Graphene‐based electrodes for flexible electronics

As ‘flexibility’ has emerged as an important issue in next‐generation electronics, many efforts to find new classes of materials have been devoted to realizing stretchable, bendable and foldable electronic devices. For these devices to be realized, graphene has been considered as one of the most promising candidates for flexible electrodes due to its extraordinary electrical, optical and mechanical properties. Particularly, recent developments in the fabrication and modification of graphene point to a bright future for graphene electrodes in flexible electronics. This mini‐review summarizes the recent progress in graphene films as flexible electrodes for various applications such as solar cells, organic light‐emitting diodes, touchscreens, transistors and supercapacitors. © 2015 Society of Chemical Industry     

Graphene and graphene oxide and their uses in barrier polymers

Currently, there is great interest in graphene‐based devices and applications because graphene has unique electronic and material properties, which can lead to enhanced material performance. Graphene may be used in a wide variety of potential applications from next‐generation transistors to lightweight and high‐strength polymeric composite materials. Graphene, which has atomic thickness and two‐dimensional sizes in the tens of micrometer range or larger, has also been considered a promising nanomaterial in gas‐ or liquid‐barrier applications because perfect graphene sheets do not allow diffusion of small gases or liquids through its plane. Recent molecular simulations and experiments have demonstrated that graphene and its derivatives can be used for barrier applications. In general, graphene and its derivatives can be applied via two major routes for barrier polymer applications. One is the transfer or coating of few‐layered, ultrathin graphene and its derivatives, such as graphene oxide (GO) and reduced graphene oxide (rGO), on polymeric substrates. The other is the incorporation of fully exfoliated GO or rGO nanosheets into the polymeric matrix. In this article, we review the state‐of‐the‐art research on the use of graphene, GO, and rGO for barrier applications, including few‐layered graphene or its derivatives in coated polymeric films and polymer nanocomposites consisting of chemically exfoliated GO and rGO nanosheets, and their gas‐barrier properties. As compared to other nanomaterials being used for barrier applications, the advantages and current limitations are discussed to highlight challenging issues for future research and the potential applications of graphene/polymer, GO/polymer, and rGO/polymer composites. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 39628.     

Graphene‐Based Bimorph Actuators with Dual‐Response and Large‐Deformation by a Simple Method

There is a high demand for the design of high‐performance soft actuators with multi‐stimuli response and easy fabrication. Here, soft bimorph actuators consisting of graphene and polypropylene are fabricated by the drop‐coating of graphene film and subsequent adhesion of polypropylene on the graphene film. The fabrication method is simple, fast, and scalable, and this bimorph actuator exhibits optically and electrically induced actuation with large and reversible deformation (angle change > 100°), fast response (≈8 s), and low driving voltage (≤7 V). The remarkable actuation performance is mainly attributed to the thermally induced expansion of the polypropylene film, bimorph structure, and the energy conversion property of the graphene. Because of the dual‐responsiveness and large‐deformation, this actuator can be used to construct diversely biomimetic devices with smart mechanical output. As an example, an artificial flower composed of four pieces of the actuator is fabricated to show optically and electrically driven blooming. These results open the way for using a simple method for the construction of soft actuators and smart devices toward practical biomimetic applications.     

Polyaniline electrochromic devices with transparent graphene electrodes

Transparent, conductive and uniform graphene films have been prepared and used as electrodes of the electrochromic devices of polyaniline. Polyaniline films on both graphene and the widely used indium tin oxide (ITO) electrodes showed similar electrochemical and spectroelectrochemical properties. However, graphene electrodes exhibited much higher electrochemical stability than ITO in aqueous acidic electrolytes. The performances of the electrochromic devices with graphene electrodes exhibited slight decrease upon voltage switching while those of the devices with ITO electrodes decreased dramatically. After 300 cycles, the electrochromic devices with graphene electrodes showed much larger optical contrast and shorter switching time than those of the devices with ITO electrodes.     

Structure and photoluminescence properties of graphene nanoflakes grown on zinc oxide films by hot filament chemical vapor deposition

Graphene nanoflakes (GNFs) were grown on silicon substrates pre-coated with different thicknesses of zinc oxide (ZnO) films by hot filament chemical vapor deposition in the methane environment. The structural, compositional and photoluminescence (PL) properties of the synthesized GNFs were systematically and extensively studied using a broad range of advanced characterization instruments including field emission scanning electron microscope, high-resolution transmission electron microscope, Raman spectroscopy, and X-ray photoelectron spectroscopy, etc. It is shown that the GNFs can be synthesized on ZnO films and that the thickness of GNFs reduces with the increased thickness of ZnO films. It is also shown that the GNFs can generate a series of strong green PL bands, but the green PL bands shift towards long wavelengths with the increased thickness of ZnO films. The change in the structural and PL properties of GNFs has been analyzed in terms of the features of graphene nanoribbons and the interfacial interaction of GLFs with ZnO films. These outcomes can contribute to the control of growth and structure of graphene-based materials and the fabrication of the optoelectronic devices relevant to graphene-based materials.     

