Controllable Bipolar Doping of Graphene with 2D Molecular Dopants

The fine control of graphene doping levels over a wide energy range remains a challenging issue for the electronic applications of graphene. Here, the controllable doping of chemical vapor deposited graphene, which provides a wide range of energy levels (shifts up to ± 0.5 eV), is demonstrated through physical contact with chemically versatile graphene oxide (GO) sheets, a 2D dopant that can be solution‐processed. GO sheets are a p‐type dopant due to their abundance of electron‐withdrawing functional groups. To expand the energy window of GO‐doped graphene, the GO surface is chemically modified with electron‐donating ethylene diamine molecules. The amine‐functionalized GO sheets exhibit strong n‐type doping behaviors. In addition, the particular physicochemical characteristics of the GO sheets, namely their sheet sizes, number of layers, and degree of oxidation and amine functionality, are systematically varied to finely tune their energy levels. Finally, the tailor‐made GO sheet dopants are applied into graphene‐based electronic devices, which are found to exhibit improved device performances. These results demonstrate the potential of GO sheet dopants in many graphene‐based electronics applications.     

Graphene‐Assisted Antioxidation of Tungsten Disulfide Monolayers: Substrate and Electric‐Field Effect

Transition metal dichalcogenides (TMDs) have emerged as promising materials to complement graphene for advanced optoelectronics. However, irreversible degradation of chemical vapor deposition‐grown monolayer TMDs via oxidation under ambient conditions limits applications of TMD‐based devices. Here, the growth of oxidation‐resistant tungsten disulfide (WS₂) monolayers on graphene is demonstrated, and the mechanism of oxidation of WS₂ on SiO₂, graphene/SiO₂, and on graphene suspended in air is elucidated. While WS₂ on a SiO₂ substrate begins oxidation within weeks, epitaxially grown WS₂ on suspended graphene does not show any sign of oxidation, attributed to the screening effect of surface electric field caused by the substrate. The control of a local oxidation of WS₂ on a SiO₂ substrate by a local electric field created using an atomic force microscope tip is also demonstrated.     

Germanium‐Assisted Direct Growth of Graphene on Arbitrary Dielectric Substrates for Heating Devices

Direct growth of graphene on dielectric substrates is a prerequisite to the development of graphene‐based electronic and optoelectronic devices. However, the current graphene synthesis methods on dielectric substrates always involve a metal contamination problem, and the direct production of graphene patterns still remains unattainable and challenging. Herein, a semiconducting, germanium (Ge)‐assisted, chemical vapor deposition approach is proposed to produce monolayer graphene directly on arbitrary dielectric substrates. By the prepatterning of a catalytic Ge layer, the graphene with desired pattern can be achieved conveniently and readily. Due to the catalysis of Ge, monolayer graphene is able to form on Ge‐covered dielectric substrates including SiO₂/Si, quartz glass, and sapphire substrates. Optimization of the process parameters leads to complete sublimation of the catalytic Ge layer during or immediately after formation of the monolayer graphene, enabling direct deposition of large‐area and continuous graphene on dielectric substrates. The large‐area, highly conductive graphene synthesized on a transparent dielectric substrate using the proposed approach has exhibited a wide range of applications, including in both defogger and thermochromic displays, as already successfully demonstrated here.     

TiO<Subscript>2</Subscript> nanocrystals grown on graphene as advanced photocatalytic hybrid materials

A graphene/TiO₂ nanocrystals hybrid has been successfully prepared by directly growing TiO₂ nanocrystals on graphene oxide (GO) sheets. The direct growth of the nanocrystals on GO sheets was achieved by a two-step method, in which TiO₂ was first coated on GO sheets by hydrolysis and crystallized into anatase nanocrystals by hydrothermal treatment in the second step. Slow hydrolysis induced by the use of EtOH/H₂O mixed solvent and addition of H₂SO₄ facilitates the selective growth of TiO₂ on GO and suppresses growth of free TiO₂ in solution. The method offers easy access to the GO/TiO₂ nanocrystals hybrid with a uniform coating and strong interactions between TiO₂ and the underlying GO sheets. The strong coupling gives advanced hybrid materials with various applications including photocatalysis. The prepared graphene/TiO₂ nanocrystals hybrid has superior photocatalytic activity to other TiO₂ materials in the degradation of rhodamine B, showing an impressive three-fold photocatalytic enhancement over P25. It is expected that the hybrid material could also be promising for various other applications including lithium ion batteries, where strong electrical coupling to TiO₂ nanoparticles is essential.     

