Devices that rely on the manipulation of excitons-bound pairs of electrons and holes-hold great promise for realizing efficient interconnects between optical data transmission and electrical processing systems. Although exciton-based transistor actions have been demonstrated successfully in bulk semiconductor-based coupled quantum wells, the low temperature required for their operation limits their practical application. The recent emergence of two-dimensional semiconductors with large exciton binding energies may lead to excitonic devices and circuits that operate at room temperature. Whereas individual two-dimensional materials have short exciton diffusion lengths, the spatial separation of electrons and holes in different layers in heterostructures could help to overcome this limitation and enable room-temperature operation of mesoscale devices. Here we report excitonic devices made of MoS-WSe van der Waals heterostructures encapsulated in hexagonal boron nitride that demonstrate electrically controlled transistor actions at room temperature. The long-lived nature of the interlayer excitons in our device results in them diffusing over a distance of five micrometres. Within our device, we further demonstrate the ability to manipulate exciton dynamics by creating electrically reconfigurable confining and repulsive potentials for the exciton flux. Our results make a strong case for integrating two-dimensional materials in future excitonic devices to enable operation at room temperature.

Long-lived interlayer excitons in van der Waals heterostructures based on transition metal dichalcogenides, together with unique spin-valley physics, make them promising for next-generation photonic and valleytronic devices. While the emission characteristics of interlayer excitons have been studied, efficient manipulation of their valley-state, a necessary requirement for information encoding, is still lacking. Here, we demonstrate comprehensive electrical control of interlayer excitons in a MoSe2/WSe2 heterostructure. Encapsulation of our well-aligned stack with hexagonal boron nitride (h-BN) allows us to resolve two separate narrow interlayer transitions with opposite helicities under circularly polarized excitation, either preserving or reversing the polarization of incoming light. By electrically controlling their relative intensities, we realize a polarization switch with tuneable emission intensity and wavelength. Finally, we demonstrate large Zeeman shifts of these two transitions upon application of an external magnetic field. These results are interpreted within the picture of moiré-induced brightening of forbidden optical transitions. The ability to control the polarization of interlayer excitons is a step forward towards the manipulation of the valley degree-of-freedom in realistic device applications.

Toward the large-area deposition of MoS 2 layers, we employ metal−organic precursors of Mo and S for a facile and reproducible van der Waals epitaxy on c-plane sapphire. Exposing c-sapphire substrates to alkali metal halide salts such as KI or NaCl together with the Mo precursor prior to the start of the growth process results in increasing the lateral dimensions of single crystalline domains by more than 2 orders of magnitude. The MoS 2 grown this way exhibits high crystallinity and optoelectronic quality comparable to singlecrystal MoS 2 produced by conventional chemical vapor deposition methods. The presence of alkali metal halides suppresses the nucleation and enhances enlargement of domains while resulting in chemically pure MoS 2 after transfer. Field-effect measurements in polymer electrolyte-gated devices result in promising electron mobility values close to 100 cm 2 V −1 s −1 at cryogenic temperatures. KEYWORDS: Chemical vapor deposition, two-dimensional transition metal dichalcogenides, nucleation and growth, microstructure engineering, FET devices T he chemical vapor deposition of two-dimensional materials is a highly promising method to produce atomically thin layers at a large scale for harnessing their attractive properties. Monolayer MoS 2 is a model 2D semiconductor that can be used to realize field-effect transistors with high current on/off ratios. 1 It is a naturally occurring material with a good chemical stability that exhibits a wide range of attractive properties such as a spin−orbit couplinginduced band splitting, 2,3 a mechanically tunable bandgap, 4−8 and a low temperature superconductivity. 9−13 Toward the large-scale synthesis of MoS 2 thin films, a conventional chemical vapor deposition method of producing MoS 2 monolayers typically involves low vapor pressure solid powder precursors such as MoO 3 and sulfur. It has been investigated for centimeter-scale deposition of polycrystalline monolayer MoS 2 with grain sizes of nanometer to micrometer and with controllable coverage. 14,15 However, low vapor pressures of the solid precursors require them to be loaded inside a heated zone of the reactor chamber leading to a limited control over the vapor phase composition and deposition rate. Thus, this synthesis approach heavily undermines the ability to control the nucleation density, thickness, and coverage.Here, we aim to address this issue by employing well-known metal−organic precursors of molybdenum, Mo(CO) 6 , which is a high vapor pressure solid, and of sulfur, H 2 S in gas phase. 16−19 This metal-organic chemical vapor deposition (MOCVD) approach allows reliably setting the concentration of precursor gases within the gaseous mixture that is transported to the substrate by controlling the evaporation rates of the solid precursor and mass flow rates. An extensive vapor phase thermodynamics study performed by Kumar et al. 19 has shown that growth temperatures above 850°C at atmospheric pressure lead to layer-by-layer growth of MoS 2 without extraneous deposition of carbon ...

Valleytronics is an appealing alternative to conventional charge-based electronics which aims at encoding data in the valley degree of freedom, i.e. the information over which extreme of the conduction or valence band carriers are occupying. The ability to create and control valleycurrents in solid state devices could therefore enable new paradigms for information processing. Transition metal dichalcogenides (TMDCs) are a promising platform for valleytronics, due to the presence of two inequivalent valleys with spin-valley locking 1 and a direct band gap 2,3 , which allows optical initialization and readout of the valley-state 4,5. Recent progresses on the control of interlayer excitons in these materials 6-8 could offer an effective way to realize optoelectronic devices based on the valley degree of freedom. Here, we show the generation and transport over mesoscopic distances of valley-polarized excitons in a device based on a type-II TMDC heterostructure. Engineering of the interlayer coupling results in enhanced diffusion of valleypolarized excitons, which can be controlled and switched electrically. Furthermore, using electrostatic traps, we can increase exciton concentration by an order of magnitude, reaching densities in the order of 10 12 cm-2 , opening the route to achieving a coherent quantum state of valley-polarized excitons via Bose-Einstein condensation. Users may view, print, copy, and download text and data-mine the content in such documents, for the purposes of academic research, subject always to the full Conditions of use:

Defects are ubiquitous in solids, often introducing new properties that are absent in pristine materials. One of the opportunities offered by these crystal imperfections is an extrinsically induced long-range magnetic ordering,1 a long-time subject of theoretical investigations.1-3 Intrinsic, two-dimensional (2D) magnetic materials4-7 are attracting increasing attention for their unique properties including layer-dependent magnetism4 and electric field modulation6. Yet, inducing magnetism into otherwise non-magnetic 2D materials remains a challenge. Here, we investigate magneto-transport properties of ultrathin PtSe 2 crystals and demonstrate unexpected magnetism. Our electrical measurements show the existence of either ferromagnetic or antiferromagnetic ground state orderings depending on the number of layers in this ultra-thin material. The change in the device resistance upon application of a ~ 25 mT magnetic field is as high as 400 Ω with a magnetoresistance (MR) value of 5%. Our first-principles calculations suggest that surface magnetism induced by the presence of Pt vacancies and the Ruderman-Kittel-Kasuya-Yosida (RKKY) exchange couplings across ultra-thin films of PtSe 2 are responsible for the observed layer-dependent magnetism. Considering the existence of such unavoidable growthrelated vacancies in 2D materials, 8,9 these findings can expand the range of 2D ferromagnets into materials that would otherwise be overlooked. Users may view, print, copy, and download text and data-mine the content in such documents, for the purposes of academic research, subject always to the full Conditions of use: