High-performance graphene-based transparent flexible heaters

Highly flexible and transparent film heaters based on polyimide films embedded with silver nanowires

Highly flexible AgNW/PI transparent film heaters with superior mechanical and thermal response behavior were fabricated using an all solution-coating method.

One-step rod coating of high-performance silver nanowire–PEDOT:PSS flexible electrodes with enhanced adhesion after sulfuric acid post-treatment

An AgNWs–PEDOT:PSS film treated with H₂SO₄ exhibited both excellent optoelectric and adhesive performance.

Thermal response of transparent silver nanowire/PEDOT:PSS film heaters

Thermal response behavior of transparent silver nanowire/PEDOT:PSS film heaters are intensively studied for manipulating heating temperature, response time, and power consumption. Influences of substrate heat capacity, heat transfer coefficient between air and heater, sheet resistance and dimension of Ag nanowire film, on the thermal response are investigated from thermodynamic analysis. Suggestion is given for practical applications that if other parameters are fixed, Ag nanowire coverage can be utilized as an effective parameter to adjust the thermal response. The heat transfer coefficient plays opposite roles on thermal response speed and achievable steady temperature. A value of ≈32 W m(-2) K(-1) is obtained from transient process analysis after correcting it by considering heater resistance variation during heating tests. Guidance of designing heaters with a given response time is provided by forming Ag nanowire film with a suitable sheet resistance on substrate of appropriate material and a certain thickness. Thermal response tests of designed Ag heaters are performed to show higher heating temperature, shorter response time, and lower power consumption (179 °C cm(2) W(-1)) than ITO/FTO heaters, as well as homogeneous temperature distribution and stability for repeated use. Potential applications of the Ag heaters in window defogging, sensing and thermochromism are manifested.

Ultrathin transparent conductive polyimide foil embedding silver nanowires

Metallic nanowires are among the most promising transparent conductor (TC) alternatives to widely used indium tin oxide (ITO) because of their excellent trade-off between electrical and optical properties, together with their mechanical flexibility. However, they tend to suffer from relatively large surface roughness, instability against oxidation, and poor adhesion to the substrate. Embedding in a suitable material can overcome these shortcomings. Here we propose and demonstrate a new TC comprising silver nanowires (AgNWs) in an ultrathin polyimide foil that presents an optical transmission in the visible larger than ITO (>90%), while maintaining similar electrical sheet resistance (15 ohm/sq). The polyimide protects the Ag against environmental agents such as oxygen and water and, thanks to its deformability and very small thickness (5 μm), provides an ideal mechanical support to the NW's network, in this way ensuring extreme flexibility (bending radius as small as at least 1 mm) and straightforwardly removing any adhesion issue. The initial AgNWs' roughness is also reduced by a factor of about 15, reaching RMS values as low as 2.4 nm, suitable for the majority of applications. All these properties together with the simple fabrication technique based on all-solution processing put the developed TC in a competitive position as a lightweight, mechanically flexible and inexpensive substrate for consumer electronic and optoelectronic devices.

Flexible and transparent gas molecule sensor integrated with sensing and heating graphene layers

Graphene leading to high surface-to-volume ratio and outstanding conductivity is applied for gas molecule sensing with fully utilizing its unique transparent and flexible functionalities which cannot be expected from solid-state gas sensors. In order to attain a fast response and rapid recovering time, the flexible sensors also require integrated flexible and transparent heaters. Here, large-scale flexible and transparent gas molecule sensor devices, integrated with a graphene sensing channel and a graphene transparent heater for fast recovering operation, are demonstrated. This combined all-graphene device structure enables an overall device optical transmittance that exceeds 90% and reliable sensing performance with a bending strain of less than 1.4%. In particular, it is possible to classify the fast (≈14 s) and slow (≈95 s) response due to sp(2) -carbon bonding and disorders on graphene and the self-integrated graphene heater leads to the rapid recovery (≈11 s) of a 2 cm × 2 cm sized sensor with reproducible sensing cycles, including full recovery steps without significant signal degradation under exposure to NO2 gas.

Novel fabrication of flexible graphene-based chemical sensors with heaters using soft lithographic patterning method

We have fabricated graphene-based chemical sensors with flexible heaters for the highly sensitive detection of specific gases. We believe that increasing the temperature of the graphene surface significantly enhanced the electrical signal change of the graphene-based channel, and reduced the recovery time needed to obtain a normal state of equilibrium. In addition, a simple and efficient soft lithographic patterning process was developed via surface energy modification for advanced, graphene-based flexible devices, such as gas sensors. As a proof of concept, we demonstrated the high sensitivity of NO2 gas sensors based on graphene nanosheets. These devices were fabricated using a simple soft-lithographic patterning method, where flexible graphene heaters adjacent to the channel of sensing graphene were utilized to control graphene temperature.

