Dewetted bimetallic AuPd nanostructures on silicon carbide surface: Effect of film thickness and annealing temperature

Abstract

Bimetallic nanostructures are, nowadays, largely investigated in several technological applications due to the synergistic combination of the properties of the two constituting metallic elements. In this regard, AuPd nanostructures are particularly promising combining the peculiar plasmonic properties of Au and the catalytic characteristics of Pd in nanoscale form. Hence, we report here on a thermal-induced dewetting process of deposited bimetallic AuPd films in order to obtain two-dimensional arrays of AuPd nanostructures on SiC surface. SiC is chosen, here, as wide band-gap model substrate enabling potential applications in optical, sensing, energy fields. In particular, we focus our study on the effect of the thickness of the AuPd film and of the annealing temperature on the characteristics of the AuPd dewetting process and on the morphology of the resulting dewetted nanostructures. We sputter-deposited, on the surface of 4H–SiC substrate, bimetallic AuPd thin films, thickness in the 4.3–48.6 nm range, and we carried out annealing processes increasing the annealing temperature from 573 K to 1173 K. As a result, we observed, by atomic force microscopy, the evolution of the AuPd films dewetting process towards the formation of two-dimensional arrays of AuPd nanostructures. In particular, we observed the sequential steps involved in the dewetting process by increasing the annealing temperature: heterogeneous voids nucleation in the films resulting in holes formation (leaving SiC surface exposed), holes growth by continuous addition of vacancies, holes coalescence and metal film retraction leaving interconnected metallic filaments, and filaments decay into isolated nanostructures. The quantification of the fraction of the AuPd-uncovered surface area (i.e. dewetted area leaving exposed SiC surface) versus the annealing temperature for each AuPd film thickness allowed us to evaluate the dewetting temperature (the temperature at which the dewetting process of the film starts) and its evolution with the film thickness. Then, the quantification of the particles average planar and vertical sizes (and the corresponding aspect ratio) was carried out versus the annealing temperature and film thickness. Finally, the temperature-dependent and thickness-dependent growth mode for the dewetted AuPd nanostructures is proposed.