The optical and electronic properties of tungsten disulfide monolayers (WS2) have been extensively studied in the last few years, yet growth techniques for WS2 remain behind other transition metal dichalcogenides (TMDCs) such as MoS2. Here we demonstrate chemical vapor deposition (CVD) growth of continuous monolayer WS2 films on mm2 scales and elucidate effects related to hydrogen (H2) gas concentration during growth. WS2 crystals were grown by reduction and sulfurization of WO3 using H2 gas and sulfur evaporated from solid sulfur powder. Several different growth formations (in-plane shapes) were observed depending on the concentration of H2. Characterization using atomic force microscopy (AFM) and scanning electron microscopy (SEM) revealed etching of the SiO2 substrate at low concentrations of H2 and in the presence of an Ar carrier gas. We attribute this to insufficient reduction of WO3 during growth. High H2 concentrations resulted in etching of the grown WS2 crystals after growth. The two dimensional X-ray diffraction (2D XRD) pattern demonstrates that the monolayer WS2 was grown with the (004) plane normal to the substrate, showing that the WS2 conforms to the growth substrate.

Monolayers of transition-metal dichalcogenides such as WSe 2 have become increasingly attractive due to their potential in electrical and optical applications. Because the properties of these 2D systems are known to be affected by their surroundings, we report how the choice of the substrate material affects the optical properties of monolayer WSe 2 . To accomplish this study, pump-density-dependent micro-photoluminescence measurements are performed with time-integrating and time-resolving acquisition techniques. Spectral information and power-dependent mode intensities are compared at 290K and 10K for exfoliated WSe 2 on SiO 2 /Si, sapphire (Al 2 O 3 ), hBN/Si 3 N 4 /Si, and MgF 2 , indicating substrate-dependent appearance and strength of exciton, trion, and biexciton modes. Additionally, one CVD-grown WSe 2 monolayer on sapphire is included in this study for direct comparison with its exfoliated counterpart. Time-resolved micro-photoluminescence shows how radiative decay times strongly differ for different substrate materials. Our data indicates exciton-exciton annihilation as a shortening mechanism at room temperature, and subtle trends in the decay rates in correlation to the dielectric environment at cryogenic temperatures. On the measureable time scales, trends are also related to the extent of the respective 2D-excitonic modes' appearance. This result highlights the importance of further detailed characterization of exciton features in 2D materials, particularly with respect to the choice of substrate.

Transition Metal Dichalcogenides (TMDs) are one of the most studied two-dimensional materials in the last 5-10 years due to their extremely interesting layer dependent properties. Despite the presence of vast research work on TMDs, the complex relation between the electro-chemical and physical properties make them the subject of further research. Our main objective is to provide a better insight into the electronic structure of TMDs. This will help us better understand the stability of the bilayer post growth homo/hetero products based on the various edge-termination, and different stacking of the two layers. In this regard, two Tungsten (W) based non-periodic chalcogenide flakes (sulfides and selenides) were considered. An in-depth analysis of their different edge termination and stacking arrangement was performed via Density Functional Theory method using VASP software. Our finding indicates the preference of chalcogenide (c-) terminated structures over the metal (m-) terminated structures for both homo and heterobilayers, and thus strongly suggests the nonexistence of the m-terminated TMDs bilayer products. generation 2D materials, Transition Metal Dichalcogenides (TMDs) are one of the most versatile materials owing to their wide variety of physicochemical, electrical, and mechanical properties that lie in between the semiconductor and metal.3-5 Moreover, the graphite-like features of the bulk-TMD crystals and possible graphene-like exfoliation of monolayer-TMDs, as well as the semi-honeycomb features, make them an attractive candidate. TMDs have the general formula MX2 (M = Mo and W; X = S, Se, and Te). The crystal structure of TMDs is three atoms thin, where one metal atom is sandwiched in between two chalcogen atoms (X-M-X) via strong covalent bonds (see Fig. 1b). These monolayer 2D TMDs are attached through weak van der Waals forces in the multilayered 3D crystal structures. Among various 2D TMDs, Molybdenum/Tungsten Sulfide/Selenides (Mo/W S2/Se2)6 are known to be stable at ambient conditions and thus are known to be useful for energy-storage, sensing, electronic and photonic devices applications.Among the monolayer (ML) TMDs, the direct band gap of MoS2, WS2, and WSe2 fall in the visible to the near-IR range and are therefore well explored for their significant electronic, optical and photocatalytic properties.3,7-11 ML MoS2 is well studied12-18 for its transistor applications, excellent carrier mobility, as well as its potential applications in spintronics, valleytronics, bio, and gas sensing applications. On the other hand, WS2 possesses similar crystal structure as of MoS2 and, shows higher quantum efficiency, wider valence band maximum splitting and, lower effective mass of the carrier due to the presence of heavier W atom as compared to the Mo atom.19-23However, the synthesis of these ML 2D materials is the first step towards its practical application.The initial step begins with the exfoliation of ML WS2 from the bulk crystal structures. Liquid exfoliation technique by Coleman et.al24 and the mecha...

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