Properties and application potential of nanoparticles produced by a non-equilibrium microwave plasma

Applications of nanomaterials may be divided into two groups. The first one, with improved properties as compared to coarse-grained materials, replaces conventional materials. The other group is based on physical properties, depending on the particle size. Typical examples therefore are the blocking temperature of superparamagnetic particles or the blue shift of luminescence of quantum dots. Most of the conventional gas phase synthesis routes, as flame processes or syntheses in hot wall reactors are leading to large particles with broad particle size distributions. They are not suited for the synthesis of nanomaterials with high requests on particle sizes and size distribution. In non-equilibrium plasmas the average plasma temperature is quite low. This makes these plasmas very interesting for technical applications regarding synthesis of nanoparticles and coated nanoparticles. The great advantage of non-equilibrium plasmas is that kinetic barriers can be overcome easily, as the components are ionized and dissociated partly. Therefore, reaction temperatures may be significantly lower than in conventional gas phase systems. Furthermore, reactions in the plasma lead to charged particles. As these particles are equally charged, they repel each other, suppressing agglomeration and thus, allowing in-situ synthesis of coated (core/shell) nanoparticles. A coating of nanoparticles becomes important, when either the coating acts as a diffusion barrier, or the coating modifies physical properties, or allows the introduction of additional functionality. The Karlsruhe Microwave Plasma Process (KMPP), a nonthermal, low-pressure process is very well suited for the synthesis of bare and core/shell nanoparticles with particle size <10 nm and very narrow particle size distribution. One of the reasons is the short residence time of the reactants in the plasma of only a few milliseconds. In combination, low temperature, short residence time in the reaction zone and equally charged- - particles reduce particle growth and formation of hard agglomerates. In this presentation examples for different types of nanoparticles, synthesized with this method, will be presented. Special attention will be drawn on the application potential for these nanomaterials. Examples will be shown from the area of gas sensing materials, superparamagnetic materials, and optical properties.