Spray coating of crack templates for the fabrication of transparent conductors and heaters on flat and curved surfaces

Transparent conducting electrodes (TCEs) have been made on flat, flexible, and curved surfaces, following a crack template method in which a desired surface was uniformly spray-coated with a crackle precursor (CP) and metal (Ag) was deposited by vacuum evaporation. An acrylic resin (CP1) and a SiO2 nanoparticle-based dispersion (CP2) derived from commercial products served as CPs to produce U-shaped cracks in highly interconnected networks. The crack width and the density could be controlled by varying the spray conditions, resulting in varying template thicknesses. By depositing Ag in the crack regions of the templates, we have successfully produced Ag wire network TCEs on flat-flexible PET sheets, cylindrical glass tube, flask and lens surface with transmittance up to 86%, sheet resistance below 11 Ω/□ for electrothermal application. When used as a transparent heater by joule heating of the Ag network, AgCP1 and AgCP2 on PET showed high thermal resistance values of 515 and 409 °C cm(2)/W, respectively, with fast response (<20 s), requiring only low voltages (<5 V) to achieve uniform temperatures of ∼100 °C across large areas. Similar was the performance of the transparent heater on curved glass surfaces. Spray coating in the context of crack template is a powerful method for producing transparent heaters, which is shown for the first time in this work. AgCP1 with an invisible wire network is suited for use in proximity while AgCP2 wire network is ideal for use in large area displays viewed from a distance. Both exhibited excellent defrosting performance, even at cryogenic temperatures.

A highly conducting graphene film with dual-side molecular n-doping

Doping is an efficient way to engineer the conductivity and the work function of graphene, which is, however, limited to wet-chemical doping or metal deposition particularly for n-doping, Here, we report a simple method of modulating the electrical conductivity of graphene by dual-side molecular n-doping with diethylenetriamine (DETA) on the top and amine-functionalized self-assembled monolayers (SAMs) at the bottom. The resulting charge carrier density of graphene is as high as -1.7 × 10(13) cm(-2), and the sheet resistance is as low as ∼86 ± 39 Ω sq(-1), which is believed to be the lowest sheet resistance of monolayer graphene reported so far. This facile dual-side n-doping strategy would be very useful to optimize the performance of various graphene-based electronic devices.

Study on Ag mesh/conductive oxide hybrid transparent electrode for film heaters

Ag mesh-indium tin oxide (ITO) hybrid transparent conductive films were fabricated and evaluated for use in film heaters. PS monolayer templates were prepared using highly mono-dispersed PS spheres (11.2 μm) obtained by a filtering process with micro-sieves. At first, three Ag meshes with different sheet resistances (20, 100, and 300 Ω sq(-1)) and transmittances (70, 73, and 76%) were evaluated for film heaters in terms of voltage and long-term stability. Subsequently, in an effort to obtain better transmittance, Ag mesh-ITO hybrid heaters were fabricated utilizing finite ITO depositions. At the optimised ITO thickness (15 nm), the sheet resistance and the transmittance were 300 Ω sq(-1) and 88%, respectively, which indicates that this material is a good potential candidate for an efficient defroster in vehicles.

A highly crystalline single Au wire network as a high temperature transparent heater

A transparent conductor which can generate high temperatures finds important applications in optoelectronics. In this article, a wire network made of Au on quartz is shown to serve as an effective high temperature transparent heater. The heater has been fabricated by depositing Au onto a cracked sacrificial template. The highly interconnected Au wire network thus formed exhibited a transmittance of ∼87% in a wide spectral range with a sheet resistance of 5.4 Ω □(-1). By passing current through the network, it could be joule heated to ∼600 °C within a few seconds. The extraordinary thermal performance and stability owe much to the seamless junctions present in the wire network. Furthermore, the wire network gets self-annealed through joule heating as seen from its increased crystallinity. Interestingly, both transmittance and sheet resistance improved following annealing to 92% and 3.2 Ω □(-1), respectively.

