Two-dimensional transition-metal-dichalcogenide semiconductors have emerged as promising candidates for optoelectronic devices with unprecedented properties and ultra-compact performances.However atomically thin materials are highly sensitive to surrounding dielectric media, which imposes severe limitations to their practical applicability. Hence for their suitable integration into devices, the development of reliable encapsulation procedures that preserve their physical properties are required.Here, the excitonic photoluminescence of monolayers is assessed, at room temperature and 10 K, on mechanically exfoliated WSe2 monolayer flakes encapsulated with SiOx and AlxOy layers employing chemical and physical deposition techniques. Conformal flakes coating on untreated -nonfunctionalized -flakes is successfully demonstrated by all the techniques except for atomic layer deposition, where a cluster-like oxide coating is observed. No significant compositional or strain state changes in the flakes are detected upon encapsulation by any of the techniques. Remarkably, our results evidence that the flakes' optical emission is strongly influenced by the quality of the encapsulating oxide -stoichiometry -. When the encapsulation is carried out with slightly sub-stoichiometric oxides two 2 remarkable phenomena are observed. First, there is a clear electrical doping of the monolayers that is revealed through a dominant trion -charged exciton -room-temperature photoluminescence. Second, a strong decrease of the monolayers optical emission is measured attributed to non-radiative recombination processes and/or carriers transfer from the flake to the oxide. Power-and temperaturedependent photoluminescence measurements further confirm that stoichiometric oxides obtained by physical deposition lead to a successful encapsulation, opening a promising route for the development of integrated two-dimensional devices.Two-dimensional (2D) semiconductor transition-metal-dichalcogenides (TMDCs) present unique physical properties, such as a native sizeable bandgap, relatively large in-plane carrier mobility up to 250 cm 2 /Vs, valley-dependent optoelectronics, strong light-matter interaction, and indirect-to-direct bandgap transition in monolayer crystals, which make them ideal candidates for ultra-compact optoelectronic, photonic and electronic applications [1][2][3][4][5] . Owing to strong spatial confinement of carriers and reduced dielectric screening of Coulomb interactions, electron and holes in monolayer TMDCs are tightly bound, thus forming exciton and multi-exciton complexes with binding energies up to 0.5-1 eV, more than one order of magnitude larger than in conventional III-V 2D quantum well nanostructures 6 .The simplest excitonic specie is the neutral exciton (X 0 ), which consists of an electron-hole pair bounded by Coulomb interaction. Singly charged excitons, or trions (X -), quasi-particles consisting of two electrons (holes) and a hole (electron), have been reported 7,8 with binding energies up to 20-40 meV relative to X 0 ...

The current work investigates the microstructure and property relations of molybdenum oxide thin films synthesized by reactive dc magnetron sputtering in an industrial-scale sputter deposition plant using a rotatable molybdenum target with varying oxygen/argon gas flow ratio. The evolution of microstructure and chemical bonds as a function of oxygen partial pressure was studied by x-ray diffraction, Raman spectroscopy, and x-ray photoelectron spectroscopy. With oxygen partial pressure raising from 0% to 100%, the film growth rate decreased from 350 to 50 nm/min, while the oxygen content within the films increased up to 75 at. %. The films were dominated by MoO2, polymorphs of Mo4O11 and Mo9O26, and MoO3 phases. The electrical properties changed from electrically conductive to insulating with increasing oxygen partial pressure. The optical properties are versatile, e.g., transmittance values up to 80%, absorbance values between 50% and 80% and reflectance values up to 55%, depending on the oxygen content. In general, it can be concluded that microstructure and properties of molybdenum oxide thin films can be adjusted by varying the oxygen/argon gas flow ratio and might thus enable their use in a wide range of optical and electronic applications.

The effects of wet chemical processing employed in device fabrication standards are studied on molybdenum oxide (MoOx) ultra-thin films. We have combined x-ray photoelectron spectroscopy (XPS), angle resolved XPS and x-ray reflectivity to gain insight into the changes in composition, structure and electronic states upon treatment of films with different initial stoichiometry prepared by reactive sputtering. Our results show significant reduction effects associated with the development of gap states in MoOx, as well as changes in the composition and structure of the films, systematically correlated with the initial oxidation state of Mo.

No abstract