Graphene film for thermal management:A review

Thermal conductivity and thermal dissipation are of great importance for modern electronics due to the increased transistor density and operation frequency of contemporary integrated circuits.Due to its exceptionally high thermal conductivity,graphene has drawn considerable interests worldwide for heat spreading and dissipation.However,maintaining high thermal conductivity in graphene laminates(the basic technological unit)is a significant technological challenge.Aiming at highly thermal conductive graphene films(GFs),this prospective review outlines the most recent progress in the production of GFs originated from graphene oxide due to its great convenience in film processing.Additionally,we also consider such issues as film assembly,defect repair and mechanical compression during the post-treatment.We also discuss the thermal conductivity in in-plane and through-plane direction and mechanical properties of GFs.Further,the current typical applications of GFs are presented in thermal management.Finally,perspectives are given for future work on GFs for thermal management.

Self-assembled activated carbon sandwiched graphene film for symmetrical supercapacitors

Activated carbon(AC)particles sandwiched reduced graphene oxide sheets(rGO)film has been successfully fabricated via a facile self-assemble approach.The as-formed AC/rGO film is self-standing,flexible and mechanically robust,allowing to be transferred to any substrate on demand without rupture.Since AC particles effectively suppressed the restacking of the rGO sheet,AC/rGO film exhibits loose layer-by-layer stacking structures with various gaps between AC particles and rGO sheets,which is different from compact structures of pure graphene films.The as-formed gaps provide fast diffusion channels for electrolyte ions and enhanced accessible surface area of rGO.Therefore,the AC/rGO electrode delivers improved electrochemical performance over the voltage range of 0.0−3.0 V.This work offers a promising strategy to design free-standing supercapacitor electrodes based on traditional nanocarbon materials.

A Novel Method of Fabricating Flexible Transparent Conductive Large Area Graphene Film

We fabricate flexible conductive and transparent graphene films on position-emission-tomography substrates and prepare large area graphene films by graphite oxide sheets with the new technical process. The multi-layer graphene oxide sheets can be chemically reduced by HNO3 and HI to form a highly conductive graphene film on a substrate at lower temperature. The reduced graphene oxide sheets show a high conductivity sheet with resistance of 476Ω/sq and transmittance of 76% at 550nm （6 layers）. The technique used to produce the transparent conductive graphene thin film is facile, inexpensive, and can be tunable for a large area production applied for electronics or touch screens.

Electrochemical study and application on rutin at chitosan/graphene films modified glassy carbon electrode

Graphene（G） was dispersed into 0.5% chitosan（Chit） solution,then the composite films were coated on glassy carbon electrode（GCE）,the electrochemical behavior of rutin on a Chit/G modified GCE was investigated and the electrochemical parameters of rutin were calculated.Rutin effectively accumulated on the Chit/G/GCE and caused a pair of redox peaks at around 408 mV and 482 mV（vs.SCE） in 0.1M phosphate buffer solution（pH 4.0）.Under optimized conditions,the anodic peak current was linear to the rutin concentration in the range of 5×107-1.04×105M.The regression equation was：y 9.9219x-0.0025,r=0.9958.The proposed method was successfully used for the determination of rutin content in tablet samples with satisfactory results.

CVD growth of fingerprint-like patterned 3D graphene film for an ultrasensitive pressure sensor

With the rapid development of wearable devices, flexible pressure sensors with high sensitivity and wide workable range are highly desired. In nature, there are many well-adapted structures developed through natural selection, which inspired us for the design of biomimetic materials or devices. Particularl3 human fingertip skin, where many epidermal ridges amplify external stimulations, might be a good example to imitate for highly sensitive sensors. In this work, based on unique chemical vapor depositions （CVD）-grown three-dimensional （3D） graphene films that mimic the morphology of fingertip skin, we fabricated flexible pressure sensing membranes, which simultaneously showed a high sensitivity of 110 （kPa）-1 for 0-0.2 kPa and wide workable pressure range （up to 75 kPa）. Hierarchical structured polydimethylsiloxane （PDMS） films molded from natural leaves were used as the supporting elastic films for the graphene films, which also contribute to the superior performance of the pressure sensors. The pressure sensor showed a low detection limit （0.2 Pa）, fast response （〈 30 ms）, and excellent stability for more than 10,000 loading/unloading cycles. Based on these features, we demonstrated its applications in detecting tiny objects, sound, and human physiological signals, showing its potential in wearable electronics for health monitoring and human/machine interfaces.

