Gas Sensing Properties of PLD Grown 2D SnS Film: Effect of Film Thickness, Metal Nanoparticle Decoration, and In Situ KPFM Investigation.

This study employs novel growth methodologies and surface sensitization with metal nanoparticles to enhance and manipulate gas sensing behavior of two-dimensional (2D)SnS film. Growth of SnS films is optimized by varying substrate temperature and laser pulses during pulsed laser deposition (PLD). Thereafter, palladium (Pd), gold (Au), and silver (Ag) nanoparticles are decorated on as-grown film using gas-phase synthesis techniques. X-ray diffraction (XRD), Raman spectroscopy, and Field-emission scanning electron microscopy (FESEM) elucidate the growth evolution of SnS and the effect of nanoparticle decoration. X-ray photoelectron spectroscopy (XPS) analyses the chemical state and composition. Pristine SnS, Ag, and Au decorated SnS films are sensitive and selective toward NO2 at room temperature (RT). Ag nanoparticle increases the response of pristine SnS from 48 to 138% toward 2 ppm NO2, which indicates electronic and chemical sensitization effect of Ag. Pd decoration on SnS tunes its selectivity toward H2 gas with a response of 55% toward 70 ppm H2 and limit of detection (LOD) < 1 ppm. In situ Kelvin probe force microscopy (KPFM) maps the work function changes, revealing catalytic effect of Ag toward NO2 in Ag-decorated SnS and direct charge transfer between Pd and SnS during H2 exposure in Pd-decorated SnS.

Synthesis of a large area ReS2 thin film by CVD for in-depth investigation of resistive switching: effects of metal electrodes, channel width and noise behaviour.

The anisotropic crystal structure and layer independent electrical and optical properties of ReS2 make it unique among other two-dimensional materials (2DMs), emphasizing a special need for its synthesis. This work discusses the synthesis and in-depth characterization of a 1 × 1 cm2 large and few layered ReS2 film. Vibrational modes and excitonic peaks observed from the Raman and photoluminescence (PL) spectra corroborated the formation of a ReS2 film with a 1.26 eV bandgap. High resolution transmission electron microscopy (HRTEM) images and selected area electron diffraction (SAED) patterns inferred the polycrystalline nature of the film, while cross-sectional field emission scanning electron microscopy (FESEM) indicated planar growth with ∼10 nm thickness. The chemical composition of the film analysed through X-ray photoelectron spectroscopy (XPS) indicated the formation of a ReS2 film with a Re : S atomic ratio of 1 : 1.75, indicating a small amount of non-stoichiometric RexSy. Following the basic characterization studies, the ReS2 film was tested for resistive switching (RS) device application in which the effects of different metal electrodes (Pt/Au and Ag/Au) and different channel widths (200, 100, and 50 µm) were studied. The highest memory window equal to 108 was obtained for the Ag/Au electrode while Pt/Au showed a memory window of 102. RS for the former was ascribed to the formation of a conducting filament (CF) because of the migration of Ag+ ions, while defect mediated charge carrier transport led to switching in the Pt/Au electrode. Furthermore, the RHRS/RLRS ratio achieved in this work (108) is also of the highest magnitude reported thus far. Furthermore, a comparison of devices with Ag/Au electrodes but with different channel widths (50, 100 and 200 µm) gave insightful results on the existence of multiple resistance states, device endurance and retention. An inverse relationship between the retention time and the device's channel width was observed, where the device with a 50 µm channel width showed a retention time of 48 hours, and the one with a 200 µm width showed stability only up to 3000 s. Furthermore, low frequency noise measurements were performed to understand the effect of defects in the low resistance state (LRS) and the high resistance state (HRS). The HRS exhibited Lorentzian noise behaviour while the LRS exhibited Lorentzian only at low current bias which converged to 1/f noise at higher current bias.

Tailoring the Vertical and Planar Growth of 2D WS2 Thin Films Using Pulsed Laser Deposition for Enhanced Gas Sensing Properties.

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.

Localized surface plasmon resonance-enhanced solar-blind Al0.4Ga0.6N MSM photodetectors exhibiting high-temperature robustness.

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 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 32µW cm-2at -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 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.

Modifying the Thermoelectric Transport of Sb2Te3 Thin Films via the Carrier Filtering Effect by Incorporating Size-Selected Gold Nanoparticles.

Hot energy carrier filtering as a means to improve the thermoelectric (TE) property in Sb2Te3 thin film samples having size-selected Au nanoparticles (NPs) is investigated in the present study. Nonagglomerated Au NPs with a very narrow size distribution grown by an integrated gas-phase synthesis setup are incorporated into the Sb2Te3 thin film synthesized by RF magnetron sputtering. TE properties have been investigated as a function of size-selected Au NP concentrations and compared with that of a nanocomposite sample having non-size-selected Au NPs. An increase in the Seebeck coefficient and power factor, along with a slight decrease in electrical conductivity, is observed for samples with a NP size of minimum variance. Further, the Kelvin probe force microscopy and conducting atomic force microscopy techniques were employed to understand the nature of the interface and charge transport across the Sb2Te3 matrix and Au NPs. The study provides an opportunity to modulate the TE properties in Sb2Te3 thin films by constructing a metal-semiconductor heterostructure through controlling the concentration and randomness to achieve a high TE performance.