Covalently bridging graphene edges for improving mechanical and electrical properties of fibers.

Assembling graphene sheets into macroscopic fibers with graphitic layers uniaxially aligned along the fiber axis is of both fundamental and technological importance. However, the optimal performance of graphene-based fibers has been far lower than what is expected based on the properties of individual graphene. Here we show that both mechanical properties and electrical conductivity of graphene-based fibers can be significantly improved if bridges are created between graphene edges through covalent conjugating aromatic amide bonds. The improved electrical conductivity is likely due to extended electron conjugation over the aromatic amide bridged graphene sheets. The larger sheets also result in improved π-π stacking, which, along with the robust aromatic amide linkage, provides high mechanical strength. In our experiments, graphene edges were bridged using the established wet-spinning technique in the presence of an aromatic amine linker, which selectively reacts to carboxyl groups at the graphene edge sites. This technique is already industrial and can be easily upscaled. Our methodology thus paves the way to the fabrication of high-performance macroscopic graphene fibers under optimal techno-economic and ecological conditions.

Multispecies-coadsorption-induced rapid preparation of graphene glass fiber fabric and applications in flexible pressure sensor.

Direct chemical vapor deposition (CVD) growth of graphene on dielectric/insulating materials is a promising strategy for subsequent transfer-free applications of graphene. However, graphene growth on noncatalytic substrates is faced with thorny issues, especially the limited growth rate, which severely hinders mass production and practical applications. Herein, graphene glass fiber fabric (GGFF) is developed by graphene CVD growth on glass fiber fabric. Dichloromethane is applied as a carbon precursor to accelerate graphene growth, which has a low decomposition energy barrier, and more importantly, the produced high-electronegativity Cl radical can enhance adsorption of active carbon species by Cl-CH2 coadsorption and facilitate H detachment from graphene edges. Consequently, the growth rate is increased by ~3 orders of magnitude and carbon utilization by ~960-fold, compared with conventional methane precursor. The advantageous hierarchical conductive configuration of lightweight, flexible GGFF makes it an ultrasensitive pressure sensor for human motion and physiological monitoring, such as pulse and vocal signals.

Waveguide-integrated twisted bilayer graphene photodetectors.

Graphene photodetectors have exhibited high bandwidth and capability of being integrated with silicon photonics (SiPh), holding promise for future optical communication devices. However, they usually suffer from a low photoresponsivity due to weak optical absorption. In this work, we have implemented SiPh-integrated twisted bilayer graphene (tBLG) detectors and reported a responsivity of 0.65 A W-1 for telecom wavelength 1,550 nm. The high responsivity enables a 3-dB bandwidth of >65 GHz and a high data stream rate of 50 Gbit s-1. Such high responsivity is attributed to the enhanced optical absorption, which is facilitated by van Hove singularities in the band structure of high-mobility tBLG with 4.1o twist angle. The uniform performance of the fabricated photodetector arrays demonstrates a fascinating prospect of large-area tBLG as a material candidate for heterogeneous integration with SiPh.

Polyacrylonitrile as an Efficient Transfer Medium for Wafer-Scale Transfer of Graphene.

The disparity between growth substrates and application-specific substrates can be mediated by reliable graphene transfer, the lack of which currently strongly hinders the graphene applications. Conventionally, the removal of soft polymers, that support the graphene during the transfer, would contaminate graphene surface, produce cracks, and leave unprotected graphene surface sensitive to airborne contaminations. In this work, it is found that polyacrylonitrile (PAN) can function as polymer medium for transferring wafer-size graphene, and encapsulating layer to deliver high-performance graphene devices. Therefore, PAN, that is compatible with device fabrication, does not need to be removed for subsequent applications. The crack-free transfer of 4 in. graphene onto SiO2/Si wafers, and the wafer-scale fabrication of graphene-based field-effect transistor arrays with no observed clear doping, uniformly high carrier mobility (≈11 000 cm2 V-1 s-1), and long-term stability at room temperature, are achieved. This work presents new concept for designing the transfer process of 2D materials, in which multifunctional polymer can be retained, and offers a reliable method for fabricating wafer-scale devices of 2D materials with outstanding performance.

