Structural and Optical Characterization of Pulsed LaserDeposited MoS2 and WS2 Layered Thin Films and Quantum Dots

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The aim of the thesis is to synthesize monolayer to multilayered MoS2 and WS2 thin films and quantum dots (QDs) by pulsed laser deposition and pulsed laser ablation in liquid and to use the as-synthesized thin films and quantum dots sample as an efficient catalyst for hydrogen generation. Mono- and a few layered MoS2 films were deposited with finely controlled by applying different numbers of laser pulses. A similar experiment was also performed to synthesize monolayer to multilayered WS2 thin films where the laser fluence was varied and other deposition parameters were kept constant. An efficient way to deposit 1T and 2H mixed-phase mono and a few layered MoS2 thin films in a very short span of time of 20 sec was achieved in a single step bottom-up growth process. The films showed a mixed-phase structure while the 2H to 1T phase ratio increased from 66 to 84% with an increase in the deposition temperature from 400 to 720 °C. An experimental investigation was carried out to identify the scaling behavior as well as the growth mechanism of 2D MoS2 thin films grown by pulsed laser deposition at different deposition time durations, using atomic force microscopy images. The growth of MoS2 thin films followed intrinsic anomalous scaling behavior. Similar work was also extended to WS2 films deposited on to corning glass and SiO2/Si substrate as well. A significantly large reverse saturation absorption and positive nonlinear refraction response were observed in all the MoS2 and WS2 films deposited at various argon pressure, as measured by the open and closed aperture Z-scan experiment under He-Ne laser at 632.8 nm. The anomalously high nonlinear optical response of the film was attributed to the continuous-wave laser-induced thermal nonlinearity dominance over optical nonlinearity. Optical limiting was also observed in the WS2 thin films. Along with layered thin films, MoS2 and WS2 QDs were also synthesized by multilevel photo-exfoliation of solid MoS2 target using pulsed laser ablation in distilled water. As an application, MoS2 and WS2 films of various thicknesses and MoS2 QDs of various sizes were used as a catalyst for hydrogen evolution reaction
Supervisor: Ashwini Kumar Sharma