Focusing on energy and optoelectronic applications: a journey for graphene and graphene oxide at large scale

Carbon is the only element that has stable allotropes in the 0th through the 3rd dimension, all of which have many outstanding properties. Graphene is the basic building block of other important carbon allotropes. Studies of graphene became much more active after the Geim group isolated "free" and "perfect" graphene sheets and demonstrated the unprecedented electronic properties of graphene in 2004. So far, no other individual material combines so many important properties, including high mobility, Hall effect, transparency, mechanical strength, and thermal conductivity. In this Account, we briefly review our studies of bulk scale graphene and graphene oxide (GO), including their synthesis and applications focused on energy and optoelectronics. Researchers use many methods to produce graphene materials: bottom-up and top-down methods and scalable methods such as chemical vapor deposition (CVD) and chemical exfoliation. Each fabrication method has both advantages and limitations. CVD could represent the most important production method for electronic applications. The chemical exfoliation method offers the advantages of easy scale up and easy solution processing but also produces graphene oxide (GO), which leads to defects and the introduction of heavy functional groups. However, most of these additional functional groups and defects can be removed by chemical reduction or thermal annealing. Because solution processing is required for many film and device applications, including transparent electrodes for touch screens, light-emitting devices (LED), field-effect transistors (FET), and photovoltaic devices (OPV), flexible electronics, and composite applications, the use of GO is important for the production of graphene. Because graphene has an intrinsic zero band gap, this issue needs to be tackled for its FET applications. The studies for transparent electrode related applications have made great progress, but researchers need to improve sheet resistance while maintaining reasonable transparency. Proposals for solving these issues include doping or controlling the sheet size and defects, and theory indicates that graphene can match the overall performance of indium tin oxide (ITO). We have significantly improved the specific capacitance in graphene supercapacitor devices, though our results do not yet approach theoretical values. For composite applications, the key issue is to prevent the restacking of graphene sheets, which we achieved by adding blocking molecules. The continued success of graphene studies will require further development in two areas: (1) the large scale and controlled synthesis of graphene, producing different structures and quantities that are needed for a variety of applications and (2) on table applications, such as transparent electrodes and energy storage devices. Overall, graphene has demonstrated performance that equals or surpasses that of other new carbon allotropes. These features, combined with its easier access and better processing ability, offer the potential basis for truly revolutionary applications and as a future fundamental technological material beyond the silicon age.     

Reliable processing of graphene using metal etchmasks

Graphene exhibits exciting properties which make it an appealing candidate for use in electronic devices. Reliable processes for device fabrication are crucial prerequisites for this. We developed a large area of CVD synthesis and transfer of graphene films. With patterning of these graphene layers using standard photoresist masks, we are able to produce arrays of gated graphene devices with four point contacts. The etching and lift off process poses problems because of delamination and contamination due to polymer residues when using standard resists. We introduce a metal etch mask which minimises these problems. The high quality of graphene is shown by Raman and XPS spectroscopy as well as electrical measurements. The process is of high value for applications, as it improves the processability of graphene using high-throughput lithography and etching techniques.     

A simple scheme of molecular electronic devices with multiwalled carbon nanotubes as the top electrodes

A simple fabrication scheme for molecular electronic junctions is presented with multiwalled carbon nanotubes (MWCNTs) as the top electrodes. Results indicate that our approach retains the molecular character of the chosen molecules [a self-assembled monolayer of octadecanethiol on gold bottom electrodes] and opens the door for studying a wide variety of organothiol candidates for molecular electronics. The fabrication scheme is designed in a way that it can be modified into all-carbon devices in the future by using graphitic carbon bottom electrodes functionalized with nitrozoabenzene, for example, and MWCNTs or graphene as the top electrodes. Alternatively, the scheme is applicable for all-gold devices with gold bottom electrodes and gold nanowire top electrodes.     

Polymer photovoltaic devices with transparent graphene electrodes produced by spin-casting

Large-area, smooth, transparent and conductive graphene films were produced by a spin-coating method using graphene solutions. Bulk heterojunction polymer organic photovoltaic devices using these pure graphene films as a transparent anode were fabricated and studied. A direct pure graphene film electrode ensured that the device fabrication cost remained low and the processing was simple. The photovoltaic device displayed a power-conversion efficiency of 0.13%.     

