Formation and Removal of Alloyed Bimetallic Au-Ag Nanoparticles from Silicon Substrates for Tunable Surface Plasmon Resonance

The formation of alloyed Au-Ag bimetallic nano-particles (NPs) by the solid-state thermal dewetting of Au-Ag bilayer thin films on Si substrates was reported in this work. Complete dewetting of the bilayer thin films to form alloyed Au- Ag bimetallic NPs can be achieved at temperatures above the Tammann temperature of at least one of the metal components. The NP size depends heavily on the total thickness of the bilayer thin films, whereas the atomic ratio of Au/Ag and the sputtering order in the bilayer films do not affect the NP size significantly. It has been demonstrated by X-ray photoelectron spectroscopy (XPS) analysis that the produced Au-Ag bimetallic NPs are alloyed and the sputtering order of Au and Ag in the initial bilayer films has no impact on the final configuration of the produced NPs. To further characterize the surface plasmon resonance (SPR) of Au-Ag NPs, a procedure involving the use of poly(vinyl alcohol) (PVA) and poly(methyl methacrylate) (PMMA) was developed to remove the bimetallic NPs from the optically opaque Si substrates. The SPR peak wavelength of the bimetallic Au-Ag alloy NPs has been shown to vary linearly with the atomic percentage of Au in the NPs, allowing for the tuning of the resonance wavelength by changing the alloy composition. The ability to produce uniform Au-Ag alloy NPs by thermal dewetting, the successful removal of the Au-Ag NPs from the Si substrates into the colloidal solution, the tunable SPR, and the excellent long-term stability of the alloy NPs in solution open up many opportunities for the potential applications of these Au-Ag alloy NPs, for example, for surface functionalization, sensing, and catalysis.