Low Dimensional Transition Metal Dichalcogenides for Nano Device applications.

Abstract

Two-Dimensional (2D) Transition Metal Dichalcogenides (2D-TMDs) materials have huge potential to next generation nano-electronic device application. The 2D-TMDs possesses the exciting electronic properties from metal to semiconductor to insulator to even topological insulator [1]. This work is motivated by experimentally realized MoSSe monolayer. This is the new category of TMDs, renamed as Janus monolayer. A stability and experimental existence of Janus monolayer is predicted through phonon band structure. External and inbuilt strain leads to change in the electronic properties. Interestingly Janus WSSe monolayer showed the electronic transition against biaxial tensile and compression strain. The electronic properties of WSSe monolayer are more sensitive to tensile strain compare to compression strain. Moreover, the point defects mediated electronic properties of Janus (Mo/W)SSe monolayers also investigated. The vacancy defects showed the non-magnetic nature while some of the antisite defects ensure the magnetic behavior. The antisite and vacancy defects based on chalcogen elements are most stable among other defects. Adsorption of H2S, NH3, NO2, and NO toxic gases are investigated on pristine and defects included Janus MoSSe and WSSe monolayers and found the better gas sensing properties compare to conventional TMDs like MoS2, MoSe2, WS2, WSe2 monolayers. The selenium surface of Janus MoSSe and WSSe monolayers are more active compare to sulfur surface for gas sensing application. Moreover, the chalcogen vacancies work as active site for adsorption of gas molecule which lead to enhancement in the gas sensing properties. From the theoretical study analysis, we have projected that WSSe and MoSSe monolayer may be the promising materials for NO2 and NO gas sensing application. We have further extended our work to explore the electronic and optical properties of WS2 and WSe2 monolayers. The reasonable band gap and high absorption coefficient makes them suitable for solar cell application. In addition, the investigated optoelectronic properties of Janus WSSe and WSeTe confirm the suitability as buffer layer for W(S/Se)2 absorber based solar cell. Performance of single junction solar cell is optimized by varying the thickness, carrier concentration, defect density and work function of absorber and buffer layer. The effect of interface defect density (n/p junction) on the performance of solar cell investigated. The maximum efficiencies of about ~ 17.73% and 18.87% are noticed for optimized single-junction WSSe/WS2 and WSSe/WSe2 solar cells. Thermoelectric properties of Janus WSSe monolayer also investigated using the density functional theory and semi-classical Boltzmann theory. Strain is the most efficient approach to tailor the electrical and thermal properties. The tensile strain reduced the lattice thermal conductivity from 25.37 (0% strain) to 9.90 Wm-1K-1 (8% strain). Biaxial tensile and compression strain assistance to tailor the valley degeneracy, which led to the improvement in the electrical conductivity and finally the figure of merit enhanced. The figure of merit is improved from 0.90 (0.74) to 1.25 (1.08) under biaxial strain for n(p)-type carriers. Thus, the thesis work carried out on Janus TMDs monolayers will provide a framework for the experimentalist to design the nano-devices for various applications such as sensors, ultrathin solar cells, and thermoelectric devices.