Relationship between material properties and transparent heater performance for both bulk-like and percolative nanostructured networks

Transparent heaters are important for many applications and in the future are likely to be fabricated from thin, conducting, nanostructured networks. However, the electrical properties of such networks are almost always controlled by percolative effects. The impact of percolation on heating effects has not been considered, and the material parameter combinations that lead to efficient performance are not known. In fact, figures of merit for transparent heaters have not been elucidated, either in bulk-like or percolative systems. Here, we develop a simple yet comprehensive model describing the operation of transparent heaters. By considering the balance of Joule heating versus power dissipated by both convection and radiation, we derive an expression for the time-dependent heater temperature as a function of both electrical and thermal parameters. This equation can be modified to describe the relationship between temperature, optical transmittance, and electrical/thermal parameters in both bulk-like and percolative systems. By performing experiments on silver nanowire networks, systems known to display both bulk-like and percolative regimes, we show the model to describe real systems extremely well. This work shows the performance of transparent heaters in the percolative regime to be significantly less efficient compared to the bulk-like regime, implying the diameter of the nanowires making up the network to be critical. The model allows the identification of figures of merit for networks in both bulk-like and percolative regimes. We show that metallic nanowire networks are most promising, closely followed by CVD graphene, with networks of solution-processed graphene and carbon nanotubes being much less efficient.

Optical impedance transformer for transparent conducting electrodes

A fundamental limitation of transparent conducting electrode design is thought to be the trade-off between photonic and electronic performances. The photonic transmission property of a transparent conducting electrode, however, is not intrinsic but depends critically on the electromagnetic environment where the electrode is located. We develop the concept of optical impedance transformation, and use this concept to design nanophotonic structures that provide broadband and omnidirectional reduction of optical loss in an ultrathin transparent conducting electrode, without compromising its electrical performance.

25th anniversary article: carbon nanotube- and graphene-based transparent conductive films for optoelectronic devices

Carbon nanotube (CNT)- and graphene (G)-based transparent conductive films (TCFs) are two promising alternatives for commonly-used indium tin oxide-based TCFs for future flexible optoelectronic devices. This review comprehensively summarizes recent progress in the fabrication, properties, modification, patterning, and integration of CNT- and G-TCFs into optoelectronic devices. Their potential applications and challenges in optoelectronic devices, such as organic photovoltaic cells, organic light emitting diodes and touch panels, are discussed in detail. More importantly, their key characteristics and advantages for use in these devices are compared. Despite many challenges, CNT- and G-TCFs have demonstrated great potential in various optoelectronic devices and have already been used for some products like touch panels of smartphones. This illustrates the significant opportunities for the industrial use of CNTs and graphene, and hence pushes nanoscience and nanotechnology one step towards practical applications.

A review of production methods of carbon nanotube and graphene thin films for electrothermal applications

Electrothermal materials transform electric energy into heat due to the Joule effect. To date, resistive wires made of heavy metal alloys have primarily been used as the heat source in many appliances surrounding us. Recent discoveries in the field of carbon nanostructures revealed that they can offer a spectrum of advantages over the traditional materials. We review the production methods of thin films composed of carbon nanotubes or graphene and depict how they can be used as conductive coatings for electrothermal applications. We screen all reports from the field up to now and highlight the features of designed nanoheaters. A particular focus is placed on the analysis of general findings of how to tune their electrothermal properties, why carbon nanostructure devices operate the way they do and in what aspects they are superior to the currently available materials on the market.

Fast synthesis of high-performance graphene films by hydrogen-free rapid thermal chemical vapor deposition

The practical use of graphene in consumer electronics has not been demonstrated since the size, uniformity, and reliability problems are yet to be solved to satisfy industrial standards. Here we report mass-produced graphene films synthesized by hydrogen-free rapid thermal chemical vapor deposition (RT-CVD), roll-to-roll etching, and transfer methods, which enabled faster and larger production of homogeneous graphene films over 400 × 300 mm(2) area with a sheet resistance of 249 ± 17 Ω/sq without additional doping. The properties of RT-CVD graphene have been carefully characterized by high-resolution transmission electron microscopy, Raman spectroscopy, chemical grain boundary analysis, and various electrical device measurements, showing excellent uniformity and stability. In particular, we found no significant correlation between graphene domain sizes and electrical conductivity, unlike previous theoretical expectations for nanoscale graphene domains. Finally, the actual application of the RT-CVD films to capacitive multitouch devices installed in the most sophisticated mobile phone was demonstrated.

