Controlling surface segregation of reactively sputtered Ag/TiOx nanocomposites

Abstract

Reactive sputtering of silver/titania nanocomposites is potentially attractive for antibacterial, photocatalytic, and optical coatings. However, previous studies on such coatings showed strong segregation of silver towards the surface, which could lead to undesirable properties. Here we present results of our investigations of the mechanism of silver surface segregation and influence of sputter parameters on the morphology of reactively sputtered Ag/TiOx nanocomposites. It was observed that silver surface segregation depends on the silver filling factor of the composite and that it is largely independent of the oxygen partial pressure used in the sputtering process. At high total pressure even sputtering of pure TiOx barriers is—unlike non-reactive sputtering of TiOx—not sufficient to prevent the strong surface segregation effect. Surface segregation is only thoroughly suppressed by reduction of the total pressure during reactive sputtering, allowing the formation of a dense composite film. Based on this principle silver surface segregation can be further reduced by deposition of TiOx barriers with very high sputtering rates. Furthermore, the relation between the morphology of the nanocomposites and their optical properties is presented. Nanocomposites produced under these conditions show a controllable surface morphology for functional applications.