Low‐Temperature Heteroepitaxy of 2D PbI2/Graphene for Large‐Area Flexible Photodetectors

Heterostructures based on graphene and other 2D atomic crystals exhibit fascinating properties and intriguing potential in flexible optoelectronics, where graphene films function as transparent electrodes and other building blocks are used as photoactive materials. However, large‐scale production of such heterostructures with superior performance is still in early stages. Herein, for the first time, the preparation of a submeter‐sized, vertically stacked heterojunction of lead iodide (PbI₂)/graphene on a flexible polyethylene terephthalate (PET) film by vapor deposition of PbI₂ on graphene/PET substrate at a temperature lower than 200 °C is demonstrated. This film is subsequently used to fabricate bendable graphene/PbI₂/graphene sandwiched photodetectors, which exhibit high responsivity (45 A W^?1 cm^?2), fast response (35 µs rise, 20 µs decay), and high‐resolution imaging capability (1 µm). This study may pave a facile pathway for scalable production of high‐performance flexible devices.     

Wafer‐Scale Patterning of Reduced Graphene Oxide Electrodes by Transfer‐and‐Reverse Stamping for High Performance OFETs

A wafer‐scale patterning method for solution‐processed graphene electrodes, named the transfer‐and‐reverse stamping method, is universally applicable for fabricating source/drain electrodes of n‐ and p‐type organic field‐effect transistors with excellent performance. The patterning method begins with transferring a highly uniform reduced graphene oxide thin film, which is pre‐prepared on a glass substrate, onto hydrophobic silanized (rigid/flexible) substrates. Patterns of the as‐prepared reduced graphene oxide films are then formed by modulating the surface energy of the films and selectively delaminating the films using an oxygen‐plasma‐treated elastomeric stamp with patterns. Reduced graphene oxide patterns with various sizes and shapes can be readily formed onto an entire wafer. Also, they can serve as the source/drain electrodes for benchmark n‐ and p‐type organic field‐effect transistors with enhanced performance, compared to those using conventional metal electrodes. These results demonstrate the general utility of this technique. Furthermore, this simple, inexpensive, and scalable electrode‐patterning‐technique leads to assembling organic complementary circuits onto a flexible substrate successfully.     

Electric Power Generation through the Direct Interaction of Pristine Graphene‐Oxide with Water Molecules

Converting ubiquitous environmental energy into electric power holds tremendous social and financial interests. Traditional energy harvesters and converters are limited by the specific materials and complex configuration of devices. Herein, it is presented that electric power can be directly produced from pristine graphene oxide (GO) without any pretreatment or additives once encountering the water vapor, which will generate an open‐circuit‐voltage of up to 0.4–0.7 V and a short‐circuit‐current‐density of 2–25 µA cm^?2 on a single piece of GO film. This phenomenon results from the directional movement of charged hydrogen ions through the GO film. The present work demonstrates and provides an extremely simple method for electric energy generation, which offers more applications of graphene‐based materials in green energy converting field.     

Graphene Oxide: Surface Activity and Two‐Dimensional Assembly

Graphene oxide (GO) is a promising precursor for preparing graphene‐based composites and electronics applications. Like graphene, GO is essentially one‐atom thick but can be as wide as tens of micrometers, resulting in a unique type of material building block, characterized by two very different length scales. Due to this highly anisotropic structure, the collective material properties are highly dependent on how these sheets are assembled. Therefore, understanding and controlling the assembly behavior of GO has become an important subject of research. In this Research News article the surface activity of GO and how it can be employed to create two‐dimensional assemblies over large areas is discussed.     

