Plasmon resonance frequency shift

Instead of precursor metal salts, metal oxides could also be used. Silver-PS coreshell particles were thus recently synthesized using a two-step procedure [346]. First, hydrogen reduction of a saturated silver(I) oxide solution at elevated temperature was carried out in the presence of commercial sulfate-functionalized PS particles (200 nm) used as support for the reduction of Ag salt into silver. The formed Ag nanoparticles were attached to the PS particles (Fig. 42a). Subsequent acetone treatment led to the encapsulation of the silver nanoparticles inside the PS spheres (Fig. 42b), accompanied by a red-shift of the characteristic plasmon resonance frequency of the particles (Fig. 42c). 

The piezo-electric effect of deformations of quartz under alternating current (at a frequency in the order of 10 MHz) is used by coating the crystal with a selectively binding substance, e. g. an antibody. When exposed to the antigen, an antibody-antigen complex will be formed on the surface and shift the resonance frequency of the crystal proportionally to the mass increment which is, in turn, proportional to the antigen concentration. A similar approach is used with surface acoustic wave detectors [142] or with the surface plasmon resonance technology (BIAcore, Pharmacia). 

For the same particles, the volume plasmon is located at very high energies (6-9 eV). The surface obviously plays a very important role for the observation of the surface plasmon resonance because it alters the boundary conditions for the polarizability of the metal and therefore shifts the resonance to optical frequencies. In this sense, the surface plasmon absorption is a small particle (or thin layer) effect but is definitely not a quantum size effect [14]. 

The Novotny group has produced similar theoretical results with calculations on spherical nanoparticles, showing that fluorescence enhancement due to metal near field effects is strongly frequency dependent and that florescence enhancement is maximized when the fluorophore emits red-shifted to the plasmon resonance peak of the nanoparticle. They also explained this result as a consequence of the slight offset of the frequency dependence of the quenching term and enhancement term. ... 

It is clear from the foregoing considerations that the surface plasmon is shifted by interaction with the oscillatory modes of the adsorbed layer, and new coupled modes are introduced. In fact, the adsorbed layer substantially changes all the dielectric response properties of the substrate in accordance with Eq.(22). In consequence of this, its optical properties are modified, in particular in surface plasmon resonance experiments (as well as in all other probes). Analysis of such modifications reflect on the nature of the oscillatoiy modes of the adsorbate, which can identify it for sensing purposes. It should be noted that the determination of the screening function K (Eq.(22), for example) not only provides the shifted coupled mode spectram in terms of its frequency poles, but it also provides the relative oscillator strengths of the various modes in terms of the residues at the poles. The analytic technique employed here for the adsorbate layer (in interaction with the substrate) can be extended to multiple layers, wire- and dot-like structures, lattices of such, as well as to the case of a few localized molecular oscillators. It can also take account of spatial nonlocality, phonons, etc., and the frequencies of the shifted surface (and other) plasmon resonances can be tuned by the application of a magnetic field. 

However, the resonance described here refers to an incident field with the electric field polarized parallel to the axis of symmetry of the spheroidal nanoparticle. There is another plasmon resonance associated with the incident electric field polarized perpendicular to the symmetry axis. This resonance is identical in frequency to the parallel resonance for a sphere, but it shifts in the opposite direction for a spheroid, i.e., blueshifting for prolate spheroids and red-shifting for oblate spheroids. The parameter x for the two cases of polarization parallel and perpendicular to the axis of the ellipsoid is given by [49] ... 

Af/f is small whenever rq,2 is close to one. Conversely, since the QCM only works well when the normalized frequency shift Af/ff is small, it makes sense to assume 1. Equation 39 shows that quartz crystals are acoustic re-flectometers. The results of QCM measurements can therefore be easily compared to data obtained with other forms of ultrasonic reflectometry [57,58]. It is well known from optical techniques such as elUpsometry [59] or surface plasmon resonance (SPR) spectroscopy [60] that a film thickness can be inferred from a measurement of the reflectivity. The same applies to acoustics. 

When the applied electric field is directed along the long axis of the ellipsoid, a further enhancement of the electric field in the space between the ellipsoids and a red shift of the resonance frequency take place due to the strong electric field coupling that cannot occur when the electric field is directed along the short axis. This coupling, which is called surface plasmon resonance (SPR), broadens the surface plasmon modes at IR wavelengths [294, 349, 350,404] and promotes the SEIRA effect. 

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]. 

Extinction calculations for aluminum spheres and a continuous distribution of ellipsoids (CDE) are compared in Fig. 12.6 the dielectric function was approximated by the Drude formula. The sum rule (12.32) implies that integrated absorption by an aluminum particle in air is nearly independent of its shape a change of shape merely shifts the resonance to another frequency between 0 and 15 eV, the region over which e for aluminum is negative. Thus, a distribution of shapes causes the surface plasmon band to be broadened, the... 

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