Role of Annealing Temperature, Time, and Composition on the Fabrication of Aux Pd1−x Nanostructures on c-Plane Sapphire by the Solid-State Dewetting of Bimetallic Thin Films

Bimetallic nanostructures (BNSs) are important components in various applications due to their elemental composition and morphology dependent catalytic, electronic and optical properties. In this paper, the systematic evolution of Au_xPd_1-x (gold-palladium) BNSs on sapphire (0001) is studied via the solid-state dewetting of sequentially deposited Au-Pd bilayers along with the systematic control of elemental composition, annealing temperature, and time. Through the annealing of Au_xPd_1-x bilayers at various temperatures between 400 and 900 °C, the fabrication of nanoparticles (NPs) on bilayer, voids, and Au-Pd BNSs are observed with various compositions (Au_0.25Pd_0.75, Au_0.5Pd_0.5, and Au_0.75Pd_0.25). The evolution process of Au_xPd_1-x BNSs is systematically analyzed based on the interdiffusion of Au and Pd and surface energy minimization mechanism. Along with the increased Au composition, the rate of dewetting is enhanced, which resulted in the significantly larger layer- top bimetallic NPs, voids, and BNSs. Furthermore, with the control of annealing time between 0 and 3600 s, the shape of Au-Pd BNSs is transform from the elongated to the round NPs due to the gradually enhanced diffusion. The reflectance (UV-VIS-NIR) spectra show the tight correlation with the surface morphology of Au_xPd_1-x BNSs and the average reflectance is gradually decreased along with the increased annealing temperature and time. The dipolar resonance peaks in the NIR region reveal a gradual blue-shift along with the increased annealing time due to the NP size reduction.