Carrier Transport Regulation of Pixel Graphene Transparent Electrodes for Active-Matrix Organic Light-Emitting Diode Display.

Integrating a graphene transparent electrode (TE) matrix with driving circuits is essential for the practical use of graphene in optoelectronics such as active-matrix organic light-emitting diode (OLED) display, however it is disabled by the transport of carriers between graphene pixels after deposition of a semiconductor functional layer caused by the atomic thickness of graphene. Here, the carrier transport regulation of a graphene TE matrix by using an insulating polyethyleneimine (PEIE) layer is reported. The PEIE forms an ultrathin uniform film (≤10 nm) to fill the gap of the graphene matrix, blocking horizontal electron transport between graphene pixels. Meanwhile, it can reduce the work function of graphene, improving the vertical electron injection through electron tunneling. This enables the fabrication of inverted OLED pixels with record high current and power efficiencies of 90.7 cd A-1 and 89.1 lm W-1 , respectively. By integrating these inverted OLED pixels with a carbon nanotube-based thin-film transistor (CNT-TFT)-driven circuit, an inch-size flexible active-matrix OLED display is demonstrated, in which all OLED pixels are independently controlled by CNT-TFTs. This research paves a way for the application of graphene-like atomically thin TE pixels in flexible optoelectronics such as displays, smart wearables, and free-form surface lighting.

Growing Nanocrystalline Graphene on Aggregates for Conductive and Strong Smart Cement Composites.

Highly conductive concrete/mortar has been long pursued to realize structural health monitoring in the development of smart-cement-based facilities. However, it remains challenging to significantly increase the electrical conductivity of concrete/mortar without lowering the compressive strength and flowability. Here, nanocrystalline-graphene-coated aggregates (termed Gr@AGs) are synthesized to break this conductivity-strength tradeoff. Admixing Gr@AGs with cement enables the construction of a conductive network of graphene that simultaneously strengthens the interfacial transition zone between aggregates and paste. As a result, high conductivity and improved mechanical properties have been simultaneously realized in Gr@AGs-based smart mortars. The significant positive effects of Gr@AGs are further enhanced by combining them with a low percentage of carbon fiber. Typically, the 28-day compressive/flexural strength of the optimized mortar increases by 12.2%/19.4%, with the electrical resistivity reduced by over 3 orders of magnitude from ∼4.6 × 105 to 182 Ω cm. On this basis, we demonstrate high-sensitivity cement-based piezoresistive sensors with a fractional change in resistivity as high as ∼25%, which is more than 1 order of magnitude higher than those reported in comparable systems. This study provides a solution to the critical issues in developing smart cementitious composites by taking full advantage of graphene's properties.

A polymer electrolyte design enables ultralow-work-function electrode for high-performance optoelectronics.

Ambient solution-processed conductive materials with a sufficient low work function are essential to facilitate electron injection in electronic and optoelectronic devices but are challenging. Here, we design an electrically conducting and ambient-stable polymer electrolyte with an ultralow work function down to 2.2 eV, which arises from heavy n-doping of dissolved salts to polymer matrix. Such materials can be solution processed into uniform and smooth films on various conductors including graphene, conductive metal oxides, conducting polymers and metals to substantially improve their electron injection, enabling high-performance blue light-emitting diodes and transparent light-emitting diodes. This work provides a universal strategy to design a wide range of stable charge injection materials with tunable work function. As an example, we also synthesize a high-work-function polymer electrolyte material for high-performance solar cells.

High-performance flexible resistive random access memory devices based on graphene oxidized with a perpendicular oxidation gradient.

The conventional strategy of fabricating resistive random access memory (RRAM) based on graphene oxide is limited to a resistive layer with homogeneous oxidation, and the switching behavior relies on its redox reaction with an active metal electrode, so the obtained RRAMs are typically plagued by inferior performance and reliability. Here, we report a strategy to develop high-performance flexible RRAMs by using graphene oxidized with a perpendicular oxidation gradient as the resistive layer. In contrast to a homogeneous oxide, this graphene together with its distinctive inter-layer oxygen diffusion path enables excellent oxygen ion/vacancy diffusion. Without an interfacial redox reaction, oxygen ions can diffuse to form conductive filaments with two inert metal electrodes by applying a bias voltage. Compared with state-of-the-art graphene oxide RRAMs, these graphene RRAMs have shown superior performance including a high on-off current ratio of ∼105, long-term retention of ∼106 s, reproducibility over 104 cycles and long-term flexibility at a bending strain of 0.6%, indicating that the material has great potential in wearable smart data-storage devices.

Pushing the conductance and transparency limit of monolayer graphene electrodes for flexible organic light-emitting diodes.