High performance of a solid-state flexible asymmetric supercapacitor based on graphene films

Solid-state flexible energy storage devices hold the key to realizing portable and flexible electronic devices. Achieving fully flexible energy storage devices requires that all of the essential components (i.e., electrodes, separator, and electrolyte) with specific electrochemical and interfacial properties are integrated into a single solid-state and mechanically flexible unit. In this study, we describe the fabrication of solid-state flexible asymmetric supercapacitors based on an ionic liquid functionalized-chemically modified graphene (IL-CMG) film (as the negative electrode) and a hydrous RuO(2)-IL-CMG composite film (as the positive electrode), separated with polyvinyl alcohol-H(2)SO(4) electrolyte. The highly ordered macroscopic layer structures of these films arising through direct flow self-assembly make them simultaneously excellent electrical conductors and mechanical supports, allowing them to serve as flexible electrodes and current collectors in supercapacitor devices. Our asymmetric supercapacitors have been optimized with a maximum cell voltage up to 1.8 V and deliver a high energy density (19.7 W h kg(-1)) and power density (6.8 kW g(-1)), higher than those of symmetric supercapacitors based on IL-CMG films. They can operate even under an extremely high rate of 10 A g(-1) with 79.4% retention of specific capacitance. Their superior flexibility and cycling stability are evident in their good performance stability over 2000 cycles under harsh mechanical conditions including twisted and bent states. These solid-state flexible asymmetric supercapacitors with their simple cell configuration could offer new design and fabrication opportunities for flexible energy storage devices that can combine high energy and power densities, high rate capability, and long-term cycling stability.     

Design of artificial nacre-like hybrid films as shielding to mitigate electromagnetic pollution

Multifunctional designs of biomimetic layered materials are in great demand for broadening their applications. Artificial hybrid films are fabricated using a simple evaporation-induced assembly method, using nacre as the structural model, two-dimensional reduced graphene oxide (RGO) and magnetic graphene (MG) as inorganic building blocks and poly(vinyl alcohol) (PVA) as glue. The nacre-like films exhibit good mechanical performance, such as high stiffness, strength and toughness. The biomimetic materials possess the shielding properties of electromagnetic pollution. MG based nacre-like films present more significant electromagnetic interference (EMI) shielding performance than RGO film, because of a synergism between dielectric loss of graphene and magnetic loss of magnetic nanoparticles. Average EMI shielding effectiveness (SE) reaches ∼20.3 dB over the frequency range of 8.2–12.4 GHz (X band) for MG hybrid film only 0.36 mm thick. The lightweight, flexible and thin MG artificial hybrid films possess good potential for EMI shielding applications.     

Multiscale modeling of heat conduction in graphene laminates

We developed a combined atomistic-continuum hierarchical multiscale approach to explore the effective thermal conductivity of graphene laminates. To this aim, we first performed molecular dynamics simulations in order to study the heat conduction at atomistic level. Using the non-equilibrium molecular dynamics method, we evaluated the length dependent thermal conductivity of graphene as well as the thermal contact conductance between two individual graphene sheets. In the next step, based on the results provided by the molecular dynamics simulations, we constructed finite element models of graphene laminates to probe the effective thermal conductivity at macroscopic level. A similar methodology was also developed to study the thermal conductivity of laminates made from hexagonal boron-nitride (h-BN) films. In agreement with recent experimental observations, our multiscale modeling confirms that the flake size is the main factor that affects the thermal conductivity of graphene and h-BN laminates. Provided information by the proposed multiscale approach could be used to guide experimental studies to fabricate laminates with tunable thermal conduction properties.     

Engineering the metal–organic interface by transferring a high-quality single layer graphene on top of organic materials

We present a new method for transferring chemical vapor deposition (CVD)-grown graphene onto the surfaces of organic materials directly. Raman and near edge X-ray absorption fine structure measurements prove that high-quality and single layer graphene/organic thin films can be obtained with minimized impurity introduction. In-situ synchrotron radiation photoemission spectroscopy combined with ultraviolet photoelectron spectroscopy experiments demonstrate that the inserted graphene can not only act as a buffer layer to reduce the interfacial chemical reactions between the deposited Al and organic materials, but also tune the metal/organic interface electronic structure significantly. This new graphene transfer technique may have a great potential in the application of engineering the metal–organic interface properties, which is one of the key technologies for the optimal design and fabrication of organic electronic and optoelectronic devices.     

