Preparation and optical characterization of core-shell bi-metallic nanoparticles

Chemical approaches allow for the synthesis of highly defined metal heterostructures, such as core-shell nanoparticles. As the material of metal nanoparticles determines the plasmon resonance-induced absorption band, the control of particle composition results in control of the absorption maximum position. Metal deposition on gold or silver nanoparticles was used to prepare core-shell particles with modified optical properties with respect to monometal nanoparticles. UV-vis spectroscopy on solution-grown and immobilized particles was conducted as ensemble measurements, complemented by single particle spectroscopy of selected structures. Increasing layers of a second metal, connected to a dominant contribution of the shell material to the extinction spectrum, lead to a shift in the absorption band. The extent of shell growth could be controlled by reaction time or the concentration of either the metal salt or the reducing agent. Additional to the optical characterization, the utilization of AFM, SEM and TEM yielded important information about the ultrastructure of the nanoparticle complexes.