Nanoscale mapping of electrical resistivity and connectivity in graphene strips and networks

Microstructural tailoring of porous few-layer graphene-like biochar from kitchen waste hydrolysis residue in molten carbonate medium: Structural evolution and conductive additive-free supercapacitor application

Biomass-derived graphene-like material is a promising candidate for supercapacitor electrodes, while it is critical to controllably convert biomass into structure-tunable graphene. Herein, few-layer graphene-like biochar (FLGBS) was successfully fabricated from waste biomass in molten carbonate medium. Molten carbonate acted as the effective catalyst for graphitizing and the liquid medium for microcrystal relinking to achieve the rearrangement of carbon structure. It was found that the stacking of graphene layer and formation of porous structure were influenced by the volume of reaction medium and biomass pre‑carbonation. Namely, increasing the dosage of molten K₂CO₃ was in favor to form few layer-type graphene structure, but excess dosage could destroy the nanopore structure to expand the aperture. In addition, pre‑carbonation at high temperature impeded the exfoliation of graphene layers. When FLGBSs were applied to fabricate conductive additive-free electrode, they displayed a superior supercapacitor performance (up to 237.4 F g^-1 at 0.5 Ag^-1). This excellent performance should be attributed to the large specific surface area, hierarchical pore structure and graphene-like structure. In short, this work could help to get insights into the structural evolution of biomass carbon to graphene-like biochar in molten carbonate medium and achieve the tailoring of microstructure for further application in energy storage.

The Physics behind the Modulation of Thermionic Current in Photodetectors Based on Graphene Embedded between Amorphous and Crystalline Silicon

In this work, we investigate a vertically illuminated near-infrared photodetector based on a graphene layer physically embedded between a crystalline and a hydrogenated silicon layer. Under near-infrared illumination, our devices show an unforeseen increase in the thermionic current. This effect has been ascribed to the lowering of the graphene/crystalline silicon Schottky barrier as the result of an upward shift in the graphene Fermi level induced by the charge carriers released from traps localized at the graphene/amorphous silicon interface under illumination. A complex model reproducing the experimental observations has been presented and discussed. Responsivity of our devices exhibits a maximum value of 27 mA/W at 1543 nm under an optical power of 8.7 μW, which could be further improved at lower optical power. Our findings offer new insights, highlighting at the same time a new detection mechanism which could be exploited for developing near-infrared silicon photodetectors suitable for power monitoring applications.

Scalable Assembly of High-Quality Graphene Films via Electrostatic-Repulsion Aligning

Assembling pristine graphene into freestanding films featuring high electrical conductivity, superior flexibility, and robust mechanical strength aims at meeting the all-around high criteria of new-generation electronics. However, voids and defects produced in the macroscopic assembly process of graphene nanosheets severely degrade the performance of graphene films, and mechanical brittleness often limits their applications in wide scenarios. To address such challenges, an electrostatic-repulsion aligning strategy is demonstrated to produce highly conductive, ultraflexible, and multifunctional graphene films. Typically, the high electronegativity of titania nanosheets (TiNS) induces the aligning of negatively charged graphene nanosheets via electrostatic repulsion in the film assembly. The resultant graphene films show fine microstructure, enhanced mechanical properties, and improved electrical conductivity up to 1.285 × 10⁵ S m^-1 . Moreover, the graphene films can withstand 5000 repeated folding without structural damage and electrical resistance fluctuation. These comprehensive improved properties, combined with the facile synthesis method and scalable production, make these graphene films a promising platform for electromagnetic interference (EMI) shielding and thermal-management applications in smart and wearable electronics.

Electrically conducting films prepared from graphite and lignin in pure water

In this study, we present electrically conducting self-standing graphite films consisting of lignin derivatives extracted by simultaneous enzymatic saccharification and comminution (SESC). Sonication of graphite powder in the presence of SESC lignin and pure water allows dispersion of the SESC-lignin-attached graphite without addition of other chemicals. The SESC-lignin-attached graphite having a diameter of several micrometers can be used as a surface electroconductive coating and molded into self-standing films by drying. The SESC-lignin-attached graphite film exhibits higher conductivity (∼2,075 S/cm) than graphite-based composites consisting of ordinary lignin derivatives. Manufacturing self-standing films of micrometer-sized graphite using SESC lignin enables high electrical conductivity of the SESC-lignin-attached graphite film. The size of the SESC-lignin-attached graphite is proportional to the conductivity of the film. The SESC-lignin-attached graphite also acts as an antiplasticizer and a conductive filler for polymer films, i.e., conductive films consisting of poly(ethylene glycol) or Li+ montmorillonite can be obtained through a water-based process.

Progress on Optical Fiber Biochemical Sensors Based on Graphene

Graphene, a novel form of the hexagonal honeycomb two-dimensional carbon-based structural material with a zero-band gap and ultra-high specific surface area, has unique optoelectronic capabilities, promising a suitable basis for its application in the field of optical fiber sensing. Graphene optical fiber sensing has also been a hotspot in cross-research in biology, materials, medicine, and micro-nano devices in recent years, owing to prospective benefits, such as high sensitivity, small size, and strong anti-electromagnetic interference capability and so on. Here, the progress of optical fiber biochemical sensors based on graphene is reviewed. The fabrication of graphene materials and the sensing mechanism of the graphene-based optical fiber sensor are described. The typical research works of graphene-based optical fiber biochemical sensor, such as long-period fiber grating, Bragg fiber grating, no-core fiber and photonic crystal fiber are introduced, respectively. Finally, prospects for graphene-based optical fiber biochemical sensing technology will also be covered, which will provide an important reference for the development of graphene-based optical fiber biochemical sensors.