Graphene has emerged as an attractive candidate for flexible transparent electrode (FTE) for a new generation of flexible optoelectronics. Despite tremendous potential and broad earlier interest, the promise of graphene FTE has been plagued by the intrinsic trade-off between electrical conductance and transparency with a figure of merit (σDC/σOp) considerably lower than that of the state-of-the-art ITO electrodes (σDC/σOp <123 for graphene vs. ∼240 for ITO). Here we report a synergistic electrical/optical modulation strategy to simultaneously boost the conductance and transparency. We show that a tetrakis(pentafluorophenyl)boric acid (HTB) coating can function as highly effective hole doping layer to increase the conductance of monolayer graphene by sevenfold and at the same time as an anti-reflective layer to boost the visible transmittance to 98.8%. Such simultaneous improvement in conductance and transparency breaks previous limit in graphene FTEs and yields an unprecedented figure of merit (σDC/σOp ∼323) that rivals the best commercial ITO electrode. Using the tailored monolayer graphene as the flexible anode, we further demonstrate high-performance green organic light-emitting diodes (OLEDs) with the maximum current, power and external quantum efficiencies (111.4 cd A-1, 124.9 lm W-1 and 29.7%) outperforming all comparable flexible OLEDs and surpassing that with standard rigid ITO by 43%. This study defines a straightforward pathway to tailor optoelectronic properties of monolayer graphene and to fully capture their potential as a generational FTE for flexible optoelectronics.

A Double Support Layer for Facile Clean Transfer of Two-Dimensional Materials for High-Performance Electronic and Optoelectronic Devices.

Clean transfer of two-dimensional (2D) materials grown by chemical vapor deposition is critical for their application in electronics and optoelectronics. Although rosin can be used as a support layer for the clean transfer of graphene grown on Cu, it has not been usable for the transfer of 2D materials grown on noble metals or for large-area transfer. Here, we report a poly(methyl methacrylate) (PMMA)/rosin double support layer that enables facile ultraclean transfer of large-area 2D materials grown on different metals. The bottom rosin layer ensures clean transfer, whereas the top PMMA layer not only screens the rosin from the transfer conditions but also improves the strength of the transfer layer to make the transfer easier and more robust. We demonstrate the transfer of monolayer WSe2 and WS2 single crystals grown on Au as well as large-area graphene films grown on Cu. As a result of the clean surface, the transferred WSe2 retains the intrinsic optical properties of the as-grown sample. Moreover, it does not require annealing to form good ohmic contacts with metal electrodes, enabling high-performance field effect transistors with mobility and ON/OFF ratio ∼10 times higher than those made by PMMA-transferred WSe2. The ultraclean graphene film is found to be a good anode for flexible organic photovoltaic cells with a high power conversion efficiency of ∼6.4% achieved.

UV-Epoxy-Enabled Simultaneous Intact Transfer and Highly Efficient Doping for Roll-to-Roll Production of High-Performance Graphene Films.

Flexible graphene transparent conductive films (TCFs) prepared by chemical vapor deposition hold great promise for next-generation wearable optoelectronic devices, but the lack of low-cost scalable intact transfer and highly efficient doping greatly limits their commercialization. Here, we report a UV-epoxy adhesive as a robust multifunctional layer for the low-cost scalable production of high-performance flexible graphene TCF. Its high solvent stability, sufficient adhesion force, and conformal contact with graphene enable the intact bubbling transfer of graphene. More importantly, a highly strong and stable p-dopant, superacid HSbF6, is in situ generated from UV-epoxy. HSbF6 substantially increases the hole concentration of pristine graphene by more than 10 times and consequently reduces its sheet resistance by up to 95% with high stability. Furthermore, it can be readily integrated with the roll-to-roll transfer process. These features enable continuous production of graphene TCFs with overall performances superior to those produced by common transfer methods and typical dopants. As an example, we demonstrate the use of this film in the capacitive multitouch panel of tablet computers.

Large-area synthesis of high-quality and uniform monolayer WS2 on reusable Au foils.

Large-area monolayer WS2 is a desirable material for applications in next-generation electronics and optoelectronics. However, the chemical vapour deposition (CVD) with rigid and inert substrates for large-area sample growth suffers from a non-uniform number of layers, small domain size and many defects, and is not compatible with the fabrication process of flexible devices. Here we report the self-limited catalytic surface growth of uniform monolayer WS2 single crystals of millimetre size and large-area films by ambient-pressure CVD on Au. The weak interaction between the WS2 and Au enables the intact transfer of the monolayers to arbitrary substrates using the electrochemical bubbling method without sacrificing Au. The WS2 shows high crystal quality and optical and electrical properties comparable or superior to mechanically exfoliated samples. We also demonstrate the roll-to-roll/bubbling production of large-area flexible films of uniform monolayer, double-layer WS2 and WS2/graphene heterostructures, and batch fabrication of large-area flexible monolayer WS2 film transistor arrays.

Repeated growth-etching-regrowth for large-area defect-free single-crystal graphene by chemical vapor deposition.