Overcoming the Incompatibility Between Electrical Conductivity and Electromagnetic Transmissivity: A Graphene Glass Fiber Fabric Design Strategy.

Conventional conductive materials such as metals are crucial functional components of conductive systems in diverse electronic instruments. However, their severe intrinsic impedance mismatch with air dielectric causes strong reflection of incident electromagnetic waves, and the resulting low electromagnetic transmissivity typically interferes with surrounding electromagnetic signal communications in modern multifunction-integrated instruments. Herein, graphene glass fiber fabric (GGFF) that merges intrinsic electrical and electromagnetic properties of graphene with dielectric attributes and highly porous macrostructure of glass fiber fabric (GFF) is innovatively developed. Using a novel decoupling chemical vapor deposition growth strategy, high-quality and layer-limited graphene is prepared on noncatalytic nonmetallic GFF in a controlled manner; this is pivotal to realizing GGFF with the desired compatibility among high conductivity, low electromagnetic reflectivity, and high electromagnetic transmissivity. At the same sheet resistance over a wide range of values (250-3000 Ω·sq-1), the GGFF exhibits significantly lower electromagnetic reflectivity (by 0.42-0.51) and higher transmissivity (by 0.27-0.62) than those of its metal-based conductive counterpart (CuGFF). The material design strategy reported herein provides a constructive solution to eliminate the incompatibility between electrical conductivity and electromagnetic transmissivity faced by conventional conductive materials, spotlighting the applicability of GGFF in electric heating scenarios in radar, antenna, and stealth systems.

Recent trends in the transfer of graphene films.

Recent years have witnessed advances in chemical vapor deposition growth of graphene films on metal foils with fine scalability and thickness controllability. However, challenges for obtaining wrinkle-free, defect-free and large-area uniformity remain to be tackled. In addition, the real commercial applications of graphene films still require industrially compatible transfer techniques with reliable performance of transferred graphene, excellent production capacity, and suitable cost. Transferred graphene films, particularly with a large area, still suffer from the presence of transfer-related cracks, wrinkles and contaminants, which would strongly deteriorate the quality and uniformity of transferred graphene films. Potential applications of graphene films include moisture barrier films, transparent conductive films, electromagnetic shielding films, and optical communications; such applications call different requirements for the performance of transferred graphene, which, in turn, determine the suitable transfer techniques. Besides the reliable transfer process, automatic machines should be well developed for the future batch transfer of graphene films, ensuring the repeatability and scalability. This mini-review provides a summary of recent advances in the transfer of graphene films and offers a perspective for future directions of transfer techniques that are compatible for industrial batch transfer.

Quasi van der Waals Epitaxy of Single Crystalline GaN on Amorphous SiO2/Si(100) for Monolithic Optoelectronic Integration.

The realization of high quality (0001) GaN on Si(100) is paramount importance for the monolithic integration of Si-based integrated circuits and GaN-enabled optoelectronic devices. Nevertheless, thorny issues including large thermal mismatch and distinct crystal symmetries typically bring about uncontrollable polycrystalline GaN formation with considerable surface roughness on standard Si(100). Here a breakthrough of high-quality single-crystalline GaN film on polycrystalline SiO2/Si(100) is presented by quasi van der Waals epitaxy and fabricate the monolithically integrated photonic chips. The in-plane orientation of epilayer is aligned throughout a slip and rotation of high density AlN nuclei due to weak interfacial forces, while the out-of-plane orientation of GaN can be guided by multi-step growth on transfer-free graphene. For the first time, the monolithic integration of light-emitting diode (LED) and photodetector (PD) devices are accomplished on CMOS-compatible SiO2/Si(100). Remarkably, the self-powered PD affords a rapid response below 250 µs under adjacent LED radiation, demonstrating the responsivity and detectivity of 2.01 × 105 A/W and 4.64 × 1013 Jones, respectively. This work breaks a bottleneck of synthesizing large area single-crystal GaN on Si(100), which is anticipated to motivate the disruptive developments in Si-integrated optoelectronic devices.