Controllable formation of periodic wrinkles in Marangoni-driven self-assembled graphene film for sensitive strain detection

Controllable formation of microstructures in the assembled graphene film could tune the physical properties and broaden its applications in flexible electronics.Many efforts have been made to control the formation of wrinkles and ripples in graphene films.However,the formation of orderly wrinkles in graphene film remains a challenge.Here,we reported a simple strategy for the fabrication of graphene film with periodic and parallel wrinkles with a pre-stretched polydimethylsiloxane substrate.The width of the wrinkles in graphene can be controlled by changing the pre-stretched strain of the substrate.The average width of wrinkles in graphene film on the substrate with pre-stretched strain of 10%,20%,and 50%was about 3.68,2.99 and 2.01µm,respectively.The morphological evolution of wrinkled double-layered graphene under mechanical deformation was observed and studied.Furthermore,a strain sensor was constructed based on the wrinkled graphene,showing high sensitivity,large working range and excellent cyclic stability.These strain sensors show great potential in real-time motion detection,health surveillance and electronic skins.

Secondary electron emission characteristics of graphene films with copper substrate

For modern and future circular accelerators, especially high-intensity proton synchrotrons or colliders, the electron cloud effect is a key issue. So, in order to reduce the electron cloud effect, exploring very low secondary electron yield （SEY） material or coating used in vacuum tubes becomes necessary. In this article, we studied the SEY characteristics of graphene films with different thicknesses which were deposited on copper substrates using chemical vapor deposition. The SEY tests were done at temperatures of 25 ℃and vacuum pressure of （2-6）x 10-9 torr. The properties of the deposited graphene films were investigated by X-ray photoelectron spectroscopy （XPS） and Raman spectroscopy. The SEY curves show that the number of graphene layers has a great effect on the SEY of graphene films. The maximum SEY of graphene films decreases with the increase of the number of layers. The maximum SEY of 6-8 layers of graphene film is 1.25. These results have a great significance for next-generation particle accelerators.

Fast scanning growth of high-quality graphene films on Cu foils fueled by dimeric carbon precursor

Carbon source precursor is a critical factor governing chemical vapor deposition growth of graphene films.Methane(CH4),has been the most commonly used precursor in the last decade,but it presents challenges in terms of decomposition efficiency and growth rate.Here we thoroughly evaluated acetylene(C2H_(2)),a precursor that is probably for providing carbon dimer(C2)species,for fast growth of large-scale graphene films.We find that the graphene growth behaviors fueled by C2H_(2) exhibit unconventional localized growth behavior with significant advantages in terms of high growth rate,which mainly ascribe to the as-decomposed C2 species.Therefore,a C2-fueled scanning growth strategy is proposed,and the fast scanning growth rate of 40 cm/min was experimentally demonstrated.This growth strategy is compatible with the approach of unidirectional growth of single-crystal graphene films,and the as-grown graphene films are of high-quality.This work demonstrates a reliable and promising strategy for the rapid synthesis of high-quality graphene film and may pave the avenue to cost-effective mass production of graphene materials in the roll-to-roll system.

Highly sensitive wearable sensor based on a flexible multi-layer graphene film antenna

The use of advanced carbon nanomaterials for flexible antenna sensors has attracted great attention due to their outstanding electromechanical properties. However, carbon nanomaterial based composites have yet to overcome drawbacks, such as low conductivity and toughness. In this work, a flexible multi-layer graphene film(FGF) with a high conductivity of 10~6 S/m for antenna based wearable sensors is investigated. A 1.63 GHz FGF antenna sensor exhibits significantly high strain sensitivity of 9.8 for compressive bending and 9.36 for tensile bending, which is super than the copper antenna sensor(5.39 for compressive bending and 4.05 for tensile bending). Moreover, the FGF antenna sensor shows very good mechanical flexibility, reversible deformability and structure stability, and thus is well suited for applications like wearable devices and wireless strain sensing.

