Processing Core/Alloy/Shell Nanoparticles: Tunable Optical Properties and Evidence for Self-Limiting Alloy Growth

The postsynthetic processing of nanomaterials may allow researchers to reach specific properties, morphologies, or phase regimes that are not accessible by simple synthesis alone. Here, we take advantage of atomic interdiffusion at nanoparticle interfaces to fabricate core/alloy and core/alloy/shell nanoparticles. Modest temperature changes were found to have profound effects for the interfacial alloying of the confined nanosystem. The alloy formation and subsequent interdiffusion allowed us to tailor nanoparticle composition and ultrastructure, as well as surface plasmon response. This processing step, which involves the layer-by-layer formation of a core/alloy/shell morphology, utilizes hydrothermal annealing provided by automated microwave irradiation to control solute deposition, as well as alloy thickness. As a proof-of-principle system, we employed a Au/AuxAg1-x/Ag nanosystem, due, in large part, to its miscible phase diagram and rich plasmonic behavior. Nanostructure morphology was characterized by TEM and STEM, and compositional analysis was performed via selective area EDX and XPS. The resulting surface plasmon resonance signatures were modeled as a function of alloy or monometallic shell thickness, as well as alloy composition, using the discrete dipole approximation method. A proposed shell growth mechanism is described, which involves the competition between a classical ripening system at low temperature and a self-limiting growth at high temperature, the latter of which may be driven by an alloy order disorder phenomena at the interface. These optical and growth models strongly correlate with the experimental results, namely, that the plasmon resonance is highly tunable on shell thickness and alloy composition and that alloy thickness and interdiffusion are highly tunable by the thermal processing.