Establishing Pinhole Deposition Mode of Zn via Scalable Monolayer Graphene Film.

The uneven texture evolution of Zn during electrodeposition would adversely impact upon the lifespan of aqueous Zn metal batteries. To address this issue, tremendous endeavors are made to induce Zn(002) orientational deposition employing graphene and its derivatives. Nevertheless, the effect of prototype graphene film over Zn deposition behavior has garnered less attention. Here, it is attempted to solve such a puzzle via utilizing transferred high-quality graphene film with controllable layer numbers in a scalable manner on a Zn foil. The multilayer graphene fails to facilitate a Zn epitaxial deposition, whereas the monolayer film with slight breakages steers a unique pinhole deposition mode. In-depth electrochemical measurements and theoretical simulations discover that the transferred graphene film not only acts as an armor to inhibit side reactions but also serves as a buffer layer to homogenize initial Zn nucleation and decrease Zn migration barrier, accordingly enabling a smooth deposition layer with closely stacked polycrystalline domains. As a result, both assembled symmetric and full cells manage to deliver satisfactory electrochemical performances. This study proposes a concept of "pinhole deposition" to dictate Zn electrodeposition and broadens the horizons of graphene-modified Zn anodes.

Tunable Adhesion for All-Dry Transfer of 2D Materials Enabled by the Freezing of Transfer Medium.

The real applications of chemical vapor deposition (CVD)-grown graphene films require the reliable techniques for transferring graphene from growth substrates onto application-specific substrates. The transfer approaches that avoid the use of organic solvents, etchants, and strong bases are compatible with industrial batch processing, in which graphene transfer should be conducted by dry exfoliation and lamination. However, all-dry transfer of graphene remains unachievable owing to the difficulty in precisely controlling interfacial adhesion to enable the crack- and contamination-free transfer. Herein, through controllable crosslinking of transfer medium polymer, the adhesion is successfully tuned between the polymer and graphene for all-dry transfer of graphene wafers. Stronger adhesion enables crack-free peeling of the graphene from growth substrates, while reduced adhesion facilitates the exfoliation of polymer from graphene surface leaving an ultraclean surface. This work provides an industrially compatible approach for transferring 2D materials, key for their future applications, and offers a route for tuning the interfacial adhesion that would allow for the transfer-enabled fabrication of van der Waals heterostructures.

Super Graphene-Skinned Materials: An Innovative Strategy toward Graphene Applications.

Super graphene-skinned materials are emerging members of the graphene composite materials family, which are produced through the high-temperature chemical deposition of continuous graphene layers on traditional engineering materials. The high-performance graphene "skin" endows the traditional engineering materials with additional functionalities, and atomically thin graphene films enter the market by hitching a ride on traditional material carriers. Beyond the physical coating of graphene powders onto engineering materials, the directly grown continuous graphene skin keeps its excellent intrinsic properties to a great extent and holds promise for future applications. Super graphene-skinned material is an innovative pathway for applications of continuous graphene films, which avoids the challenging peeling-transfer process and solves the non-self-supporting issue of ultrathin graphene film. It is a big family, including graphene-skinned powders, fibers, foils, and foams. With further processing and molding, we can obtain graphene-dispersed bulk materials, especially for metal-based graphene-skinned materials, which provides a creative pathway for uniformly dispersing graphene into a metal matrix. In practical applications, graphene-skinned materials would exhibit excellent performance with perfect processing compatibility with current engineering materials and be pushed to real industrial applications relying on the broad market of engineering materials.

Controlled Growth of Single-Crystal Graphene Wafers on Twin-Boundary-Free Cu(111) Substrates.