Thermoacoustic theory of graphene films considering heat transfer of substrate materials

Based on thermoacoustic theory, a coupled thermal-mechanical model for graphene films is established, and the analytical solutions for thermal-acoustic radiation from a graphene thin film are obtained. The sound pressure of the graphene film generator on different substrates is measured, and the measurement data is compared with the theoretical results. The frequency response from the experimental results is consistent with the theoretical ones, while the measured values are slightly lower than the theoretical ones. Therefore, the accuracy of the proposed theoretical model is verified. It is shown that thermal-acoustic radiation from a graphene thin film reveals a wide frequency response. The sound pressure level increases with the frequency in the low frequency range, while the sound pressure varies smoothly with frequency in the high frequency range. Thus it can be used as excellent thermal generator. When the thermal effusivity of the substrate is smaller, then the sound pressure of grapheme films will be higher. Furthermore, the sound pressure decreases with the increase of heat capacity per unit area of grapheme films. Results will contribute to the mechanism of graphene films generator and its applications in the design of loudspeaker and other related areas.

Direct fabrication of flexible tensile sensors enabled by polariton energy transfer based on graphene nanosheet films

A fundamental problem in the direct manufacturing of flexible devices is the low melting temperature of flexible substrates,which hinders the development of flexible electronics.Proposed here is an electron-cyclotron-resonance sputtering system that can batch-fabricate devices directly on flexible substrates under a low temperature by virtue of the polariton energy transfer between the plasma and the material.Flexible graphene nanosheet-embedded carbon(F-GNEC)films are manufactured directly on polyimide,polyethylene terephthalate,and polydimethylsiloxane,and how the substrate bias(electron energy),microwave power(plasma flux and energy),and magnetic field(electron flux)affect the nanostructure of the F-GNEC films is investigated,indicating that electron energy and flux contribute to the formation of standing graphene nanosheets in the film.The films have good uniformity of distribution in a large size(17 mm×17 mm),and tensile and angle sensors with a high gauge factor(0.92)and fast response(50 ms)for a machine hand are obtained by virtue of the unique nanostructure of the F-GNEC film.This work sheds light on the quantum manufacturing of carbon sensors and its applications for intelligent machine hands and virtual-reality technology.

A 3D graphene/polyimide fiber framework with improved thermal conductivity and mechanical performance

The integration of electronic components and the popularity of flexible devices have come up with higher expectations for the heat dissipation capability and comprehensive mechanical performance of thermal management materials.In this work,after the modification of polyimide(PI)fibers through oxidation and amination,the obtained PDA@OPI fibers(polydopamine(PDA)-modified pre-oxidized PI fibers)with abundant amino groups were mixed into graphene oxide(GO)to form uniform GO-PDA@OPI composites.Followed by evaporation,carbonization,graphitization and mechanical compaction,the G-gPDA@OPI films with a stable three-dimensional(3D)long-range interconnected covalent structure were built.In particular,due to the rich covalent bonds between GO layers and PDI@OPI fibers,the enhanced synergistic graphitization promotes an ordered graphitized structure with less interlayer distance between adjacent graphene sheets in composite film.As a result,the optimized G-gPDA@OPI film displays an improved tensile strength of 78.5 MPa,tensile strain of 19.4%and thermal conductivity of 1028 W/(m·K).Simultaneously,it also shows superior flexibility and high resilience.This work provides an easily-controlled and relatively low-cost route for fabricating multifunctional graphene heat dissipation films.