Graphene-Based Composite Membrane Prepared from Solid Carbon Source Catalyzed by Ni Nanoparticles

Emerging as a light, flexible and highly thermally conductive material, graphene-based membranes have attracted extensive attention in thermal management field. However, the preparation of high-quality graphene-based membranes usually involves complex processes and thermal annealing at ultra-high temperature, which limits their large-scale application in thermal management field. In our study, reduced graphene oxide-Ni-hydroxypropyl methyl cellulose (RGO-Ni-HPMC) composite membrane was prepared from catalytic pyrolysis of hydroxypropyl methyl cellulose (HPMC) with Ni nanoparticles to generate multilayer graphene and form phonon transport channels between adjacent graphene layers. Further, our study shows that the RGO-Ni-HPMC composite membrane has a good heat dissipation effect at the hot spots at high temperature. The average temperature of hot spots is reduced by 11.5 °C. It is expected to solve the heat dissipation problem of high-power electronic equipment.

2D Colloids: Size- and Shape-Controlled 2D Materials at Fluid-Fluid Interfaces

Advances in synthesis of model 3D colloidal particles with exotic shapes and physical properties have enabled discovery of new 3D colloidal phases not observed in atomic systems, and simulations and quasi-2D studies suggest 2D colloidal systems have an even richer phase behavior. However, a model 2D (one-atom-thick) colloidal system has yet to be experimentally realized because of limitations in solution-phase exfoliation of 2D materials and other 2D particle fabrication technologies. Herein, we use a photolithography-based methodology to fabricate size- and shape-controlled monolayer graphene particles, and then transfer the particles to an air-water interface to study their dynamics and self-assembly in real-time using interference reflection microscopy. Results suggest the graphene particles behave as "hard" 2D colloidal particles, with entropy influencing the self-assembled structures. Additional evidence suggests the stability of the self-assembled structures manifests from the edge-to-edge van der Waals force between 2D particles. We also show graphene discs with diameters up to 50 μm exhibit significant Brownian motion under optical microscopy due to their low mass. This work establishes a facile methodology for creating model experimental systems of colloidal 2D materials, which will have a significant impact on our understanding of fundamental 2D physics. Finally, our results advance our understanding of how physical particle properties affect the interparticle interactions between monolayer 2D materials at fluid-fluid interfaces. This information can be used to guide the development of scalable synthesis techniques (e.g., solution-phase processing) to produce bulk suspensions of 2D materials with desired physical particle properties that can be used as building blocks for creating thin films with desired structures and properties via interfacial film assembly.

COMSOL Simulation for Design of Induction Heating System in VULCAN Facility

The experimental facility VULCAN was setup to study the fuel-coolant interaction (FCI) phenomena in a postulated severe accident of light water reactors. The heating system is important for the facility to prepare molten material in a crucible. This article is concerned with the design of the heating system, which applies electromagnetic induction heating method. The COMSOL code was employed to simulate the induction heating characteristics of a graphite crucible under different current and frequency of the work coil (inductor). Given a frequency, the relationship between the crucible’s average temperature and the inductor’s current is obtained, which is instrumental to select the power supply of the induction heating system. Meanwhile, the skin effect of induction heating is analyzed to guide the choice of frequency and inductor of the heating system. According to the simulation results, the induction heating system of frequency 47 kHz is suitable for the experiment, with a good agreement in temperature between the measured and the predicted.

Functionalized and Platinum-Decorated Multi-Layer Oxidized Graphene as a Proton, and Electron Conducting Separator in Solid Acid Fuel Cells

In the present article, electrodes containing a composite of platinum on top of a plasma-oxidized multi-layer graphene film are investigated as model electrodes that combine an exceptional high platinum utilization with high electrode stability. Graphene is thereby acting as a separator between the phosphate-based electrolyte and the platinum catalyst. Electrochemical impedance measurements in humidified hydrogen at 240 °C show area-normalized electrode resistance of 0.06 Ω·cm−2 for a platinum loading of ∼60 µgPt·cm−2, resulting in an outstanding mass normalized activity of almost 280 S·mgPt−1, exceeding even state-of-the-art electrodes. The presented platinum decorated graphene electrodes enable stable operation over 60 h with a non-optimized degradation rate of 0.15% h−1, whereas electrodes with a similar design but without the graphene as separator are prone to a very fast degradation. The presented results propose an efficient way to stabilize solid acid fuel cell electrodes and provide valuable insights about the degradation processes which are essential for further electrode optimization.

Graphene and water-based elastomer nanocomposites - a review

Water-based elastomers (WBEs) are polymeric elastomers in aqueous systems. WBEs have recently continued to gain wide acceptability by both academia and industry due to their remarkable environmental and occupational safety friendly nature, as a non-toxic elastomeric dispersion with low-to-zero volatile organic compound (VOC) emission. However, their inherent poor mechanical and thermal properties remain a drawback to these sets of elastomers. Hence, nano-fillers such as graphene oxide (GO), reduced graphene oxide (rGO) and graphene nanoplatelets (GNPs) are being employed for the reinforcement and enhancement of this set of elastomers. This work is geared towards a critical review and summation of the state-of-the-art developments of graphene enhanced water-based elastomer composites (G-WBEC), including graphene and composite production processes, properties, characterisation techniques and potential commercial applications. The dominant production techniques, such as emulsion mixing and in situ polymerisation processes, which include Pickering emulsion, mini-emulsion and micro-emulsion, as well as ball-milling approach, are systematically evaluated. Details of the account of mechanical properties, electrical conductivity, thermal stability and thermal conductivity enhancements, as well as multifunctional properties of G-WBEC are discussed, with further elaboration on the structure-property relationship effects (such as dispersion and filler-matrix interface) through effective and non-destructive characterisation tools like Raman and XRD, among others. The paper also evaluates details of the current application attempts and potential commercial opportunities for G-WBEC utilisation in aerospace, automotive, oil and gas, biomedicals, textiles, sensors, electronics, solar energy, and thermal management.

