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...

Layered materials can be assembled vertically to fabricate a new class of van der Waals heterostructures a few atomic layers thick, compatible with a wide range of substrates and optoelectronic device geometries, enabling new strategies for control of light–matter coupling. Here, we incorporate molybdenum diselenide/hexagonal boron nitride (MoSe2/hBN) quantum wells in a tunable optical microcavity. Part-light–part-matter polariton eigenstates are observed as a result of the strong coupling between MoSe2 excitons and cavity photons, evidenced from a clear anticrossing between the neutral exciton and the cavity modes with a splitting of 20 meV for a single MoSe2 monolayer, enhanced to 29 meV in MoSe2/hBN/MoSe2 double-quantum wells. The splitting at resonance provides an estimate of the exciton radiative lifetime of 0.4 ps. Our results pave the way for room-temperature polaritonic devices based on multiple-quantum-well van der Waals heterostructures, where polariton condensation and electrical polariton injection through the incorporation of graphene contacts may be realized.

Monolayers of molybdenum and tungsten dichalcogenides are direct bandgap semiconductors, which makes them promising for opto-electronic applications. In particular, van der Waals heterostructures consisting of monolayers of MoS 2 sandwiched between atomically thin hexagonal boron nitride (hBN) and graphene electrodes allows one to obtain light emitting quantum wells (LEQW's) with low-temperature external quantum efficiency (EQE) of 1%. However, the EQE of MoS 2 and MoSe 2 -based LEQW's shows behavior common for many other materials: it decreases fast from cryogenic conditions to room temperature, undermining their practical applications. Here we compare MoSe 2 and WSe 2 LEQW's. We show that the EQE of WSe 2 devices grows with temperature, with room temperature EQE reaching 5%, which is 250x more than the previous best performance of MoS 2 and MoSe 2 quantum wells in ambient conditions. We attribute such a different temperature dependences to the inverted sign of spin-2 orbit splitting of conduction band states in tungsten and molybdenum dichalcogenides, which makes the lowest-energy exciton in WSe 2 dark.

Atomically thin materials such as graphene or MoS are of high in-plane symmetry. Crystals with reduced symmetry hold the promise for novel optoelectronic devices based on their anisotropy in current flow or light polarization. Here, we present polarization-resolved optical transmission and photoluminescence spectroscopy of excitons in 1T'-ReSe. On reducing the crystal thickness from bulk to a monolayer, we observe a strong blue shift of the optical band gap from 1.37 to 1.50 eV. The excitons are strongly polarized with dipole vectors along different crystal directions, which persist from bulk down to monolayer thickness. The experimental results are well reproduced by ab initio calculations based on the GW-BSE approach within LDA+GdW approximation. The excitons have high binding energies of 860 meV for the monolayer and 120 meV for bulk. They are strongly confined within a single layer even for the bulk crystal. In addition, we find in our calculations a direct band gap in 1T'-ReSe regardless of crystal thickness, indicating weak interlayer coupling effects on the band gap characteristics. Our results pave the way for polarization-sensitive applications, such as optical logic circuits operating in the infrared spectral region.