Reducing nucleation density and healing structural defects are two challenges for fabricating large-area high-quality single-crystal graphene, which is essential for its electronic and optoelectronic applications. We have developed a method involving chemical vapor deposition (CVD) growth followed by repeated etching-regrowth, to solve both problems at once. Using this method, we can obtain single-crystal graphene domains with a size much larger than that allowed by the nucleation density in the initial growth and efficiently heal structural defects similar to graphitization but at a much lower temperature, both of which are impossible to realize by conventional CVD. Using this method with Pt as a growth substrate, we have grown ∼3 mm defect-free single-crystal graphene domains with a carrier mobility up to 13,000 cm2 V(-1) s(-1) under ambient conditions.

Repeated and controlled growth of monolayer, bilayer and few-layer hexagonal boron nitride on Pt foils.

Atomically thin hexagonal boron nitride (h-BN), as a graphene analogue, has attracted increasing interest because of many fascinating properties and a wide range of potential applications. However, it still remains a great challenge to synthesize high-quality h-BN with predetermined number of layers at a low cost. Here we reported the controlled growth of h-BN on polycrystalline Pt foils by low-cost ambient pressure chemical vapor deposition with ammonia borane as the precursor. Monolayer, bilayer and few-layer h-BN domains and large-area films were selectively obtained on Pt by simply changing the concentration of ammonia borane. Moreover, using a bubbling method, we have achieved the nondestructive transfer of h-BN from Pt to arbitrary substrates and the repeated use of the Pt for h-BN growth, which not only reduces environmental pollution but also decreases the production cost of h-BN. The monolayer and bilayer h-BN obtained are very uniform with high quality and smooth surfaces. In addition, we found that the optical band gap of h-BN increases with decreasing number of layers. The repeated growth of large-area, high-quality monolayer and bilayer h-BN films, together with the successful growth of graphene, opens up the possibility for creating various functional heterostructures for large-scale fabrication and integration of novel electronics.

Tuning the electrical and optical properties of graphene by ozone treatment for patterning monolithic transparent electrodes.

Tunable electrical and optical properties of graphene are vital to promote its use as film electrodes in a variety of devices. We developed an etching-free ozone treatment method to continuously tune the electrical resistance and optical transmittance of graphene films by simply varying the time and temperature of graphene exposure to ozone. Initially, ozone exposure dramatically decreases the electrical resistance of graphene films by p-doping, but this is followed by increases in the resistance and optical transmittance as a result of surface oxidation. The rate of resistance increase can be significantly increased by raising the treatment temperature. The ozone-oxidized graphene is not removed but is gradually transformed to graphene oxide (GO). On the basis of such effects of ozone treatment, we demonstrate a well-defined graphene pattern by using ozone photolithography, in which the ozone-treated graphene electrodes are monolithic but separated by insulating GO regions. Such a monolithic graphene pattern shows low optical contrast, a clean and more hydrophilic surface, indicating the promising use of ozone treatment to achieve high-performance graphene-based optoelectronic devices.

Repeated growth and bubbling transfer of graphene with millimetre-size single-crystal grains using platinum.

Large single-crystal graphene is highly desired and important for the applications of graphene in electronics, as grain boundaries between graphene grains markedly degrade its quality and properties. Here we report the growth of millimetre-sized hexagonal single-crystal graphene and graphene films joined from such grains on Pt by ambient-pressure chemical vapour deposition. We report a bubbling method to transfer these single graphene grains and graphene films to arbitrary substrate, which is nondestructive not only to graphene, but also to the Pt substrates. The Pt substrates can be repeatedly used for graphene growth. The graphene shows high crystal quality with the reported lowest wrinkle height of 0.8 nm and a carrier mobility of greater than 7,100 cm(2) V(-1) s(-1) under ambient conditions. The repeatable growth of graphene with large single-crystal grains on Pt and its nondestructive transfer may enable various applications.

Advanced materials for energy storage.

Popularization of portable electronics and electric vehicles worldwide stimulates the development of energy storage devices, such as batteries and supercapacitors, toward higher power density and energy density, which significantly depends upon the advancement of new materials used in these devices. Moreover, energy storage materials play a key role in efficient, clean, and versatile use of energy, and are crucial for the exploitation of renewable energy. Therefore, energy storage materials cover a wide range of materials and have been receiving intensive attention from research and development to industrialization. In this Review, firstly a general introduction is given to several typical energy storage systems, including thermal, mechanical, electromagnetic, hydrogen, and electrochemical energy storage. Then the current status of high-performance hydrogen storage materials for on-board applications and electrochemical energy storage materials for lithium-ion batteries and supercapacitors is introduced in detail. The strategies for developing these advanced energy storage materials, including nanostructuring, nano-/microcombination, hybridization, pore-structure control, configuration design, surface modification, and composition optimization, are discussed. Finally, the future trends and prospects in the development of advanced energy storage materials are highlighted.