Selectively coupled small Pd nanoparticles on sp2-hybridized domain of graphene-based aerogel with enhanced catalytic activity and stability

The precisely coupling of metal nanoparticles with support domain are crucial to enhance the catalytic activity and stability of supported metal nanoparticle catalysts (MNPs). Here we selectively anchor Pd nanoparticles to the sp² domain in graphene-based aerogel constructed with base-washed graphene oxide (BGO) by removing oxidative debris (OD). The effects of OD on the size and chemical composition of Pd nanoparticles in aerogels are initially unveiled. The removal of OD nanoparticles prompt selective coupling of Pd nanoparticles to the exposed sp²-hybridized domain on BGO nanosheets, and then prevent it from agglomeration. As a result, the Pd nanoparticle size of self-assembled Pd/BGA is 4.67 times smaller than that of traditional Pd/graphene oxide aerogel (Pd/GA). The optimal catalytic activity of Pd/BGA for the model catalytic reduction of 4-nitrophenol is 15 times higher than that of Pd/GA. Pd/BGA could maintain its superior catalytic activity and achieves 98.72% conversion in the fifth cycle. The superior catalytic performance could be ascribed to the small Pd nanoparticles and high percentage of Pd(0) in Pd/BGA, and the enhanced electronic conductivity of Pd/BGA. These integrated merits of Pd/BGA as heterogeneous catalysts are attributed to selectively anchor Pd nanoparticles on sp²-hybridized domain of graphene-based aerogel, and strongly coupled interaction of MNPs with support. The structure-regulated BGO nanosheets could serve as versatile building blocks for fabricating MNPs/graphene aerogels with superior performance for catalytic transformation of water pollutants.

Comprehensive Enhancement in Thermomechanical Performance of Melt-Extruded PEEK Filaments by Graphene Incorporation

A simple and scalable fabrication process of graphene nanoplatelets (GnPs)-reinforced polyether ether ketone (PEEK) filaments with enhanced mechanical and thermal performance was successfully demonstrated in this work. The developed PEEK-GnP nanocomposite filaments by a melt-extrusion process showed excellent improvement in storage modulus at 30 °C (61%), and significant enhancement in tensile strength (34%), Young's modulus (25%), and elongation at break (37%) when GnP content of 1.0 wt.% was used for the neat PEEK. Moreover, the GnPs addition to the PEEK enhanced the thermal stability of the polymer matrix. Improvement in mechanical and thermal properties was attributed to the improved dispersion of GnP inside PEEK, which could form a stronger/robust interface through hydrogen bonding and π-π* interactions. The obtained mechanical properties were also correlated to the mechanical reinforcement models of Guth and Halpin-Tsai. The GnP layers could form agglomerates as the GnP content increases (>1 wt.%), which would decline neat PEEK's crystallinity and serve as stress concentration sites inside the composite, leading to a deterioration of the mechanical performance. The results demonstrate that the developed PEEK-GnP nanocomposites can be used in highly demanding engineering sectors like 3D printing of aerospace and automotive parts and structural components of humanoid robots and biomedical devices.

Recent Progress in Graphene/Polymer Nanocomposites

Nanocomposites, multiphase solid materials with at least one nanoscaled component, have been attracting ever-increasing attention because of their unique properties. Graphene is an ideal filler for high-performance multifunctional nanocomposites in light of its superior mechanical, electrical, thermal, and optical properties. However, the 2D nature of graphene usually gives rise to highly anisotropic features, which brings new opportunities to tailor nanocomposites by making full use of its excellent in-plane properties. Here, recent progress on graphene/polymer nanocomposites is summarized with emphasis on strengthening/toughening, electrical conduction, thermal transportation, and photothermal energy conversion. The influence of the graphene configuration, including layer number, defects, and lateral size, on its intrinsic properties and the properties of graphene/polymer nanocomposites is systematically analyzed. Meanwhile, the role of the interfacial interaction between graphene and polymer in affecting the properties of nanocomposites is also explored. The correlation between the graphene distribution in the matrix and the properties of the nanocomposite is discussed in detail. The key challenges and possible solutions are also addressed. This review may provide a constructive guidance for preparing high-performance graphene/polymer nanocomposite in the future.

Accurate Monitoring of Small Strain for Timbre Recognition via Ductile Fragmentation of Functionalized Graphene Multilayers

Sensitivity and linearity are two key parameters of flexible strain sensors. Although the introduction of microstructures (e.g., channel crack inspired by the geometry of the spider's slit organ) can effectively improve the sensitivity, the sudden breakage of the conductive path in turn leads to poor linearity. In practical applications, in order to achieve precise detection of subtle strains, high sensitivity and high linearity are required simultaneously. Here, we report a strain sensor design strategy based on the ductile fragmentation of functionalized graphene multilayers (FGMs) in which the conductive path is gradually broken to ensure high sensitivity while greatly improving the linear response of the sensor. The presence of oxygen-containing functional groups plays a key role in the deformation and fracture behaviors of the sensitive layer. High sensitivity (gauge factor ∼ 200) and high linearity (adjusted R-square ∼ 0.99936) have been achieved simultaneously in the strain range of 0-2.5%. In addition, the sensor also shows an ultralow detection limit (ε < 0.001%), an ultrafast response (response time ∼ 50 μs), good stability, and good patterning capability compatible with complex curved surface manufacturing. These outstanding performances allow the FGM-based strain sensors to accurately distinguish the sound amplitude and frequency, highlighting the sensor's potential as smart devices for human voice detection. Such sensors have potential applications in the fields of smart skin, wearable electronics, robotics, and so on.

