Pristine Graphene Films Research Articles

Optimizing Atmospheric Ion Harvesting Electrodes with Graphene for Clean Energy Generation Based on Capacitive Properties and Energy Storage

The atmosphere is rich in positive ions, rendering it electrically more positive than the Earth's surface. This characteristic presents the atmosphere as a potential source of renewable energy through ion harvesting. This study harnesses the electrical properties by optimizing ion harvesting electrodes using pristine graphene and graphene-Au thin films to generate clean electricity. Research methods included Raman Spectroscopy and Cyclic Voltammetry (CV) to assess the surface characteristics and capacitance of the graphene samples, along with laboratory-scale ion harvesting simulations to evaluate the energy data produced in the ion harvesting process. The samples used in this study were identified as bilayer graphene, as confirmed by Raman Spectroscopy. CV testing yielded capacitance values of 0.40288 F for pristine graphene and 0.44879 F for graphene-Au samples. According to ion harvesting simulations, graphene-Au generated approximately 6.8 times more energy than pristine graphene and five times more energy than copper alone. The respective energy outputs for graphene-Au, pristine graphene, and pure copper were 1.376 mW, 1.157 mW, and 0.374 mW. These results demonstrate that adding a graphene layer to the atmospheric ion-harvesting electrode can optimize the electricity generation process.

Highly efficient sers performance from the silver nanoparticles/graphene nanoribbons/ cellulose paper

Introduction: Metal/graphene heterojunction structure has been one of the most crucial tools in the growth of high-performance Surface-enhanced Raman spectroscopy (SERS) platform, which is appropriate for sensing applications. In this research, we developed a SERS platform, graphene nanoribbons (GNRs) decorated silver nanoparticles (AgNPs) on cellulose paper substrate, in which GNRs was synthesized by wet chemical based on unzipping process of Multi-walled carbon nanotubes (MWCNTs), then GNRs hybridized with AgNPs through magnetron sputtering method.
 Methods: The morphology of graphene nanoribbons coated-Ag (AgNPs@GNRs) was analyzed by field emission scanning electronic microscopy (FESEM) and transmission electron microscopy (TEM). The quality and thermal stability of GNRs were characterized using thermogravimetric analysis. Besides, its structure and quality were also characterized by Raman spectroscope.
 Results: The results show that the unzipping process of MWCNTs to form GNRs was strongly affected by dispersing time and stirring temperature. The suitable condition creating the Graphene Nanoribbons using MWCNTs was the dispersing time of 10 mins in an acid environment, stirring in 30 mins at room temperature and in 45 min at 100°C. Moreover, SERS platform of AgNPs@GNRs exhibit the outstanding SERS signal enhancement with rhodamine 6G (R6G) low concentration of 10-5 M compared with pristine graphene and silver thin film. This could be attributed to the synergistic effect between AgNPs, GNRs and analyze molecules based on the enhancement of electromagnetic mechanism (EM) and chemical mechanism (CM), which plays a vital role in promoting the improvement SERS behavior.
 Conclusion: Ag NPs assembled onto graphene nanoribbons/ cellulose paper substrate could also serve as SERS active substrates for practical applications in various fields at trace levels.

Superhigh Electromagnetic Interference Shielding of Ultrathin Aligned Pristine Graphene Nanosheets Film.

Ultrathin, lightweight, high-strength, and thermally conductive electromagnetic interference (EMI) shielding materials with high shielding effectiveness (SE) are highly desired for next-generation portable and wearable electronics. Pristine graphene (PG) has a great potential to meet all the above requirements, but the poor processability of PG nanosheets hinders its applications. Here, efficient synthesis of highly aligned laminated PG films and nacre-like PG/polymer composites with a superhigh PG loading up to 90 wt% by a scanning centrifugal casting method is reported. Due to the PG-nanosheets-alignment-induced high electrical conductivity and multiple internal reflections, such films show superhigh EMI SE comparable to the reported best synthetic material, MXene films, at an ultralow thickness. An EMI SE of 93 dB is obtained for the PG film at a thickness of ≈100 µm, and 63 dB is achieved for the PG/polyimide composite film at a thickness of ≈60 µm. Furthermore, such PG-nanosheets-based films show much higher mechanical strength (up to 145 MPa) and thermal conductivity (up to 190 W m-1 K-1 ) than those of their MXene counterparts. These excellent comprehensive properties, along with ease of mass production, pave the way for practical applications of PG nanosheets in EMI shielding.

