatible with ionogel electrolyte, achieving a high voltage of 3.4 V and resulting in competitive energy density of 14.5 mW h cm−3, outperforming most reported graphene-based MSCs. The MSCs show excellent long cycling performance retaining nearly 100% capacitance over 10,000 cycles. We also demonstrate the mechanical flexibility and integration of the MSCs connected in series and parallel for powering future microscale flexible electronics.

Exploring graphene and graphene/nanoparticles as fillers to enhance atomic oxygen corrosion resistance of polyimide films

Graphene and graphene/nanoparticles were explored to enhance the atomic oxygen (AO) corrosion resistance of polyimide (PI) composites. Graphene flakes (EGs) were produced by electrochemical method using alternating current (AC). A series of EGs/PI composite films were prepared by in-situ polymerization. After exposure to AO, although all EGs/PI composite films were resistant to AO, we found a much greater enhancement, i.e. adding 1.5 wt% EGs can reduce the mass loss of the composite by 42%. On this basis, EGs/SiO 2 and EGs/Al 2 O 3 co-doping fillers were firstly used to further enhance the AO corrosion resistance of PI. The sample co-doped 1.5 wt% EGs with 3 wt% SiO 2 could achieve a 71% decrease in the composite’ mass loss. Bonding effect and barrier effect of graphene fillers are responsible for the enhanced protective performance. At the same time, the “synergistic effect” between EGs and nanoparticles also plays a role in the enhancement. These preliminary but interesting results pave a novel way for the study of AO corrosion resistance. • High-quality few-layer graphene was successfully exfoliated by electrochemical method via alternating current. • A series of EGs/PI composite films were compared. • EGs/SiO 2 and EGs/Al 2 O 3 co-doping fillers were firstly used to further enhance the AO corrosion resistance of PI. • The “synergistic effect” between EGs and nanoparticles plays a role in the enhancement.

Microwave plasma-based high temperature dehydrogenation of hydrocarbons and alcohols as a single route to highly efficient gas phase synthesis of freestanding graphene

Understanding underlying processes behind the simple and easily scalable graphene synthesis methods enables their large-scale deployment in the emerging energy storage and printable device applications. Microwave plasma decomposition of organic precursors forms a high-temperature environment, above 3000 K, where the process of catalyst-free dehydrogenation and consequent formation of C2 molecules leads to nucleation and growth of high-quality few-layer graphene (FLG). In this work, we show experimental evidence that a high-temperature environment with a gas mixture of H2 and acetylene, C2H2, leads to a transition from amorphous to highly crystalline material proving the suggested dehydrogenation mechanism. The overall conversion efficiency of carbon to FLG reached up to 47%, three times as much as for methane or ethanol, and increased with increasing microwave power (i.e. with the size of the high-temperature zone) and hydrocarbon flow rate. The yield decreased with decreasing C:H ratio while the best quality FLG (low D/G–0.5 and high 2D/G–1.5 Raman band ratio) was achieved for C:H ratio of 1:3. The structures contained less than 1 at% of oxygen. No additional hydrogen was necessary for the synthesis of FLG from higher alcohols having the same stoichiometry, 1-propanol and isopropanol, but the yield was lower, 15%, and dependent on the atom arrangement of the precursor. The prepared FLG nanopowder was analyzed by scanning electron microscopy, Raman, x-ray photoelectron spectroscopy, and thermogravimetry. Microwave plasma was monitored by optical emission spectroscopy.

(Invited) Porphyrinoid-Carbon Nanostructure Ensembles

The development of new chromophoric receptors able to bind curved carbon nanostructures is at the epicenter of the quest for fullerene-based organic photovoltaics with improved performances. Herein, a subphthalocyanine-based multicomponent ensemble consisting of two electron-rich SubPc-monomers rigidly attached to the convex surface of a electron-poor SubPc-dimer has been synthesized and characterized. Such a unique conformation, especialy in terms of the two SubPc-monomers, together with the overall stiffness of the tether endows the multicomponent system with a well-defined tweezer-like topology to efficiently embrace a fullerene in its inner cavity. On the other hand, the selective on-surface synthesis of a porphyrin-graphene nanoribbon hybrid has been accomplished. The atomically precise structure of the hybrid has been unambiguously characterized by bond-resolved scanning tunneling microscopy and noncontact atomic force microscopy. Finally, a novel electron donor-acceptor hybrid consisting of a NIR absorbing azulenocyanine as electron donor and few-layer graphene as electron acceptor was prepared. The extended aromatic core of azulenocyanine assist in the exfoliation of graphite and allows formation of a very high-quality few-layer graphene azulenocyanine hybrid system.

