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PRINTED ELECTRONICS nanoporous printing stamps comprising dots, whose chemical potential and mutual Nanotube resolution vertically grown carbon nanotubes tunnel coupling are tuned independently. Sci. Adv. 2, e1601660 (2016) embedded in a polymeric matrix. The Three additional collinear dots are also pore size is chosen in such a way that it is defined and used as single-electron charge larger than the ink particles but appreciably detectors. By performing measurements of smaller than the printed features. Complex conductance and pulsed-gate spectroscopy, printing designs are achieved via controlled the researchers observe narrow value nanotube growth on photolithographically distributions for the charging and orbital- pre-patterned silicon substrates. Once excited energies of the nine dots considered. loaded with an ink, the stamp coated by Sensitive real-time detection capabilities for mechanically robust carbon nanotube tunnelling events, with effective bandwidth arrays comes into uniform contact with up to 30 kHz are also demonstrated. the target substrate and enables the The researchers perform single-shot printing of high resolution images. This readouts of an electron spin trapped technique is readily scalable to roll-to-roll in one of the dots, quantifying spin- manufacturing and offers at least tenfold relaxation times in the order of hundreds resolution improvement over current of milliseconds. Remarkably, they also industrial flexographic printing. OB demonstrate a strong capacitive coupling 500 µm between adjacent dot couples —

AAAS QUANTUM COMPUTATION a convenient property in view of the Qubits in a row implementation of high-frequency Printing electronic devices with very Phys. Rev. Appl. 6, 054013 (2016) computing tasks. GP small features in a short time and in a reproducible way over large areas represents Gate-defined quantum dots in ARTIFICIAL PHOTOSYNTHESIS a significant technological challenge. Some semiconductors have been identified as Adaptable kind printing techniques achieve nanoscale hosts for high-fidelity quantum bits, storing Nano Lett. 16, 7461–7466 (2016) structures, yet they are not compatible the information in the spin state of the with high-throughput production. Relief trapped charges. However, reproducing In conventional energy harvesting devices, printing, also known as flexography, is one the physical properties of the dots is still steady power output is achieved via active of the most common techniques used in a challenging aspect of their fabrication, feedback components such as voltage roll-to-roll manufacturing. The smallest undermining the realistic prospects for the converters. The same requirement applies feature size that can be obtained by the implementation of quantum-computing to solar cells that are not equipped with any method is tens of micrometres, which is tasks based on these devices. internal regulation mechanism to account a serious limitation for applications in Now, Zajac et al. report on the for natural variations in the incident flexible electronics. fabrication of nine collinear quantum solar power. This long standing issue of Now, Kim et al. report on a new dots with reproducible properties. The inefficient energy storage has now been flexography method with sub-micrometre researchers pattern an undoped Si/SiGe addressed by Arp and colleagues, who resolution that works on both rigid and heterostructure with aluminium-based propose a model describing a quantum flexible surfaces. The method is based on screening layers and electrodes defining the heat engine photocell with an intrinsic mechanism for the suppression of BIOFUEL CELLS energy fluctuations. Similar to the case of photosynthesis in Angew. Chem. Int. Ed. 55, 15434–15438 (2016) What’s in store green plants, a heat engine with quantized energy levels converts the energy of solar A biofuel cell converts chemical energy stored in the chemical bonds of a biofuel, usually photons into useful work. In the simplest glucose, into electrical energy using enzymes. In the anode, glucose is oxidized to case, a nanoscale light-harvesting photocell produce gluconolactone and give out electrons that are then routed through an external consists of two photon-absorbing channels circuit to power a device. In the cathode, the electrons reduce oxygen to form water. with equal charge transfer probability but When the fuel supply ends, the fuel cell stops working. Now, Pankratov et al. describe a different energy input. When exposed to charge-storing biofuel cell, a device in which the charges produced at the anode and the fluctuating solar energy, the quantum cathode form an integrated supercapacitor. heat engine photocell stochastically Like a biofuel cell, the device is composed of an anode where glucose is oxidized and a switches between high and low power separated cathode where oxygen is reduced. Electrons produced in the anode reduce an channels to yield steady-state output. osmium(iii) compound to osmium(ii), whereas the reduction of oxygen in the cathode is Simulations reveal that this suppression tied to the oxidation of the same osmium couple. As a result, a concentration (or, better, mechanism is effective over a wide range an activity) difference and therefore a voltage difference is generated between the two of the solar spectrum with the exception of electrodes, akin to a supercapacitor. To discharge the supercapacitor one connects the the green region, in which the regulatory anode and the cathode through an external circuit when needed. benefits vanish and the two-channel The device proposed by Pankratov et al. can generate an open-circuit voltage of 0.45 V, quantum heat engine essentially behaves as meaning that virtually all chemical energy is stored as charges in the supercapacitor a one-channel photocell. OB (no leakage currents). At discharge, the power density generated by this charge-storing biofuel cell exceeds that of the corresponding biofuel cell by a factor of 8. AM Written by Olga Bubnova, Alberto Moscatelli and Giacomo Prando.