Wetting‐Assisted Crack‐ and Wrinkle‐Free Transfer of Wafer‐Scale Graphene onto Arbitrary Substrates over a Wide Range of Surface Energies

The polymer‐supported wet transfer of chemical vapor deposition‐grown graphene provides high‐quality large‐area graphene on a target substrate. The transfer‐induced defects that result from these processes, such as micrometer‐scale folds and cracks, have been regarded as an inevitable problem. Here, the transfer processes are thoroughly examined stage‐by‐stage and it is found that lamination wrinkles, which cause defects in the graphene, are generated as a result of the high contact angles of the trapped transfer medium liquids. Systematic theoretical and experimental studies demonstrate that a liquid droplet with a low surface tension trapped between the polymer/graphene film and the substrate minimizes lamination wrinkles during the transfer process by completely wetting the target substrate, regardless of the surface energy. In connection with these results, a simple and broadly applicable transfer method is developed using an organic liquid with a low surface tension to uniformly transfer high‐quality graphene onto arbitrary substrates, even onto superhydrophobic substrate. The graphene obtained using the proposed organic liquid transfer method displays better electrical and mechanical properties than the graphene transferred by the conventional method using water. This effective and practical transfer method provides an approach to obtaining high‐quality graphene for use in graphene‐based devices.     

Direct Graphene Transfer and Its Application to Transfer Printing Using Mechanically Controlled,Large Area Graphene/Copper Freestanding Layer

Direct graphene transfer is an attractive candidate to prevent graphene damage, which is a critical problem of the conventional wet transfer method. Direct graphene transfer can fabricate the transferred graphene film with fewer defects by using a polymeric carrier. Here a unique direct transfer method is proposed using a 300 nm thick copper carrier as a suspended film and a transfer printing process by using the polydimethylsiloxane (PDMS) stamp under controlled peeling rate and modulus. Single and multilayer graphene are transferred to flat and curved PDMS target substrate directly. With the transfer printing process, the transfer yield of a trilayer graphene with 1000 µm s^?1 peeling rate is 68.6% of that with 1 µm s^?1 peeling rate. It is revealed that the graphene transfer yield is highly related to the storage modulus of the PDMS stamp: graphene transfer yield decreases when the storage modulus of the PDMS stamp is lower than a specific threshold value. The relationship between the graphene transfer yield and the interfacial shear strain of the PDMS stamp is studied by finite‐element method simulation and digital image correlation.     

Defect‐Tolerant Nanocomposites through Bio‐Inspired Stiffness Modulation

A biologically inspired, multilayer laminate structural design is deployed into nanocomposite films of graphene oxide‐poly(methyl methacrylate) (GO‐PMMA). The resulting multilayer GO‐PMMA films show greatly enhanced mechanical properties compared to pure‐graphene‐oxide films, with up to 100% increases in stiffness and strength when optimized. Notably, a new morphology is observed at fracture surfaces: whereas pure‐graphene‐oxide films show clean fracture surfaces consistent with crack initiation and propagation perpendicular to the applied tensile load, the GO‐PMMA multilayer laminates show terracing consistent with crack stopping and deflection mechanisms. As a consequence, these macroscopic GO‐PMMA films become defect‐tolerant and can maintain their tensile strengths as their sample volumes increase. Linear elastic fracture analysis supports these observations by showing that the stiffness modulation introduced by including PMMA layers within a graphene oxide film can act to shield or deflect cracks, thereby delaying failure and allowing the material to access more of its inherent strength. Together, these data clearly demonstrate that desirable defect‐tolerant traits of structural biomaterials can indeed be incorporated into graphene‐ oxide‐based nanocomposites.     