Facile and scalable synthesis of high-quality few-layer graphene from biomass by a universal solvent-free approach

Graphene has attracted tremendous interest for its applications in numerous fields. However, there remain challenges to the scalable and commercially viable production of graphene for practical applications. In this study we present a universal and solvent-free approach to preparing few-layer graphene flakes from various low-cost biomass precursors via a facile and scalable solvent-free shearing/graphitization method. The introduction of Fe3+ and Cl− into diverse biomass precursors during the shearing process enables the formation of graphene during the subsequent catalytic carbothermal graphitization reaction. The size of the obtained graphene flakes is generally determined by the shearing time and rate. The as-synthesized biomass-derived few-layer graphene flakes benefit from the relatively high Hall mobility (1.17 × 105 cm2/V∙s), sheet resistance (0.36 kΩ/sq), and electrical conductivity (1.39 × 104 S/m). Its comparable conductivity value and ease of preparation make it a suitable support for many important technological applications, including as a suitable catalyst support for electrocatalytic oxygen reduction reactions. Moreover, there are many readily available sources of biomass for use as the precursor, and even biowaste precursors can be used. Therefore, this concise solvent-free shearing/graphitization method has potential as a universal approach to the scalable synthesis of biomass-derived high-quality graphene materials for a wide variety of applications.

Facile Synthesis of Few-Layer Graphene Oxide from Cinnamomum camphora

This study presents a facile synthesis technique to produce few-layer graphene oxide from Cinnamomum camphora (Camphor L.). Camphor upon carbonization and chemical oxidation leads to the formation of few-layer graphene oxide sheets of around ten layers with a lateral size of 4.14 nm and stacking height of 3.10 nm. AFM and SEM analysis results reveal the wrinkled morphology of the graphene oxide sheets formed. The sharp G band and the relative intensity of the defect to the graphitic band in the Raman spectrum indicate the formation of nanocrystalline graphene oxide sheets with fewer defects. The FTIR spectrum and the deconvoluted C 1s XPS spectrum of graphene oxide synthesized reveal the presence of predominant sp2 hybridized carbon along with carbon bound to various oxygen functionalities. In brief, the formation of high-quality few-layer graphene oxide from an abundant, low-cost, and readily available botanical precursor is herein reported.

High-Quality Few-Layer Graphene on Single-Crystalline SiC thin Film Grown on Affordable Wafer for Device Applications.

Graphene is promising for next-generation devices. However, one of the primary challenges in realizing these devices is the scalable growth of high-quality few-layer graphene (FLG) on device-type wafers; it is difficult to do so while balancing both quality and affordability. High-quality graphene is grown on expensive SiC bulk crystals, while graphene on SiC thin films grown on Si substrates (GOS) exhibits low quality but affordable cost. We propose a new method for the growth of high-quality FLG on a new template named “hybrid SiC”. The hybrid SiC is produced by bonding a SiC bulk crystal with an affordable device-type wafer and subsequently peeling off the SiC bulk crystal to obtain a single-crystalline SiC thin film on the wafer. The quality of FLG on this hybrid SiC is comparable to that of FLG on SiC bulk crystals and much higher than of GOS. FLG on the hybrid SiC exhibited high carrier mobilities, comparable to those on SiC bulk crystals, as anticipated from the linear band dispersions. Transistors using FLG on the hybrid SiC showed the potential to operate in terahertz frequencies. The proposed method is suited for growing high-quality FLG on desired substrates with the aim of realizing graphene-based high-speed devices.

