Dimers of silver nanospheres: facile synthesis and their use as hot spots for surface-enhanced Raman scattering

This paper describes a simple, one-pot method that generates dimers of silver nanospheres in one step without any additional assembly steps. The dimers are consisted of single-crystal silver nanospheres approximately 30 nm in diameter and separated by a gap of 1.8 nm wide. The key to the success of this method lies in the control of colloidal stability and oxidative etching by optimizing the amount of chloride added to a polyol synthesis. The dimers provide a well-defined system for studying the hot spot phenomenon (hot spot: the gap region of a pair of strongly coupled silver or gold nanoparticles), an extremely important but poorly understood subject in surface-enhanced Raman scattering (SERS). Because of the relatively small size of the silver nanospheres, only those molecules trapped in the hot spot region are expected to contribute to the detected SERS signals. By correlating SERS measurements with SEM imaging, we found that the SERS enhancement factor within the hot spot region of such a dimer was on the order of 2 x 10(7).