Plasmons, surface enhanced model

One may classify the various proposed models in several ways. One way is to differentiate between models that focus on the role of the electric field E and the emission G terms (these two are related), on the one hand, and those that emphasize the role of changes in the Raman polarizability tensor, on the other. The former discuss the enhancement in terms of amplified fields, due to the presence of the surface, which act on the scattering molecule and its emission being further amplified by the surface. These are the local field and emission enhancement models (LFE). The difference between the various models which belong to this group is in the identification of the specific excitation in the solid which is responsible for the amplification plasmon polaritons, shape resonances, electron holes, etc. 

The phenomenon of surface-enhanced infrared absorption (SEIRA) spectroscopy involves the intensity enhancement of vibrational bands of adsorbates that usually bond through contain carboxylic acid or thiol groups onto thin nanoparticulate metallic films that have been deposited on an appropriate substrate. SEIRA spectra obey the surface selection rule in the same way as reflection-absorption spectra of thin films on smooth metal substrates. When the metal nanoparticles become in close contact, i.e., start to exceed the percolation limit, the bands in the adsorbate spectra start to assume a dispersive shape. Unlike surface-enhanced Raman scattering, which is usually only observed with silver, gold and, albeit less frequently, copper, SEIRA is observed with most metals, including platinum and even zinc. The mechanism of SEIRA is still being discussed but the enhancement and shape of the bands is best modeled by the Bruggeman representation of effective medium theory with plasmonic mechanism pla dng a relatively minor role. At the end of this report, three applications of SEIRA, namely spectroelectrochemical measurements, the fabrication of sensors, and biochemical applications, are discussed. 

Earlier observations by Cesario et al. [60] of a decay in fluorescence for arrays of Au nanoparticles spaced above a Ag film by a Si02 layer of increasing thickness, were interpreted as due to the finite vertical extent of the evanescent fields associated with a surface plasmon. In this model the coupling results in an enhanced interaction between individual localized plasmons at individual nanostructures [61] and thus an enhancement in the radiative efficiency increasing the spacer layer thickness moves the nanowires out of the evanescent field of the surface plasmon. A possible physical mechanism for the experimentally observed decay involves nonradiative decay of the excited states. The aluminum oxide deposited in these experiments was likely to be nonstoichio-metric, and defects in the oxide could act as recombination centers. Thicker oxides would result in higher areal densities of defects, and decay in fluorescence. A definitive assignment of the mechanism for the observed fall off of fluorescence would require determination of the complex dielectric function of our oxides (as deposited onto an Ag film), and inclusion in the field-square calculations. Finally it should be noted that at very small thicknesses quenching of the fluorescence is expected [38,62] consistent with observations of an optimum nanowire-substrate spacer thickness. 

The optimal enhancement effect is observed when the localized surface plasmon resonance is tuned to the emission wavelength of a locally situated fluorophore [86]. This is consistent with the model suggesting a greatly increased efficiency for energy transfer from fluorophores to surface plasmons [78]. Since resonance energy transfer is involved, the important factors affecting the intensity of fluorescence emission must also be the orientation of the dye dipole moments relative to the... 

Dye molecule orientation at the surface of a NP has a significant influence on the enhancement factor. For an ideal conductor, the electric field is always perpendicular to the surface. As discussed above, in order to demonstrate plasmonic enhancement as a function of NP radius, NPs were coated with a silica shell prior to attachment of a dye in order to minimize dye quenching. The presence of this shell slightly modifies the electric field but it is still almost perpendicular to the surface. It follows from this and from Equation 4 that molecules, which aie oriented normally, are considerably more excited than those oriented tangential to the surface. For this model, we assume that there is random dipole orientation at the NP surface. The average intensity over all possible dipole positions due to a EM field E is given by 1/3 Ef. Hence the averaged enhancement factor can be written as ... 

