Synthesis and Property Engineering of the 2D Oxides and Chalcogenides

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Two-dimensional materials’ “all-surface” architecture presents a new paradigm for investigations into electron confinement effects and surface phenomena. However, synthesizing, characterizing, and ultimately engineering the properties of 2D materials represents a formidable challenge. This thesis presents several cases of isolating novel 2D materials via vapor-based syntheses. Vapor-based syntheses allow for reproducible growth of monolayer and few-layer materials with electronic-grade quality. I and my colleagues focus on the group VI oxides and chalcogenides, a materials class with many synthetically-accessible layered compounds and a plethora of materials properties. Specifically, we apply an evaporative thinning technique to Bi2Se3-xTex to produce a single monolayer. We show the rapid stoichiometry changes that can take place during evaporative thinning, to produce bismuth-rich, heteroanion compounds. For the MoO3 system, we present a physical vapor deposition technique to produce few-layer, electronic-grade nanosheets of MoO3. The development of crystal doping techniques, allowing for precise electronic property engineering of Ge and Si crystals, contributed to the Nobel Prize-winning discovery of solid-state transistors at Bell Labs. For 2D chalcogenides and oxides to be integrated into devices, taking advantage of their unique physics, similar property engineering control is required. This thesis presents several property engineering techniques to control doping levels in 2D materials. For MoO3, we demonstrate an electron beam dose technique to precisely introduce oxygen vacancies into the MoO3-x structure, thereby n-type doping the MoO3-x sheet. Chapters 4-6 focus on engineering the properties of the transition metal dichalcogenides (TMDs). First, we introduce a platform chemical vapor deposition synthesis for both monolayer and heterostructure compounds. Building on this platform, we present a lithium-intercalation technique to n-type dope and engineer the on-chip monolayer phase of MoSe2. However, we also show challenges with this technique, due to the instability of the intercalated structure and the resulting lack of reliability for device applications. As such, we direct the field to more fruitful paths. Based on these results, we present an alternative property engineering technique, using the charge transfer dopant AuCl3 to p-type dope MoS2. We provide structural and chemical insights into the doping process and outline a polymer pen lithography (PPL) technique to pattern the AuCl3 dopant and resulting Au nanoparticles. This thesis concludes with perspectives on future research to advance the field. Finally, the appendix focuses on broader impacts. With the decline of materials research in corporate labs such as Bell Labs, universities have a greater responsibility to understand and apply the commercialization process to materials discoveries. The appendix examines the advanced materials commercialization process with a focus on battery materials.

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  • 02/18/2019
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