No abstract

Two-dimensional transition metal dichalcogenides offer exciting opportunities to tailor their electrical and optoelectronic properties, which provide them with a multitude of applications, following which their large-area synthesis holds high significance. Herein, we demonstrate NaCl-assisted centimeter-scale (1.5 × 1 cm 2 ) growth of monolayer WS 2 using an atmospheric-pressure chemical vapor deposition technique. We attempt to explicate the growth mechanism with the Volmer− Weber, Stranski−Krastanov, and Frank−van der Merwe modes by incorporating the importance of NaCl as a growth promoter and other parameters like the quantity of sulfur, temperature, gas flow rate, and hold time. The parameters optimized for large-area monolayer growth were found at a NaCl to WO 3 w/w ratio of 1:2 and a sulfur to WO 3 w/w ratio of 3:1, with a 1 min hold time at 820 °C in a 120 sccm argon gas flow. The optical microscope images along with the corresponding Raman and photoluminescence spectra were employed to examine the growth of the WS 2 film at different sets of synthesis parameters. The photodetector fabricated on the as-grown film showed a high responsivity and specific detectivity of 4.27 mA/W and 1.27 × 10 10 Jones, respectively, in the visible region and 261 mA/W and 7.72 × 10 11 Jones, respectively, in the ultraviolet (UV) region. These results support its application in UV−visible photodetection.

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.

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.

The appealing properties of tunable direct wide bandgap, high-temperature robustness and chemical hardness, make AlxGa1-xN a promising candidate for fabricating robust solar-blind photodetectors (PDs). In this work, we have utilized the optical phenomenon of localized surface plasmon resonance (LSPR) in metal nanoparticles (NPs) to significantly enhance the performance of solar-blind Al0.4Ga0.6N metal-semiconductor-metal (MSM) PDs that exhibit high-temperature robustness. We demonstrate that the presence of palladium (Pd) NPs leads to a remarkable enhancement by nearly 600, 300, and 462%, respectively, in the photo-to-dark current ratio (PDCR), responsivity, and specific detectivity of the Al0.4Ga0.6N PD at the wavelength of 280 nm. Using the optical power density of only 32 μWcm−2 at −10 V, maximum values of ~3×103,2.7 AW−1, and 2.4×1013Jones are found for the PDCR, responsivity and specific detectivity, respectively. The experimental observations are supported by finite difference time domain (FDTD) simulations, which clearly indicate the presence of LSPR in Pd NPs decorated on the surface of Al0.4Ga0.6N. The mechanism behind the enhancement is investigated in detail, and is ascribed to the LSPR induced effects, namely, improved optical absorption, enhanced local electric field and LSPR sensitization effect. Moreover, the PD exhibits a stable operation up to 400 K, thereby exhibiting the high-temperature robustness desirable for commercial applications.