电泳沉积氧化石墨烯的碳纤维表面改性及其增强环氧树脂复合材料界面性能

采用超声辅助电泳沉积法,以异丙醇作为溶剂,在连续碳纤维(CF)表面沉积一层氧化石墨烯(GO),对CF表面进行改性。再经200℃高温处理来增强(GO)与CF之间的黏合性,从而增加CF/环氧树脂(EP)复合材料的界面结合强度。利用SEM和AFM对改性前后CF的表面形貌及微观结构变化进行了表征,通过XPS对改性前后CF表面官能团的变化进行了检测。结果表明,在CF表面沉积GO并经200℃处理后,有效地部分还原了GO(RGO),填补或桥联了CF表面缺陷,使改性后CF的拉伸强度提高了34.58%。同时,高温处理使RGO与CF之间生成牢固的化学键,从而提高了RGO与CF之间的结合强度,最终使RGO-CF/EP复合材料的界面剪切强度(IFSS)提高了69.9%。      

Hydrothermal synthesis of graphene-ZnS quantum dot nanocomposites

ZnS nanodots highly dispersed on the surfaces of graphene sheets were successfully synthesized via an easy hydrothermal method by using Na₂S as reducing agent as well as sulfide source. The reduction of graphite oxide (GO) to graphene was accompanied by the deposition of ZnS particles on the surface of graphene sheets. The results of X-ray diffraction (XRD) and Fourier transform infrared (FT-IR) demonstrate the efficient reduction of GO to graphene sheets. The morphology characterization of the sample shows a wrinkled paper-like structure of the graphene sheets decorated with ZnS nanoparticles. Moreover, photoluminescence (PL) measurement shows a smooth spectrum, indicating fewer defects in the composite.     

Photocatalytic Reduction of Graphene Oxide–TiO2 Nanocomposites for Improving Resistive‐Switching Memory Behaviors

Graphene oxide (GO)‐based resistive‐switching (RS) memories offer the promise of low‐temperature solution‐processability and high mechanical flexibility, making them ideally suited for future flexible electronic devices. The RS of GO can be recognized as electric‐field‐induced connection/disconnection of nanoscale reduced graphene oxide (RGO) conducting filaments (CFs). Instead of operating an electrical FORMING process, which generally results in high randomness of RGO CFs due to current overshoot, a TiO₂‐assisted photocatalytic reduction method is used to generate RGO‐domains locally through controlling the UV irradiation time and TiO₂ concentration. The elimination of the FORMING process successfully suppresses the RGO overgrowth and improved RS memory characteristics are achieved in graphene oxide–TiO₂ (Go‐TiO₂) nanocomposites, including reduced SET voltage, improved switching variability, and increased switching speed. Furthermore, the room‐temperature process of this method is compatible with flexible plastic substrates and the memory cells exhibit excellent flexibility. Experimental results evidence that the combined advantages of reducing the oxygen‐migration barrier and enhancing the local‐electric‐field with RGO‐manipulation are responsible for the improved RS behaviors. These results offer valuable insight into the role of RGO‐domains in GO memory devices, and also, this mild photoreduction method can be extended to the development of carbon‐based flexible electronics.     

A Cut‐and‐Paste Approach to 3D Graphene‐Oxide‐Based Architectures

Properly cut sheets can be converted into complex 3D structures by three basic operations including folding, bending, and pasting to render new functions. Folding and bending are extensively employed in crumpling, origami, and pop‐up fabrications for 3D structures. Pasting joins different parts of a material together, and can create new geometries that are fundamentally unattainable by folding and bending. However, it has been much less explored, likely due to limited choice of weldable thin film materials and residue‐free glues. Here it is shown that graphene oxide (GO) paper is one such suitable material. Stacked GO sheets can be readily loosened up and even redispersed in water, which upon drying, restack to form solid structures. Therefore, water can be utilized to heal local damage, glue separated pieces, and release internal stress in bent GO papers to fix their shapes. Complex and dynamic 3D GO architectures can thus be fabricated by a cut‐and‐paste approach, which is also applicable to GO‐based hybrid with carbon nanotubes or clay sheets.     

Magnetically Controlled Growth‐Factor‐Immobilized Multilayer Cell Sheets for Complex Tissue Regeneration

