Nanoparticles and Nanocomposites in RF Plasma
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Nanoparticles and Nanocomposites in RF Plasma Jin Cao and Themis Matsoukas Department of Chemical Engineering, The Pennsylvania State University University Park, PA 16802, U.S.A. ABSTRACT The use of low pressure radio-frequency (rf) plasma for nanoparticle formation and the deposition of thin film on particulate substrates are reported. Plasma polymer particles are synthesized in a capacitively-coupled Ar/monomer discharge at rf power of 15-30 W. A variety of particle structures are observed, including monodispersed nanospheres and liquid-like viscous nano-droplets. Styrene in particular is observed to produce hollow nanospheres. By manipulating the process parameters, films of plasma polymers can be deposited onto suspended submicron particles. We take advantage of the electrostatic trapping of “dusty plasma” to suspend small particles in plasma for extended periods of time until the desired coating thickness is achieved. Sub-micron silica particles introduced into a low pressure rf Ar/monomer plasma are coated with film of thickness ranging from 2 to 70 nm.
INTRODUCTION Most efforts in gas phase thin film deposition have been directed towards flat substrates. While it is very difficult to apply a homogeneous and low-thickness coating to particles by physico-chemical or purely chemical approaches such as spraying, dipping, or fluidization [1], polymer films as thin as several tens of angstroms can be deposited in plasmas [2]. High molecular weight solid deposits formed in plasma are termed “plasma polymers.” High energy electrons in plasma sever chemical bonds and ionize neutral species, making the polymerization process atomic in nature. The final product can be controlled via process parameters [3], and materials with many unique and useful features have been obtained [4-11]. Almost any compound that can be brought into the plasma can undergo polymerization. This leads to a wide variety of products not observed from thermally activated reactions. Plasma polymers are generally highly cross-linked and can therefore deposit as pinhole free film. Plasma polymers also deposit in the form of particles depending on the monomer and reaction conditions [12, 13], giving even more varieties in plasma polymer coating. However, only film deposition of plasma polymerization has been extensively studied thus far. This work reports our study of the particle formation during plasma polymerization, since these particles may be used as substrates for the coated nanocomposites of our interest. The key to thin film deposition on particles in plasma is a controlled manner of fluidization of the substrates. The present work demonstrates the new concept of fluidizing particles using “dusty plasma,” i.e., plasma with trapped particles, for surface coating. We have successfully fabricated nanocomposites with a silica core and plasma polymer coating using this method. There have been only very few efforts of thin film deposition on particles in plasma to date[1, 14, 15]. In those cases, either a tumbling device was used to keep the partic
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