Nanogranular SnO2 Layers for Gas Sensing Applications by In Situ Deposition of Nanoparticles Produced by the Karlsruhe Microwave Plasma Process

Abstract

To improve the performance of the central element of the Karlsruhe Micronose—a gas-sensor microarray—the gas sensitive layer is modified toward a highly porous nanogranular layer. To synthesize those layers, the Karlsruhe Microwave Plasma Process which is in its native form a precursor-based process to produce nanoparticles with diameters below 10 nm, was modified for in situ tin-dioxide layer deposition. The produced layers have due to their structure a very large active surface area. The process parameters were optimized to generate thin layers with high surface homogeneity. This was mostly established by significantly reducing the precursor feed and therefore reducing the primary particle size to below 2 nm. The layers were analyzed for their mechanical stability, structural, and chemical properties. It is shown that the precursor residue can be completely removed by applying a default annealing step. The structure of the layers reminds of little clubs starting on top of the substrate growing wider toward the surface. Prototype sensors were fabricated and tested for their gas sensory properties in comparison to a standard gas-sensor microarray with a sputtered tin-dioxide layer. The gas-sensor microarrays with nanogranular layer show an increased signal response of up to one order of magnitude to isopropanol. The time of response is equal in both sensor systems while the time of recovery is nearly doubled for the sensors with nanogranular layer due to increased surface area and gas absorption.