Single layers of WS2 are direct gap semiconductors with high photoluminescence (PL) yield holding great promise for emerging applications in optoelectronics. The spatial confinement in a 2D monolayer together with the weak dielectric screening lead to huge binding energies for the neutral excitons as well as other excitonic complexes, such as trions and biexcitons whose binding energies scale accordingly. Here, we report on the existence of biexcitons in mechanically exfoliated WS2 flakes from 78 K up to room temperature. Performing temperature and power dependent PL measurements, we identify the biexciton emission channel through the superlinear behavior of the integrated PL intensity as a function of the excitation power density. On the contrary, neutral and charged excitons show a linear to sublinear dependence in the whole temperature range. From the energy difference between the emission channels of the biexciton and neutral exciton, a biexciton binding energy of 65-70 meV is determined. *Corresponding authors: gnk@materials.uoc.gr ; stratak@iesl.forth.gr 2 The interest on two-dimensional (2D) materials has been steadily increasing since the discovery of graphene, a material with fascinating properties and great potential for various applications 1 . Transition metal dichalcogenides (TMDs) with the form MX2 (M = Mo, W, Ti, etc and X = S, Se, Te) exhibit a structure very similar to that of graphene and have attracted significant attention of the scientific community 2-5 for a number of reasons. Among the most attractive features of TMD compounds is that their electronic properties can vary from metallic to those of a wide band gap semiconductor depending on the structure, composition and dimensionality, while their band structure can be easily tuned by applying stress 6 . Owing to such layer-dependent electronic structure, TMDs exhibit extraordinary optoelectronic properties 7,8 as well as potential for enhanced performance in applications such as photodetectors 9 , photovoltaics 10,11 and non-linear optical components 12 .Among their prominent optical properties, TMD monolayers show strong PL in the visible and near-infrared spectral range, due to the transition from indirect band gap semiconductors in their bulk and few-layer forms, to direct band gap semiconductors in the monolayer limit 13 . Besides this, the spatial confinement of carriers in a 2D monolayer lattice and the weak dielectric screening, give rise to unusually strong excitonic effects and high binding energies [14][15][16] . These properties favor the stability of a variety of excitonic quasiparticles with extremely large binding energies, including neutral excitons with binding energies of several hundred meV 17,18 , charged excitons (trions) was shown that the biexciton emission in WS2 can be electrically tuned 27 , which is of great importance for TMDs-based photonic devices.The unusually strong excitonic effects in TMDs strongly suggest an enhanced thermal stability of biexciton emission. While neutral and charged excitons have...

Chlorine-doped tungsten disulfide monolayer (1L-WS2) with tunable charge carrier concentration has been realized by pulsed laser irradiation of the atomically thin lattice in a precursor gas atmosphere. This process gives rise to a systematic shift of the neutral exciton peak towards lower energies, indicating reduction of the crystal's electron density. The capability to progressively tune the carrier density upon variation of the exposure time is demonstrated; this implicates that the Fermi level shift is directly correlated to the respective electron density modulation due to the chlorine species. Notably, this electron withdrawing process enabled the determination of the trion binding energy of the intrinsic crystal, found to be as low as 20 meV, in accordance to theoretical predictions. At the same time, it is found that the effect can be reversed upon continuous wave laser scanning of the monolayer in air. Scanning Auger Microscopy and X-ray photoelectron spectroscopy are used to link the actual charge carrier doping to the different chlorine configurations in the monolayer lattice. The spectroscopic analyses, complemented by density functional theory calculations, reveal that chlorine physisorption is responsible for the carrier density modulation induced by the pulsed laser photochemical reaction process. Such bidirectional control of the Fermi level, coupled with the capability offered by lasers to process at pre-selected locations, can be advantageously used for spatially resolved doping modulation in 1L-WS2 with micrometric resolution. This method can also be extended for the controllable doping of other TMD monolayers. Monolayer (1L) transition metal dichalcogenides (TMDs) are promising materials for future 2D nanoelectronics due to their unique electrical, 1 optical 2,3 and mechanical properties. 4 Indeed, TMDs have been demonstrated to be ideal candidates for field-effect transistors, photovoltaic cells, lightemitting diodes, single-atom storage, molecule sensing, quantum-state metamaterials and electrocatalytic water splitting applications. 5-7 Carrier modulation is an important parameter in the study of the electronic properties of semiconductors and at the heart of many applications in microelectronics. Tuning the charge carrier density, i.e. doping, can be realized chemically, via bonding of foreign atoms to the crystal structure, 8-10 or electrostatically, by inducing a charge accumulation. 11-13 Electrical doping

We report on the successful growth of strained core-shell GaAs/InGaAs nanowires on Si (111) substrates by molecular beam epitaxy. The as-grown nanowires have a density in the order of 108 cm−2, length between 3 and 3.5 μm, and diameter between 60 and 160 nm, depending on the shell growth duration. By applying a range of characterization techniques, we conclude that the In incorporation in the nanowires is on average significantly smaller than what is nominally expected based on two-dimensional growth calibrations and exhibits a gradient along the nanowire axis. On the other hand, the observation of sharp dot-like emission features in the micro-photoluminescence spectra of single nanowires in the 900–1000-nm spectral range highlights the co-existence of In-rich enclosures with In content locally exceeding 30 %.

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