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We report on the intrinsic optoelectronic response of high-quality dual-gated monolayer and bilayer graphene p-n junction devices. Local laser excitation (of wavelength 850 nanometers) at the p-n interface leads to striking six-fold photovoltage patterns as a function of bottom-and top-gate voltages. These patterns, together with the measured spatial and density dependence of the photoresponse, provide strong evidence that nonlocal hot carrier transport, rather than the photovoltaic effect, dominates the intrinsic photoresponse in graphene. This regime, which features a long-lived and spatially distributed hot carrier population, may offer a path to hot carrier-assisted thermoelectric technologies for efficient solar energy harvesting.

The richness of optical and electronic properties of graphene attracts enormous interest. Graphene has high mobility and optical transparency, in addition to flexibility, robustness and environmental stability. So far, the main focus has been on fundamental physics and electronic devices. However, we believe its true potential lies in photonics and optoelectronics, where the combination of its unique optical and electronic properties can be fully exploited, even in the absence of a bandgap, and the linear dispersion of the Dirac electrons enables ultrawideband tunability. The rise of graphene in photonics and optoelectronics is shown by several recent results, ranging from solar cells and light-emitting devices to touch screens, photodetectors and ultrafast lasers. Here we review the state-of-the-art in this emerging field.

A wafer-scale graphene circuit was demonstrated in which all circuit components, including graphene field-effect transistor and inductors, were monolithically integrated on a single silicon carbide wafer. The integrated circuit operates as a broadband radio-frequency mixer at frequencies up to 10 gigahertz. These graphene circuits exhibit outstanding thermal stability with little reduction in performance (less than 1 decibel) between 300 and 400 kelvin. These results open up possibilities of achieving practical graphene technology with more complex functionality and performance.

A practical solution The best electronic and optoelectronic devices are built via semiconductor crystal growth on a single-crystal substrate. Over 100 papers have been published in recent years in Nature on alternative devices, produced instead from the solution phase. They have some advantages over conventional crystalline semiconductor devices: ease of fabrication, physical flexibility and — most important — low cost. The problem was the poor electronic performance of solution-processed devices, compared with single-crystal counterparts. But that could change now: a team from the University of Toronto reports that one such system — colloidal quantum dots of lead sulphide — can actually outperform the state-of-the-art crystalline alternative. A solution-processed electronic device that uses colloidal quantum dots of lead sulphide outperforms the state-of-the-art crystalline alternatives, with ease of fabrication, physical flexibility, large device areas and low cost among its benefits. Solution-processed electronic 1 and optoelectronic 2 , 3 , 4 , 5 devices offer low cost, large device area, physical flexibility and convenient materials integration compared to conventional epitaxially grown, lattice-matched, crystalline semiconductor devices. Although the electronic or optoelectronic performance of these solution-processed devices is typically inferior to that of those fabricated by conventional routes, this can be tolerated for some applications in view of the other benefits. Here we report the fabrication of solution-processed infrared photodetectors that are superior in their normalized detectivity ( D *, the figure of merit for detector sensitivity) to the best epitaxially grown devices operating at room temperature. We produced the devices in a single solution-processing step, overcoating a prefabricated planar electrode array with an unpatterned layer of PbS colloidal quantum dot nanocrystals. The devices showed large photoconductive gains with responsivities greater than 10 3  A W -1 . The best devices exhibited a normalized detectivity D * of 1.8 × 10 13  jones (1 jones = 1 cm Hz 1/2  W -1 ) at 1.3 µm at room temperature: today's highest performance infrared photodetectors are photovoltaic devices made from epitaxially grown InGaAs that exhibit peak D * in the 10 12  jones range at room temperature, whereas the previous record for D * from a photoconductive detector lies at 10 11  jones. The tailored selection of absorption onset energy through the quantum size effect, combined with deliberate engineering of the sequence of nanoparticle fusing and surface trap functionalization, underlie the superior performance achieved in this readily fabricated family of devices.