Nitzan and Brus fust suggested the possibility of siuface plasmon enhanced photochemistry in 1981 [13, 14]. They studied a phenomenological model for a molecule interacting with a small spherical metal nanoparticle that can support a SP resonance when irradiated. The model was derived in detail in a subsequent paper by Gersten and Nitzan [15]. I will thus refer to the model as the Nitzan-Brus-Gerstan or NBG model. The calculations based on the NBG model showed that both UV photodissociation and IR multiphoton absorption could be surface plasmon enhanced [13, 14]. 

Nitzan and Brus developed an analytical formula for the molecular absorption cross section given the model defined above [14]. Figure 9.2 is taken fi"om Ref. [13] and shows the calculated absorption cross section based on the model associated with the photodissociation of I2. (The I2 formed through the absorption process is very short lived.) Photodissociation predicted to be enhanced as the molecule is placed near a silver metal nanoparticle of radius a - 50 nm near the electronic transition resonance position of cat) 22,200 cm . If e eiai(co) is the dielectric fiinction for the metal, a small metal nanoparticle plasmon in air will have its dipolar surface plasmon resonance at frequency <24 such that [1]... 

Figure 19.1 (A) 2D projection of the calculated local field intensity distribution around a pair of 15 nm diameter silver nanoparticles excited with Xi = 400 nm light polarized along the interpaiticle axis. The edge-to-edge particle separation is 2 nm and the free space incident light intensity Ej,x P taken to be unity. The local field intensity near the pair is shown in false color. The calculation was done using dipole-dipole approximation (DDA) method with each dipole unit being a square with sides of 0.2 nm. (B) Model of the photophysics of a molecule represented by a three level system and how the excitation and decay dynamics are affected by plasmon enhancement of radiative rates and the introducticm of a rate for quenching Icq of the excited state due to proximity to the metal surface. E (X ) and E (X2) are the field enhancements at the position of the molecule for the excitation and emission wavelengths respectively, kn and kMR represent the radiative and non-radiative decay rates of the molecule in the absence of plasmon enhancement.

Understanding the field enhancement of radiative rates is insufficient to predict how molecular photophysical properties such as enhancement of fluorescence quantum yield will be affected by interactions of the molecule with plasmons. A more detailed model of the photophysics that accounts for non-radiative rates is necessary to deduce effects on photoluminescence (PL) yields. Such a model must include decay pathways present in the absence of metal nanoparticles as well as additional pathtvays such as charge transfer quenching that are associated with the introduction of the metal particles. Schematically, we depict the simplest conceivable model in Figure 19. IB. Note that both the contributions of radiative rate enhancement and the excited state quenching by proximity to the metal surface will depend on distance of the chromophore from the metal assembly. In most circumstances, one expects the optimal distance of the chromophores from the surface to be dictated by the competition between quenching when it is too close and reduction of enhancement when it is too far. The amount of PL will be increased both due to absorption enhancement and emissive rate enhancement. Hence, it is possible to increase PL substantially even for molecules with 100 % fluorescence yield in the absence of metal nanoparticles. 

For reasons clear from the introduction, enhancement of phosphorescence is a particularly attractive application of plasmonics to OLED technology. Since carriers are injected into an OLED from separate contracts, their spins are uncorrelated and spin statistics dictate preferential formation of triplet excited states. Since these are generally poor emitters at ambient temperature, metallic enhancement of the phosphorescent rate would be desirable. Moreover, triplet states are typically long-lived and prone to oxidation reactions so that reduction of the triplet lifetime could potentially improve stability of the phosphors. PtOEP is a model phosphor and its application to electroluminescence was pioneered by the groups of Forrest and Thomson (46-47). We have investigated plasmonic enhancement of the PtOEP phosphorescence on silver surfaces prepared using the Tollens reaction. Dilute PtOEP in a polymer binder was spin cast onto substrates with various densities of nanotextured silver and assumed to deposit conformally, the spin speed being used to control the approximate thickness of the overlayer. 

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