Micro-patterned metal current collectors for high aspect ratio flexible graphene supercapacitors

When graphene thin films are used as high aspect ratio supercapacitor electrodes, their performance can be limited due to graphene’s relatively low electrical conductivity, compared with metals. The integration of the graphene film with a metal current collector can overcome this limited conductivity. However, poor adhesion between pristine graphene and metal films adversely affects a supercapacitor’s flexibility. In this study, a metal current collector film was patterned on a flexible substrate in the form of a microgrid, via selective laser sintering of silver nanoparticles. The delamination of the graphene film was mitigated by interlocking it with the substrate using laser transmission welding. A specific capacitance of 5.8 mF/cm2 was obtained at 1.5 mA/cm2 with PVA- H3PO4 electrolyte, and the capacitance was retained at up to 94% when the electrode length was increased by six times. In addition, a thin gold (Au) layer was electrodeposited to the silver grid to improve its electrochemical stability. As a result, the capacitance change remained below 10% for 10,000 charging/discharging cycles. The electrodeposition of pseudocapacitive manganese dioxide (MnO2) nanoparticles was also performed, further increasing the capacitance to 49.1 mF/cm2 at 1 mA/cm2 with 0.5 M Na2SO4 electrolyte.

Mosquito bite prevention through graphene barrier layers

Graphene-based materials are being developed for a variety of wearable technologies to provide advanced functions that include sensing; temperature regulation; chemical, mechanical, or radiative protection; or energy storage. We hypothesized that graphene films may also offer an additional unanticipated function: mosquito bite protection for light, fiber-based fabrics. Here, we investigate the fundamental interactions between graphene-based films and the globally important mosquito species, Aedes aegypti, through a combination of live mosquito experiments, needle penetration force measurements, and mathematical modeling of mechanical puncture phenomena. The results show that graphene or graphene oxide nanosheet films in the dry state are highly effective at suppressing mosquito biting behavior on live human skin. Surprisingly, behavioral assays indicate that the primary mechanism is not mechanical puncture resistance, but rather interference with host chemosensing. This interference is proposed to be a molecular barrier effect that prevents Aedes from detecting skin-associated molecular attractants trapped beneath the graphene films and thus prevents the initiation of biting behavior. The molecular barrier effect can be circumvented by placing water or human sweat as molecular attractants on the top (external) film surface. In this scenario, pristine graphene films continue to protect through puncture resistance-a mechanical barrier effect-while graphene oxide films absorb the water and convert to mechanically soft hydrogels that become nonprotective.

Inkjet printed pseudocapacitive electrodes on laser-induced graphene for electrochemical energy storage

Pseudocapacitance boosts the capacitance of supercapacitors by introducing additional redox sites with fast faradaic reactions. Here, we demonstrate the preparation of pseudocapacitive electrodes by inkjet printing bis-terpyridyl based molecular cobalt complexes as pseudocapacitive additives on laser induced graphene films. The substrate temperature during inkjet printing can effectively tune the morphology of the pseudocapacitive additives by overcoming the influence of coffee ring effect. Under optimum conditions, the capacitance of the pseudocapacitive electrodes was enhanced for 75 times over pristine graphene films. This approach can also be employed for the deposition of other functional materials via inkjet printing.

Flexible cellulose nanofibril/pristine graphene nanocomposite films with high electrical conductivity

Cellulose nanofibrils extracted from wood are natural polymer nanomaterials with high specific surface area, low density, high strength, and high elastic modulus. However, they are insulating materials and cannot be directly used in the electronic devices. Herein, pristine graphene (PG) exfoliated by triethanolamine was used to composite with TEMPO-oxidized cellulose nanofibril (TOCN) to prepare the conductive nanocomposite films in aqueous system. As a reference, the graphene oxide (GO) and reduced graphene oxide (RGO) were also composited with TOCN to prepared the TOCN−GO and TOCN−RGO composite films. The maximum electrical conductivity of the TOCN−PG film was 568 S/cm with 20 wt% PG, achieving great enhancement than the TOCN film with insulation. It was also much higher than the TOCN−GO films (3.44 × 10−11 S/cm) and about 185 times more than TOCN−RGO composite films (3.06 S/cm). Moreover, the TOCN−PG composites containing 10 wt% PG exhibited excellent tensile strength, elastic modulus and the elongation at break of 389 MPa, 8.0 GPa and about 20%, respectively, much higher than those of the neat TOCN film.