Alkylamine capped metal nanoparticle "inks" for printable SERS substrates, electronics and broadband photodetectors

We report a facile and general method for the preparation of alkylamine capped metal (Au and Ag) nanoparticle "ink" with high solubility. Using these metal nanoparticle "inks", we have demonstrated their applications for large scale fabrication of highly efficient surface enhanced Raman scattering (SERS) substrates by a facile solution processing method. These SERS substrates can detect analytes down to a few nM. The flexible plastic SERS substrates have also been demonstrated. The annealing temperature dependent conductivity of the nanoparticle films indicated a transition temperature above which high conductivity was achieved. The transition temperature could be tailored to the plastic compatible temperatures by using proper alkylamine as the capping agent. The ultrafast electron relaxation studies of the nanoparticle films demonstrated that faster electron relaxation was observed at higher annealing temperatures due to stronger electronic coupling between the nanoparticles. The applications of these highly concentrated alkylamine capped metal nanoparticle inks for the printable electronics were demonstrated by printing the oleylamine capped gold nanoparticles ink as source and drain for the graphene field effect transistor. Furthermore, the broadband photoresponse properties of the Au and Ag nanoparticle films have been demonstrated by using visible and near-infrared lasers. These investigations demonstrate that these nanoparticle "inks" are promising for applications in printable SERS substrates, electronics, and broadband photoresponse devices.     

Inorganic nanostructures grown on graphene layers

This article presents a review of current research activities on the hybrid heterostructures of inorganic nanostructures grown directly on graphene layers, which can be categorized primarily as zero-dimensional nanoparticles; one-dimensional nanorods, nanowires, and nanotubes; and two-dimensional nanowalls. For the hybrid structures, the nanostructures exhibit excellent material characteristics including high carrier mobility and radiative recombination rate as well as long-term stability while graphene films show good optical transparency, mechanical flexibility, and electrical conductivity. Accordingly, the versatile and fascinating properties of the nanostructures grown on graphene layers make it possible to fabricate high-performance optoelectronic and electronic devices even in transferable, flexible, or stretchable forms. Here, we review preparation methods and possible device applications of the hybrid structures consisting of various types of inorganic nanostructures grown on graphene layers.     

Controlled growth of carbon nanotube-graphene hybrid materials for flexible and transparent conductors and electron field emitters

We report a versatile synthetic process based on rapid heating and cooling chemical vapor deposition for the growth of carbon nanotube (CNT)-graphene hybrid materials where the thickness of graphene and density of CNTs are properly controlled. Graphene films are demonstrated as an efficient barrier layer for preventing poisoning of iron nanoparticles, which catalyze the growth of CNTs on copper substrates. Based on this method, the opto-electronic and field emission properties of graphene integrated with CNTs can be remarkably tailored. A graphene film exhibits a sheet resistance of 2.15 kΩ sq(-1) with a transmittance of 85.6% (at 550 nm), while a CNT-graphene hybrid film shows an improved sheet resistance of 420 Ω sq(-1) with an optical transmittance of 72.9%. Moreover, CNT-graphene films are demonstrated as effective electron field emitters with low turn-on and threshold electric fields of 2.9 and 3.3 V μm(-1), respectively. The development of CNT-graphene films with a wide range of tunable properties presented in this study shows promising applications in flexible opto-electronic, energy, and sensor devices.     

Multifunctional Shape Memory Films for a Flexible Electrical Sensor

Shape memory polymers exhibit great potential for applications in aeronautics, astronautics, biomedicine, intelligent robots, and electronics. Based on traditional fabrication methods, as-prepared shape memory micro-/nanodevices are brittle with poor deformability. For instance, these devices shatter or crack when they are bent, which greatly limit their practical applications. In particular, there has been little use of shape memory films for high performance sensing devices in electrical circuits. Here, a novel polyvinyl alcohol (PVA) film is fabricated from a mixture of graphene nanosheets (GNs), sisal cellulose microcrystals (CNC), polyamine-functionalized perylene bisimide derivative (APBI), and PVA through a simple electrospinning technique. These novel PVA films exhibit excellent thermal, mechanical, and shape memory properties. Moreover, after spin-coating with Ag nanowires, the PVA films show excellent conductivity and flexibility in shape memory recovery cycles, and are used as a flexible sensing device in the circuit.     

石墨烯分散液的制备与应用研究进展

石墨烯是一种二维蜂窝状碳质新材料,其具有优异的热学、电学、机械性能。制备高浓度稳定分散且性能优良的石墨烯分散液在透明导电薄膜,纳米复合材料领域有巨大的前景。但石墨烯片与片之间存在范德华力在分散液中容易发生团聚。本文综述了国内外多种石墨烯分散液的先进制备方法,以及石墨烯分散液在光电器件以及生物方面的重要应用。     

Electrical properties of acrylic resin composite thin films with graphene/silver nanowires

Silver nanowires and graphene were used to form networks within acrylic resin to improve its toughness and conductivity through silane coupling agent. Meanwhile, acrylic resin was favorable to the adhesion of graphene to glass substrates and the connection among graphene sheets to form films. Experimental results indicate that after annealing at 400°C, sheet resistances of graphene‐silver nanowire films were lower than those graphene films without silver nanowires. The findings in this study provide helpful information on the fabrication of graphene‐based electronic devices. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42387.