"Flash" preparation of strongly coupled metal nanoparticle clusters with sub-nm gaps by Ag soldering: toward effective plasmonic tuning of solution-assembled nanomaterials.

Noble metal nanoparticle oligomers are important in applications including plasmonics, catalysis, and molecular sensing. These nanostructural units featuring abundant inter-particle junctions are helpful for a physical/chemical understanding of structure-activity relationships of self-assembled metamaterials. A simple, rapid, and potentially general strategy for the preparation of monodisperse nanoparticle clusters in a homogeneous solution is highly desired for fundamental research toward liquid metamaterials and chemical/biological applications, but this is however very challenging. Here we report an Ag soldering strategy to prepare strongly coupled plasmonic (Au) and catalytic (Pt, Au@Pd (Au core with a Pd shell)) nanoparticle clusters almost instantly (<1 min) in a solution without special synthetic efforts, complicated surface decorations, or structure-directing templates. The resulting clusters are isolatable by agarose gel electrophoresis, resulting in mechanically stable products in high purity. The optical extinctions of Au nanodimers (the simplest and most basic form of a coupled structure) exhibit prominent longitudinal plasmonic coupling for nanoparticles down to 13.3 nm in diameter. Theoretical simulations attribute the strong coupling to the existence of a sub-nm gap ( 0.76 nm) between soldered particles, suggesting an ideal (stable, soluble, monodisperse, and weakly passivated) substrate for surface enhanced Raman scattering (SERS) applications.