Wetting-Induced Climbing for Transferring Interfacially Assembled Large-Area Ultrathin Pristine Graphene Film.

Owing to inherent 2D structure, marvelous mechanical, electrical, and thermal properties, graphene has great potential as a macroscopic thin film for surface coating, composite, flexible electrode, and sensor. Nevertheless, the production of large-area graphene-based thin film from pristine graphene dispersion is severely impeded by its poor solution processability. In this study, a robust wetting-induced climbing strategy is reported for transferring the interfacially assembled large-area ultrathin pristine graphene film. This strategy can quickly convert solvent-exfoliated pristine graphene dispersion into ultrathin graphene film on various substrates with different materials (glass, metal, plastics, and cloth), shapes (film, fiber, and bulk), and hydrophobic/hydrophilic patterns. It is also applicable to nanoparticles, nanofibers, and other exfoliated 2D nanomaterials for fabricating large-area ultrathin films. Alternate climbing of different ultrathin nanomaterial films allows a layer-by-layer transfer, forming a well-ordered layered composite film with the integration of multiple pristine nanomaterials at nanometer scale. This powerful strategy would greatly promote the development of solvent-exfoliated pristine nanomaterials from dispersions to macroscopic thin film materials.

Comparing Structural and Electrical Properties of Fluorinated Graphene, Graphene Oxide, and Graphene Films Functionalized with N-Methylpyrrolidone

The article presents comparison of structural and electrical properties of fluorinated graphene (FG), graphene oxide (GO), and graphene films functionalized with N-methylpyrrolidone (G-NMP). The obtained functionalized graphene films were continuous, having no ruptures, their thickness was 20–50 nm. Fluorinated films are formed from fluorinated areas and corrugated graphene islets. The size and shape of microstructures on G-NMP surfaces depend on the duration of NMP treatment. GO films demonstrate a rippled surface morphology. The resistance of all films of functionalized graphene exceeds that of pristine graphene films (several kilohms). GO and FG films exhibit dielectric properties. Current-voltage characteristics of FG demonstrate two features: stepwise current increase and negative differential resistance (NDR). Functionalized graphene can be used in flexible electronics, particularly in planar printing technologies.

Cleaning of graphene surfaces by low-pressure air plasma.

The polymer residues still present on a chemical vapour-deposited graphene surface after its wet transfer by the poly(methyl methacrylate) method to the arbitrary substrates, tend to cause problems such as electrical degradation and unwanted intentional doping. In this study, by using an effective cleaning method for the graphene surface by air-assisted plasma, the graphene surface was cleaned significantly without damaging the graphene network, which resulted in the reduction (approx. 71.11%) of polymer residues on its surface. The analysis reveals that this approach reduced the D-band (impurities, polymer residues) formation while maintaining the π-bonding of the graphene, which affects conductivity. By characterizations of the optical microscope, Raman spectroscopy and atomic force microscopy, we obtained a significantly cleaner graphene surface (roughness of 4.1 nm) compared to pristine graphene (roughness of 1.2 nm) on a SiO2 substrate. In addition, X-ray photoelectron spectroscopy data revealed that the C1s peak of the air-assisted graphene film was higher than the one of a pristine graphene film, indicating that a cleaner graphene surface was obtained.

Growth of boron-doped few-layer graphene by molecular beam epitaxy

We investigated the growth of boron-doped few-layer graphene on α-Al2O3 (0001) substrates by molecular beam epitaxy using two different growth approaches: one where boron was provided during the entire graphene synthesis and the second where boron was provided only during the second half of the graphene growth run. Electrical measurements show a higher p-type carrier concentration for samples fabricated utilizing the second approach, with a remarkable modulation in the carrier concentration of almost two orders of magnitude in comparison to the pristine graphene film. The results concerning the influence of the boron flux at different growth stages of graphene on the electrical and physicochemical properties of the films are presented.

