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GROWTH AND TEM INVESTIGATION OF TWO DIMENSIONAL TRANSITION METAL DICHALCOGENIDES AND THEIR HETEROSTRUCTURES
Restricted (Penn State Only)
Materials Science and Engineering
Master of Science
Date of Defense:
April 26, 2017
Nasim Alem, Thesis Advisor
two dimensional materials
transmission electron microscopy
transition metal dichalcogenides
In this thesis, a systematic study on the synthesis, transfer and TEM characterization of 2D TMDs is done to provide an understanding for growth approaches aiming to obtain high quality TMDs film and defect-engineering, which can greatly facilitate the exploration of fundamental phenomena and novel applications in these fascinating 2D systems. Using a combined experimental and numerical approach, we studied the key parameters for the planar and vertical growth of 2D materials and demonstrated the possibility for engineering their morphology by controlling the concentration and flow profiles. This provides a new mechanism for controllable growth of 2D MoS2 with different orientations and morphologies for applications in energy, catalysis, electronic devices and ultrathin integrated circuits. Epitaxial MoS2/hBN vdW heterostructure is synthesized via powder vapor transport (PVT) process, scanning/tramission electron nicroscopy S/TEM imaging and spectroscopy are used to study the atomic structure and chemical composition of the as-grown heterostructure, we also experimentally verify the crucial role of substrate defects on the nucleation and growth of MoS2/hBN heterostructure. A universal and etching-free transfer technique is developed which enables transfer of the TMDs films and flakes onto arbitrary substrates with high fidelity. We demonstrate a systematic investigation of morphology and intrinsic defects in the metal organic chemical vapor deposition-(MOCVD) grown WSe2 film, including point defects, dislocations and grain boundaries using aberration-corrected (S)TEM imaging. The orientation and stacking of as-grown WSe2 film is explored and the presence of interlayer strain is also revealed via dark field imaging. A new type of vacancy defect is also observed along the grain boundaries of WSe2 with reconstructed mirror symmetry, showing great possibilities in tunable devices.
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