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Raman spectroscopy in transition metal dichalcogenides (TMDCs) helps determine their structural information and layer dependency. Because it is non‐destructive and fast, it is an archetypal spectroscopic technique to investigate the structure and defects in TMDCs. In our earlier study, we used a metal‐dielectric coating to enhance Raman signal of WS2 because the Raman Spectra measured from WS2 coated on the standard Si/SiO2 was significantly lower. This metal‐dielectric coating allowed access to the otherwise unavailable E12g and A1g modes of WS2. In this study, we compare the Raman spectra of WS2 on a Si/SiO2 to that of metal layers (Au [200 nm] and Al [200 nm]). A significant enhancement in the Raman signal of 2‐3L WS2 is observed for both the Au and Al coatings. Although 200 nm Au coating enhances the Raman Signal better than the 10 nm Au coating, it does not resolve the other signature vibrations other than E12g and A1g. Whereas, coating based on Al (200 nm) and Al (10 nm) both enhance the Raman signal of WS2. In the case of Al coating, we report well‐resolved vibrational modes such as the ones below <300 cm−1in a few layered WS2. This enhancement allows us to quantify the number of layers and also investigate the presence of defects. Photoluminescence measurements on 2–3L and 15L WS2 on Al (200 nm) confirm this. [ABSTRACT FROM AUTHOR]

Layered transition metal dichalcogenides (TMDCs) are considered among the next-generation materials for gas sensing. Here, we report exfoliated 2 H-WS 2 nanosheets for the fabrication of highly performing NO 2 sensors. Thermal annealing at several temperatures was performed to investigate the oxidation of WS 2. The long-term stability of 2 H-WS 2 bulk was verified. Using droplet variation method, three batches of conductometric sensors from 2 H-WS 2 dispersions were fabricated on electrical transducers, namely two layers (2 L), five layers (5 L) and ten layers (10 L) WS 2 nanosheets. These sensors were tested towards low NO 2 concentrations at different temperatures (Room Temperature (20 ℃), 50 ℃ and 100 ℃) and relative humidity (RH) levels (20%, 40%, 60%, 80% and 90% RH). 2 L-WS 2 based sensor showed the highest response at room temperature (RT). Excellent repeatability (4 cycles) towards 1 ppm NO 2 and long-term stability (more than two months) were achieved. Full selectivity towards NO 2 (1 ppm) at RT was observed over NH 3 (15 ppm), H 2 S (15 ppm), ethanol (30 ppm) and acetone (30 ppm). Our results confirm that low-power consumption devices with high sensitivity (even at high RH), long-term stability and excellent selectivity towards NO 2 were fabricated using 2 H-WS 2 nanosheets. [Display omitted] • Layered 2 H-WS 2 are considered among the next-generation materials for gas sensing. • Long-term stability of 2 H-WS 2 bulk (More than one year) was proved. • 2 H-WS 2 layers show a high response towards low concentration of NO 2 at RT. • Excellent stability at high humidity (up to 90%) and full selectivity at RT. • Energy-efficient NO 2 sensors with excellent sensing performance were fabricated. [ABSTRACT FROM AUTHOR]

Transition-metal di-chalcogenides (TMDCs) are van der Waals (vdW) layered materials with unique properties, important for the semiconductor industry. Generally, MX 2 (where M = metal and X = chalcogen atoms, such as S, Se, or Te) type TMDCs contain easily exfoliable layers, turning to essentially 2D materials. Planar structural defects, such as stacking faults (SFs), change the atomic arrangement, further improving electronic properties, making these materials useful for various applications (optoelectronic devices, spintronics, gas sensing, catalysis, energy storage, thermoelectrics, etc.) Previous reports on TMDC synthesis with specific SFs highlight the challenge in obtaining them. Even more, for a successful application, these SFs should be stable and consistent under experimental conditions. Our previous work on facile preparation of 2H-WSe 2 on W foil in an ambient-pressure chemical vapor deposition (APCVD) exhibited inherent selenium vacancies forming during the synthesis. In current research, we present detailed characterization of SFs forming during this synthesis in various 2H structures: (tungsten di-selenide) WSe 2 , (molybdenum di-selenide) MoSe 2 and (tungsten di-sulfide) WS 2. The stability of these defects as a function of post-annealing treatments is also discussed. [Display omitted] • Ambient pressure CVD–based synthesis of 2H transition metal di-chalcogenides is shown. • Presence of planar structural defects. • Detailed characterization of stacking faults (SFs) is presented. • Intrinsic stacking fault type-I is dientified. • Stability of SFs as a function of post-annealed 2H transition metal di-chalcogenides is discussed. [ABSTRACT FROM AUTHOR]