Nanostructured Graphene Surfaces Promote Different Stages of Bone Cell Differentiation

Nanostructured graphene films were used as platforms for the differentiation of Saos-2 cells into bonelike cells. The films were grown using the plasma-enhanced chemical vapor deposition method, which allowed the production of both vertically and horizontally aligned carbon nanowalls(CNWs). Modifications of the techniqueallowed control of the density of the CNWs and their orientation after the transfer process. The influence of two different topographies on cell attachment, proliferation,and differentiation was investigated. First, the transferred graphene surfaces were shown to be noncytotoxic and were able to support cell adhesion and growth for over 7 days.Second, early cell differentiation(identified by cellular alkaline phosphatase release) was found to be enhanced on the horizontally aligned CNW surfaces, whereas mineralization(identified by cellular calcium production), a later stage of bone cell differentiation, was stimulated by the presence of the vertical CNWs on the surfaces. These results show that the graphene coatings, grown using the presented method, are biocompatible. And their topographies have an impact on cell behavior, which can be useful in tissue engineering applications.

Catalyst-Free Growth of Graphene by Microwave Surface Wave Plasma Chemical Vapor Deposition at Low Temperature

Catalyst-free graphene films has been synthesized by microwave (MW) surface wave plasma (SWP) chemical vapor deposition (CVD) using hydrogenated carbon source on silicon substrates at low temperature (500℃). The synthesized process is simple, low-cost and possible for application on transparent electrodes, gas sensors and thin film resistors. Analytical methods such as Raman spectroscopy, transmission electron microscopy (TEM) and four points prove resistivity measurement and UV-VIS-NIR spectroscopy were employed to characterize properties of the graphene films. The formation of multilayer of graphene on silicon substrate was confirmed by Raman spectroscopy and TEM. It is possible to grow graphene directly on silicon substrate (without using catalyst) due to high radical density of MW SWP CVD. In addition, we also observed that the hydrogen had significant role for quality of graphene.

Characteristic modification by inserted metal layer and interface graphene layer in ZnO-based resistive switching structures

ZnO-based resistive switching device Ag/ZnO/TiN, and its modified structure Ag/ZnO/Zn/ZnO/TiN and Ag/graphene/ZnO/TiN, were prepared. The effects of inserted Zn layers in ZnO matrix and an interface graphene layer on resistive switching characteristics were studied. It is found that metal ions, oxygen vacancies, and interface are involved in the RS process. A thin inserted Zn layer can increase the resistance of HRS and enhance the resistance ratio. A graphene interface layer between ZnO layer and top electrode can block the carrier transport and enhance the resistance ratio to several times. The results suggest feasible routes to tailor the resistive switching performance of ZnO-based structure.

Wide temperature range,air stable,transparent,and self-powered photodetectors enabled by a hybrid film of graphene and singlewalled carbon nanotubes

Transparent photovoltaic devices(TPVDs)have attracted increasing attention in emerging electronic devices.As the application scenarios extend,there raise higher requirements regarding the stability and operating temperature range of TPVDs.In this work,a unique preparation strategy is proposed for air stable TPVD with a wide operating temperature range,i.e.,a nanoscale architecture termed as H-TPVD is constructed that integrates a free-standing and highly transparent conductive hybrid film of graphene and single-walled carbon nanotubes(G-SWNT TCF for short)with a metal oxide NiO/TiO_(2)heterojunction.The preparation approach is suitable for scaling up.Thanks to the excellent transparent conductivity of the freestanding G-SWNT hybrid film and the ultrathin NiO/TiO_(2)heterojunction(100 nm),H-TPVD selectively absorbs the ultraviolet(UV)band of sunlight and has a transparency of up to 71%in the visible light.The integrated nanoscale architecture manifests the significant holecollecting capability of the G-SWNT hybrid film and the efficient carrier generation and separation within the ultrathin NiO/TiO_(2)heterojunction,resulting in excellent performance of the H-TPVD with a specific detectivity of 2.7×10^(10) Jones.Especially,the freestanding G-SWNT TCF is a super stable and non-porous two-dimensional film that can insulate gas molecules,thereby protecting the surface properties of NiO/TiO_(2)heterojunctions and enhancing the stability of H-TPVD.Having subjected to 20,000 cycles and storage in air for three months,the performance parameters such as photo-response signal,output power,and specific detectivity show no noticeable degradation.In particular,the as-fabricated self-powered H-TPVD can operate over a wide temperature range from −180 to 300℃,and can carry out solar-blind UV optical communication in this range.In addition,the 4×4 array H-TPVD demonstrates clear optical imaging.These results make it possible for H-TPVD to expand its potential application scenarios.