Epitaxial synthesis of graphene on 4H-SiC by microwave plasma chemical vapor deposition

Epitaxial graphene (EG) on semi-insulating SiC prepared by a thermal decomposition method is the most promising strategy for graphene application in large-scale integrated circuits due to compatibility with current semiconductor processes. In this study, high-quality few-layer graphene (FLG) was epitaxially grown on the semi-insulating on-axis 4H-SiC by microwave plasma chemical vapor deposition (MPCVD). Both sides of the SiC substrate were etched with H2 plasma at ∼1000 °C to promote the nucleation and the growth of graphite before epitaxial growth. The surface morphology and properties of EG on SiC (0001) were measured by energy-dispersive x-ray (EDX), x-ray photoelectron spectrogram (XPS), and atomic force microscope (AFM). The qualities of EG grown on the two surfaces of SiC at various temperatures were checked by Raman spectroscopy. Furthermore, the EG growth was controlled by the effect of Ar plasma in the MPCVD and the few-layer (1–3 layers) high-quality EG films were formed on SiC (000). The room temperature Hall mobility of EGs up to 2790 cm2 V−1 s−1 on SiC(000) was realized.

Direct chemical vapor deposition growth of graphene on Ni particles using solid carbon sources

Graphene has attained a considerable amount of popularity as an attractive ultra-thin reinforcement for nickel (Ni) matrix composites in recent years. However, its excellent reinforcement efficiency is suffered from the agglomeration of graphene nanosheets in manufacturing process and the poor bonding strength of graphene with Ni matrix. To overcome these two problems, one of the efficient strategies is to in-situ grow graphene reinforcements on Ni particles for powder metallurgy. This work aims to synthesize uniform graphene@Ni composite particles by using polymethyl methacrylate (PMMA) as the solid sources for chemical vapor deposition (CVD) process. The results demonstrate that few-layer or multilayer graphene with different morphologies can be grown on the particles by controlling the PMMA content and annealed temperature, respectively. The optimum condition for the formation of high-quality few-layer graphene is 1.0 mg·ml−1 PMMA and 900 °C. A competition mechanism rises from the growth kinetic, and the spatial confinement effect has led to the formation of graphene with different microstructures and morphologies.

Characterization of few-layer graphene aerosols by laser-induced incandescence

Gas-phase synthesis is a promising route for producing large amounts of high quality few-layer graphene (FLG) nanoparticles economically, but optimizing these processes requires a detailed understanding of the formation kinetics, which in turn demands diagnostics for characterizing this material in situ. This work reports the first laser-induced incandescence measurements on FLG aerosols. Temporally- and spectrally-resolved incandescence signals from FLG particles are measured and used to calculate pyrometric temperatures. Differences between incandescence signals and pyrometric temperatures obtained from FLG and aerosolized soot nanoaggregates are attributed to the larger absorption cross-section and specific surface area of FLG compared to soot. LII signal intensity is found to vary linearly with particle number concentration measured independently by a condensation particle counter. Overall, these results demonstrate the potential for laser-induced incandescence to measure FLG nanoparticle mass (volume) fraction and active surface area in situ, as well as to differentiate graphene from other types of carbonaceous nanomaterials online.

(Invited) Covalent and Supramolecular Multicomponent Porphyrinoid-Carbon Nanostructure Ensembles

Herein, a novel electron donor-acceptor hybrid consisting of a NIR absorbing azulenocyanine as electron donor and few-layer graphene as electron acceptor was prepared. The extended aromatic core of azulenocyanine assist in the exfoliation of graphite and allows formation of a very high-quality few-layer graphene azulenocyanine hybrid system. The development of new chromophoric receptors able to bind curved carbon nanostructures is at the epicenter of the quest for fullerene-based organic photovoltaics with improved performances. Herein, a subphthalocyanine-based multicomponent ensemble consisting of two electron-rich SubPc-monomers rigidly attached to the convex surface of a electron-poor SubPc-dimer has been synthesized and characterized. Such a unique conformation, especialy in terms of the two SubPc-monomers, together with the overall stiffness of the tether endows the multicomponent system with a well-defined tweezer-like topology to efficiently embrace a fullerene in its inner cavity. The selective on-surface synthesis of a porphyrin-graphene nanoribbon hybrid has been accomplished. The atomically precise structure of the hybrid has been unambiguously characterized by bond-resolved scanning tunneling microscopy and noncontact atomic force microscopy.