Vapor-phase molecular doping of graphene for high-performance transparent electrodes

Doping is an essential process to engineer the conductivity and work-function of graphene for higher performance optoelectronic devices, which includes substitutional atomic doping by reactive gases, electrical/electrochemical doping by gate bias, and chemical doping by acids or reducing/oxidizing agents. Among these, the chemical doping has been widely used due to its simple process and high doping strength. However, it also has an instability problem in that the molecular dopants tend to gradually evaporate from the surface of graphene, leading to substantial decrease in doping effect with time. In particular, the instability problem is more serious for n-doped graphene because of undesirable reaction between dopants and oxygen or water in air. Here we report a simple method to tune the electrical properties of CVD graphene through n-doping by vaporized molecules at 70 °C, where the dopants in vapor phase are mildly adsorbed on graphene surface without direct contact with solution. To investigate the dependence on functional groups and molecular weights, we selected a series of ethylene amines as a model system, including ethylene diamine (EDA), diethylene triamine (DETA), and triethylene tetramine (TETA) with increasing number of amine groups showing different vapor pressures. We confirmed that the vapor-phase doping provides not only very high carrier concentration but also good long-term stability in air, which is particularly important for practical applications.

Large area defrosting windows based on electrothermal heating of highly conducting and transmitting Ag wire mesh

Low power electrothermal heating behavior of junctionless transparent Ag mesh is explored for large area defrosting application.

25 GHz embedded-gate graphene transistors with high-k dielectrics on extremely flexible plastic sheets

Despite the widespread interest in graphene electronics over the past decade, high-performance graphene field-effect transistors (GFETs) on flexible substrates have been rarely achieved, even though this atomic sheet is widely understood to have greater prospects for flexible electronic systems. In this article, we report detailed studies on the electrical and mechanical properties of vapor synthesized high-quality monolayer graphene integrated onto flexible polyimide substrates. Flexible graphene transistors with high-k dielectric afforded intrinsic gain, maximum carrier mobilities of 3900 cm(2)/V·s, and importantly, 25 GHz cutoff frequency, which is more than a factor of 2.5 times higher than prior results. Mechanical studies reveal robust transistor performance under repeated bending, down to 0.7 mm bending radius, whose tensile strain is a factor of 2-5 times higher than in prior studies. In addition, integration of functional coatings such as highly hydrophobic fluoropolymers combined with the self-passivation properties of the polyimide substrate provides water-resistant protection without compromising flexibility, which is an important advancement for the realization of future robust flexible systems based on graphene.

Thermo-optic mode extinction modulator based on graphene plasmonic waveguide

We developed a thermo-optic (TO) mode extinction modulator based on graphene plasmonic waveguide. For compact device design and fabrication, the graphene plasmonic waveguide and heating element are configured all-in-one. Thermally induced inhomogeneous refractive-index distribution of the polymer near the microribbon cut off the long-range surface plasmon polariton (LRSPP) stripe mode propagating along a graphene microribbon. Numerical modeling are conducted on the time-dependent temperature (and hence the refractive-index) distribution by resistive heating element inside the graphene TO modulator. Experimental results demonstrate 30 dB attenuation with 12 mW electrical power injection at a telecom wavelength and exhibit a good agreement with the thermal modeling.

A transparent and stretchable graphene-based actuator for tactile display

A tactile display is an important tool to help humans interact with machines by using touch. In this paper, we present a transparent and stretchable graphene-based actuator for advanced tactile displays. The proposed actuator is composed of transparent and compliant graphene electrodes and a dielectric elastomer substrate. Since the electrode is coated onto the appointed region of the substrate layer by layer, only the area of the dielectric elastomer substrate with electrodes bumps up in response to the input voltage, which consequently produces actuation. The actuator is proven to be operable while preserving its electrical and mechanical properties even under 25% stretching. Also, the simple fabrication of the proposed actuator is cost-effective and can easily be extended to multiple arrays. The actuator is expected to be applicable to various applications including tactile displays, vari-focal lenses etc.

Enhanced optical response of hybridized VO₂/graphene films

Application of graphene as transparent electrodes is an active research area due to its excellent electrical and optical properties. Vanadium dioxide (VO2) is an attractive material since it is a thermochromic material that undergoes a structural phase transition when heat is applied. The phase transition results in the change of electrical and optical characteristics. We report optical characteristics of hybrid materials of graphene and VO2. We observed a 12% improvement in infrared transmittance with VO2 films deposited on graphene sapphire substrates compared to that of bare sapphire substrates. We also found that the phase transition temperature decreases as the number of graphene layers on the substrates increases. In the case of VO2 films on the substrate that was coated with four layers of graphene, the mean phase transition temperature was lowered to ∼56 °C.

Thermoacoustic sound generation from monolayer graphene for transparent and flexible sound sources

Transparent and flexible loudspeakers are realized with large-area monolayer graphene. The acoustic performances are characterized according to the supporting substrate effect and geometrical configurations. The substrate effect on the thermoacoustic sound generation from graphene is studied by controlling the surface porosity of various substrates.

