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Direct Synthesis of Semiconducting Transition Metal Dichalcogenide Monolayers and Heterojunctions by Chemical Vapor Deposition

Abstract

Since the isolation of graphene in 2004, interest regarding two-dimensional materials properties and their synthesis has exploded. Group VIb transition metal dichalcogenides (MX2: M=Mo, W; X = S, Se) are a class of layered semiconductors that show unique layer-number dependent electronic and optical properties. For example, bulk three-dimensional MoS2 is an indirect band gap semiconductor with a 1.2 eV band gap, while monolayer MoS2 nanohseet is a direct band gap semiconductor with a 1.8 eV energy gap. This exciting change in electronic structure opens several possibilities towards implementing these materials in novel device assemblies such as field effect transistors, electroluminescent devices, and flexible optoelectronic devices. While these materials offer great promise, isolating transition metal dichalcogenide monolayers requires tedious mechanical exfoliations using scotch tape, which is neither practical nor scalable for production. In the first part of my dissertation, we investigated the synthesis of MoS2 and WS2 monolayers using chemical vapor deposition. The isolated nanosheets were single crystal, triangular, and had edge lengths up to 100 μm. In the next chapter, by utilizing H2 gas in a chemical vapor deposition apparatus, MoSe2 monolayers and few-layers were also synthesized. The MoSe2 nanosheets exhibited thickness-dependent vibrational and optical properties, and a notable intense photoluminescence emission from the direct band gap monolayer region. In the following chapter, we describe an alternative method to produce WSe2 nanosheets by physically vaporizing of WSe2 powder at a high temperature. Using WSe2 powder directly is advantageous since under optimal conditions we can selectively grow single-layer WSe2 domains or single-layer films up to several square centimeters. Lastly, we combine the synthesis of the transition metal chalcogenides previously described into lateral and vertical heterostructures grown in situ by chemical vapor deposition. The lateral MoS2-MoSe2 and WS2-WSe2 heterostructures formed stitched monolayer heterojunctions confirmed, as confirmed by photoluminescence and Raman spectroscopy studies. Vertical MoS2-MoSe2 and WS2-WSe2 heterojunctions were two layers thick and had vibrational and emission confirmations of their composition. This dissertation lays the foundation for the rational synthesis of two-dimensional transition metal dichalcogenide monolayer and heterostructure, which represents the key challenge to apply these exciting materials systems into functional optoelectronic devices.

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