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

Highly sensitive SERS substrates based on MoS2-Au nanocomposites for detection of hazardous dyes and infectious bacteria

Surface functionalization of two-dimensional materials with noble metal nanoparticles has unlocked new possibilities in Raman-based sensing by leveraging both chemical and electromagnetic enhancement effects. In this work, the optimized morphology of nanosheets of MoS2 adorned with Au NPs has been utilized for sensing of hazardous molecules Rhodamine B, N719 dye, and S. aureus, E. coli bacteria samples. MoS2 nanosheets were prepared by facile hydrothermal method and Au NPs were decorated onto the nanosheets' surface by reducing chloroauric acid solution. The Au nanoparticles concentration was optimized by altering the concentration of chloroauric acid solution. Rhodamine B and N719 dyes are known to be toxic and carcinogenic, if inhaled or indigested, whereas S. aureus and E. coli bacteria can cause skin infections, sepsis, food poisoning and severe diarrhoea. Therefore, detecting even trace concentrations of these molecules in the environment is critically important. The prepared SERS substrate successfully detects the Rhodamine B and N719 dyes up to 10-15 and 10-9 M concentrations. The highest enhancement factor obtained for Rhodamine B and N719 dyes are 5.2 × 107 and 2.1 × 107, respectively. The nanocomposite SERS substrate exhibits excellent signal uniformity and reproducibility with relative standard deviation value of around 10 %. Further, the nanocomposite substrate was employed for the sensing of infectious S. aureus and E. coli bacteria down to 102 cfu/mL. A charge transfer mechanism is also proposed between N719 dye and MoS2, along with the role of Au NPs, which produces the synergistic enhancement of the SERS signal.

Thin Films of Tungsten Disulfide Grown by Sulfurization of Sputtered Metal for Ultra-Low Detection of Nitrogen Dioxide Gas

Tungsten disulfide (WS2) is attractive for the development of chemiresistive sensors due to its favorable band gap, as well as its mechanical strength and chemical stability. In this work, we elaborate a procedure for the synthesis of thin films consisting of vertically and/or horizontally oriented WS2 nanoparticles by sulfurizing nanometer-thick tungsten layers deposited on oxidized silicon substrates using magnetron sputtering. According to X-ray photoelectron spectroscopy and Raman scattering data, WS2 films grown in an H2-containing atmosphere at 1000 °C are almost free of tungsten oxide. The WS2 film's thickness is controlled by varying the tungsten sputtering duration from 10 to 90 s. The highest response to nitrogen dioxide (NO2) at room temperature was demonstrated by the film obtained using a tungsten layer sputtered for 30 s. The increased sensitivity is attributed to the high surface-to-volume ratio provided by the horizontal and vertical orientation of the small WS2 nanoparticles. Based on density functional calculations, we conclude that the small in-plane size of WS2 provides many high-energy sites for NO2 adsorption, which leads to greater charge transfer in the sensor. The detection limit of NO2 calculated for the best sensor (WS2-30s) is 15 ppb at room temperature and 8 ppb at 125 °C. The sensor can operate in a humid environment and is significantly less sensitive to NH3 and a mixture of H2, CO, and CO2 gases.

High-Performance Visible-to-SWIR Photodetector Based on the Layered WS2 Heterojunction with Light-Trapping Pyramidal Black Germanium

This study presents a layered transition metal dichalcogenide/black germanium (b-Ge) heterojunction photodetector that exhibits superior performance across a broad spectrum of wavelengths spanning from visible (vis) to shortwave infrared (SWIR). The photodetector includes a thin layer of b-Ge, which is created by wet etching of germanium (Ge) wafer to form submicrometer pyramidal structures. On top of this b-Ge layer, the WS2 thin film is deposited using pulsed laser deposition. In comparison to conventional germanium, b-Ge absorbs about 25% more light between 850 and 1750 nm wavelengths. The WS2/b-Ge photodetector has a peak photoresponsivity of 0.65 A/W, which is more than twice the photoresponsivity of the WS2/Ge photodetector at 1540 nm. Additionally, it shows better responsivity and response speed compared with other similar state-of-the-art photodetectors. Such an improvement in the performance of the device is credited to the light-trapping effect enabled by the germanium pyramids. Theoretical simulations employing the finite-difference time-domain technique help validate the concept. This novel photodetector holds promise for efficient detection of light across the vis to SWIR spectrum.

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.

Modification of WS2thin film properties using high dose gamma irradiation

The tunability of the transition metal dichalcogenide properties has gained attention from numerous researchers due to their wide application in various fields including quantum technology. In the present work, WS2has been deposited on fluorine doped tin oxide substrate and its properties have been studied systematically. These samples were irradiated using gamma radiation for various doses, and the effect on structural, morphological, optical and electrical properties has been reported. The crystallinity of the material is observed to be decreased, and the results are well supported by x-ray diffraction, Raman spectroscopy techniques. The increase in grain boundaries has been supported by the agglomeration observed in the scanning electron microscopy micrographs. The XPS results of WS2after gamma irradiation show evolution of oxygen, carbon, C=O, W-O and SO4-2peaks, confirming the addition of impurities and formation of point defect. The gamma irradiation creates point defects, and their density increases considerably with increasing gamma dosage. These defects crucially altered the structural, optical and electrical properties of the material. The reduction in the optical band gap with increased gamma irradiation is evident from the absorption spectra and respective Tauc plots. TheI-Vgraphs show a 1000-fold increase in the saturation current after 100 kGy gamma irradiation dose. This work has explored the gamma irradiation effect on the WS2and suggests substantial modification in the material and enhancement in electrical properties.

Highly Selective Room Temperature Detection of NH3 and NOx Using Oxygen-Deficient W18O49-Supported WS2 Heterojunctions

In this paper, we reported the controlled synthesis of tungsten disulfide/reduced tungsten oxide (WS₂/W₁₈O₄₉) heterojunctions for highly efficient room temperature NO_x and ammonia (NH₃) sensors. X-ray diffraction analysis revealed the formation of the oxygen-deficient W₁₈O₄₉ phase along with WS₂. Field-emission scanning electron microscopy and transmission electron microscopy displayed the formation of WS₂ flakes over W₁₈O₄₉ nanorods. X-ray photoelectron spectroscopy showed the presence of tungsten in W⁴⁺, W⁵⁺, and W⁶⁺ oxidation states corresponding to WS₂ and W₁₈O₄₉, respectively. The WS₂/W₁₈O₄₉ heterojunction sensor exhibited sub-ppm level sensitivity to NO_x and NH₃ at room temperature. The heterojunction sensor detected 0.6 ppm NO_x and 0.5 ppm NH₃, with a corresponding response of 7.1 and 3.8%, respectively. The limit of detection of the sensor was calculated to be 0.05 and 0.17 ppm for NH₃ and NO_x, respectively. The cyclic stability test showed that the sensor exhibited high stability even after 24 cycles for the detection of NH₃ and 14 cycles for NO_x. Compared to pristine WO₃ and WS₂, the WS₂/W₁₈O₄₉ heterojunction showed high selectivity toward NO_x and NH₃. The results could be useful for the development of room temperature NO_x and NH₃ sensors.