Graphene transfer: key for applications

The first micrometer-sized graphene flakes extracted from graphite demonstrated outstanding electrical, mechanical and chemical properties, but they were too small for practical applications. However, the recent advances in graphene synthesis and transfer techniques have enabled various macroscopic applications such as transparent electrodes for touch screens and light-emitting diodes (LEDs) and thin-film transistors for flexible electronics in particular. With such exciting potential, a great deal of effort has been put towards producing larger size graphene in the hopes of industrializing graphene production. Little less than a decade after the first discovery, graphene now can be synthesized up to 30 inches in its diagonal size using chemical vapour deposition methods. In making this possible, it was not only the advances in the synthesis techniques but also the transfer methods that deliver graphene onto target substrates without significant mechanical damage. In this article, the recent advancements in transferring graphene to arbitrary substrates will be extensively reviewed. The methods are categorized into mechanical exfoliation, polymer-assisted transfer, continuous transfer by roll-to-roll process, and transfer-free techniques including direct synthesis on insulating substrates.

How to optically count graphene layers

The total thickness of a graphene sample depends upon the number of individually stacked graphene layers. The corresponding surface plasmon resonance (SPR) reflectance alters the SPR angle, depending on the number of graphene layers. Thus, the correlation between the SPR angle shift and the number of graphene layers allows for a nonintrusive, real-time, and reliable counting of graphene layers. A single-layer graphene (SLG) is synthesized by means of chemical vapor deposition, followed by physical transfer to a thin gold film (48 nm) repeatedly, so that multilayer graphene samples with one, three, and five layers can be prepared. Both the measured SPR angles and the entire reflectance curve profiles successfully distinguish the number of graphene layers.

Highly stable air working bimorph actuator based on a graphene nanosheet/carbon nanotube hybrid electrode

A RGO/CNT hybrid electrode of porous structure is prepared through a surfactant-free solution method to construct a bimorph ionic actuator, showing wide frequency range responsive and highly repeatable (over a million times) and stable bending actuation performance.

Efficient transfer of large-area graphene films onto rigid substrates by hot pressing

Graphene films grown on metal substrates by chemical vapor deposition (CVD) method have to be safely transferred onto desired substrates for further applications. Recently, a roll-to-roll (R2R) method has been developed for large-area transfer, which is particularly efficient for flexible target substrates. However, in the case of rigid substrates such as glass or wafers, the roll-based method is found to induce considerable mechanical damages on graphene films during the transfer process, resulting in the degradation of electrical property. Here we introduce an improved dry transfer technique based on a hot-pressing method that can minimize damage on graphene by neutralizing mechanical stress. Thus, we enhanced the transfer efficiency of the large-area graphene films on a substrate with arbitrary thickness and rigidity, evidenced by scanning electron microscope (SEM) and atomic force microscope (AFM) images, Raman spectra, and various electrical characterizations. We also performed a theoretical multiscale simulation from continuum to atomic level to compare the mechanical stresses caused by the R2R and the hot-pressing methods, which also supports our conclusion. Consequently, we believe that the proposed hot-pressing method will be immediately useful for display and solar cell applications that currently require rigid and large substrates.

Nanostructured hybrid transparent conductive films with antibacterial properties

Here, we demonstrate that the assembly of nanostructures with different dimensionalities yields "multicomponent hybrid" transparent conductive films (TCFs) with sheet resistance and optical transmittance comparable to that of indium tin oxide (ITO) films. It was shown that sheet resistance of single-component Ag nanowire (NW) films can be further decreased by introducing gold-decorated reduced graphene oxide (RG-O) nanoplatelets that bridge the closely located noncontacting metal NWs. RG-O nanoplatelets can act as a protective and adhesive layer for underneath metal NWs, resulting in better performance of hybrid TCFs compared to single-component TCFs. Additionally, these hybrid TCFs possess antibacterial properties, demonstrating their multifunctional characteristics that might have a potential for biomedical device applications. Further development of this strategy paves a way toward next generation TCFs composed of different nanostructures and characterized by multiple (or additional) functionalities.

Transparent Film Heaters based on Silver Nanowire Random Networks

We present the fabrication and characterization of transparent thin film heaters (TTFHs) based on silver nanowires. The goal is to develop a simple process for the production of transparent heating elements by large area printing techniques. The TTFHs are based on recently developed random networks of silver nanowires. Thanks to the very low sheet resistance achievable with silver nanowires, we show that it is possible to obtain high heating rates and good steady state temperatures at low voltages, typically below 12 V.