Chemical Vapor Deposition Grown MoS2 for Sensing Applications.

Thumbnail Image
Kumar, Rahul
Kumar, Mahesh
Journal Title
Journal ISSN
Volume Title
Indian Institute of Technology Jodhpur
Since the isolation of graphene in 2004 by Geim and Novoselov, two-dimensional (2D) materials have been opened new avenues for developing next-generation electronics devices. 2D semiconducting MoS2 with a tunable bandgap, being the frontrunner of layered transition metal dichalcogenides (TMDCs) family has grabbed the renewed interest of the research community by providing unprecedented device performance at the atomic scale. This thesis work is focused on most vibrant gas and light sensing applications of chemical vapor deposition (CVD) grown 2D MoS2.Different nanostructures including horizontal flakes, vertical flakes and nanowires network of the MoS2 were synthesized by using the CVD process and examined their gas sensing characteristics to explore the role of different adsorption sites of MoS2. It was observed that edge sites of MoS2 exhibit higher gas adsorption compared to that of terrace sites on the basal plane of MoS2 because edge sites have a large number of dangling bonds as well as high d-orbital electron density. In this context, the MoS2 nanowires sensor with high edge sites-to-volume ratio exhibited better sensitivity with detection limit to 4.2 ppb NO2 as compared to that of horizontal and vertical aligned MoS2. Further, defect and interface engineering were simultaneously utilized for improving the gas sensing performance of the MoS2 sensor. Optimal sulfur vacancies as defects were deliberately created in vertically aligned MoS2 via thermal annealing and then, rGO nanoparticles were loaded on the sulfur vacancy containing MoS2 (Sv-MoS2) for forming rGO/Sv-MoS2 heterojunctions. P-type rGO changed the intrinsic n-type semiconducting behaviour of MoS2 into p-type via enhancing charge transfer through chemical bonding in between rGO and Sv-MoS2. As a result, p-type rGO/Sv-MoS2 sensor exhibited excellent sensitivity to NO2 with complete recovery at low temperature (50 °C) by exploiting electronic and chemical sensitization.Despite the high sensitivity to NO2 gas at room temperature, slow response and incomplete recovery restrict MoS2 usage on a commercial gas sensing platform. To address slow response/recovery kinetics, MoS2 gas sensor was tested under thermal and optical energy sources. The temperature was capable to achieve full recovery with the expense of sensitivity, however, the sensor showed enhanced sensitivity with complete recovery at room temperature under UV light irradiation. In addition, nucleation controlled one-step CVD process to synthesize MoS2–MoO3 hybrid micro flowers using vapor transport process was developed. The MoS2–MoO3 hybrid sensor showed good response to NO2 gas with complete recovery at room temperature without using any extra energy source (temperature or UV light). This research work helps to remove the microheater from commercial metal-oxide gas sensor technology due to gas detection at room temperature.In the last part of the thesis, 2D MoS2 is also explored in optoelectronics application via fabricating a photodetector using a van der Waals heterostructure of the MoS2 and rGO. This vertical out-of-plane rGO/MoS2 heterojunctions exhibited high responsitivity and detectivity in visible range wavelengths with excellent stability in air ambient as a result of the synergistic effect of enhanced photoexcited carrier density and photogating effect. The potential challenges and future perspectives in the emerging MoS2 research for sensing applications are also discussed.
Kumar, Rahul. (2020). Chemical Vapor Deposition Grown MoS2 for Sensing Applications (Doctor's thesis). Indian Institute of Technology Jodhpur, Jodhpur.