Date of Award
Doctor of Philosophy (PhD)
Nonequilibrium plasma (NEP) is an extraordinary environment for material synthesis. NEP is comprised of hot electrons with temperatures greater than 10000 K and of cold ions and neutrals that are usually at few hundred kelvins above room temperature. Due to this large difference in species’ temperatures, the assumption of local thermal equilibrium does not hold in NEP. Therefore, NEP can act as a unique processor of mass, and it can transform materials along pathways that are not accessible by methods wherein local thermal equilibrium is valid. For decades, NEPs have been employed in the semiconductor industry to manufacture many thin film electronic devices. In the last 15 years, NEPs have been translated to the synthesis of free-standing semiconductor nanocrystals (NCs) with success. It was shown that in low-pressure NEPs NCs could be made monodisperse with tunable size and optical properties, which is the level of control required for the synthesis of functional nanomaterials. Although successful synthesis of group IV and metal oxide semiconductor NCs has been reported in literature, the mechanisms responsible for the growth of high-quality nanocrystals are currently unknown. Furthermore, the lack of reports on the characterization of NEP reactors makes it difficult to determine the operating parameters that are necessary for synthesizing materials with desired properties. In order to move away from empiricisms in the design of NEP reactors and processes, and to develop new approaches for synthesizing broader classes of semiconductors, experiments and measurements that elucidate the fundamentals of plasma-aerosol interactions are needed.
The first aim of this dissertation is to shed light on the growth dynamics of NCs in NEP. A simple platform for investigating plasma-aerosol interactions was developed. The platform allowed for experiments in which a pre-made aerosol was sent into the plasma as a probe, and the plasma-treated aerosol was collected downstream to monitor the changes in the size and shape of the aerosol particles. During these experiments, a new mechanism of growth was discovered. It was found that the aerosol rapidly vaporizes in the plasma, despite the low background gas temperature. Aerosol dynamics that follow the vaporization process was observed to cause a significant size-focusing in the aerosol. A simplified scheme of dynamics for the experimentally observed growth was proposed, and the scheme was tested by setting up a sectional aerosol dynamics model. Computations were complemented by a thorough characterization of the tubular flow-through reactor, which revealed a characteristic zone with elevated ion density and gas temperature that can induce intense ion bombardment on the particles.
The second aim of this dissertation is to advance NEP synthesis by providing a means to produce compound semiconductor nanocrystals that currently do not have an established method of synthesis. A new method that works in a way that is similar to the platform described above was developed. The method, named nonequilibrium plasma aerotaxy, uses elemental aerosols as precursors, and it employs NEP as the reactor. With this method, free-standing nanocrystals of gallium nitride, indium nitride and gallium antimonide were synthesized in gas-phase for the first time. The optical properties of these materials and their growth mechanisms were investigated in detail.
Mikhail Berezin, Pratim Biswas, Milorad Dudukovic, Kenneth Kelton,