Microwave plasma synthesis of graphene and its laser-optical in situ characterization

Graphene and its derivatives possess exceptional electrical, mechanical, and thermal properties, which enable the use of these materials in numerous existing and emerging applications. Accordingly, large-scale graphene production is needed to fulfill the expected demand for this material. However, the current examples of graphene mass fabrication through top-down scalable approaches, e.g., liquid-phase exfoliation and chemical graphene oxide reduction, are incapable of consistently producing graphene at the quality levels needed to exploit its unique properties in applications.

Gas-phase microwave-assisted synthesis of free-standing few-layer graphene is a promising technique for the economical production of high-quality material. The gas-phase approach makes the method continuous, scalable, and highly tunable. However, little is known about the underlying physical and chemical processes, e.g., the kinetics of graphene formation and growth or the flakes morphology in the aerosol phase. This information is required to characterize and optimize production. Currently, the synthesis parameters are usually found empirically, through a  combination of trial-and-error parametric studies and ex situ materials  characterization, which is time-consuming and often intrusive. There is a corresponding need to develop measurement tools that can help to build a fundamental understanding of the synthesis process, monitor the quality of produced graphene in situ, and ultimately find ways to increase yield while maintaining graphene high quality. On the other hand, the increasing production rates of graphene aerosols lead to occupational exposure and potential adverse health effects. Hence, reliable aerosol morphology characterization is critical to the regulation of graphene aerosol production and handling.

This thesis seeks to deploy laser-optical in situ diagnostics to assess the aforementioned challenges. Timeresolved laser-induced incandescence (TiRe-LII) technique is applied to graphene aerosols for the first time to measure the graphene volume fraction and specific surface area in situ. Moreover, the TiRe-LII enables distinguishing between graphene flakes and soot particles, which are often formed during the synthesis as the undesired by-product. Fourier-transform infrared (FTIR) absorption spectroscopy is used as an in situ line-of-sight technique to measure gaseous species involved in graphene synthesis. The laser-optical diagnostics were complemented with chemical kinetics simulations and mass spectrometry measurements to reveal the factors playing a major role in graphene formation kinetics. Additionally, the optical properties and morphology of synthesized graphene flakes were characterized using aerosol diagnostics and numerical simulations. Consequently, the connection between the graphene morphology and optical properties was established, and the spectroscopic model of
crumpled graphene flakes was developed.


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