Zunfeng Liu

Evaluation of Solution-Processed Reduced Graphene Oxide Films as Transparent Conductors

Processable, single-layered graphene oxide (GO) is an intriguing nanomaterial with tremendous potential for electronic applications. We spin-coated GO thin-films on quartz and characterized their sheet resistance and optical transparency using different reduction treatments. A thermal graphitization procedure was most effective, producing films with sheet resistances as low as 10(2) -10(3) Omega/square with 80% transmittance for 550 nm light. Our experiments demonstrate solution-processed GO films have potential as transparent electrodes.

Organic Light-Emitting Diodes on Solution-Processed Graphene Transparent Electrodes

Theoretical estimates indicate that graphene thin films can be used as transparent electrodes for thin-film devices such as solar cells and organic light-emitting diodes, with an unmatched combination of sheet resistance and transparency. We demonstrate organic light-emitting diodes with solution-processed graphene thin film transparent conductive anodes. The graphene electrodes were deposited on quartz substrates by spin-coating of an aqueous dispersion of functionalized graphene, followed by a vacuum anneal step to reduce the sheet resistance. Small molecular weight organic materials and a metal cathode were directly deposited on the graphene anodes, resulting in devices with a performance comparable to control devices on indium-tin-oxide transparent anodes. The outcoupling efficiency of devices on graphene and indium-tin-oxide is nearly identical, in agreement with model predictions.

Organic photovoltaic cells based on an acceptor of soluble graphene

In this paper, an organic photovoltaic device based on an acceptor of solution-processable functionalized graphene was designed. A short circuit current density (Jsc) of 4.0mAcm−2, open circuit voltage (Voc) of 0.72V, and a solar power conversion efficiency of 1.1% were obtained for the device of indium tin oxide/poly(ethylene dioxythiophene) doped with polystyrene sulfonic acid (40nm)/poly(3-hexylthiophene-1, 3-diyl):graphene (graphene 10wt%, 100nm)/LiF (1nm)/Al (70nm) after an annealing treatment under simulated AM1.5G 100mW illumination in air. Because of the low price, ease of preparation, and inertness against ambient conditions, soluble graphene will be a promising candidate used for acceptor materials in the photovoltaic applications.

Hierarchically buckled sheath-core fibers for superelastic electronics, sensors, and muscles

Composite stretchable conducting wires Think how useful a stretchable electronic “skin” could be. For example you could place it over an aircraft fuselage or a body to create a network of sensors, processors, energy stores, or artificial muscles. But it is difficult to make electronic interconnects and strain sensors that can stretch over such surfaces. Liu et al. created superelastic conducting fibers by depositing carbon nanotube sheets onto a prestretched rubber core (see the Perspective by Ghosh). The nanotubes buckled on relaxation of the core, but continued to coat it fully and could stretch enormously, with relatively little change in resistance. Science, this issue p. 400; see also p. 382 Rubber fibers coated with sheets of carbon nanotubes form highly stretchable conducting wires. [Also see Perspective by Ghosh] Superelastic conducting fibers with improved properties and functionalities are needed for diverse applications. Here we report the fabrication of highly stretchable (up to 1320%) sheath-core conducting fibers created by wrapping carbon nanotube sheets oriented in the fiber direction on stretched rubber fiber cores. The resulting structure exhibited distinct short- and long-period sheath buckling that occurred reversibly out of phase in the axial and belt directions, enabling a resistance change of less than 5% for a 1000% stretch. By including other rubber and carbon nanotube sheath layers, we demonstrated strain sensors generating an 860% capacitance change and electrically powered torsional muscles operating reversibly by a coupled tension-to-torsion actuation mechanism. Using theory, we quantitatively explain the complementary effects of an increase in muscle length and a large positive Poisson’s ratio on torsional actuation and electronic properties.

Fabrication and evaluation of solution-processed reduced graphene oxide electrodes for p- and n-channel bottom-contact organic thin-film transistors.

Reduced graphene oxide (RGO) is an electrically conductive carbon-based nanomaterial that has recently attracted attention as a potential electrode for organic electronics. Here we evaluate several solution-based methods for fabricating RGO bottom-contact (BC) electrodes for organic thin-film transistors (OTFTs), demonstrate functional p- and n-channel devices with such electrodes, and compare their electrical performance with analogous devices containing gold electrodes. We show that the morphology of organic semiconductor films deposited on RGO electrodes is similar to that observed in the channel region of the devices and that devices fabricated with RGO electrodes have lower contact resistances compared to those fabricated with gold contacts. Although the conductivity of RGO is poor compared to that of gold, RGO is still an enticing electrode material for organic electronic devices possibly owing to the retention of desirable morphological features, lower contact resistance, lower cost, and solution processability.