Single-crystal graphene (SCG) wafers are needed to enable mass-electronics and optoelectronics owing to their excellent properties and compatibility with silicon-based technology. Controlled synthesis of high-quality SCG wafers can be done exploiting single-crystal Cu(111) substrates as epitaxial growth substrates recently. However, current Cu(111) films prepared by magnetron sputtering on single-crystal sapphire wafers still suffer from in-plane twin boundaries, which degrade the SCG chemical vapor deposition. Here, it is shown how to eliminate twin boundaries on Cu and achieve 4 in. Cu(111) wafers with ≈95% crystallinity. The introduction of a temperature gradient on Cu films with designed texture during annealing drives abnormal grain growth across the whole Cu wafer. In-plane twin boundaries are eliminated via migration of out-of-plane grain boundaries. SCG wafers grown on the resulting single-crystal Cu(111) substrates exhibit improved crystallinity with >97% aligned graphene domains. As-synthesized SCG wafers exhibit an average carrier mobility up to 7284 cm2 V-1 s-1 at room temperature from 103 devices and a uniform sheet resistance with only 5% deviation in 4 in. region.

High Moisture-Barrier Performance of Double-Layer Graphene Enabled by Conformal and Clean Transfer.

Graphene films that can theoretically block almost all molecules have emerged as promising candidate materials for moisture barrier films in the applications of organic photonic devices and gas storage. However, the current barrier performance of graphene films does not reach the ideal value. Here, we reveal that the interlayer distance of the large-area stacked multilayer graphene is the key factor that suppresses water permeation. We show that by minimizing the gap between the two monolayers, the water vapor transmission rate of double-layer graphene can be as low as 5 × 10-3 g/(m2 d) over an A4-sized region. The high barrier performance was achieved by the absence of interfacial contamination and conformal contact between graphene layers during layer-by-layer transfer. Our work reveals the moisture permeation mechanism through graphene layers, and with this approach, we can tailor the interlayer coupling of manually stacked two-dimensional materials for new physics and applications.

Fast synthesis of large-area bilayer graphene film on Cu.

Bilayer graphene (BLG) is intriguing for its unique properties and potential applications in electronics, photonics, and mechanics. However, the chemical vapor deposition synthesis of large-area high-quality bilayer graphene on Cu is suffering from a low growth rate and limited bilayer coverage. Herein, we demonstrate the fast synthesis of meter-sized bilayer graphene film on commercial polycrystalline Cu foils by introducing trace CO2 during high-temperature growth. Continuous bilayer graphene with a high ratio of AB-stacking structure can be obtained within 20 min, which exhibits enhanced mechanical strength, uniform transmittance, and low sheet resistance in large area. Moreover, 96 and 100% AB-stacking structures were achieved in bilayer graphene grown on single-crystal Cu(111) foil and ultraflat single-crystal Cu(111)/sapphire substrates, respectively. The AB-stacking bilayer graphene exhibits tunable bandgap and performs well in photodetection. This work provides important insights into the growth mechanism and the mass production of large-area high-quality BLG on Cu.

Graphene/Cholesteric Liquid-Crystal-Based Electro-Driven Thermochromic Light Modulators toward Wide-Gamut Dynamic Light Color-Tuning-Related Applications.

Light filters are ubiquitous in projection and display techniques, illumination engineering, image sensing, photography, etc., while those enabling wide-gamut dynamic light color tuning are still lacking. Herein, by combining the electro-heating capability of graphene and unique optical properties (thermochromism and circular dichroism) of small-molecule-weight cholesteric liquid crystal (ChLC), a brand-new thermochromic light modulator is constructed as actively tunable color filter. Transparent graphene/glass hybrid with reasonably high conductivity serves both as a high-performance heater for actuating the thermochromism of temperature-responsive ChLC and as neutral light attenuator for brightness control. Thanks to the temperature- and polarization-dependent spectral properties of the ChLC, widely tunable hue and saturation properties of transmission light color are achieved, respectively. Several intriguing applications, e.g., color-variable smart windows for backlight color tuning and color-variable filters for photography, are also demonstrated. This work hereby provides new paradigms for promoting the applications of graphene/ChLC-based light modulators in next-generation light-management-related scenarios.

Oxidization-Temperature-Triggered Rapid Preparation of Large-Area Single-Crystal Cu(111) Foil.

