Real time optical observation of the synthesis of novel 2D materials and investigation of their fundamental properties

Kasırga, Talip Serkan
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Bilkent University
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Two-dimensional transition metal dichalcogenide (2D TMDC) with superb phys-ical and chemical properties, used as the active material for various devices. The on-going primary focus is their reliable high-throughput synthesis using processes compatible with the current semiconductor technology. At present, among the common approaches, chemical vapor deposition (CVD) has been considered as the most promising method for preparing large-area high-quality 2D materials. However, the lack of in-situ information during the growth in conventional CVD systems, makes it impractical to realize high-temperature phenomena. In this thesis, we developed a novel CVD chamber that allows real time optical obser-vation and control of the crystal growth. Using this new CVD method, which we call real time optical-CVD, RTO-CVD in short, we elaborated the involved mechanisms in salt-assisted synthesis of TMDCs and their vertical/lateral het-erostructures. Through direct visualization of WSe2 monolayer growth, we iden-tified that both vapour-solid-solid and vapour-liquid-solid growth routes are in an interplay. Then, we focused our attention to synthesize novel 2D materials such as V2O3 and K-MnO2 nanosheets. We succeeded synthesis route in favor of high-quality single-crystalline V2O3 nanoplates whose 2D characteristic allows us to study their peculiar electrical and physical properties such as metal-insulator transition (MIT) and supercritical state. The electrical properties of both as-grown and transferred V2O3 crystals were investigated with respect to the V2O3 phase-stability diagram. We observed emergence of a novel crystal structure upon electron beam heating in selected area electron diffraction (SAED) experi-ments and correlated it to the supercritical state by means of high-temperature Raman spectroscopy. Finally, we introduced large-area ultra-thin layered MnO2 crystals, spontaneously intercalated by potassium ions during the synthesis. The charge transport in 2D K-MnO2 devices was shown to be dominated by the in-plane ionic conductivity through the motion of hydrated K ions in the interlayer space. The K-MnO2 crystals exhibited reversible layered-to-spinel phase tran-sition accompanied by an optical contrast change based on the electrical and optical modulation of the potassium and the interlayer water concentration. We used the electric-field driven ionic motion in K-MnO2 devices to demonstrate the memristive properties and elucidated the resistance-switching mechanisms via real-time analyses upon the measurements. K-MnO2 memristors were artificially able to emulate neuromorphic synapse-like behaviors, namely short and long-term potentiation/depression as well as ionic coupling effects.

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2D materials, Transition metal dichalcogenide, CVD growth, In-situ annalysis, V2O3 nanoplate, K-MnO2 birnessite, 2D memristors, Synaptic emulation
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