Ultra-Small Graphene Oxide Functionalized with Polyethylenimine (PEI) for Very Efficient Gene Delivery in Cell and Zebrafish Embryos

AbstractEfficient DNA delivery is essential for introducing new genes into living cells. However, effective virus-based systems carry risks and efficient synthetic systems that are non-toxic remain to be discovered. The bottle-neck in synthetic systems is cytotoxicity, caused by the high concentration of DNA-condensing compounds required for efficient uptake of DNA. Here we report a polyethyleneimine (PEI) grafted ultra-small graphene oxide (PEI-g-USGO) for transfection. By removing the free PEI and ensuring a high PEI density on small sized graphene, we obtained very high transfection efficiencies combined with very low cytotoxicity. Plasmid DNA could be transfected into mammalian cell lines with up to 95% efficiency and 90% viability. Transfection in zebrafish embryos was 90%, with high viability, compared to efficiencies of 30% or lower for established transfection technologies. This result suggests a novel approach to the design of synthetic gene delivery vehicles for research and therapy.

Harvesting electrical energy from carbon nanotube yarn twist

Making the most of twists and turns The rise of small-scale, portable electronics and wearable devices has boosted the desire for ways to harvest energy from mechanical motion. Such approaches could be used to provide battery-free power with a small footprint. Kim et al. present an energy harvester made from carbon nanotube yarn that converts mechanical energy into electrical energy from both torsional and tensile motion. Their findings reveal how the extent of yarn twisting and the combination of homochiral and heterochiral coiled yarns can maximize energy generation. Science, this issue p. 773 Twisted and coiled carbon nanotubes can harvest electrical energy from mechanical motion. Mechanical energy harvesters are needed for diverse applications, including self-powered wireless sensors, structural and human health monitoring systems, and the extraction of energy from ocean waves. We report carbon nanotube yarn harvesters that electrochemically convert tensile or torsional mechanical energy into electrical energy without requiring an external bias voltage. Stretching coiled yarns generated 250 watts per kilogram of peak electrical power when cycled up to 30 hertz, as well as up to 41.2 joules per kilogram of electrical energy per mechanical cycle, when normalized to harvester yarn weight. These energy harvesters were used in the ocean to harvest wave energy, combined with thermally driven artificial muscles to convert temperature fluctuations to electrical energy, sewn into textiles for use as self-powered respiration sensors, and used to power a light-emitting diode and to charge a storage capacitor.

Downsized Sheath–Core Conducting Fibers for Weavable Superelastic Wires, Biosensors, Supercapacitors, and Strain Sensors

Hair-like-diameter superelastic conducting fibers, comprising a buckled carbon nanotube sheath on a rubber core, are fabricated, characterized, and deployed as weavable wires, biosensors, supercapacitors, and strain sensors. These downsized sheath-core fibers provide the demonstrated basis for glucose sensors, supercapacitors, and electrical interconnects whose performance is undegraded by giant strain, as well as ultrafast strain sensors that exploit strain-dependent capacitance changes.

DNA electrochemical sensor based on an adduct of single-walled carbon nanotubes and ferrocene

A novel electrochemical sandwich-type gene sensing system was designed by using a DNA probe (DNA-probe1) immobilized on a gold electrode, the target DNA, and another DNA probe (DNA-probe2) conjugated on a single-walled carbon nanotubes/ferrocene (Fc–SWNT) adduct. In this sandwich-type gene-sensing electrode, the Fc–SWNT adduct could significantly amplify the electrochemical response of the reduction of H2O2. The target DNA could be detected selectively and sensitively based on the much enhanced electrochemical catalytic property of the Fc–SWNT adduct toward H2O2 reduction.

A photovoltaic device based on a poly(phenyleneethynylene)/SWNT composite active layer

Although research on the use of single walled carbon nanotubes (SWNTs) as the acceptor in polymer photovoltaic cells is currently making great progress, their poor dispersion in a polymer matrix has greatly hindered the overall performance of the devices. Here a novel bulk heterojunction structure based on a poly(phenyleneethynylene)/SWNT composite was designed to improve the dispersion of SWNTs in the composite based on their structural similarity and strong interaction. Better dispersion and higher performance are achieved compared with a common control device based on a poly(3-octylthiophene)/SWNT composite layer.