The controlled preparation of single-crystal Cu(111) is intensively investigated owing to the superior properties of Cu(111) and its advantages in synthesizing high-quality 2D materials, especially graphene. However, the accessibility of large-area single-crystal Cu(111) is still hindered by time-consuming, complicated, and high-cost preparation methods. Here, the oxidization-temperature-triggered rapid preparation of large-area single-crystal Cu(111) in which an area up to 320 cm2 is prepared within 60 min, and where low-temperature oxidization of polycrystalline Cu foil surface plays a vital role, is reported. A mechanism is proposed, by which the thin Cux O layer transforms to a Cu(111) seed layer on the surface of Cu to induce the formation of a large-area Cu(111) foil, which is supported by both experimental data and molecular dynamics simulation results. In addition, a large-size high-quality graphene film is synthesized on the single-crystal Cu(111) foil surface and the graphene/Cu(111) composites exhibit enhanced thermal conductivity and ductility compared to their polycrystalline counterpart. This work, therefore, not only provides a new avenue toward the monocrystallinity of Cu with specific planes but also contributes to improving the mass production of high-quality 2D materials.

Rapid and Scalable Transfer of Large-Area Graphene Wafers.

Recently, scalable production of large-area graphene films on metal foils with promising qualities is successfully achieved by eliminating grain boundaries, wrinkles, and adlayers. The transfer of graphene from growth metal substrates onto functional substrates remains one inescapable obstacle on the road to the real commercial applications of chemical vaport deposition (CVD) graphene films. Current transfer methods still require time-consuming chemical reactions, which hinders its mass production, and produces cracks and contamination that strongly impede performance reproducibility. Therefore, graphene transfer techniques with fine intactness and cleanness of transferred graphene, and improved production efficiency would be ideal for the mass production of graphene films on destination substrates. Herein, through the engineering of interfacial forces enabled by sophisticated design of transfer medium, the crack-free and clean transfer of 4-inch-sized graphene wafers onto silicon wafers within only 15 min is realized. The reported transfer method is an important leap over the long-lasting obstacle of the batch-scale graphene transfer without degrading the quality of graphene, bringing the graphene products close to the real applications.

Rationalized Electroepitaxy toward Scalable Single-Crystal Zn Anodes.

Electroepitaxy is recognized as an effective approach to prepare metal electrodes with nearly complete reversibility. Nevertheless, large-scale manipulation is still not attainable owing to complicated interfacial chemistry. Here, the feasibility of extending Zn electroepitaxy toward the bulk phase over a mass-produced mono-oriented Cu(111) foil is demonstrated. Interfacial Cu-Zn alloy and turbulent electroosmosis are circumvented by adopting a potentiostatic electrodeposition protocol. The as-prepared Zn single-crystalline anode enables stable cycling of symmetric cells at a stringent current density of 50.0 mA cm-2 . The assembled full cell further sustaines a capacity retention of 95.7% at 5.0 A g-1 for 1500 cycles, accompanied by a controllably low N/P ratio of 7.5. In addition to Zn, Ni electroepitaxy can be realized by using the same approach. This study may inspire rational exploration of the design of high-end metal electrodes.

Scalable Fabrication of Dual-Function Fabric for Zero-Energy Thermal Environmental Management through Multiband, Synergistic, and Asymmetric Optical Modulations.

Solar heating and radiative cooling techniques have been proposed for passive space thermal management to reduce the global energy burden. However, the currently used single-function envelope/coating materials can only achieve static temperature regulation, presenting limited energy savings and poor adaption to dynamic environments. In this study, a sandwich-structured fabric, composed of vertical graphene, graphene glass fiber fabric, and polyacrylonitrile nanofibers is developed, with heating and cooling functions integrated through multiband, synergistic, (solar spectrum and mid-infrared ranges) and asymmetric optical modulations on two sides of the fabric. The dual-function fabric demonstrates high adaption to the dynamic environment and superior performance in a zero-energy-input temperature regulation. Furthermore, it demonstrates ≈15.5 and ≈31.1 MJ m-2 y-1 higher annual energy savings compared to those of their cooling-only and heating-only counterparts, corresponding to ≈173.7 MT reduction in the global CO2 emission. The fabric exhibits high scalability for batch manufacturing with commercially abundant raw materials and facile technologies, providing a favorable guarantee of its mass production and use.