All-Carbon Electrodes for Flexible Solar Cells

Transparent electrodes based on carbon nanomaterials have recently emerged as new alternatives to indium tin oxide (ITO) or noble metal in organic photovoltaics (OPVs) due to their attractive advantages, such as long-term stability, environmental friendliness, high conductivity, and low cost. However, it is still a challenge to apply all-carbon electrodes in OPVs. Here, we report our efforts to develop all-carbon electrodes in organic solar cells fabricated with different carbon-based materials, including carbon nanotubes (CNTs) and graphene films synthesized by chemical vapor deposition (CVD). Flexible and semitransparent solar cells with all-carbon electrodes are successfully fabricated. The best power conversion efficiency achieved for the devices with all-carbon electrodes is 0.63%, comparable to the reported performance of OPVs using pristine CVD graphene films as anodes on rigid substrates (glass). Moreover, the current densities of as-obtained devices are comparable to those assembled with all-carbon active layers and standard electrodes (e.g., ITO and metal), which indicates that the all-carbon electrodes made of CNT and graphene films are suitably effective for carrier collection and extraction. Our results present the feasibility and potential of applying all-carbon electrodes based on graphitic nanomaterials in next-generation carbon-based photovoltaics.

Enhanced Nonlinear Saturable Absorption of MoS2/Graphene Nanocomposite Films

MoS2/graphene nanocomposite films are fabricated by vacuum filtering with liquid-phase exfoliated MoS2/graphene suspension. The nanocomposite films are characterized by Raman spectroscopy, UV–vis spectroscopy, and atomic force microscopy, indicating the optical films with a large scale and high optical homogeneity. The enhanced saturable absorption of MoS2/graphene nanocomposite films compared with pristine MoS2 film and graphene film is investigated using an open-aperture Z-scan technique with a femtosecond laser at 800 nm. The nonlinear absorption coefficient of MoS2/graphene nanocomposite film is ∼ −1217.8 cm/GW, which is larger than that of MoS2 film (∼ −136.1 cm/GW) and graphene film (∼ −961.6 cm/GW) at the same condition. The imaginary part of the third-order nonlinear optical susceptibility of the nanocomposite film can reach Imχ(3) ∼ 10–9 esu with a figure of merit ∼10–14 esu cm, low saturable intensity (∼157.0 GW/cm2), and high modulation length (∼32%). A coupling model is considered in order to ...

The Effect of Varying Ultrafast Pulse Laser Energies on the Electrical Properties of Reduced Graphene Oxide Sheets in Solution

Laser treatment of graphene oxide solution among other techniques is a well-established technique for producing reduced graphene sheets. However, production of high-quality ultra-low sheet resistance reduced graphene oxide (rGO) sheets in solution has been a challenge due to their high degree of randomness, defect-rich medium, and lack of controlability. Recent studies lack an in-depth analytic comparison of laser treatment parameters that yield the highest quality rGO sheets with a low defect ratio. Hence, in this study, we implement a comprehensive comparison of laser treatment parameters and their effect on the yielded rGO sheets from an electronic and physical standpoint. Ultra-low sheet resistance graphene oxide sheets were fabricated using ultrafast laser irradiation with different laser pulse energies in the range of 0.25–2 mJ. Laser treatment for 10 min using a pulse energy of 1 mJ resulted in an increase in the defect spacing, accompanied by a large red shift in the optical absorption of the C=C bond, indicating significant restoration of the sp 2 carbon bonds. These enhancements resulted in a significant reduction in the electrical resistance of the rGO flakes (up to 2 orders of magnitude), raising the electron mobility of the films produced using the irradiated graphene oxide a step closer to that of pristine graphene films. From this study, we can also deduce which exposure regimes result in the fabrication of quantum dots and continuous defect-free films.

