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dc.contributor.authorRehman, Gul-
dc.description.abstractTwo-dimensional (2D) materials have attracted enormous interest for both, modern technological application and fundamental academic research, due to their unique electrical and optical properties. Beyond graphene, semiconductor 2D layered materials such as transition metal dichalcogenides (TMDCs), hexagonal-boron nitride (h-BN), MXenes, phosphorenes, etc. are recently emerged as potential candidates for electronic, optoelectronic and light harvesting applications. Stacking 2D materials form van der Waals heterostructures (vdWHs) can provide endless opportunities in high tech applications of the constituent layers. A large number of promising electronic and optoelectronic devices based on vdWHs have been successfully fabricated, e.g. transistors, ultra-fast photodetectors, solar cells, light emitting diodes and much more. The aim of this research work is to achieve type-II band alignment by creating freestanding vdWHs of 2D materials, which leads to efficient charge separation for light detection and harvesting. It is well established that excitons govern the optical transition in 2D isolated monolayers and vdWHs semiconductors. Furthermore, the performance of vdWHs can be engineered with the application of moderate strain (tensile/compressive), i.e. transitions among type-I-to-typeII-to-type-III band alignments and metal-to-semiconductor-to-metal, and band gap modulation. In this work first principle methodology is employed to systematically investigate structural, electronic, optical, and photocatalytic properties of MSe2/Zr2CT2 (M= Mo, W and T=O, F), MX2/Hf2CO2 (X= S, Se) and BlueP/Sc2CX2 (X = O, F, OH) vdWHs, as well as their corresponding monolayers using HSE06 hybrid ii functional. The stability of all the considered vdWHs are confirmed by means of binding energies and absence of imaginary frequencies in phonon dispersion. Mo(W)Se2/Zr2CO2 and MX2/Hf2CO2 vdWHs are found to be indirect band gap semiconductors, whereas MoSe2/Zr2CF2 and WSe2/Zr2CF2 are metallic in nature. Type-II band alignment is observed in WSe2/Zr2CO2, MoS2/Hf2CO2 and WS(Se)2/Hf2CO2, while in case of MoSe2/Zr2CO2 a moderate 2% tensile and compressive strains can easily transform it from type-I into type-II band alignment. The formation of heterostructure enhanced the optical absorption in visible light region as compared to their parent monolayer, whereas blue and shift are observed in the absorption spectra under compressive and tensile strains in heterostructures. Photocatalytic properties show that Mo(W)Se2/Zr2CO2, MoS2/Hf2CO2 and WS(Se)2/Hf2CO2 heterostructures can oxidize H22O/O2 to O2, whereas MoSe2/Hf2CO2 is promising candidate for photocatalytic water splitting reaction. BlueP/Sc2CX2 (X = O, F, OH) vdWHs are found to be indirect semiconductors with type-II band alignment. A small charge transfer from BlueP to Sc2CF2 and from Sc2CO2 (Sc2C(OH)2) to BlueP causing its p- and n-doped respectively. The formation of heterostructure also enhanced the optical absorption in visible light region as compared to their parent monolayer, particularly in BlueP/Sc2CF2 and BlueP/Sc2C(OH)2 heterostructures. This study reveals that BlueP/Sc2CX2 heterostructures are ideal candidates for novel optical/electronic device applications. Heterostructures show excellent light absorption efficiencies (70-80%) from infra-red to ultraviolet spectrum of light. The findings of this thesis work suggest that MSe2/Zr2CT2 (M= Mo, W and T=O, F), MX2/Hf2CO2 (X= S, Se) and BlueP/Sc2CX2 (X = O, F, OH) freestanding vdWHs are promising candidates for nanoelectronic, optoelectronics, photocatalysis and photovoltaic device applications.en_US
dc.description.sponsorshipHigher Education Commission Pakistanen_US
dc.publisherUniversity of Malakand, Malakanden_US
dc.subjectPhysical Sciencesen_US
dc.titleFreestanding Van Der Waals Heterostructures of Two Dimensional Materialsen_US
Appears in Collections:PhD Thesis of All Public / Private Sector Universities / DAIs.

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