Optical Properties of Nanoparticles on a Surface

In modem sensing applications, nanoparticles are immobilized on a surface so they present the maximum detection surface to the analyte. The sensing signal is the optical absorption spectmm. This configuration is well known to researchers in the surface science community as surface quantum dots or supported thin-film islands, and their optical properties have been studied for a while. Specifically, the Marton-Schlesinger method and the Bedeaux Vlieger methods have provided both quantitative calculations of the optical properties of nanoparticles on a surface. One big advantage of the latter method is the effect of the substrate is naturally built into the formalism (see Fig. 1). A limitation of these methods is that the 

Effect of Si02 substrate on LSPR of 14nm AuNPs 

Figuie 1. Calculated absorption spectrum of a hemispherical gold nanoparticle of radius 7 nm with and without a Si02 substrate. Inset. Geometry of calculation. 

Optical spectrum ef hemispherical gold nanoparticle dinner of radius 7nm, and center to center separation 17.5nm 

Despite the limitations of the DDA method, it is best suited for applications where it is important to know the local electric field on the surface, or when the nanoparticle itself has a composite structure. For example, the second feature in the two-nanoparticle absorption spectrum shown in Fig. 2 can be explained by plotting the electric field map. The map reveals that the presence of a dielectric can mediate the overlap of evanescent fields, an effect that was hitherto unknown.  

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