Effects of Reduced Dimensional Scales on the Electrochemical Reversible Li-Storage and Associated Integrity of Well-Ordered Graphitic/Graphenic Carbon

Even though graphitic carbon is the most commonly used anode material for Li-ion battery, it possesses low gravimetric capacity and rate capability; which has stimulated research towards alternate anode materials. Recent investigations have indicated that graphene, the two dimensional building block of graphitic carbon, may possess enhanced Li-storage capacity, along with improved rate capability. However, the mechanism of reversible Li-storage and mechanical/structural integrity of graphene upon repeated Li-insertion/removal are yet to be understood; with too many contradictory reports are available in the literature concerning the Li-storage reversible capacity/behavior and cyclic stability of graphenic carbon [1, 2].Therefore, using fairly pristine and well-ordered CVD-grown few layers graphene (FLG) film as model material and performing extensive electrochemical/structural investigations, also involving in-situ studies, we have been able to develop better insights into the inter-related phenomena concerning lithiation/delithiation mechanisms leading to higher Li-capacity, stress developments at different states of charges and mechanical integrity upon lithiation/delithiation of such nanoscaled ‘graphitic’ carbon. Presence of distinct features corresponding to formation of Li-intercalated graphite compounds below 0.3 V against Li/Li+ in chronopotentiograms (viz., potential plateaus) and cyclic voltammograms (viz., current peaks) obtained with FLG (~7 graphene layers), similar to thicker (~450 nm) bulk graphite (TBG) film, indicates that Li-intercalation/de-intercalation in the interlayer spaces does take place even for such reduced (nano)dimensional form of ‘graphitic’ carbon. Furthermore, the capacity obtained for FLG within just the same lower potential window (i.e., below 0.3 V) and even after subtracting entire possible contribution from Cu substrate underneath, is ~3-4 times greater than the overall specific capacity obtained with TBG (i.e., ~380 mAh/g) [3]. Analysis based on the electrochemical response indicated that this excess capacity in the case of FLG (or Li-storage in FLG below 0.25 V) has contributions from both diffusion controlled ‘classical’ bulk processes of Li-storage, as well as surface phenomena. In this context, simulation based on density functional theory indicated that the excess Li-capacity of graphene (beyond the LiC6 composition) is associated with additional stable Li-storage on the surface of the outer or topmost graphene layer in the form of two Li-layers (but different from Li-plating) and segregation of Li close to the ‘stepped’ exposed edges of the inner graphene layers; the relative contributions from which would go down with increase in dimensional scale [3]. Even though the nanoscaled FLG possess higher specific capacity, as well as improved rate capability, with respect to the thicker TBG, cyclic stability of FLG over the full range of lithiation/delithiation (i.e., between 2 and 0.001 V against Li/Li+) was observed to be inferior w.r.t. TBG. Interestingly, the cyclic stability, when compared with respect to the capacities obtained just within the lower potential window (i.e., below 0.25 V), was not inferior to that for TBG. In order to gain further insights into this, in-plane stress developments in the FLG was monitored in-situ during galvanostatic cycling. The net in-plane stress development upon full lithiation in FLG agrees reasonably well with the stress magnitude expected based on the net dilation along graphite a-axis upon ‘classical’ bulk Li-intercalation up to LiC6 composition. This tends to additionally indicate that the mechanisms of excess Li-storage (i.e., other than the ‘classical’ bulk Li-intercalation) leads to much lesser in-plane dimensional change (if any). Another interesting observation was that of ‘stress release’ in the stress-time profile of FLG, just within the potential window of 0.5-0.25 V (against Li/Li+) [4] where pristine graphene/graphite, dilute stages I and IV co-exist. Based on the evidences obtained from structural characterizations (including changes in Raman spectra after cycling within different potential windows) and geometrical modelling, such stress release is believed to be associated with mechanical/structural degradation taking place primarily during the initial stages of lithiation and later stages of delithiation as a result of stretching of individual graphene layers beyond fracture strain around intercalated Li at interfaces between the concerned co-existing stages [4]. Accordingly, the observed ‘stress release’ within the concerned potential window above 0.25 V and the as-believed mechanism for mechanical/structural degradation in the case of FLG tends to agree well with the fading of the overall capacity.

Direct Observation of Poly(Methyl Methacrylate) Removal from a Graphene Surface

Poly(methyl methacrylate) (PMMA) is commonly used as a temporary support layer for chemical vapor deposition (CVD) graphene transfer; it is then removed by a chemical or thermal treatment. Regardless of the method used for PMMA removal, polymer residues are left on the graphene surface, which alter its intrinsic properties. A method based on isotope labeling of PMMA and time-of-flight secondary ion mass spectrometry (ToF-SIMS) has now been developed to identify, locate, and quantify these residues. It is shown that vacuum annealing does not completely remove the PMMA residues but, instead, transforms them into amorphous carbon. In contrast, air annealing under optimized conditions generates a PMMA-free surface with limited damage to the graphene structure. This cleaned graphene surface demonstrates low friction which is comparable with that of pristine graphene film.