Photoquantum Hall Effect and Light‐Induced Charge Transfer at the Interface of Graphene/InSe Heterostructures

The transfer of electronic charge across the interface of two van der Waals crystals can underpin the operation of a new class of functional devices. Among van der Waals semiconductors, an exciting and rapidly growing development involves the “post‐transition” metal chalcogenide InSe. Here, field effect phototransistors are reported where single layer graphene is capped with n‐type InSe. These device structures combine the photosensitivity of InSe with the unique electrical properties of graphene. It is shown that the light‐induced transfer of charge between InSe and graphene offers an effective method to increase or decrease the carrier density in graphene, causing a change in its resistance that is gate‐controllable and only weakly dependent on temperature. The charge transfer at the InSe/graphene interface is probed by Hall effect and photoconductivity measurmentes and it is demonstrated that light can induce a sign reversal of the quantum Hall voltage and photovoltaic effects in the graphene layer. These findings demonstrate the potential of light‐induced charge transfer in gate‐tunable InSe/graphene phototransistors for optoelectronics and quantum metrology.     

Interfacial Sliding and Buckling of Monolayer Graphene on a Stretchable Substrate

The nonlinear mechanical response of monolayer graphene on polyethylene terephthalate (PET) is characterised using in‐situ Raman spectroscopy and atomic force microscopy. While interfacial stress transfer leads to tension in graphene as the PET substrate is stretched, retraction of the substrate during unloading imposes compression in the graphene. Two interfacial failure mechanisms, shear sliding under tension and buckling under compression, are identified. Using a nonlinear shear‐lag model, the interfacial shear strength is found to range between 0.46 and 0.69 MPa. The critical strain for onset of interfacial sliding is ～0.3%, while the maximum strain that can be transferred to graphene ranges from 1.2% to 1.6% depending on the interfacial shear strength and graphene size. Beyond a critical compressive strain of around ?0.7%, buckling ridges are observed after unloading. The results from this work provide valuable insight and design guidelines for a broad spectrum of applications of graphene and other 2D nanomaterials, such as flexible and stretchable electronics, strain sensing, and nanocomposites.     

Highly Conductive and Environmentally Stable Organic Transparent Electrodes Laminated with Graphene

Improving the lifetime and the operational and thermal stability of organic thin‐film materials while maintaining high conductivity and mechanical flexibility is critical for flexible electronics applications. Here, it is reported that highly conductive and environmentally stable organic transparent electrodes (TEs) can be fabricated by mechanically laminating poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) films containing dimethylsulfoxide and Zonyl fluorosurfactant (PDZ films) with a monolayer graphene barrier. The proposed lamination process allows graphene to be coated onto the PDZ films uniformly and conformally with tight interfacial binding, free of wrinkles and air gaps. The laminated films exhibit an outstanding room‐temperature hole mobility of ≈85.1 cm² V^?1 s^?1 since the graphene can serve as an effective bypass for charge carriers. The significantly improved stability of the graphene‐laminated TEs against high mechanical/thermal stress, humidity, and ultraviolet irradiation is particularly promising. Furthermore, the incorporation of the graphene barrier increases the expected lifetime of the TEs by more than two orders of magnitude.     

Enhancement of Thermal Energy Transport Across Graphene/Graphite and Polymer Interfaces: A Molecular Dynamics Study

Understanding thermal energy transport in polymeric nanocomposite materials is important to the engineering of polymer composites with better engineered heat transfer properties. Interfacial thermal resistance between the filling particles and the polymer matrices is a major bottleneck for the thermal conductivity improvement of polymer composite materials. Here, thermal energy transport in graphene/graphite‐polymer (paraffin wax‐C₃₀H₆₂) composite systems are systematically studied using molecular dynamics simulations. The influences of graphene size, interfacial bonding strength, and polymer density on the interfacial thermal transport are studied. According to the simulation results, approaches to improve interfacial thermal transport are proposed. Spectral analysis is performed to explore the mechanism of thermal transport. It is found that thermal energy transport across graphene/graphite‐polymer interfaces can be enhanced by increasing the polymer density and graphene size or forming covalent bonds between the graphite edges and polymer molecules. The results offer valuable guidance on improving thermal transport properties of polymeric nanocomposite.     

