Linker-molecule-free gold nanorod layer-by-layer films for surface-enhanced Raman scattering

This paper reports a methodology for synthesizing and ordering gold nanorods into two-dimensional arrays at a water/hexane interface. This preparation method allows the systematic control of the nanoparticle film thickness. An investigation into the thickness-dependent surface-enhanced Raman scattering (SERS) of the adsorbed molecules revealed the nanorod (NR) films to have 1 order of magnitude stronger SERS enhancement than the nanosphere (NS) under similar experimental conditions. The exposed surface areas of the prepared NR and NS films were analyzed using electrochemical methods, and it was found that they had comparable exposed surface areas. Therefore, the order of magnitude difference in the enhancement factor was not due to the differences in surface area but to their intrinsic difference in the optical coupling of each film. The difference was attributed to the high density of junction points with the NR films in comparison with the corresponding NS films. Scanning emission microscopy showed that the NR films have line contacts with each other but the NS films have point contacts, which can explain the difference in SERS intensity between the NR and NS films.