Synthesis and Characterization of Two-Dimensional Transition Metal Dichalcogenide Materials



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Two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs), such as MoS2 and WS2, have attract extensive attention due to their exceptional band structure, optical and vibrational properties. Different from graphene, mono- and few- layer MS2 (M =Mo/W) with bandgap 1-2 eV exhibit significant potential applications in electronic and optoelectronic devices. To realize these applications, synthesis of large-area and high quality TMD films is important. In this dissertation, we focus on the synthesis and characterization of MoS2 and WS2 semiconductor materials. We investigated the optical and vibrational properties of MoS2 and WS2. The Raman spectra of MoS2 and WS2 are thickness dependent. Therefore, the number of MoS2 and WS2 layers can be determined by Raman measurement. We introduce a novel method to synthesize WS2 grains from patterned W films and address the bandgap variation induced from residue strain and stress during the WS2 synthesis. By controlling the introduction timing of sulfur vapors, monolayer single crystal WS2 grains larger than 100 µm were synthesized at ~1000°C. All PL mapping taken from large WS2 grains presents a strong and uniform intensity among all samples. In an effort to reduce the synthesis temperature, we developed a modified CVD process with the addition of H2O to synthesize monolayer WS2 at temperature as low as 800°C. With the presence of H2O vapor, W is transported viaWO2(OH)2, which is the most volatile form of hydrated oxides and can significantly enhance the transport of W to the nucleation site to form WS2. The modified CVD process was also applied to the synthesis of MoS2 from Mo precursors. The monolayer MoS2 can be synthesized at temperatures ~700°C. In addition, we combine our modified CVD method with the seeded growth technology to synthesize monolayer single crystal WS2 and MoS2 grains at predetermined locations. By optimizing the reaction variables, the array of monolayer MoS2/WS2 in-plane heterostructure can be obtained at 950°C.



Two-dimensional TMDs, MoS2, WS2, CVD, Raman, PL