Rolf Ochs

Nanoparticle SnO 2 films as gas sensitive membranes

The Karlsruhe Microwave Plasma Process (KMPP), a versatile gas-phase process is applied to produce SnO 2 and core shell SnO 2 /SiO 2 nanoparticles which are, respectively, deposited in-situ on preheated Si-Substrates. These substrates are already equipped with an electrode microarray. The proof of sensor concept shows, that mechanically stable, nanoscaled and nanogranular gas sensing layers can be produced. In a first step synthesis and deposition parameters of SnO 2 are elaborated, and gas-sensitivity tests are performed. Additionally, annealing experiments are done. The morphology and struc-ture of nanoparticles is characterized by X-ray diffraction and TEM-methods. The layers are in-vestigated by SEM techniques and by XPS. The sensitivity of the nanogranular layer is deter-mined in comparison with a standard microarray equipped with sputtered layers. Particles crys-tallize in the tetragonal cassiterite structure. It is found that a precursor concentration of 3×10 −6 mol/l leads to particles with crystallite size in the region of 2nm, whereas a concentration of 5.5×10 −4 mol/l results in approximately 5nm particles. With the precursor concentration, columnar porous layers of 200nm thickness are obtained after a deposition time of 1min. This thickness is comparable to the one of sputtered layers. First sensor tests show 10 times higher sensitivity to isopropanol, compared to the standard array. The time of response is equivalent. The grain growth observed for bare and core/shell nanoparticles at 300°C is marginal.

New Core/Shell Ta 2 O 5 -PMMA Nanocomposites for Applications as Polymer Waveguides

To realize ceramic/polymer nanocomposites for polymer waveguides, PMMA-coated Ta 2 O 5 nanoparticles are synthesized as core/shell particles. Therefor a gas-phase process is used: the Karlsruhe Microwave Plasma Process. The organic coating is supposed to reduce the agglomeration of the ceramic cores and should facilitate the incorporation into the polymer resin. TEM investigations of the nanoparticles exhibit crystalline and amorphous Ta 2 O 5 with sizes around 3 to 6 nm, confirmed by electron diffraction. Although the polymer coating is not visible in TEM imaging, electron energy loss spectroscopy (EELS) exhibits a significant C-edge, proofing the organic coating. The Ta 2 O 5 /PMMA nanoparticles are incorporated with different weight fractions to a maximum of 1 wt% by a dissolver stirrer into PMMA resin. The optical properties (refractive index, transmission) are determined as a function of the nanoparticle content. Compared to the pure polymer, the refractive index of the modified composite, measured at 633 nm, is increased by 0.001 and 0.004 at 0.1 wt% and 0.7 wt%, respectively. A similar tendency is observed at 1550 nm. The transmission in the near infrared (NIR) is similar to that of PMMA up to a content of 0.3 wt%. At higher nanoparticle contents transmission is reduced. The reduction in transmission is due to the presence of agglomerates larger then 1/10 of the applied wavelength, confirmed by TEM. The concept of incorporating inorganic/organic hybrid nanoparticles with intrinsic high refractive index in polymer matrices is very promising. A suitable effect in refractive index for application of ceramic nanoparticle/polymer nanocomposites as polymer waveguides could be observed even with low particle concentration.

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.