Synthesis of metal nanoparticles. Scanning electron microscopy (SEM) images of a gold nanospheres, b nanotriangles and c nanostars. d SERS spectra of R6G molecules measured from three different nanoparticles. e HRTEM image of a nanobridged nanogap particle. f Comparison of the SERS signal intensity produced in the presence or absence of a nanogap inside of a nanoparticle. g Calculated electric field intensity distributions at the metallic inner nanogap.