Plasmon/plasmonic shift

Nanoparticles of different shape affect plasmon characteristics, since the shape changes the way particle plasmons are excited. Overall trends of the plasmon shift are consistent with Sect. 2.1.4.4. For example, stronger plasmons tend to be induced in nanoparticles with comers. In other words, isolated LSPs may produce a more significant shift. 

Fig. 1 presents the EELS data for silicon growth (Vsi=0.17 nm/min at substrate temperature 150°C) atop 2D Mg2Si with structure (2/3)V3-R30°. It is apparent, that surface phase does not destroy at Si overgrowth and 2 nm of Si completely cover the silicide phase. However, the surface plasmon shifted to lower energy at 20 nm of Si thickness, while position of bulk plasmon corresponded to the monocrystalline silicon. The main cause of the given difference is the strong surface relief Therefore in this case the known relation between bulk and surface plasmons for atomically clean surface is not valid. 

Fig. 4.7. (a) Zero field cooled and field cooled surface plasmon, shifted due to capping by Fe. 

This universal scaling behavior permits a derivation of a simple empirical equation, which can be used to determine the distance between both nanostructures. Distances in biological systems are of special importance. AA/Aq is the fractional plasmon shift, r the intemanoparticle edge-to-edge separation, and D the particle diameter. ki and are empirical factors and in biological systems is equal to approximately 0.2. 

B. K. Juluri et al.. Effects of geometry and composition on charge-induced plasmonic shifts in gold nanoparticles. Journal of Physical Chemistry C, 112(19), 7309-7317 (2008). 

The substrate has long been known to play a role in the excitation of localized particle plasmons, shifting the resonant frequencies due to a change in the permittivity of the medium on which the metal nanostructures are located [28]. In addition, recent work has shown that certain substrates play an active role in light-plasmon coupling [24,27,43]. 

When the silver nanocrystals are organized in a 2D superlattice, the plasmon peak is shifted toward an energy lower than that obtained in solution (Fig. 6). The covered support is washed with hexane, and the nanoparticles are dispersed again in the solvent. The absorption spectrum of the latter solution is similar to that used to cover the support (free particles in hexane). This clearly indicates that the shift in the absorption spectrum of nanosized silver particles is due to their self-organization on the support. The bandwidth of the plasmon peak (1.3 eV) obtained after deposition is larger than that in solution (0.9 eV). This can be attributed to a change in the dielectric constant of the composite medium. Similar behavior is observed for various nanocrystal sizes (from 3 to 8 nm). 

The UV-visible spectrum (Fig. 6) of the aggregates described earlier shows a 0.25-eV shift toward lower energy of the plasmon peak with a slight decrease in the bandwidth (0.8 eV) compared to that observed in solution (0.9 eV). As observed earlier with monolayers, by washing the support, the particles are redispersed in hexane and the absorption spectrum remains similar to that of the colloidal solution used to make the self-assemblies. 

Averitt, R.D., Sarkar, D. and Halas, N.J. (1997) Plasmon resonance shifts of Au-coated AU2S nanoshells Insight into multicomponent nanopartide growth. Physical Review Letters, 78, 4217-4220. 

Remarkably the position of the final plasmon peak of the alloy particles is dependent on the molar ratio of gold to silver nanoparticles. When the ratio is shifted favoring either metal, an alloy of any desired composition can be formed. This alloying phenomenon indicates that it is possible for true tuneability of the properties of a set of nanoparticles. 

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