Despite several years of research into graphene electronics, sufficient on/off current ratio / on // off in graphene transistors with conventional device structures has been impossible to obtain. We report on a three-terminal active device, a graphene variable-barrier \"barristor\" (GB), in which the key is an atomically sharp interface between graphene and hydrogenated silicon. Large modulation on the device current (on/off ratio of 10⁵) is achieved by adjusting the gate voltage to control the graphene-silicon Schottky barrier. The absence of Fermi-level pinning at the interface allows the barrier's height to be tuned to 0.2 electron volt by adjusting graphene's work function, which results in large shifts of diode threshold voltages. Fabricating GBs on respective 150-mm wafers and combining complementary p-and n-type GBs, we demonstrate inverter and half-adder logic circuits.

By monitoring the environment of a superconducting quantum bit in real time, the quantum bit can be maintained in a pure state and its time evolution, as described by its ‘quantum trajectory’, can be tracked. To stabilize quantum systems — measure them A quantum state, such as a particle's superposition between two energy levels, quickly reverts to a classically described state on contact with the environment. To avoid this 'decoherence', large efforts are usually made to decouple quantum devices from their surroundings. But there is another way. Kater Murch et al . show that quantum coherence can be preserved by continuous, accurate monitoring of the environmental fluctuations. They studied a qubit consisting of a superconducting device embedded in a microwave cavity with fluctuations likely to cause decoherence. The act of accurately measuring either phase or amplitude of the fluctuations was found to steer the qubit's state along random trajectories that are purely quantum in nature. This work suggests a new type of control, harnessing action at a distance through measurement, for the manipulation of quantum systems in complex environments, ranging from biological systems to quantum computers. The length of time that a quantum system can exist in a superposition state is determined by how strongly it interacts with its environment. This interaction entangles the quantum state with the inherent fluctuations of the environment. If these fluctuations are not measured, the environment can be viewed as a source of noise, causing random evolution of the quantum system from an initially pure state into a statistical mixture—a process known as decoherence. However, by accurately measuring the environment in real time, the quantum system can be maintained in a pure state and its time evolution described by a ‘quantum trajectory’ 1 , 2 determined by the measurement outcome. Here we use weak measurements to monitor a microwave cavity containing a superconducting quantum bit (qubit), and track the individual quantum trajectories 3 of the system. In this set-up, the environment is dominated by the fluctuations of a single electromagnetic mode of the cavity. Using a near-quantum-limited parametric amplifier 4 , 5 , we selectively measure either the phase or the amplitude of the cavity field, and thereby confine trajectories to either the equator or a meridian of the Bloch sphere. We perform quantum state tomography at discrete times along the trajectory to verify that we have faithfully tracked the state of the quantum system as it diffuses on the surface of the Bloch sphere. Our results demonstrate that decoherence can be mitigated by environmental monitoring, and validate the foundation of quantum feedback approaches based on Bayesian statistics 6 , 7 , 8 . Moreover, our experiments suggest a new means of implementing ‘quantum steering’ 9 —the harnessing of action at a distance to manipulate quantum states through measurement.

Unidirectional electric current flow, such as that found in a diode, is essential for modern electronics. It usually occurs at asymmetric interfaces such as p-n junctions or metal/semiconductor interfaces with Schottky barriers. We report on a diode effect associated with the direction of bulk electric polarization in BiFeO3: a ferroelectric with a small optical gap edge of approximately 2.2 electron volts. We found that bulk electric conduction in ferroelectric monodomain BiFeO3 crystals is highly nonlinear and unidirectional. This diode effect switches its direction when the electric polarization is flipped by an external voltage. A substantial visible-light photovoltaic effect is observed in BiFeO3 diode structures. These results should improve understanding of charge conduction mechanisms in leaky ferroelectrics and advance the design of switchable devices combining ferroelectric, electronic, and optical functionalities.

