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

Abstract

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.

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

“…Also, the positive charge transfer implies the charge transferred from WS 2 to the analyte and the negative charge transfer indicates the charge flow from the analyte molecule to WS 2 . It is evident from the above DFT calculations ,, that NO 2 is highly reactive to WS 2 and NH 3 is the next most reactive to WS 2 which is consistent with our enhanced response to NO x and NH 3 , as presented in Figure .…”

γ-Ray-Induced Surface-Charge Redistribution and Change of the Surface Morphology in Monolayer WS2

“…These peaks characterize the 6+ oxidation state of the tungsten atom, which can be attributed to formation of the W−O bond. 35 As discussed before, γ radiation tends to induce preferential sputtering of sulfur atoms by colliding the film with highlyenergy 60 Co photons via the photoelectric effect and Compton scattering. As a result, highly reactive dangling bonds are created at the vacant sites, making them favorable as binding centers for species present in the surrounding atmosphere.…”

“…In addition, Figure 8a depicts a shift in the peak position of the S 2p spectra toward lower binding energy, which further substantiates p-type doping. 7…”

“…6 Bisht et al studied the electronic charge-transfer mechanism in a WS 2 film as a function of its growth orientation. 7 Obviously, modulation of the electronic properties will affect the performance of a device, and studies of the strategies adopted for achieving the required changes hold immense significance.…”

“…Bai et al tuned the tunnel current and conduction charge polarity (from n-type to bipolar) in a graphene–WS 2 –graphene field-effect tunnelling transistor by the application of an electric field . Bisht et al studied the electronic charge-transfer mechanism in a WS 2 film as a function of its growth orientation . Obviously, modulation of the electronic properties will affect the performance of a device, and studies of the strategies adopted for achieving the required changes hold immense significance.…”