Large‐Area,Periodic, Hexagonal Wrinkles on Nanocrystalline Graphitic Film

Sinusoidal wrinkles develop in compressively stressed film as a means to release stored elastic energy. Here, a simple way to fabricate large‐area, periodic, hexagonal wrinkled pattern on nanocrystalline graphitic films grown on c‐plane sapphire (<50 nm thick) by the spontaneous delamination–buckling of the as‐grown film during cooling is reported. According to the continuum mechanics calculation, strain‐relief pattern adopting the hexagonal wrinkled pattern has a lower elastic energy than that of the telephone cord wrinkle at thickness regime below 50 nm. A high‐fidelity transfer method is developed to transfer the hexagonal wrinkled films onto arbitrary substrates. Nanoindentation studies show that hexagonal wrinkle film engineered this way may act as shock absorber. The hexagonal wrinkled carbon film is able to selectively promote the differentiation of human mesenchymal stem cell toward the osteogenic lineage in the absence of osteogenic inducing medium.     

Functionally Antagonistic Hybrid Electrode with Hollow Tubular Graphene Mesh and Nitrogen‐Doped Crumpled Graphene for High‐Performance Ionic Soft Actuators

Ionic soft actuators, which exhibit large mechanical deformations under low electrical stimuli, are attracting attention in recent years with the advent of soft and wearable electronics. However, a key challenge for making high‐performance ionic soft actuators with large bending deformation and fast actuation speed is to develop a stretchable and flexible electrode having high electrical conductivity and electrochemical capacitance. Here, a functionally antagonistic hybrid electrode with hollow tubular graphene meshes and nitrogen‐doped crumpled graphene is newly reported for superior ionic soft actuators. Three‐dimensional network of hollow tubular graphene mesh provides high electrical conductivity and mechanically resilient functionality on whole electrode domain. On the contrary, nitrogen‐doped wrinkled graphene supplies ultrahigh capacitance and stretchability, which are indispensably required for improving electrochemical activity in ionic soft actuators. Present results show that the functionally antagonistic hybrid electrode greatly enhances the actuation performances of ionic soft actuators, resulting in much larger bending deformation up to 620%, ten times faster rise time and much lower phase delay in a broad range of input frequencies. This outstanding enhancement mostly attributes to exceptional properties and synergistic effects between hollow tubular graphene mesh and nitrogen‐doped crumpled graphene, which have functionally antagonistic roles in charge transfer and charge injection, respectively.     

Thermally Controlled,Patterned Graphene Transfer Printing for Transparent and Wearable Electronic/Optoelectronic System

Graphene has been highlighted as a platform material in transparent electronics and optoelectronics, including flexible and stretchable ones, due to its unique properties such as optical transparency, mechanical softness, ultrathin thickness, and high carrier mobility. Despite huge research efforts for graphene‐based electronic/optoelectronic devices, there are remaining challenges in terms of their seamless integration, such as the high‐quality contact formation, precise alignment of micrometer‐scale patterns, and control of interfacial‐adhesion/local‐resistance. Here, a thermally controlled transfer printing technique that allows multiple patterned‐graphene transfers at desired locations is presented. Using the thermal‐expansion mismatch between the viscoelastic sacrificial layer and the elastic stamp, a “heating and cooling” process precisely positions patterned graphene layers on various substrates, including graphene prepatterns, hydrophilic surfaces, and superhydrophobic surfaces, with high transfer yields. A detailed theoretical analysis of underlying physics/mechanics of this approach is also described. The proposed transfer printing successfully integrates graphene‐based stretchable sensors, actuators, light‐emitting diodes, and other electronics in one platform, paving the way toward transparent and wearable multifunctional electronic systems.     

