Do nanomaterials form truly homogeneous alloys?

The application of gold and silver nanoparticles to areas such as medical research, is based on optical properties which are a direct consequence of a surface plasmon resonance (SPR) occurring at visible frequencies. The exact frequency of the SPR can be "tuned" by varying the relative concentrations of gold and silver within an alloy nanoparticle. The common practice of determining the frequency of alloy nanoparticles using techniques that average over the multiple particles such as ultraviolet-visible (UV-Vis) spectroscopy cannot detect variation in the composition of individual alloy nanoparticles. Here we demonstrate that by using electron energy loss spectroscopy the SPR can be determined for individual gold/silver alloy nanoparticles, for the purpose of determining the extent of their homogeneity. Importantly, we observe dramatic variation in SPR frequency between particles and even within the same particle, indicative of large variations in alloy composition.