Quantifying the influence of graphene film nanostructure on the macroscopic electrical conductivity

Graphene films have emerged as a promising nanostructured material class to exploit graphene's outstanding nanoscopic properties on the macroscale. Their potential applications include solar cells (Eda et al 2008 Appl. Phys. Lett. 92, 233305; Müllen et al 2008 Nano Lett. 8, 323–7), antennas (Zhang et al 2018 Electronics 7, 285; Song et al 2018 Carbon 130, 164–9), or electromagnetic interference shielding (Zhou et al 2017 Nanoscale 9, 18613–8; Wan et al 2017 Carbon 122, 74–81; Wang et al 2018 Small 14, 1704332), all of which require a high electrical conductivity. While an outstanding electrical conductivity is a key feature of pristine graphene monolayers, the transfer to the macroscale is challenging. Here, we combined theory and experiment to quantify the impact of specific structural graphene film properties. We synthesized graphene films with systematically varied flake sizes, studied their electrical conductivities, and found excellent agreement to simulations with a three-dimensional random resistor network model. In a further percolation-type study, we computed the critical share of non-conductive elements in a graphene film θ c = 10% where a substantial loss of electrical conductivity occurs. We prepared mixed films from graphene and graphene oxide to validate the threshold experimentally. In combination, experiments and simulations provide a coherent picture of how the graphene film microstructure is related to the macroscopic electrical conductivity (Rizzi et al 2018 ACS Appl. Mater. Interfaces 10 43088–94; Rizzi et al 2019 Comput. Mater. Sci. 161, 364–70). Our findings provide valuable insights for the production of highly conductive graphene-based macro-materials.

Calculating the Electrical Conductivity of Graphene Nanoplatelet Polymer Composites by a Monte Carlo Method

Electrical conductivity is one of several outstanding features of graphene–polymer nanocomposites, but calculations of this property require the intricate features of the underlying conduction processes to be accounted for. To this end, a novel Monte Carlo method was developed. We first established a randomly distributed graphene nanoplatelet (GNP) network. Then, based on the tunneling effect, the contact conductance between the GNPs was calculated. Coated surfaces (CSs) were next set up to calculate the current flow from the GNPs to the polymer. Using the equipotential approximation, the potentials of the GNPs and CSs met Kirchhoff’s current law, and, based on Laplace equation, the potential of the CSs was obtained from the potential of the GNP by the walk-on-spheres (WoS) method. As such, the potentials of all GNPs were obtained, and the electrical conductivity of the GNP polymer composites was calculated. The barrier heights, polymer conductivity, diameter and thickness of the GNP determining the electrical conductivity of composites were studied in this model. The calculated conductivity and percolation threshold were shown to agree with experimental data.

Fluid-Assisted Sorted Assembly of Graphene on Polymer

The significant size distribution of as-synthesized nanomaterials presents a challenge for reproducable and reliable applications. In this paper, we report a fluidic-assisted sorted assembly method in which nanomaterial sorting and enhanced assembly can be achieved simultaneously. As a proof of concept, a two-dimensional (2D) graphene flake, with a large size variation, was chosen as the target nanomaterial system. This study synergizes a novel fluidic assembly design, suspending a rotating disk over a polydimethylsiloxane (PDMS) substrate, and a computational fluid dynamics (CFD) model using Ansys CFX to disclose the mechanism of sorted assembly. By controlling the rotating speed and the gap between the disk and the substrate, the flow field is altered. In contrast to centrifugal sorting, where larger particles move outward, in this study, the size of assembled graphene flake (average lateral size, X_c) reduces significantly from the center (X_c = 3 μm) to the edge of the disk (X_c = 2 μm). The particle sorting process is dictated by the fluid shear-stress, with higher shear-stress leading to smaller particles, while the assembly process is mainly dominated by the pressure field with higher pressure magnitude leading to better assembly. Near the edge of the disk, enhanced particle sorting is coupled with an enhanced assembly where a continuous graphene film with smaller X_c can be formed. To prove the potential application of this method, an ultrasensitive strain sensor with one of the lowest detection limits, 0.02%, is demonstrated. This research presents a novel route toward large-scale and cost-effective manufacturing of nanomaterial-based flexible electronics.

Temperature and Size Effect on the Electrical Properties of Monolayer Graphene based Interconnects for Next Generation MQCA based Nanoelectronics

Graphene interconnects have been projected to out-perform Copper interconnects in the next generation Magnetic Quantum-dot Cellular Automata (MQCA) based nano-electronic applications. In this paper a simple two-step lithography process for patterning CVD monolayer graphene on SiO₂/Si substrate has been used that resulted in the current density of one order higher magnitude as compared to the state-of-the-art graphene-based interconnects. Electrical performances of the fabricated graphene interconnects were evaluated, and the impact of temperature and size on the current density and reliability was investigated. The maximum current density of 1.18 ×10⁸ A/cm² was observed for 0.3 μm graphene interconnect on SiO₂/Si substrate, which is about two orders and one order higher than that of conventionally used copper interconnects and CVD grown graphene respectively, thus demonstrating huge potential in outperforming copper wires for on-chip clocking. The drop in current at 473 K as compared to room temperature was found to be nearly 30%, indicating a positive temperature coefficient of resistivity (TCR). TCR for all cases were studied and it was found that with decrease in width, the sensitivity of temperature also reduces. The effect of resistivity on the breakdown current density was analysed on the experimental data using Matlab and found to follow the power-law equations. The breakdown current density was found to have a reciprocal relationship to graphene interconnect resistivity suggesting Joule heating as the likely mechanism of breakdown.

