The fabrication of core-shell nanostructures by dusty plasma chemical vapor deposition

by 1972- Cao, Jin

iii Ultrafine particles have found extensive use in many industrial applications due to their interesting properties. These properties can be best preserved when the particles are coated to avoid aggregation. The coating of such particles (200 nm – 5 µm) has been successfully achieved by the use of a technique called dusty plasma chemical vapor deposition (DPCVD). Ultra-thin films only several nanometers in thickness have been attained on the particles. This technique uses simultaneously the phenomenon of particle trapping in the plasma for particle suspension and the process of plasma polymerization for film deposition. The plasma used in this work is a low pressure non-equilibrium radio-frequency discharge in a capacitively coupled parallel-plate reactor. DPCVD has been used to coat silica particles of various sizes with 2-propanol plasma polymer. The major experimental findings are: a) the film deposition rate increases significantly with the uncoated particle size, b) the particle size distribution broadens considerably with film deposition at a rate also dependent on the uncoated particle size, and c) the coating uniformity on the particle decreases with film deposition at a rate again dependent on the uncoated particle size. Models have been developed to successfully explain these experimental findings. These models have shown that 1) the particle size dependent depletion of electron density in the plasma is the major factor influencing the film deposition rate and 2) the broadening of the particle size distribution is a result of the spatial distribution of deposition rates. In addition, the second model has provided useful insights into the deposition environment in a reactive dusty plasma. iv DPCVD has also been used for the first time to synthesize hollow plasma polymer particles. A total of 16 organic precursors of different chemistry were tested and it was found that aromaticity and a cyclohexane ring seem conducive to the appearance of an empty core in the particles. It was found that all the precursors share the same physical mechanism of hollow particle formation. A plausible model for the hollow particle formation has been proposed.