Tunable Broadband Nanocarbon Transparent Conductor by Electrochemical Intercalation.

Optical transparent and electrical conducting materials with broadband transmission are important for many applications in optoelectronic, telecommunications, and military devices. However, studies of broadband transparent conductors and their controlled modulation are scarce. In this study, we report that reversible transmittance modulation has been achieved with sandwiched nanocarbon thin films (containing carbon nanotubes (CNTs) and reduced graphene oxide (rGO)) via electrochemical alkali-ion intercalation/deintercalation. The transmittance modulation covers a broad range from the visible (450 nm) to the infrared (5 μm), which can be achieved only by rGO rather than pristine graphene films. The large broadband transmittance modulation is understood with DFT calculations, which suggest a decrease in interband transitions in the visible range as well as a reduced reflection in the IR range upon intercalation. We find that a larger interlayer distance in few-layer rGO results in a significant increase in transparency in the infrared region of the spectrum, in agreement with experimental results. Furthermore, a reduced plasma frequency in rGO compared to few-layer graphene is also important to understand the experimental results for broadband transparency in rGO. The broadband transmittance modulation of the CNT/rGO/CNT systems can potentially lead to electrochromic and thermal camouflage applications.

Influences of Co Doping Effect on the Structural Properties of a Single Graphene Film.

The effect of cobalt doping on the structural properties of a single graphene film were determined by use of SEM, temperature-dependent Raman technique, XRD and XPS measurements. XRD and SEM measurements revealed that Co nanoparticles formed on the surface of the single layer of graphene. The XPS results also revealed the existence of bivalent Cobalt ions. Raman spectroscopy revealed enhanced disorder effects, and a large red shift in the 2D peak after the Co deposition process. With decreasing temperature of the pristine single graphene film, the G peak stiffened and sharpened. In contrast, the G peak of the graphene film after Co deposition was not dependent on temperature. This phenomenon indicates that the inhomogeneous local disorder effect is induced by the Co atoms embedded in the graphene film. Moreover, a larger disorder effect induced by Co deposition can be found in single graphene films with higher original vacancies, suggesting the possible formation of Co atom-vacancy complexes. The magnetic Co nanoparticles and magnetic Co atom-vacancy complexes in graphene produced in this experiment could be used in graphene-based spintronic device applications.

Graphene oxide/graphene vertical heterostructure electrodes for highly efficient and flexible organic light emitting diodes.

The relatively high sheet resistance, low work function and poor compatibility with hole injection layers (HILs) seriously limit the applications of graphene as transparent conductive electrodes (TCEs) for organic light emitting diodes (OLEDs). Here, a graphene oxide/graphene (GO/G) vertical heterostructure is developed as TCEs for high-performance OLEDs, by directly oxidizing the top layer of three-layer graphene films with ozone treatment. Such GO/G heterostructure electrodes show greatly improved optical transmittance, a large work function, high stability, and good compatibility with HIL materials (MoO3 in this work). Moreover, the conductivity of the heterostructure is not sacrificed compared to the pristine three-layer graphene electrodes, but is significantly higher than that of pristine two-layer graphene films. In addition to high flexibility, OLEDs with different emission colors based on the GO/G heterostructure TCEs show much better performance than those based on indium tin oxide (ITO) anodes. Green OLEDs with GO/G heterostructure electrodes have the maximum current efficiency and power efficiency, as high as 82.0 cd A(-1) and 98.2 lm W(-1), respectively, which are 36.7% (14.8%) and 59.2% (15.0%) higher than those with pristine graphene (ITO) anodes. These findings open up the possibility of using graphene for next generation high-performance flexible and wearable optoelectronics with high stability.

Electrical current stimulated desorption of carbon dioxide adsorbed on graphene based structures

The objective of this study was to investigate Joule heating/electric swing adsorption (ESA) as a mode of regeneration and to compare the carbon dioxide (CO2) adsorption capacity of pristine graphene films and reduced graphene oxide (rGO) aerogels.