2The class of 2D atomic crystals 1 , which started with graphene 2 now includes a large variety of materials. However, the real diversity can be achieved if one starts to combine several such crystals in van der Waals heterostructures 3,8 . Most attractive and powerful is the idea of band-structure engineering, where by combining several different 2D crystals one can create a designer potential landscape for electrons to live in. Rendering the band-structure with atomic precision allows tunnel barriers, QWs and other devices, based on the broad choice of 2D materials.Such band-structure engineering has previously been exploited to create LEDs and lasers based on semiconductor heterostructures grown by molecular beam epitaxy 9 . Here we demonstrate that using graphene as a transparent conductive layer, hBN as tunnel barriers and different transition metal dichalcogenides (TMDC) 1,10 as the materials for QWs, we can create efficient LEDs; Fig. 1F. In our devices, electrons and holes are injected to a layer of TMDC from the two graphene electrodes.Because of the long lifetime of the quasiparticles in the QWs (determined by the height and thickness of the neighbouring hBN barriers), electrons and holes recombine, emitting a photon. The emission wavelength can be fine-tuned by the appropriate selection of TMDC and quantum efficiency (QE) can be enhanced by using multiple QWs (MQWs).We chose TMDC because of wide choice of such materials and the fact that monolayers of many TMDC are direct band gap semiconductors [11][12][13][14][15] . Until now, electroluminescence (EL) in TMDC devices has been reported only for lateral monolayer devices and attributed to thermally assisted processes arising from impact ionization across a Schottky barrier 16 and formation of p-n junctions 15,17,18 /hBN. (H-J) Band diagrams for the case of zero applied bias (H), intermediate applied bias (I) and high bias (J) for heterostructure presented in (G). 4For brevity we concentrate on current-voltage (I-V) characteristics, photoluminescence (PL) and EL spectra from symmetric devices based on MoS 2 , Fig At low V b , the PL in Fig. 2A is dominated by the neutral A exciton, X 0 , peak 12 at 1.93 eV. We attribute the two weaker and broader peaks at 1.87 and 1.79 eV to bound excitons 22,23 . At certain V b , the PL spectrum changes abruptly with another peak emerging at 1.90 eV. This transition is correlated with an increase in the differential conductivity ( Fig. 2A). We explain this transition as being due to the fact that at this voltage the Fermi level in Gr B rises above the conduction band in MoS 2 , allowing injection of electrons into the QW (Fig. 1I). This allows us to determine the band alignment between In contrast to PL, EL starts only at V b above a certain threshold, Figs. 2B. We associate such behaviour with the Fermi level of Gr T being brought below the edge of the valence band so that holes can be injected to MoS 2 from Gr T (in addition to electrons already injected from Gr B ) as sketched in Fig. 1J. This creates con...

There are hundreds of companies worldwide claiming to produce "graphene," showing a large variation in its properties. A systematic and reliable protocol is developed to test graphene quality using electron microscopy, atomic force microscopy, Raman spectroscopy, elemental analysis, X-ray photoelectron spectrometry, and scanning and transmission electron microscopy, which is used to study graphene from 60 producers. The statistical nature of the liquid-phase exfoliation of graphite is established. It is shown that the current classification of graphene flakes used in the market is erroneous. A new classification is proposed in terms of distribution functions for number of layers and flake size. It is shown unequivocally that the quality of the graphene produced in the world today is rather poor, not optimal for most applications, and most companies are producing graphite microplatelets. This is possibly the main reason for the slow development of graphene applications, which usually require a customized solution in terms of graphene properties. It is argued that the creation of stringent standards for graphene characterization and production, taking into account both the physical properties, as well as the requirements from the particular application, is the only way forward to create a healthy and reliable worldwide graphene market.

Printed electronics offer a breakthrough in the penetration of information technology into everyday life. The possibility of printing electronic circuits will further promote the spread of the Internet of Things applications. Inks based on graphene have a chance to dominate this technology, as they potentially can be low cost and applied directly on materials like textile and paper. Here we report the environmentally sustainable route of production of graphene ink suitable for screen-printing technology. The use of non-toxic solvent Dihydrolevoglucosenone (Cyrene) significantly speeds up and reduces the cost of the liquid phase exfoliation of graphite. Printing with our ink results in very high conductivity (7.13 × 104 S m−1) devices, which allows us to produce wireless connectivity antenna operational from MHz to tens of GHz, which can be used for wireless data communication and energy harvesting, which brings us very close to the ubiquitous use of printed graphene technology for such applications.

Controlled transport of water molecules through membranes and capillaries is important in areas as diverse as water purification and healthcare technologies. Previous attempts to control water permeation through membranes (mainly polymeric ones) have concentrated on modulating the structure of the membrane and the physicochemical properties of its surface by varying the pH, temperature or ionic strength. Electrical control over water transport is an attractive alternative; however, theory and simulations have often yielded conflicting results, from freezing of water molecules to melting of ice under an applied electric field. Here we report electrically controlled water permeation through micrometre-thick graphene oxide membranes. Such membranes have previously been shown to exhibit ultrafast permeation of water and molecular sieving properties, with the potential for industrial-scale production. To achieve electrical control over water permeation, we create conductive filaments in the graphene oxide membranes via controllable electrical breakdown. The electric field that concentrates around these current-carrying filaments ionizes water molecules inside graphene capillaries within the graphene oxide membranes, which impedes water transport. We thus demonstrate precise control of water permeation, from ultrafast permeation to complete blocking. Our work opens up an avenue for developing smart membrane technologies for artificial biological systems, tissue engineering and filtration.