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Title: First Principle Investigation of Electronic Structure and Thermoelectric Properties of Layered Transition Metal Dichalcogenides
Authors: Khan, Fawad
Keywords: Physical Sciences
Issue Date: 2020
Publisher: University of Malakand, Malakand
Abstract: In this thesis, van der Waals heterostructures (vdW-h) consisting of MX2 (M= Zr, Hf and X= S, Se) monolayers are modeled and their physical properties are explored. The favorable stacking and stability of the modeled monolayer heterostructures are confirmed through binding energy and phonon dispersion calculations. After confirming stability, the electronic and thermoelectric properties of these compounds are explored using the first-principles calculations combined with semi-classical Boltzmann transport theory. It is found that type-II band alignment in ZrS2-HfSe2 facilitates charge separation for optoelectronics and solar energy conversion. All studied heterostructures show remarkably higher electrical conductivity than corresponding monolayers, responsible for large power factor values, especially at 1200 K. These findings indicate that the creation of vdW-h from MX2 may be promising for efficient optoelectronic and thermoelectric devices. The modeled vdW-h of monolayer TMDCs MX2 (M= Zr, Hf and X= S, Se), and their Janus materials with heterostructure are further investigated. Through binding energy the stacking and stability of modeled Janus monolayers and Janus heterostructures are confirmed. Using first-principles calculations the electronic properties of the stable materials were calculated. Mulliken electronegativity is used to find the band edge energies to determine the photocatalytic properties. It is concluded that the band edges of mono-layer ZrS2, HfS2, ZrSSe and HfSSe straddle the redox potentials of water at pH 0, vi in case of heterostructure only in ZrS2-HfS2 the band edges straddles, which make these five materials promising for photocatalytic water splitting. Even better functionalities can be achieved in transition metal diachalcogenides heterostructure than their individual monolayers. The electronic structure and band alignment controls the different device designs. For example type I band alignment is beneficial for high emission efficiency in light-emitting diodes, type II band alignment is required in light harvesting photovoltaic cells and the broken-gap alignment to boost the performance of next generation electronics like vertical tunneling field-effect transistors. For possible future electronic applications the understanding of band alignment in these tailored TMDCs heterostructure is very important. Novel vdW-h of monolayer TMDCs MX2 (M= Zr, Hf, Pt and X= S, Se) are modeled. Different stacking were theoretically investigated, from the binding energies most stable stacking were checked out for all the heterostructures through PBE and HSE06 and the electronic nature of these heterostructure are calculated. Type-II band alignment in these heterostructures was confirmed which facilitates charge separation for optoelectronics and solar energy conversion.
Gov't Doc #: 20955
Appears in Collections:PhD Thesis of All Public / Private Sector Universities / DAIs.

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