UCLA

Transition-metal dichalcogenides (TMDCs) with the formula MX2 (M: transition-metal from groups 4-6, X: S, Se, and Te) are a class of two-dimensional (2D) layered van der Waals (vdW) materials consisting of covalently bonded X-M-X sheets coupled to each other by vdW forces. TMDCs demonstrate crystallinity, composition, and thickness dependent properties – Group 4, 5 TMDCs can be metallic while Group 6 TMDCs are semiconducting with thickness dependent band structure. This dissertation aims to develop novel synthesis and customization strategies to tune TMDC layer composition and crystallinity. By investigating the effects of increasing partial pressure (0.01%, 0.1%, and 1%) of the reactive gas (H2S) during reactive sputter deposition of a Mo target in Ar/H2S gas mixtures, I have traced the evolution of MoSx layer stoichiometry and microstructure and developed a recipe that yields highly 000l oriented, nearly stoichiometric hexagonal structured MoS2 layers on Al2O3(0001) single-crystal substrates. I have also carried out growth of MoS2 with a vdW solid – hexagonal boron nitride (hBN) – as a buffer layer, to develop an approach that tunes the crystallinity of sputter deposited MoS2 overlayers. I have found that MoS2 layers deposited on hBN covered Al2O3(0001) substrates are of superior crystallinity compared to layers deposited on bare Al2O3(0001) substrates, indicating the potential of vdW solids in heteroepitaxial growth. I have also synthesized, by reactive high power impulse magnetron sputtering of a VNbTaMoW target in Ar/H2S gas mixtures, highly 000l oriented layers of a high entropy multi-cation TMDC alloy – (VNbTaMoW)Sx on bare and MoSx covered Al2O3(0001) substrates, developing a TMDC customization strategy based on the compositional degree of freedom. I have found that the crystallinity of (VNbTaMoW)Sx layers on bare Al2O3(0001) substrates is better than on MoSx covered Al2O3(0001) substrates suggesting that vdW materials are not all equally suitable as templates for heteroepitaxial growth. Finally, by employing a first-principles density functional theory approach, I have extended the high entropy TMDC idea and determined that over 100 high entropy TMDC alloys consisting of 5 transition metals from groups 4-6 and sulfur, are thermodynamically stable at experimentally realizable temperatures.