Title

Crystallization of Two-Dimensional Transition Metal Dichalcogenides for Tailored Optical Properties

Author

Rachel H. Rai

Date of Award

1-1-2019

Degree Name

Ph.D. in Materials Engineering

Department

Department of Chemical, Materials and Bioengineering

Advisor/Chair

Advisor: Christopher Muratore

Abstract

Two dimensional (2D) semiconducting transition metal dichalcogenides (TMD) are new materials that exhibit unique and useful combinations of physical properties, such as photoluminescence (PL) in the visible to NIR frequencies coupled with mechanical flexibility. Such properties make 2D TMDs attractive candidates for the next generation of compact, unobtrusive, and low-cost opto-electronic technologies. However, the incorporation of 2D TMDs in commercial applications and products is currently limited by the absence of synthesis approaches yielding uniform, large area films with desired properties. Thus, this work encompasses innovative techniques to tailor optical properties of TMD thin films by controlling their area, thickness, grain size, defect density and uniformity during and after processing. These new approaches start with magnetron sputtering of ultra-thin amorphous TMD films on either flexible or rigid substrates. The amorphous precursor films were then subjected to illumination with energy beams to induce crystallization, including laser light, broadband radiation from a xenon lamp, and nanoscale electron beams. Magnetron sputtering was selected as the precursor deposition technique due to the large area capability coupled with low processing temperatures, allowing deposition directly on polymer substrates. Furthermore, modulation of the energy flux to the growing film during magnetron sputtering (by controlling the flux of incident energetic particles) provided an opportunity to control the density of pre-existing nuclei in the amorphous material for an added measure of structural control upon illumination. The structure, composition and optical properties of crystalline 2D TMD materials on flexible and rigid substrates after illumination, were then correlated to the pre-existing amorphous structure and process conditions for selected TMD compositions. Important conclusions from the work include significant insight on the mechanisms of crystallization kinetics, as well as new correlations of structure to optical properties, including a dependence on photoluminescence intensity accompanied by a change in crystal edge density, correlating well to theory. In summary, this work employs various crystallization techniques to understand nucleation and growth of 2D TMD materials, and application of nucleation and growth mechanisms to control optical properties.

Keywords

Materials Science, Nanoscience, Nanotechnology, two-dimensional transition metal dichalcogenides, crystallization, in situ spectroscopy, photoluminescence

Rights Statement

Copyright 2019, author

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