Preparation of La MnO_3/graphene thin films and their photocatalytic activity

A novel photocatalyst of La MnO3/graphene thin films with the perovskite-type was synthesized by sol-gel process assisted with spin-coating methods on glass substrates.The prepared samples were characterized by scanning electron microscopy（SEM）, transmission electron microscopy（TEM）, X-ray diffraction（XRD）, Brumauer-Emmett-Teller（BET） surface area analyzer, X-ray photoelectron spectroscopy（XPS） and UV-vis diffuse reflectance spectroscopy.Results showed that after the introduction of graphene, the perovskite structure was unchanged and the size of La MnO3 particles was about 22 nm, which uniformed growth in graphene sheet.Determination of contact angle indicated that the contact angle of glass substrate decreased and the hydrophilicity improved after treating with H2SO4 and APTES.The UV-Vis photocatalytic activity of the photocatalysts was evaluated by the degradation of diamine green B.La MnO3/graphene thin films had better photocatalytic ability than La MnO3 and Ti O2 films.The obtained k was 0.5627 and 0.3441 h–1 corresponding to La MnO3/graphene films and Ti O2 films, respectively.

Ultraclean transfer of graphene by mechanically exfoliating polymer with modified crosslink density

The transfer of graphene from metallic substrates onto application-specific substrates is usually inevitable for the applications of high-quality graphene films derived from chemical vapour deposition(CVD)approaches.Commonly used to support the graphene films during the transfer,the coating of the polymer would produce the surface contaminations and hinder the industrially compatible transfer.In this work,through the thermal imidization of polyamide acid(PAA)to polyimide(PI)and tuning of the concentration of dangling chains,we achieved the ultraclean and crack-free transfer of graphene wafers with high electronic quality.The resulting contamination-free and hydrophilic surface also enabled the observed improved cell viability in a biomedical applications.By avoiding aqueous etching or the usage of strong bases,our proposed transfer method is industrially compatible for batch transfer of graphene films towards the real applications.

High conductive graphene assembled films with porous micro-structure for freestanding and ultra-low power strain sensors

Graphene emerges as an ideal material for constructing high-performance strain sensors,due to its superior mechanical property and high conductivity.However,in the process of assembling graphene into macroscopic materials,its conductivity decreases significantly.Also,tedious fabrication process hinders the application of graphene-based strain sensors.In this work,we report a freestanding graphene assembled film(GAF)with high conductivity((2.32±0.08)×105 S m-1).For the sensitive materials of strain sensors,it is higher than most of reported carbon nanotube and graphene materials.These advantages enable the GAF to be an ultra-low power consumption strain sensor for detecting airflow and vocal vibrations.The resistance of the GAF remains unchanged with increasing temperature(20-100℃),exhibiting a good thermal stability.Also,the GAF can be used as a strain sensor directly without any flexible substrates,which greatly simplifies the fabrication process in comparison with most reported strain sensors.Additionally,the GAF used as a pressure sensor with only^4.7μW power is investigated.This work provides a new direction for the preparation of advanced sensors with ultra-low power consumption,and the development of flexible and energy-saving electronic devices.

Optical super-resolution effect induced by nonlinear characteristics of graphene oxide films

In this work, we focus on the optical super-resolution effect induced by strong nonlinear saturation absorption(NSA) of graphene oxide(GO) membranes. The third-order optical nonlinearities are characterized by the canonical Z-scan technique under femtosecond laser(wavelength: 800 nm, pulse width: 100 fs) excitation. Through controlling the applied femtosecond laser energy, NSA of the GO films can be tuned continuously. The GO film is placed at the focal plane as a unique amplitude filter to improve the resolution of the focused field. A multi-layer system model is proposed to present the generation of a deep sub-wavelength spot associated with the nonlinearity of GO films. Moreover, the parameter conditions to achieve the best resolution(~λ/6) are determined entirely. The demonstrated results here are useful for high density optical recoding and storage, nanolithography, and super-resolution optical imaging.