Characterization of core/shell nanoparticle thin films for gas analytical applications

Gas analytical microsystems such as the Karlsruhe micro nose (KAMINA) can be improved toward higher sensitivity when using SnO 2 nanoparticle thin films instead of well-established sputtered layers. For achieving long-term stability, such nanoparticles must be prevented from growing and agglomerating. A very promising approach is to coat SnO 2 nanoparticles with an open-pored SiO 2 shell, simply acting as a spacer, while preserving electrical contact between adjacent core particles. Such nanoparticulate thin films can easily be realized by the gas-phase Karlsruhe microwave plasma process (KMPP). This provides an all-in-one approach to synthesize particles with diameters less than 10 nm, to coat them in situ with a protective ultrathin SiO 2 shell in a downstream step and to finally deposit the corelshell particles solvent-free onto prefabricated micro devices. The present study focuses on the surface analytical characterization of 200 nm thin films consisting of SnO 2 core/SiO 2 shell nanoparticles with various shell designs. For this purpose, the SiO 2 shell thickness was systematically increased while keeping the SnO 2 core size constant. X-ray photoelectron spectroscopy (XPS) provides information concerning chemical binding states and shell thickness in a nondestructive manner. Angle-resolved XPS together with transmission electron microscopy (TEM) validate the achieved core/shell structure. In the case of the desired ultrathin SiO 2 shells, low-energy ion scattering (LEIS) is solely the suitable means to prove that Sn and Si are the constituents of the outermost monolayer of the spherical particles and it demonstrates the attainability of open-pored coatings.