Carbon‐Based Photothermal Actuators

Actuators that can convert environmental stimuli into mechanical work are widely used in intelligent systems, robots, and micromechanics. To produce robust and sensitive actuators of different scales, efforts are devoted to developing effective actuating schemes and functional materials for actuator design. Carbon‐based nanomaterials have emerged as preferred candidates for different actuating systems because of their low cost, ease of processing, mechanical strength, and excellent physical/chemical properties. Especially, due to their excellent photothermal activity, which includes both optical absorption and thermal conductivities, carbon‐based materials have shown great potential for use in photothermal actuators. Herein, the recent advances in photothermal actuators based on various carbon allotropes, including graphite, carbon nanotubes, amorphous carbon, graphene and its derivatives, are reviewed. Different photothermal actuating schemes, including photothermal effect–induced expansion, desorption, phase change, surface tension gradient creation, and actuation under magnetic levitation, are summarized, and the light‐to‐heat and heat‐to‐work conversion mechanisms are discussed. Carbon‐based photothermal actuators that feature high light‐to‐work conversion efficiency, mechanical robustness, and noncontact manipulation hold great promise for future autonomous systems.     

Fatigue‐Resistant Bioinspired Graphene‐Based Nanocomposites

Graphene is an attractive building block for constructing functional materials of flexible electronic devices, due to its extraordinary mechanical and electrical properties. Up to now, large amounts of high‐performance graphene‐based nanocomposites are fabricated. However, the fatigue behavior of graphene‐based nanocomposites, a key parameter for flexible electronic devices, is rarely investigated. According to the fatigue mechanisms of thermosetting polymer composites, the fatigue resistance of graphene‐based nanocomposites can be significantly improved by effectively restricting the crack growth. Natural nacre demonstrates unique multisuppression of crack propagation, which is attributed to its sophisticated interfacial architecture over multiple length scales, resulting in remarkable fracture toughness. The crack suppression mechanisms corresponding to different interfacial design strategies within bioinspired graphene‐based nanocomposites (BGBNs) are summarized in this feature article. The static mechanical properties, electrical conductivity, and fatigue resistance of these BGBNs are compared and discussed. The synergistic effect from various interfacial interactions and building blocks is highlighted to serve as the guidance for constructing novel fatigue‐resistant BGBNs. The promising applications of fatigue‐resistant BGBNs in flexible electronic devices are reviewed, and several challenges and corresponding solutions are proposed. The perspective of fatigue‐resistant BGBNs for fundamental research and commercial application is depicted.     

CaO‐Templated Growth of Hierarchical Porous Graphene for High‐Power Lithium–Sulfur Battery Applications

Structural hierarchy plays an important role in the biological world and for functional materials with optimized properties and high efficiency. As promising candidates for various energy storage systems, hierarchical porous carbon/graphene materials have been intensively investigated over the past decades, while the favorable regulation of their hierarchical porosity remains a challenge. Herein, porous CaO serves as both the catalyst and template for a versatile chemical vapor deposition (CVD) of hierarchical porous graphene. The gas atmosphere during CVD and nanostructure of adopted catalysts impact significantly on the graphitization degree and hierarchical porosity of resultant materials. The as‐fabricated material exhibits abundant microsized in‐plane vacancies, mesosized wrinkled pores, and macrosized strutted cavities, thereby contributing to a strong surface entrapment, short ion diffusion pathways, rapid mass transport, low interfacial resistance, and robust framework. It is demonstrated as a favorable scaffold for lithium–sulfur battery cathodes with superior rate capability, high coulombic efficiency, and excellent stability. A high capacity of 357 (656 ) is manifested at the current rate of 5.0 C, exhibiting a 74% retention of the capacity at 0.1 C. The first use of CaO‐templated CVD growth of graphene reported herein opens up new perspectives on the effective fabrication of hierarchical porous graphene materials on metal oxide catalysts with promising applications in energy storage, catalysis, adsorption, drug delivery, and so on.     