The fabrication of transistors using vertical, six-sided core–multishell indium gallium arsenide nanowires with an all-surrounding gate on a silicon substrate combines the advantages of a three-dimensional gate architecture with the high electron mobility of the III–V nanowires, drastically enhancing the on-state current and transconductance. Fast multishell alloy transisitors In the continuing drive to improve and miniaturize transistors, the microelectronics industry has recently adopted three-dimensional electronic gate structures. Another way of improving transistors is to use semiconductor materials with higher electron mobility than silicon, although this presents significant fabrication challenges. Katsuhiro Tomioka et al . combine the two approaches; they grow, with high precision, vertical, six-sided core–multishell indium gallium arsenide nanowires with an all-surrounding gate on a silicon substrate. The resulting devices demonstrate superior transistor performance with excellent on/off switching behaviour and fast operation. Silicon transistors are expected to have new gate architectures, channel materials and switching mechanisms in ten years’ time 1 , 2 , 3 , 4 . The trend in transistor scaling has already led to a change in gate structure from two dimensions to three, used in fin field-effect transistors, to avoid problems inherent in miniaturization such as high off-state leakage current and the short-channel effect. At present, planar and fin architectures using III–V materials, specifically InGaAs, are being explored as alternative fast channels on silicon 5 , 6 , 7 , 8 , 9 because of their high electron mobility and high-quality interface with gate dielectrics 10 . The idea of surrounding-gate transistors 11 , in which the gate is wrapped around a nanowire channel to provide the best possible electrostatic gate control, using InGaAs channels on silicon, however, has been less well investigated 12 , 13 because of difficulties in integrating free-standing InGaAs nanostructures on silicon. Here we report the position-controlled growth of vertical InGaAs nanowires on silicon without any buffering technique and demonstrate surrounding-gate transistors using InGaAs nanowires and InGaAs/InP/InAlAs/InGaAs core–multishell nanowires as channels. Surrounding-gate transistors using core–multishell nanowire channels with a six-sided, high-electron-mobility transistor structure greatly enhance the on-state current and transconductance while keeping good gate controllability. These devices provide a route to making vertically oriented transistors for the next generation of field-effect transistors and may be useful as building blocks for wireless networks on silicon platforms.

Although electrochemical capacitors (ECs), also known as supercapacitors or ultracapacitors, charge and discharge faster than batteries, they are still limited by low energy densities and slow rate capabilities. We used a standard LightScribe DVD optical drive to do the direct laser reduction of graphite oxide films to graphene. The produced films are mechanically robust, show high electrical conductivity (1738 Siemens per meter) and specific surface area (1520 square meters per gram), and can thus be used directly as EC electrodes without the need for binders or current collectors, as is the case for conventional ECs. Devices made with these electrodes exhibit ultrahigh energy density values in different electrolytes while maintaining the high power density and excellent cycle stability of ECs. Moreover, these ECs maintain excellent electrochemical attributes under high mechanical stress and thus hold promise for high-power, flexible electronics.

Thin-film transistors in print Printing electronic devices using semiconducting 'ink' is seen as a promising route to cheap, large-area and flexible electronics, but the performance of such devices suffers from the relatively poor crystallinity of the printed material. Hiromi Minemawari and colleagues have developed an inkjet-based printing technique involving controlled mixing on a surface of two solutions — the semiconductor (C8-BTBT) in its solvent and a liquid in which the semiconductor is insoluble. The products of this antisolvent crystallization technique are thin semiconductor films with exceptionally high and uniform crystallinity. The use of single crystals has been fundamental to the development of semiconductor microelectronics and solid-state science 1 . Whether based on inorganic 2 , 3 , 4 , 5 or organic 6 , 7 , 8 materials, the devices that show the highest performance rely on single-crystal interfaces, with their nearly perfect translational symmetry and exceptionally high chemical purity. Attention has recently been focused on developing simple ways of producing electronic devices by means of printing technologies. ‘Printed electronics’ is being explored for the manufacture of large-area and flexible electronic devices by the patterned application of functional inks containing soluble or dispersed semiconducting materials 9 , 10 , 11 . However, because of the strong self-organizing tendency of the deposited materials 12 , 13 , the production of semiconducting thin films of high crystallinity (indispensable for realizing high carrier mobility) may be incompatible with conventional printing processes. Here we develop a method that combines the technique of antisolvent crystallization 14 with inkjet printing to produce organic semiconducting thin films of high crystallinity. Specifically, we show that mixing fine droplets of an antisolvent and a solution of an active semiconducting component within a confined area on an amorphous substrate can trigger the controlled formation of exceptionally uniform single-crystal or polycrystalline thin films that grow at the liquid–air interfaces. Using this approach, we have printed single crystals of the organic semiconductor 2,7-dioctyl[1]benzothieno[3,2- b ][1]benzothiophene (C 8 -BTBT) (ref. 15 ), yielding thin-film transistors with average carrier mobilities as high as 16.4 cm 2  V −1  s −1 . This printing technique constitutes a major step towards the use of high-performance single-crystal semiconductor devices for large-area and flexible electronics applications.