Carbonized Dehydroascorbic Acid: Aim for Targeted Repair of Graphene Defects and Bridge Connection of Graphene Sheets with Small Size

The thermal dissipation issue of electronics devices becomes increasingly prominent as they evolve to smaller sizes and more complicated structures. Therefore, the development of materials with excellent heat conduction properties and light weight turns out to be an urgent demand to solve the heat transfer problem of electronics devices with high performance. For this purpose, we put forward an innovative strategy that carbonized dehydroascorbic acid (CDA) be applied to graphene layers for the targeted repair of defects among them and bridge connection of the layers to produce graphene heat conduction materials with excellent properties. Firstly, hydrogen bonds formed from dehydroascorbic acid (DHAA, products of the oxidation of vitamin C) and each of ketone, carboxyl, and oxhydryl groups on graphene layers were absorbed at targeted locations where oxidation graphene produces defects, then targeted repair was conducted for those defects to be filled and for the graphene layers of a small size to grow into large sheet materials with improved continuity by CDA generated in thermally pressing reduction reaction at 800 °C. In our investigation, the planar thermal conductivity of rGO/VC membrane reached 1031.9 ± 10.2 Wm⁻¹K⁻¹, while the added mass content of vitamin C (VC) was 15%. Being a reference, the planar thermal conductivity of primitive graphene membrane was only 610.7 ± 11.7 Wm⁻¹K⁻¹.

1-Pyrenemethanol derived nanocrystal reinforced graphene films with high thermal conductivity and flexibility

Miniaturization and integration of electronic components lead to increasing challenges of thermal management. Ultrathin materials with excellent thermal and flexibility are urgently required for portable electronic devices. In this study, the 1-pyrenemethanol (PyM) modified graphene oxide (GO) (GO-PyM) films were prepared in ethanol solution by an evaporation-induced assembly method. The PyM interacts with the GO sheets by hydrogen bonds and π-π interactions. The GO-PyM films were further graphitized at 3000 °C and roll compressed to fabricate the graphene films (GFs), by which, the PyM was transformed into nanosized graphite crystals (PNGCs). The PNGCs filled the voids between the graphene sheets of GFs and linked the graphene sheets, which enhanced the interaction between the graphene sheets, restricted the slippage of the graphene sheets under tension, increased the number of paths for electrons and phonons, and reduced the interface thermal resistance resulted from the existed voids. The resulting GFs showed excellent flexibility of a large elongation up to 14% and an elastic zone up to 3%, a tensile strength of 30.4 MPa, a thermal conductivity of 1316.32 W m^-1 K^-1, and an electrical conductivity of 6.48 × 10⁵ S m^-1. These integrated excellent properties of GFs will promote their applications in thermal management.

Coordinating capillary infiltration with anodic oxidation: a multi-functional strategy for electrochemical fabrication of graphene

Coordinating the capillarity infiltration with anodic oxidation enables electrochemical fabrication of various graphene materials at different temperatures.

Chemically exfoliated highly conductive layer-tunable graphene by simply controlling the exfoliating temperature

The development of effective strategies for the massive production of layer-number tunable graphene is of great importance to satisfy the requirements in versatile applications such as energy storage, thermal management, photocatalysis. However, how to prepare the layer-tunable graphene by a simple and efficient way is still a great challenge. Herein, an attempt has been made to exfoliate graphite into layer-tunable graphene by simply soaking the graphite in a binary-component solution composed of H₂SO₄ and (NH₄)₂S₂O₈. In this one-step method, we demonstrate that the layer-number for the as-prepared graphene can be significantly reduced by increasing the exfoliating temperature. An average thickness of ∼20, ∼10, and ∼3 atomic layers can be obtained for the graphene samples exfoliated at the temperature of 30 °C, 60 °C, and 90 °C, respectively. Meanwhile, higher exfoliating temperature not only facilitates the higher efficiency in the exfoliation of graphite, but also achieves a superior conductivity for the prepared graphene. We have demonstrated for the first time that controlling in a sole factor of temperature can effectively tune the layer-number of graphene by a one-step chemical exfoliation method, which will find its great potential in the practical application where the designated property of graphene is required.

Graphene oxide nanohybrids for electron transfer-mediated antimicrobial activity

The rapid increase in the prevalence of antibiotic-resistant bacterial strains poses a global health risk. In this scenario, alternative strategies are needed to combat the alarming rise in multidrug-resistant bacterial populations. For example, metal-incorporated graphene derivatives have emerged as model nanomaterials owing to their intrinsic antibacterial activity together with their biocompatibility. Interestingly, photon-activated phthalocyanine sensitizers have also shown promising physiochemical biocidal effects against pathogenic bacteria populations when conjugated with diverse nanomaterials. Herein, we report the facile synthesis of graphene oxide incorporated zinc phthalocyanine (ZnPc-GO) nanohybrids showing bactericidal activity against Gram-negative Escherichia coli (E. coli) cells, in the absence of any photo-excitation. The ZnPc-GO hybrid nanomaterials were synthesized by the in situ deposition of GO flakes on ZnPc-coated indium tin oxide (ITO) substrates. Two types of morphologically different ZnPc molecules, potato-chip-like α-phase ZnPc, namely ZnPc(A), and nanorod-like β-phase ZnPc(B), were used for the synthesis of the ZnPc(A/B)-GO nanocomposites. The interactions of GO with the underlying ZnPc(A/B) entities in the ZnPc-GO systems were investigated using multiple characterization techniques. It was observed that the GO flakes in the ZnPc(B)-GO nanocomposite possess stronger π-π interactions and thus show a more efficient electron transfer mechanism when compared with the ZnPc(A) counterpart. Furthermore, the E. coli bacterial cells with an electronegative surface demonstrated a profound adherence to the electron-withdrawing ZnPc(B)-GO surface. The death kinetics of bacteria with ZnPc(B)-GO were further investigated using surface potential mapping and Kelvin probe force microscopy (KPFM) analysis. Upon direct contact with ZnPc(B)-GO, the adhered bacterial cells showed outer cell deformation and membrane protein leakage, induced by a proposed charge-transfer mechanism between negatively charged cells and the electron-withdrawing ZnPc(B)-GO surface. These new findings may provide insights into the design of potential ZnPc-GO-based novel antimicrobial nanomaterials or surface coatings.