A comparative study for producing few-layer graphene sheets via electrochemical and microwave-assisted exfoliation from graphite powder

Graphene’s astonishing properties drew attention of many scientists to dedicate a lot of their time to find out more about this extraordinary material. However, challenges continue to produce high-quality graphene in large quantities by using inexpensive and readily available methods. In this study, three different graphite powders have been used as starting materials to produce few-layer graphene sheets, which are pure natural graphite (NGr) and two different electrochemically treated expanded graphite EE1 and EE2. Two simple and time-effective techniques have been applied on the samples interchangeably to investigate the order effect on producing graphene sheets in few-layer form. These techniques are sonication in dimethylformamide (DMF) for one hour and rapid microwave irradiation for 30 s. The study suggests that if the graphite powder is treated first with a strong exfoliation reagent followed by microwave irradiation, the obtained graphene will be high-quality few-layer (~ 5 layers). Sonication in DMF has worked to increase the inter-planar spacing between graphite layers, while microwave irradiation has worked to decrease the defect density ratio that resulted after sonication process. Our work suggests a novel route to prepare high-quality few-layer graphene sheets, not only with time efficiency, low-cost, and without using harmful chemicals, but also an adequate method for large-scale high-efficiency production of graphene materials.

Design and construction of few-layer graphene cathode for ultrafast and high-capacity aluminum-ion batteries

Graphite is a key cathode material for aluminum-ion batteries (AIBs), but appears poor structural stability and cyclability due to the slow kinetics of intercalation and usage of conductive additives. So far, reasonable design and construction of high-conductive, flexible, binder-free graphitic film cathodes with fast kinetics of anion intercalation mechanism still remains challenging for high-power and high-energy AIBs. Herein, we report the reasonable design for facile construction of high-conductive and mechanically flexible film cathode of high-quality few-layer graphene (FLG) nanosheets for high capacity and fast AIBs, thanks to the strong synergy of ionic surface adsorption/desorption of graphene and ionic intercalation/deintercalation of graphite. The FLG nanosheets prepared by electrochemical cathodic exfoliation from graphite, exhibit predominantly 3–5 layers, extremely low ID/IG ratio of <0.05, ultrahigh C/O atom ratio of 43, and excellent solution-processability, which allow for large-area production of highly conducting (1300 ​S ​cm−1), binder-free and densely stacked FLG films as extremely stable cathodes for ultrafast AIBs. It’s worth mentioning that the resulting AIBs exhibit high capacity of 173 mAh g−1 at 1 ​A ​g−1, outstanding rate capability with exceptionally large capacity of 101 mAh g−1 at 10 ​A ​g−1 that outperforms the most reported graphitic materials, long-term cyclability without noticeable capacity loss after 10000 cycles, and good performance at wide range of temperatures. The multiple characterizations and theoretical simulation further confirm that the unique performance is attributed to the synergistic mechanisms of diffusion-controlled intercalation and surface adsorption of FLG film. Therefore, this work will open intriguing opportunities for the development of advanced AIBs.

The role of exfoliating solvents for control synthesis of few-layer graphene-like nanosheets in energy storage applications: Theoretical and experimental investigation

Control synthesis of graphene is the need of the hour for versatile applications in the energy sector. A novel yet facile mechanical exfoliation technique has been developed here for the synthesis of high-quality few-layer graphene (FLG)-like nanosheets from a biowaste material, peanut shells. Biowastes, on the other hand, is one of the most abundant natural resources for carbon, which needs to be recycled. Therefore, careful selection of biomass as well as proper synthesis engineering can provide a sustainable route to attain mass-scale graphene-like nanosheet production. Liquid-phase mechanical exfoliation in various solvent systems such as an organic solvent, isopropyl alcohol (IPA), water and hydrogen peroxide (as the best medium for cavitation), as well as an inorganic acidic solution of phosphoric acid which caused the exfoliation by intercalation, has been explored. Depending on the nature of solvents, reduction in dimensions of the as-derived exfoliated peanut-shell derived graphene-like nanosheets (EPSGs) occur, which ensures the number of layers of the sheets. In order to elucidate the role of exfoliating solvents, DFT-based computational studies have also been performed in detail. The as-synthesized EPSGs were tested for their subsequent applications in supercapacitor. The electrochemical results are consistent with the theoretical finding that organic solvents, like IPA, has a better exfoliating effect in the liquid-phase exfoliation and has shown excellent specific capacitance (323 F g−1) as well as energy density (101 W h kg−1).

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.