Transparent conducting electrodes (TCEs) require high transparency and low sheet resistance for applications in photovoltaics, photodetectors, fl at panel displays, touch screen devices and imagers. Indium tin oxide (ITO), or other transparent conductive oxides, have typically been used, and provide a baseline sheet resistance ( R S ) vs. transparency ( T ) relationship. However, ITO is relatively expensive (due to limited abundance of Indium), brittle, unstable, and infl exible; moreover, ITO transparency drops rapidly for wavelengths above 1000 nm. Motivated by a need for transparent conductors with comparable (or better) R S at a given T , as well as fl exible structures, several alternative material systems have been investigated. Single-layer graphene (SLG) or few-layer graphene provide suffi ciently high transparency ( ≈ 97% per layer) to be a potential replacement for ITO. However, large-area synthesis approaches, including chemical vapor deposition (CVD), typically yield fi lms with relatively high sheet resistance due to small grain sizes and high-resistance grain boundaries (HGBs). In this paper, we report a hybrid structure employing a CVD SLG fi lm and a network of silver nanowires (AgNWs): R S as low as 22 Ω /ٗ (stabilized to 13 Ω /ٗ after 4 months) have been observed at high transparency (88% at λ = 550 nm) in hybrid structures employing relatively low-cost commercial graphene with a starting R S of 770 Ω /ٗ. This sheet resistance is superior to typical reported values for ITO, comparable to the best reported TCEs employing graphene and/or random nanowire networks, and the fi lm properties exhibit impressive stability under mechanical pressure, mechanical bending and over time. The design is inspired by the theory of a co-percolating network where conduction bottlenecks of a 2D fi lm (e.g., SLG, MoS 2 ) are circumvented by a 1D network (e.g., AgNWs, CNTs) and vice versa. The development of these high-performance hybrid structures provides a route towards robust, scalable and low-cost approaches for realizing high-performance TCE.

Emerging research on printed and flexible graphene-based electronics is beginning to show tremendous promise for a wide variety of fields including wearable sensors and thin film transistors. However, post-print annealing/reduction processes that are necessary to increase the electrical conductivity of the printed graphene degrade sensitive substrates (e.g., paper) and are whole substrate processes that are unable to selectively anneal/reduce only the printed graphene-leaving sensitive device components exposed to damaging heat or chemicals. Herein a pulsed laser process is introduced that can selectively irradiate inkjet printed reduced graphene oxide (RGO) and subsequently improve the electrical conductivity (Rsheet∼0.7 kΩ□(-1)) of printed graphene above previously published reports. Furthermore, the laser process is capable of developing 3D petal-like graphene nanostructures from 2D planar printed graphene. These visible morphological changes display favorable electrochemical sensing characteristics-ferricyanide cyclic voltammetry with a redox peak separation (ΔEp) ≈ 0.7 V as well as hydrogen peroxide (H2O2) amperometry with a sensitivity of 3.32 μA mM(-1) and a response time of <5 s. Thus this work paves the way for not only paper-based electronics with graphene circuits, it enables the creation of low-cost and disposable graphene-based electrochemical electrodes for myriad applications including sensors, biosensors, fuel cells, and theranostic devices.

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Inkjet printed graphene (IPG) has recently shown tremendous promise in reducing the cost and complexity of graphene circuit fabrication. Herein we demonstrate, for the first time, the fabrication of an ion selective electrode (ISE) with IPG. A thermal annealing process in a nitrogen ambient environment converts the IPG into a highly conductive electrode (sheet resistance changes from 52.8 ± 7.4 MΩ/□ for unannealed graphene to 172.7 ± 33.3 Ω/□ for graphene annealed at 950 °C). Raman spectroscopy and field emission scanning electron microscopy (FESEM) analysis reveals that the printed graphene flakes begin to smooth at an annealing temperature of 500 °C and then become more porous and more electrically conductive when annealed at temperatures of 650 °C and above. The resultant thermally annealed, IPG electrodes are converted into potassium ISEs via functionalization with a poly(vinyl chloride) (PVC) membrane and valinomycin ionophore. The developed potassium ISE displays a wide linear sensing range (0.01−100 mM), a low detection limit (7 μM), minimal drift (8.6 × 10 −6 V/s), and a negligible interference during electrochemical potassium sensing against the backdrop of interfering ions [i.e., sodium (Na), magnesium (Mg), and calcium (Ca)] and artificial eccrine perspiration. Thus, the IPG ISE shows potential for potassium detection in a wide variety of human fluids including plasma, serum, and sweat.