Comparative study of the assemblies and the resulting plasmon fields of Langmuir-Blodgett assembled monolayers of silver nanocubes and gold nanocages

Silver nanocubes (72 nm) and gold nanocages (75 nm) were assembled by varying the average interstitial particle distances by the Langmuir-Blodgett technique. The shape of the surface pressure-area isotherms for the two types of nanoparticles are discussed in terms of the degree of surface capping of each nanoparticle which depends on the surface quality. Monolayer films formed at different surface pressures were transferred by the vertical dipping method to silicon and glass substrates where their optical spectra and SEM. images were examined. A red-shift of the strong plasmon peak maximum was observed as the average particle distance was decreased. From the SEM images, the morphology as well as the percent area of the substrate surface covered at different pressures was determined. A linear relationship was observed between the percent of covered area and the plasmon peak maximum, the slope of which reflects the average plasmon field felt by the nanoparticles. The increase in the average field with increasing percent covered area is a result of the decrease in the average nanoparticle separation and/or the increase in the number of nanoparticles in each cluster in the distribution formed. The strength of the plasmon field resulting from gold cages is found to be much higher than that from the silver cubes and increases as the gold wall thickness decreases. These results are discussed in terms of the coupling between the two separate fields of the cage wall surfaces.