Larger Assemblies of Nanoparticles

In many-body assemblies of nanoparticles, a number of inter-particle near-field interactions are involved and may give further unique properties. Many-particle assemblies have been paid much attention in relation to the development of the 

18 (a) Topography of single and aggregated gold nanoparticles, (b) Near-field Raman spectra taken at dimer 1 and the isolated particle indicated in (a). Excitation wavelength is 785 nm. (c) Polarized near-field Raman excitation images of the dimer 1 and dimer 2. Arrows indicate the incident polarization. Solid curves approximate shape of the dimer 

SERS active substrates. Various protocols have been reported for the preparation of the assemblies with close-packed forms [119-121], Most of them give moderate SERS enhancements on the order of 10 -10, which are much lower than those needed for single-molecule level sensitivity. The results may indicate that a simple increase of the particle number in the assembly is not enough to attain the ultimate field enhancement. To clarify the reason for this finding, visualization of the optical field is one of fhe most straightforward methods. 

From close inspection of the image, it is found that the highest enhancement occurs at the isolated dimer (A), and the enhancement observed at the rim or at the defect sites in the assembly is lower. This observation implies that the enhancement is the highest in the dimer and becomes lower with increasing size of the assembly. The finding may have some correlation with the relatively low SERS enhancement reported in close-packed assemblies. Plasmon localization at the rim of the assembly may have some similarities with photon localizations at the boundaries of photonic 

Near-field observation of the optical field distribution in the nanostructures reveals a wealth of information about the enhancement mechanism of the optical field and will give useful guidelines to design a desirable SERS substrate. It is also practically important to consider the potential applications of the confined optical fields to nano-optical devices, bioimaging, and photochemical reactions. 

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