Nanoparticle Shape Effects on the SERS Properties of Small Conductive Bridges

Surface-enhanced Raman scattering (SERS) allows for the vibrational spectroscopy study of a very small number of molecules, especially those located in the gap region separating two metal surfaces, where near-field enhancements are considerably large─the so-called SERS hot spots. The small number of molecules in the very localized hot spots permits the study of fundamental properties associated with SERS, such as the effect of plasmonic coupling on near-field enhancements. The engineering of hot spots can be achieved through molecular junctions. The careful analysis of the effects of hot spot shape and molecular conductances is of paramount importance for the correct interpretation of experimental results and the design of composite systems (nanoparticles and molecular junctions) for different experimental applications. In this work, we investigate these parameters for Au nanospheres and Au nanorods connected to a Au film by oligophenylenedithiol (OPD) molecules of different sizes. Our study permits the interpretation of hot spot shape (created by different nanoparticles) and molecular bridge conductance effects on SERS intensities as a function of gap size.