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The development of light-emitting diodes with improved efficiency, spectral properties, compactness and integrability is important for lighting, display, optical interconnect, logic and sensor applications. Monolayer transition-metal dichalcogenides have recently emerged as interesting candidates for optoelectronic applications due to their unique optical properties. Electroluminescence has already been observed from monolayer MoS2 devices. However, the electroluminescence efficiency was low and the linewidth broad due both to the poor optical quality of the MoS2 and to ineffective contacts. Here, we report electroluminescence from lateral p-n junctions in monolayer WSe2 induced electrostatically using a thin boron nitride support as a dielectric layer with multiple metal gates beneath. This structure allows effective injection of electrons and holes, and, combined with the high optical quality of WSe2, yields bright electroluminescence with 1,000 times smaller injection current and 10 times smaller linewidth than in MoS2 (refs 17,18). Furthermore, by increasing the injection bias we can tune the electroluminescence between regimes of impurity-bound, charged and neutral excitons. This system has the required ingredients for new types of optoelectronic device, such as spin- and valley-polarized light-emitting diodes, on-chip lasers and two-dimensional electro-optic modulators.

Semi-metallic graphene and semiconducting monolayer transition-metal dichalcogenides are the most intensively studied two-dimensional materials of recent years. Lately, black phosphorus has emerged as a promising new two-dimensional material due to its widely tunable and direct bandgap, high carrier mobility and remarkable in-plane anisotropic electrical, optical and phonon properties. However, current progress is primarily limited to its thin-film form. Here, we reveal highly anisotropic and strongly bound excitons in monolayer black phosphorus using polarization-resolved photoluminescence measurements at room temperature. We show that, regardless of the excitation laser polarization, the emitted light from the monolayer is linearly polarized along the light effective mass direction and centres around 1.3 eV, a clear signature of emission from highly anisotropic bright excitons. Moreover, photoluminescence excitation spectroscopy suggests a quasiparticle bandgap of 2.2 eV, from which we estimate an exciton binding energy of ∼0.9 eV, consistent with theoretical results based on first principles. The experimental observation of highly anisotropic, bright excitons with large binding energy not only opens avenues for the future explorations of many-electron physics in this unusual two-dimensional material, but also suggests its promising future in optoelectronic devices. [ABSTRACT FROM AUTHOR]

Single quantum emitters (SQEs) are at the heart of quantum optics and photonic quantum-information technologies. To date, all the demonstrated solid-state single-photon sources are confined to one-dimensional (1D; ref. 3) or 3D materials. Here, we report a new class of SQEs based on excitons that are spatially localized by defects in 2D tungsten-diselenide (WSe2) monolayers. The optical emission from these SQEs shows narrow linewidths of ∼130 μeV, about two orders of magnitude smaller than those of delocalized valley excitons. Second-order correlation measurements revealed a strong photon antibunching, which unambiguously established the single-photon nature of the emission. The SQE emission shows two non-degenerate transitions, which are cross-linearly polarized. We assign this fine structure to two excitonic eigenmodes whose degeneracy is lifted by a large ∼0.71 meV coupling, probably because of the electron-hole exchange interaction in the presence of anisotropy. Magneto-optical measurements also reveal an exciton g factor of ∼8.7, several times larger than those of delocalized valley excitons. In addition to their fundamental importance, establishing new SQEs in 2D quantum materials could give rise to practical advantages in quantum-information processing, such as an efficient photon extraction and a high integratability and scalability. [ABSTRACT FROM AUTHOR]