Electronic performance of printed PEDOT:PSS lines correlated to the physical and chemical properties of coated inkjet papers

PEDOT:PSS organic printed electronics chemical interactions with the ink-receiving layer (IRL) of monopolar inkjet paper substrates and coating color composition were evaluated through Raman spectroscopy mapping in Z (depth) and (XY) direction, Fourier transform infrared spectroscopy (FTIR) and energy dispersive X-ray spectroscopy (EDS). Other evaluated properties of the IRLs were pore size distribution (PSD), surface roughness, ink de-wetting, surface energy and the impact of such characteristics on the electronics performance of the printed layers. Resin-coated inkjet papers were compared to a multilayer coated paper substrate that also contained an IRL but did not contain the plastic polyethylene (PE) resin layer. This substrate showed better electronic performance (i.e., lower sheet resistance), which we attributed to the inert coating composition, higher surface roughness and higher polarity of the surface which influenced the de-wetting of the ink. The novelty is that this substrate was rougher and with somewhat lower printing quality but with better electronic performance and the advantage of not having PE in their composite structure, which favors recycling.

PEDOT:PSS ink chemical interactions with the coated surface of inkjet papers and their composition were evaluated through Raman, FTIR and EDS. Morphology of the pores and surface energy were also evaluated and how these impact sheet resistance.

Inductive Heating Using a High-Magnetic-Field Pulse to Initiate Chemical Reactions to Generate Composite Materials

Induction heating is efficient, precise, cost-effective, and clean. The heating process is coupled to an electrically conducting material, usually a metal. As most polymers are dielectric and non-conducting, induction heating is not applicable. In order to transfer energy from an electromagnetic field into polymer induction structures, conducting materials or materials that absorb the radiation are required. This report gives a brief overview of induction heating processes used in polymer technology. In contrast to metals, most polymer materials are not affected by electromagnetic fields. However, an unwanted temperature rise of the polymer can occur when a radio frequency field is applied. The now available high-field magnetic sources provide a new platform for induction heating at very low frequencies, avoiding unwanted thermal effects within the material. Using polycarbonate and octadecylamine as an example, it is demonstrated that induction heating performed by a magnetic-field pulse with a maximum flux density of 59 T can be used to initiate chemical reactions. A 50 nm thick Ag loop, with a mean diameter of 7 mm, placed in the polymer-polymer interface acts as susceptor and a resistive heating element. The formation of urethane as a linker compound was examined by infrared spectroscopic imaging and differential scanning calorimetry.

A Sprayed Graphene Pattern-Based Flexible Strain Sensor with High Sensitivity and Fast Response

Flexible strain sensors have a wide range of applications in biomedical science, aerospace industry, portable devices, precise manufacturing, etc. However, the manufacturing processes of most flexible strain sensors previously reported have usually required high manufacturing costs and harsh experimental conditions. Besides, research interests are often focused on improving a single attribute parameter while ignoring others. This work aims to propose a simple method of manufacturing flexible graphene-based strain sensors with high sensitivity and fast response. Firstly, oxygen plasma treats the substrate to improve the interfacial interaction between graphene and the substrate, thereby improving device performance. The graphene solution is then sprayed using a soft PET mask to define a pattern for making the sensitive layer. This flexible strain sensor exhibits high sensitivity (gauge factor ~100 at 1% strain), fast response (response time: 400–700 μs), good stability (1000 cycles), and low overshoot (<5%) as well. Those processes used are compatible with a variety of complexly curved substrates and is expected to broaden the application of flexible strain sensors.

Percolation Effects in Electrolytically Gated WS2/Graphene Nano:Nano Composites

Mixed networks of conducting and nonconducting nanoparticles show promise in a range of applications where fast charge transport is important. While the dependence of network conductivity on the loading level of conductive additive is well understood, little is known about the loading dependence of mobility and carrier density. This is particularly important as the addition of graphene might lead to increases in the mobility of semiconducting nanosheet network transistors. Here, we use electrolytic gating to investigate the transport properties of spray-coated composite networks of graphene and WS₂ nanosheets. As the graphene loading is increased, we find that both conductivity and carrier density increase in line with the percolation theory with percolation thresholds (∼8 vol %) and exponents (∼2.5) consistent with previous reporting. Perhaps surprisingly, we find the mobility increases modestly from ∼0.1 cm²/V s (for a WS₂ network) to ∼0.3 cm²/V s (for a graphene network) which we attribute to the similarity between WS₂-WS₂ and graphene-graphene junction resistances. In addition, we find both the transistor on- and off-currents to scale with loading according to the percolation theory, changing sharply at the percolation threshold. Through fitting, we show that only the current in the WS₂ network changes significantly upon gating. As a result, the on-off ratio falls sharply at the percolation threshold from ∼10⁴ to ∼2 at higher loading. Reflecting on these results, we conclude that the addition of graphene to a semiconducting network is not a viable strategy to improve transistor performance as it reduces the on:off ratio far more than it improves the mobility.

Polarization-Insensitive Phase Modulators Based on an Embedded Silicon-Graphene-Silicon Waveguide

A polarization-insensitive phase modulator concept is presented, based on an embedded silicon-graphene-silicon waveguide. Simulation results show that the effective mode index of both transverse electric (TE) and transverse magnetic (TM) modes in the silicon-graphene-silicon waveguide undergoes almost the same variations under different biases across a broad wavelength range, in which the real-part difference is less than 1.2 × 10−3. Based on that, a polarization-insensitive phase modulator is demonstrated, with a 3-dB modulation bandwidth of 135.6 GHz and a wavelength range of over 500 nm. Moreover, it has a compact size of 60 μm, and a low insertion loss of 2.12 dB. The proposed polarization-insensitive waveguide structure could be also applied to Mach-Zehnder modulators and electro-absorption modulators.

Multidimensional Ternary Hybrids with Synergistically Enhanced Electrical Performance for Conductive Nanocomposites and Prosthetic Electronic Skin

Graphene and silver nanowires (AgNWs) are ideal fillers for conductive polymer composites, but they tend to aggregate in the polymer matrix due to the lack of surface functional groups and large specific surface area, which is hard for the polymer composites filled with them to reach their full potential. Here, ternary hybrids with multidimensional architectures including 3D polystyrene (PS) microspheres, 2D reduced graphene oxide (RGO) nanosheets, and 1D AgNWs are obtained using a simple, but effective, electrostatic attraction strategy. The electrical conductivity (136.25 S m^-1) of the ternary hybrid conductive nanocomposites filled with RGO and AgNWs is significantly higher than that of the nanocomposites containing only RGO (3.255 S m^-1) at the same total filler loading due to the synergistic effect of RGO and AgNWs. The conductive nanocomposites simultaneously present a low percolation threshold of 0.159 vol % and a maximum electrical conductivity of 1230 S m^-1 at 3.226 vol % filler loading. Moreover, a flexible electronic skin based on the multidimensional ternary hybrids is presented, and it exhibits large stretchability, high gauge factor, and excellent cyclic working durability, which is successfully demonstrated in monitoring prosthetic finger motions.

Structure of graphene and its disorders: a review

Monolayer graphene exhibits extraordinary properties owing to the unique, regular arrangement of atoms in it. However, graphene is usually modified for specific applications, which introduces disorder. This article presents details of graphene structure, including sp2 hybridization, critical parameters of the unit cell, formation of σ and π bonds, electronic band structure, edge orientations, and the number and stacking order of graphene layers. We also discuss topics related to the creation and configuration of disorders in graphene, such as corrugations, topological defects, vacancies, adatoms and sp3-defects. The effects of these disorders on the electrical, thermal, chemical and mechanical properties of graphene are analyzed subsequently. Finally, we review previous work on the modulation of structural defects in graphene for specific applications.

Flexible and Stretchable Bio-Integrated Electronics Based on Carbon Nanotube and Graphene

Scientific and engineering progress associated with increased interest in healthcare monitoring, therapy, and human-machine interfaces has rapidly accelerated the development of bio-integrated multifunctional devices. Recently, compensation for the cons of existing materials on electronics for health care systems has been provided by carbon-based nanomaterials. Due to their excellent mechanical and electrical properties, these materials provide benefits such as improved flexibility and stretchability for conformal integration with the soft, curvilinear surfaces of human tissues or organs, while maintaining their own unique functions. This review summarizes the most recent advanced biomedical devices and technologies based on two most popular carbon based materials, carbon nanotubes (CNTs) and graphene. In the beginning, we discuss the biocompatibility of CNTs and graphene by examining their cytotoxicity and/or detrimental effects on the human body for application to bioelectronics. Then, we scrutinize the various types of flexible and/or stretchable substrates that are integrated with CNTs and graphene for the construction of high-quality active electrode arrays and sensors. The convergence of these carbon-based materials and bioelectronics ensures scalability and cooperativity in various fields. Finally, future works with challenges are presented in bio-integrated electronic applications with these carbon-based materials.

Ultimate Decoupling between Surface Topography and Material Functionality in Atomic Force Microscopy Using an Inner-Paddled Cantilever

Atomic force microscopy (AFM) has been widely utilized to gain insight into various material and structural functionalities on the nanometer scale, leading to numerous discoveries and technologies. Despite the phenomenal success in applying AFM to the simultaneous characterization of topological and functional properties of materials, it has continuously suffered from the crosstalk between the observables, causing undesirable artifacts and complicated interpretations. Here, we introduce a two-field AFM probe, namely an inner-paddled cantilever integrating two discrete pathways such that they respond independently to the variations in surface topography and material functionality. Hence, the proposed design allows reliable and potentially quantitative determination of functional properties. In this paper, the efficacy of the proposed design has been demonstrated via piezoresponse force microscopy of periodically poled lithium niobate and collagen, although it can also be applied to other AFM methods such as AFM-based infrared spectroscopy and electrochemical strain microscopy.

