It is customary, and necessary to avoid oxidation, to protect thin-film samples with capping layers measuring a few tens of nanometers in thickness at most. However, this practice should be made far more stringent when it comes to protecting nanoparticle films.
Inspired by previous work by Hillenkamp and colleagues published in Nanotechnology, the team has checked the chemical stability of heavily protected cobalt cluster films prepared by low-energy deposition of preformed, gas-aggregated, particles in the secondary chamber where they are capped by either RF sputtering or thermal evaporation with as much as 200 nm of copper or titainum.
It turns out that such films oxidize in the timescale of hours-days in spite of their thick capping layer. The same can be said for capping layers approaching a thickness of a micron, according to follow-up experiments. The oxidation was apparent both from the logarithmic loss of magnetization and from the appearance of the exchange-bias effect typical of FM-AFM (here Co-CoO) nanocomposites. The single-phase nature of the hysteresis loops indicates the homogeneous oxidation of the particle film.
Although a transversal TEM study would be needed for confirmation, the authors suggest that the extremely high porosity of the particle films (see TEM picture above) propagates up into the cover layer and provides a route for oxygen penetration. Once oxygen arrives at the cluster film, the lack of chemical bonding between the clusters and the capping layer facilitates the oxidation of the cobalt particles.
The group presented its work in Nanotechnology.