The scaffold‐free cell‐sheet technique plays a significant role in stem‐cell‐based regeneration. Furthermore, growth factors are known to direct stem cell differentiation and enhance tissue regeneration. However, the absence of an effective means to incorporate growth factors into the cell sheets hinders further optimization of the regeneration efficiency. Here, a novel design of magnetically controlled “growth‐factor‐immobilized cell sheets” is reported. A new Fe₃O₄ magnetic nanoparticle (MNP) coated with nanoscale graphene oxide (nGO@Fe₃O₄) is developed to label stem cells and deliver growth factors. First, the nGO@Fe₃O₄ MNPs can be easily swallowed by dental‐pulp stem cells (DPSCs) and have no influence on cell viability. Thus, the MNP‐labeled cells can be organized via magnetic force to form multilayered cell sheets in different patterns. Second, compared to traditional Fe₃O₄ nanoparticles, the graphene oxide coating provides plenty of carboxyl groups to bind and deliver growth factors. Therefore, with these nGO@Fe₃O₄ MNPs, bone‐morphogenetic‐protein‐2 (BMP2) is successfully incorporated into the DPSCs sheets to induce more bone formation. Furthermore, an integrated osteochondral complex is also constructed using a combination of DPSCs/TGFβ3 and DPSCs/BMP2. All these results demonstrate that the new cell‐sheet tissue‐engineering approach exhibits promising potential for future use in regenerative medicine.     

A Simple Route to Reduced Graphene Oxide‐Draped Nanocomposites with Markedly Enhanced Visible‐Light Photocatalytic Performance

Nanocomposites (denoted RGO/ZnONRA) comprising reduced graphene oxide (RGO) draped over the surface of zinc oxide nanorod array (ZnONRA) were produced via a simple low‐temperature route, dispensing with the need for hydrothermal growth, electrochemical deposition or other complex treatments. The amount of deposited RGO can be readily tuned by controlling the concentration of graphene oxide (GO). Interestingly, the addition of Sn²⁺ not only enables the reduction of GO, but also functions as a bridge that connects the resulting RGO and ZnONRA. Remarkably, the incorporation of RGO improves the visible‐light absorption and reduces the bandgap of ZnO, thereby leading to the markedly improved visible‐light photocatalytic performance. Moreover, RGO/ZnONRA nanocomposites exhibit a superior stability as a result of the surface protection of ZnONRA by RGO. The mechanism on the improved photocatalytic performance based on the cophotosensitizations under the visible‐light irradiation has been proposed. This simple yet effective route to the RGO‐decorated semiconductor nanocomposites renders the better visible‐light utilization, which may offer great potential for use in photocatalytic degradation of organic pollutants, solar cells, and optoelectronic materials and devices.     

Nanomechanical and Charge Transport Properties of Two‐Dimensional Atomic Sheets

The materials properties of graphene and other two‐dimensional atomic sheets are influenced by atomic‐scale defects, mechanical deformation, and microstructures. Thus, for graphene‐based applications, it is essential to uncover the roles of atomic‐scale defects and domain structures of two‐dimensional layers in charge transport properties. This review highlights recent studies of nanomechanical and charge transport properties of two‐dimensional atomic sheets, including graphene, MoS₂, and boron nitrides. Because of intrinsic structural differences, two‐dimensional atomic sheets give rise to unique nanomechanical properties, including a dependence on layer thickness and chemical modification that is in contrast to three‐dimensional continuum media. Mapping of local conductance and nanomechanical properties on a graphene layer can be used to image the domain and microstructures of two‐dimensional atomic layers. This paper also reviews recent experimental and theoretical findings on the role of bending, defects, and microstructures on nanomechanical and transport properties of graphene‐derived materials.     

Conjugated‐Polyelectrolyte‐Functionalized Reduced Graphene Oxide with Excellent Solubility and Stability in Polar Solvents

Conjugated‐polyelectrolyte (CPE)‐functionalized reduced graphene oxide (rGO) sheets are synthesized for the first time by taking advantage of a specially designed CPE, PFVSO₃, with a planar backbone and charged sulfonate and oligo(ethylene glycol) side chains to assist the hydrazine‐mediated reduction of graphene oxide (GO) in aqueous solution. The resulting CPE‐functionalized rGO (PFVSO₃‐rGO) shows excellent solubility and stability in a variety of polar solvents, including water, ethanol, methanol, dimethyl sulfoxide, and dimethyl formamide. The morphology of PFVSO₃‐rGO is studied by atomic force microscopy, X‐ray diffraction, and transmission electron microscopy, which reveal a sandwich‐like nanostructure. Within this nanostructure, the backbones of PFVSO₃ stack onto the basal plane of rGO sheets via strong π–π interactions, while the charged hydrophilic side chains of PFVSO₃ prevent the rGO sheets from aggregating via electrostatic and steric repulsions, thus leading to the solubility and stability of PFVSO₃‐rGO in polar solvents. Optoelectronic studies show that the presence of PFVSO₃ within rGO induces photoinduced charge transfer and p‐doping of rGO. As a result, the electrical conductivity of PFVSO₃‐rGO is not only much better than that of GO, but also than that of the unmodified rGO.     

