Strain engineering is widely used in material science to tune the (opto-)electronic properties of materials and enhance the performance of devices. Two-dimensional atomic crystals are a versatile playground to study the influence of strain, as they can sustain very large deformations without breaking. Various optical techniques have been employed to probe strain in two-dimensional materials, including micro-Raman and photoluminescence spectroscopy. Here we demonstrate that optical second harmonic generation constitutes an even more powerful technique, as it allows extraction of the full strain tensor with a spatial resolution below the optical diffraction limit. Our method is based on the strain-induced modification of the nonlinear susceptibility tensor due to a photoelastic effect. Using a two-point bending technique, we determine the photoelastic tensor elements of molybdenum disulfide. Once identified, these parameters allow us to spatially image the two-dimensional strain field in an inhomogeneously strained sample.

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Light emission from higher-order correlated excitonic states has been recently reported in hBN-encapsulated monolayer WSe 2 and WS 2 upon optical excitation. These exciton complexes are found to be bound states of excitons residing in opposite valleys in momentum space, a promising feature that could be employed in valleytronics or other novel optoelectronic devices. However, electrically-driven light emission from such exciton species is still lacking. Here we report electroluminescence from bright and dark excitons, negatively charged trions and neutral and negatively charged biexcitons, generated by a pulsed gate voltage, in hexagonal boron nitride encapsulated monolayer WSe 2 and WS 2 with graphene as electrode. By tailoring the pulse parameters we are able to tune the emission intensity of the different exciton species in both materials. We find the electroluminescence from charged biexcitons and dark excitons to be as narrow as 2.8 meV.

Single-photon emitters play a key role in present and emerging quantum technologies. Several recent measurements have established monolayer WSe2 as a promising candidate for a reliable single photon source. The origin and underlying microscopic processes have remained, however, largely elusive. We present a multi-scale tight-binding simulation for the optical spectra of WSe2 under nonuniform strain and in the presence of point defects employing the Bethe-Salpeter equation. Strain locally shifts excitonic energy levels into the band gap where they overlap with localized intra-gap defect states. The resulting hybridization allows for efficient filing and subsequent radiative decay of the defect states. We identify inter-valley defect excitonic states as the likely candidate for antibunched single-photon emission. This proposed scenario is shown to account for a large variety of experimental observations including brightness, radiative transition rates, the variation of the excitonic energy with applied magnetic and electric fields as well as the variation of the polarization of the emitted photon with the magnetic field.Transition Metal Dichalcogenides (TMDs) have attracted considerable interest over the last decade. A direct band gap in the mono layer case [1,2], extremely large excitonic binding energies in the order of 300-500 meV [3,5,11] and valley as well as spin selective optical transitions due to the D 3h symmetry make these materials very promising candidates for optical devices [6,7]. Single photon emitters (SPEs) in WSe 2 are among the most intriguing candidates for such future optical applications attracting considerable attention in the field of two-dimensional materials [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28]. Single-photon emitters promising photon emission "on demand" are key building blocks for optoelectronic and photonic-based quantum-technological devices, e.g., for generating entangled photons [26].SPEs in WSe 2 emit antibunched light from highly localized spots in suspended WSe 2 flakes featuring a narrow linewidth (down to 100 µeV ) and an intricate fine structure (for a review see [27]). A large number of experimental investigations have provided key insight to help unraveling the puzzle of the microscopic origin of SPEs. The prominent observation of SPEs in regions of enhanced strain, for example close to pillars suspending the WSe 2 membrane [19][20][21]25], points to the crucial role of locally non-uniform strain. The large defect density in WSe 2 also seems to play a role in the formation of SPEs [21]. The appearance of doublets in the optical spectra -i.e., single photon emission lines with energy spacing up to 1 meV -has been attributed to the exchange interaction between excitons but the underlying mechanism has remained an open question. While in some early studies few SPEs were found to be only weakly dependent on the magnetic field, in most measurements an unexpectedly large effective g-factor ranging from 8 to 13 was observed [13-15, 17, 23, 24, 28]. S...

Paper is the ideal substrate for the development of flexible and environmentally sustainable ubiquitous electronic systems, which, combined with two-dimensional materials, could be exploited in many Internet-of-Things applications, ranging from wearable electronics to smart packaging. Here we report high-performance MoS 2 field-effect transistors on paper fabricated with a "channel array" approach, combining the advantages of two large-area techniques: chemical vapor deposition and inkjet-printing. The first allows the pre-deposition of a pattern of MoS 2 ; the second, the printing of dielectric layers, contacts, and connections to complete transistors and circuits fabrication. Average I ON /I OFF of 8 × 10 3 (up to 5 × 10 4) and mobility of 5.5 cm 2 V −1 s −1 (up to 26 cm 2 V −1 s −1) are obtained. Fully functional integrated circuits of digital and analog building blocks, such as logic gates and current mirrors, are demonstrated, highlighting the potential of this approach for ubiquitous electronics on paper.