Hierarchical Graphene–Carbon Fiber Composite Paper as a Flexible Lateral Heat Spreader

As a low dimensional crystal, graphene attracts great attention as heat dissipation material due to its unique thermal transfer property exceeding the limit of bulk graphite. In this contribution, flexible graphene–carbon fiber composite paper is fabricated by depositing graphene oxide into the carbon fiber precursor followed by carbonization. In this full‐carbon architecture, scaffold of one‐dimensional carbon fiber is employed as the structural component to reinforce the mechanical strength, while the hierarchically arranged two‐dimensional graphene in the framework provides a convenient pathway for in‐plane acoustic phonon transmission. The as‐obtained hierarchical carbon/carbon composite paper possesses ultra‐high in‐plane thermal conductivity of 977 W m^?1 K^?1 and favorable tensile strength of 15.3 MPa. The combined mechanical and thermal performances make the material highly desirable as lateral heat spreader for next‐generation commercial portable electronics.     

Superior Fatigue Resistant Bioinspired Graphene‐Based Nanocomposite via Synergistic Interfacial Interactions

Excellent fatigue resistance is a prerequisite for flexible energy devices to achieve high and stable performance under repeated deformation state. Inspired by the sophisticated interfacial architecture of nacre, herein a super fatigue‐resistant graphene‐based nanocomposite with integrated high tensile strength and toughness through poly(dopamine)‐nickel ion (Ni²⁺) chelate architecture that mimics byssal threads is demonstrated. These kind of synergistic interfacial interactions of covalent and ionic bonding effectively suppress the crack propagation in the process of fatigue testing, resulting in superhigh fatigue life of this bioinspired graphene‐based nanocomposite (BGBN). In addition, the electrical conductivity is well kept after fatigue testing. The proposed synergistic interfacial interactions could serve as a guideline for fabricating high‐performance multifunctional BGBNs with promising applications in flexible energy devices, such as flexible electrodes for supercapacitors and lithium batteries, etc.     

Integration of Stiff Graphene and Tough Silk for the Design and Fabrication of Versatile Electronic Materials

The production of structural and functional materials with enhanced mechanical properties through the integration of soft and hard components is a common approach to Nature's material design. However, directly mimicking these optimized design routes in the lab for practical applications remains challenging. For example, graphene and silk are two materials with complementary mechanical properties that feature ultrahigh stiffness and toughness, respectively. Yet, no simple and controllable approach is developed to homogeneously integrate these two components into functional composites, mainly due to the hydrophobicity and chemical inertness of graphene. In this study, well‐dispersed and highly stable graphene/silk fibroin (SF) suspension systems are developed, which are suitable for processing to fabricate polymorphic materials, such as films, fibers, and coatings. The obtained graphene/SF nanocomposites maintain the electronic advantages of graphene, and they also allow tailorable mechanical performance to form including ultrahigh stretchable (with a strain to failure to 611 ± 85%), or high strength (339 MPa) and high stiffness (7.4 GPa) material systems. More remarkably, the electrical resistances of these graphene/SF materials are sensitive to material deformation, body movement, as well as humidity and chemical environmental changes. These unique features promise their utility as wearable sensors, smart textiles, intelligent skins, and human–machine interfaces.     

The Role of Nanowrinkles in Mass Transport across Graphene‐Based Membranes

Laminar membranes stacked by 2D materials are an emerging selective unit in separating processes across disciplines for their controllable mass transport properties. In general, parallel nanochannels formed between neighboring layers, owing to their adjustable size and surface chemistry, are considered the dominant transport regulator. Besides these flat interlayer channels, wrinkled morphology has also existed in 2D membranes, but the structure and potential transporting role of such curved channel remain largely unexplored. This study demonstrates that nanowrinkles are intrinsically formed in graphene‐based membranes, featuring an arc‐like shape with around 2.5 nm high center and two narrow wedge corners. By a facile “solvent‐treatment” during assembly, the membranes are tuned to possess different wrinkle density. In transport tests involving water and ions, the appearance of more wrinkles yields higher water permeation yet has limited effect on ion passage. These findings suggest that nanowrinkles by themselves serve as fast transporting ways while their connection with narrow interlayer channels can form a selective network. Results here are expected to deepen the understanding of mass transport mechanisms in current laminar membranes (e.g., graphene‐based) and provide strategies for designing future 2D membranes via wrinkle engineering.     

Rewritable,Moldable, and Flexible Sticker‐Type Organic Memory on Arbitrary Substrates

Fabrication of functional devices on arbitrary non‐conventional substrates has significant advantages for broadening devices applications and the development of soft electronic systems such as flexible, stretchable, wearable, and epidermal electronic modules. Information storage device is one of crucial electronic elements in modern digital circuitries. Herein, a re‐writable, transferable, and flexible sticker‐type organic memory on universal substrates is demonstrated through a facile and cost‐effective one‐step strategy. The organic memory sticker based on the graphene electrode grown by chemical vapor deposition consists of a blending composite of polymer (poly (methyl methacrylate) (PMMA):poly (3‐hexylthiophene) (P3HT) in chlorobenzene (CB) fabricated by mature solution processes and facilities. By combining with the mechanical elastic of organic material and graphene electrode, the sticker‐type organic memory can be easily tagged on non‐planar or flexible substrates after etching away the supporting metal. Particularly, the new attachable sticker‐type memory processes a unique feature of re‐programmable capability. It is believed that the universal substrate selectivity of the sticker‐type organic memory with re‐writable characteristic revealed here may greatly enlarge information storage devices in immense areas and advance the future functional soft circuitries.     

Molecular simulation on the material/interfacial strength of the low-dielectric materials

In this paper, the material stiffness of amorphous/porous low-k material and interfacial strength between amorphous silica and low-k have been simulated by the molecular dynamics (MD) methods. Due to the low stiffness of the low-k material, the interfaces which include this material are critical for the most delamination and reliability issues around the IC back-end structure. MD simulation technique is applied to elucidate the crack/delamination mechanism at these critical interfaces. However, due to the amorphous nature of the low-k material (e.g., SiOC:H), the atomic modeling technique of the amorphous/porous silica is first established. Through the experimental validation, the accuracy of this amorphous modeling technique is obtained, and the results show that this algorithm can represent the trend of the mechanical stiffness change due to different chemical composition of low-k material. A novel interfacial modeling technique, which model the status of chemical bonds at interface during the delamination loading, is developed. Afterward, the simulation of the mechanical strength of the amorphous silica/SiOC:H interface, is implemented. The simulation depicts that the existence of the strong Si–O covalent bond will significantly enhance the adhesive strength of the interface. Instead of the covalent bond at interface, the simulation results also reveal the multiple atomic scaled crack path within the material during the interfacial delamination. Hence, improving the material stiffness of the soft low-k material and preventing the pore at interface can increase the adhesive strength of the silica/low-k interfacial system.     

Wide‐Area Strain Sensors based upon Graphene‐Polymer Composite Coatings Probed by Raman Spectroscopy

Functional graphene optical sensors are now viable due to the recent developments in hand‐held Raman spectroscopy and the chemical vapor deposition (CVD) of graphene films. Herein, the strain in graphene/poly (methyl methacrylate) sensor coatings is followed using Raman band shifts. The performance of an “ideal” mechanically‐exfoliated single crystal graphene flake is compared to a scalable CVD graphene film. The dry‐transferred mechanically exfoliated sample has no residual stresses, whereas the CVD sample is in compression following the solvent evaporation during its transfer. The behavior of the sensors under cyclic deformation shows an initial breakdown of the graphene‐polymer interface with the interface then stabilizing after several cycles. The Raman 2D band shift rates per unit strain of the exfoliated graphene are ≈35% higher than CVD graphene making the former more strain sensitive. However, for practical wide‐area applications, CVD graphene coatings are still viable candidates as a Raman system can be used to read the strain in any 5 μm diameter spot in the coating to an absolute accuracy of ≈0.01% strain and resolution of ≈27 microstrains (μs), which compares favorably to commercial photoelastic systems.