Hollow-Structured Graphene-Silicone-Composite-Based Piezoresistive Sensors: Decoupled Property Tuning and Bending Reliability

A versatile flexible piezoresistive sensor should maintain high sensitivity in a wide linear range, and provide a stable and repeatable pressure reading under bending. These properties are often difficult to achieve simultaneously with conventional filler-matrix composite active materials, as tuning of one material component often results in change of multiple sensor properties. Here, a material strategy is developed to realize a 3D graphene-poly(dimethylsiloxane) hollow structure, where the electrical conductivity and mechanical elasticity of the composite can be tuned separately by varying the graphene layer number and the poly(dimethylsiloxane) composition ratio, respectively. As a result, the sensor sensitivity and linear range can be easily improved through a decoupled tuning process, reaching a sensitivity of 15.9 kPa^-1 in a 60 kPa linear region, and the sensor also exhibits fast response (1.2 ms rising time) and high stability. Furthermore, by optimizing the density of the graphene percolation network and thickness of the composite, the stability and repeatability of the sensor output under bending are improved, achieving a measurement error below 6% under bending radius variations from -25 to +25 mm. Finally, the potential applications of these sensors in wearable medical devices and robotic vision are explored.

Structure-Based Selective Adsorption of Graphene on a Gel Surface: Toward Improving the Quality of Graphene Nanosheets

Top-down graphene production via exfoliation from graphite produces a mass of graphene with structural variation in terms of the number of layers, sheet size, edge type, and defect density. All of these characteristics affect its electronic structure. To develop useful applications of graphene, structural separation of graphene is necessary. In this study, we investigate the adsorption behavior of different types of graphene fragments using a multicolumn gel chromatography system with a view to developing an efficient method for separating high-quality graphene. The graphene was dispersed in an aqueous sodium dodecyl sulfate (SDS) surfactant solution and flown through allyl-dextran-based gel columns connected in series. In the chromatographic operation, we observed that the small-sized or oxidized graphene fragments tended to bind to the gel and the relatively large-sized graphene with a low oxygen content eluted from the gel column. In this system, the adsorbed SDS molecules on the graphitic surface prevented graphitic materials from binding to the gel and the oxygen functional groups on the graphene oxide or at the abundant edge of small-sized graphene hindered SDS adsorption. We hypothesize that the reduced SDS adsorption density results in the preferential adsorption of small-sized or oxidized graphene fragments on the gel. This type of chromatographic separation is a cost-effective and scalable method for sorting nanomaterials. The structural separation of graphene based on the adsorption priority found in this study will improve the quality of graphene nanosheets on an industrial scale.

Mapping the process dependent conductivity of carbon nanotube thin-films using a non-invasive contact probing system

We report on a non-invasive contact probing (NICP) system for measuring the distribution of local surface conductivity of macroscopic thin-films of carbon nanotubes. Using the NICP system, we were able to obtain the local sheet resistance of the conducting thin-films continuously at ∼10 μm resolution over few centimeters which would not have been possible using conventional contact probing methods. Measurements performed on carbon nanotube thin-films with various nanotube densities, physical, and chemical treatments revealed that the local variation in electrical characteristics was not reflected in global conductance measurements. This demonstrated the usefulness of the NICP system for evaluating the effect of processing on the electrical uniformity of conducting thin-films made using nanomaterials.

Optimally conductive networks in randomly dispersed CNT:graphene hybrids

A predictive model is proposed that quantitatively describes the synergistic behavior of the electrical conductivities of CNTs and graphene in CNT:graphene hybrids. The number of CNT-to-CNT, graphene-to-graphene, and graphene-to-CNT contacts is calculated assuming a random distribution of CNTs and graphene particles in the hybrids and using an orientation density function. Calculations reveal that the total number of contacts reaches a maximum at a specific composition and depends on the particle sizes of the graphene and CNTs. The hybrids, prepared using inkjet printing, are distinguished by higher electrical conductivities than that of 100% CNT or graphene at certain composition ratios. These experimental results provide strong evidence that this approach involving constituent element contacts is suitable for investigating the properties of particulate hybrid materials.

Sodium-Ion Intercalated Transparent Conductors with Printed Reduced Graphene Oxide Networks

In this work, we report for the first time that Na-ion intercalation of reduced graphene oxide (RGO) can significantly improve its printed network's performance as a transparent conductor. Unlike pristine graphene that inhibits Na-ion intercalation, the larger layer-layer distance of RGO allows Na-ion intercalation, leading to simultaneously much higher DC conductivity and higher optical transmittance. The typical increase of transmittance from 36% to 79% and decrease of sheet resistance from 83k to 311 Ohms/sq in the printed network was observed after Na-ion intercalation. Compared with Li-intercalated graphene, Na-ion intercalated RGO shows much better environmental stability, which is likely due to the self-terminating oxidation of Na ions on the RGO edges. This study demonstrated the great potential of metal-ion intercalation to improve the performance of printed RGO network for transparent conductor applications.

Effect of microwave irradiation on reduction of graphene oxide films

In this study, the effect of microwave irradiation on reduction of graphene oxide films was investigated.

Nanoelectrical analysis of single molecules and atomic-scale materials at the solid/liquid interface

Evaluating the built-in functionality of nanomaterials under practical conditions is central for their proposed integration as active components in next-generation electronics. Low-dimensional materials from single atoms to molecules have been consistently resolved and manipulated under ultrahigh vacuum at low temperatures. At room temperature, atomic-scale imaging has also been performed by probing materials at the solid/liquid interface. We exploit this electrical interface to develop a robust electronic decoupling platform that provides precise information on molecular energy levels recorded using in situ scanning tunnelling microscopy/spectroscopy with high spatial and energy resolution in a high-density liquid environment. Our experimental findings, supported by ab initio electronic structure calculations and atomic-scale molecular dynamics simulations, reveal direct mapping of single-molecule structure and resonance states at the solid/liquid interface. We further extend this approach to resolve the electronic structure of graphene monolayers at atomic length scales under standard room-temperature operating conditions.