Variation in surface plasmonic response due to the reorganization of Au nanoparticles in Langmuir-Blodgett film

Abstract

Layer-by-layer structures of dodecanethiol-encapsulated Au nanoparticles have been formed on Si(001) and quartz substrates at different surface pressures by Langmuir-Blodgett method. Optical absorption spectra and out-of-plane structural information have been obtained from UV-Vis spectroscopy and X-ray reflectivity measurements, respectively. With time the thickness of the film decreases keeping the layered structure unchanged but finally monolayer like structure forms. Localized surface plasmon resonance peaks obtained from the UV-Vis spectra show that coupling between Au nanoparticles occurs at the initial stage of the reorganization process as the interparticle distance decreases and as a result, a redshift in the plasmon peak wavelength takes place. Maximum redshift occurs for the monolayer and the peak shift linearly decreases for the multilayer structures. After prolonged reorganization when all layered structures transform into monolayer like structure again redshift occurs but in this process the redshift is reverse with respect to the previous one. In the later process, redshift is minimum for the monolayer structure and increases nearly linearly for the multilayer structures. Two different mechanisms responsible for these two processes are proposed.