Graphene‐Contacted Ultrashort Channel Monolayer MoS2 Transistors

2D semiconductors are promising channel materials for field‐effect transistors (FETs) with potentially strong immunity to short‐channel effects (SCEs). In this paper, a grain boundary widening technique is developed to fabricate graphene electrodes for contacting monolayer MoS₂. FETs with channel lengths scaling down to ≈4 nm can be realized reliably. These graphene‐contacted ultrashort channel MoS₂ FETs exhibit superior performances including the nearly Ohmic contacts and excellent immunity to SCEs. This work provides a facile route toward the fabrication of various 2D material‐based devices for ultrascaled electronics.     

Quantum‐Confined‐Superfluidics‐Enabled Moisture Actuation Based on Unilaterally Structured Graphene Oxide Papers

The strong interaction between graphene oxides (GO) and water molecules has trigged enormous research interest in developing GO‐based separation films, sensors, and actuators. However, sophisticated control over the ultrafast water transmission among the GO sheets and the consequent deformation of the entire GO film is still challenging. Inspired from the natural “quantum‐tunneling‐fluidics‐effect,” here quantum‐confined‐superfluidics‐enabled moisture actuation of GO paper by introducing periodic gratings unilaterally is reported. The folded GO nanosheets that act as quantum‐confined‐superfluidics channels can significantly promote water adsorption, enabling controllable and sensitive moisture actuation. Water‐adsorption‐induced expansion along and against the normal direction of a GO paper is investigated both theoretically and experimentally. Featuring state‐of‐the‐art of ultrafast response (1.24 cm^?1 s^?1), large deformation degree, and complex and predictable deformation, the smart GO papers are used for biomimetic mini‐robots including a creeping centipede and a smart leaf that can catch a living ladybug. The reported method is simple and universal for 2D materials, revealing great potential for developing graphene‐based smart robots.     

Direct Growth of High Mobility and Low‐Noise Lateral MoS2–Graphene Heterostructure Electronics

Reliable fabrication of lateral interfaces between conducting and semiconducting 2D materials is considered a major technological advancement for the next generation of highly packed all‐2D electronic circuitry. This study employs seed‐free consecutive chemical vapor deposition processes to synthesize high‐quality lateral MoS₂–graphene heterostructures and comprehensively investigated their electronic properties through a combination of various experimental techniques and theoretical modeling. These results show that the MoS₂–graphene devices exhibit an order of magnitude higher mobility and lower noise metrics compared to conventional MoS₂–metal devices as a result of energy band rearrangement and smaller Schottky barrier height at the contacts. These findings suggest that MoS₂–graphene in‐plane heterostructures are promising materials for the scale‐up of all‐2D circuitry with superlative electrical performance.     

A High‐On/Off‐Ratio Floating‐Gate Memristor Array on a Flexible Substrate via CVD‐Grown Large‐Area 2D Layer Stacking

Memristors such as phase‐change memory and resistive memory have been proposed to emulate the synaptic activities in neuromorphic systems. However, the low reliability of these types of memories is their biggest challenge for commercialization. Here, a highly reliable memristor array using floating‐gate memory operated by two terminals (source and drain) using van der Waals layered materials is demonstrated. Centimeter‐scale samples (1.5 cm × 1.5 cm) of MoS₂ as a channel and graphene as a trap layer grown by chemical vapor deposition (CVD) are used for array fabrication with Al₂O₃ as the tunneling barrier. With regard to the memory characteristics, 93% of the devices exhibit an on/off ratio of over 10³ with an average ratio of 10⁴. The high on/off ratio and reliable endurance in the devices allow stable 6‐level memory applications. The devices also exhibit excellent memory durability over 8000 cycles with a negligible shift in the threshold voltage and on‐current, which is a significant improvement over other types of memristors. In addition, the devices can be strained up to 1% by fabricating on a flexible substrate. This demonstration opens a practical route for next‐generation electronics with CVD‐grown van der Waals layered materials.