Reasons for specific kinetics of switchable mirrors based on magnesium nickel films

Abstract

Switchable mirrors are thin films of metals or alloys which show a reversible transition from a reflecting metallic state to a transparent semiconducting state upon hydrogen exposure. We investigated the parameters which influence the kinetics of thin films of magnesium nickel alloys capped with a thin catalytic palladium layer. We found that the kinetics are strongly dependent on the composition of the films which is demonstrated by measurement of the transmittance and the reflectance of the substrate and the film sides during switching. In the case of the hydrogenation reaction this is because of the different hydride nucleation behavior of the layers. It is shown that this behavior is controlled by interface reactions during the deposition of the layers and by structural features. Nucleation at the substrate interface is believed to be due to heterogeneous nucleation of magnesium nickel hydride on MgO particles. This reaction is suppressed by alloying of magnesium and palladium at the magnesium palladium interface. Nucleation in the layer volume seems to be inhibited in the case of non crystalline magnesium nickel layers. All these features lead to fast hydrogenation kinetics and good optical properties in the hydride state for intermediate layer compositions.