In this study, pulsed laser deposition has been utilized for the controllable synthesis of WS2 thin films with growth orientation ranging from vertically to horizontally aligned layers, and the effect of growth parameters has been investigated. The growth of thin films on SiO2 substrates at three different pressures (30, 50, and 70 mTorr) and three different temperatures (400, 500, and 600 °C) has been studied. Detailed characterizations carried out on the as-grown layers clearly show the formation of the 2H-WS2 phase and its morphological evolution with deposition conditions. Atomic force microscopy and cross-sectional transmission electron microscopy have been used to deduce the growth mechanism of the vertical and planar films with different deposition parameters. The samples grown with a combination of lower temperatures and higher pressures exhibit a vertical flake-like growth with a flake thickness of ∼2–5 nm. However, at higher temperatures and lower pressures, the film growth is observed to be rather planar. The gas sensing parameters and the underlying mechanism have been observed to be quite different for vertically and horizontally grown layers. The vertical layers showed a selective response toward NO2 gas at room temperature (RT) with a limit of detection less than 50 ppb. In comparison, a very subdued and poor gas sensing response was recorded for the planar film at RT. A large specific area and abundance of active edge sites along with the flat basal plane present in the vertically grown layers seem to be responsible for efficient gas sensing toward NO2.

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This work reports the effect of γ radiation on the surface morphology and surface-charge redistribution in a monolayer WS2 film by comparing the film before and after irradiation (1, 50, 100, 200, and 400 kGy dosage). The surface morphology was monitored through optical microscopy and atomic force microscopy. Raman and photoluminescence spectroscopy were used to study the effect on phonon modes and excitonic properties. The results indicated p-type doping and increased trion-to-exciton transitions. Because of the high energy and lower atomic mass of sulfur atoms, γ irradiation induces sulfur vacancies, which creates dangling bonds at vacant sites. The adsorption of oxygen at these reactive sites results in a charge-transfer mechanism, in which electrons get transferred from the WS2 film to the adsorbed oxygen, which forms oxides and induces p-type doping. An increase in the work function of the film from 4.50 eV for a pristine film to 4.82 eV for an irradiated film (at 200 kGy) was calculated from Kelvin probe force microscopy, which indicates shifting of the Fermi level toward the valence band (VB) maxima. Further, VB spectra deduced from X-ray photoelectron spectroscopy showed a red shift of 0.17 eV after irradiation and confirms p-type doping.

Efficient thermoelectric (TE) conversion of waste heat to usable energy is a holy grail promising to address major societal issues related to energy crisis and global heat management. For these to be economical, synthesis of a solid‐state material with a high figure‐of‐merit (ZT) values is the key, with characterization methods quantifying TE and heat transport properties being indispensable for guiding the development of such materials. In the present study, a large enhancement of the TE power factor in Sb2Te3/MoS2 multilayer structures is reported. A new approach is used to simultaneously experimentally determine the values of in‐plane (kxy) and out‐of‐pane (kz) thermal conductivities for multilayer samples with characteristic layer thickness of few nanometres, essential for the quantification of the ZT, the key parameter for the TE material. Combining simultaneous enhancement in the value of in‐plane power factor (to (4.9 ± 0.4) × mWm−1 K−2) and reduction of the in‐plane value of the thermal conductivity (to 0.7 ± 0.1 Wm−1 K−1) for Sb2Te3/MoS2 multilayer sample led to high values of ZT of 2.08 ± 0.37 at room temperature. The present study, therefore, sets the foundation for a new methodology of exploiting the properties of 2D/3D interfaces for the development of novel fully viable thermoelectric materials.

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