Metallic nanoparticles, plasmon excitation

Figure 7.4 Schematic view of plasmon excitation within metallic nanoparticle via interaction with electromagnetic field...

The decay of the nanoparticle plasmons can be either radiative, ie by emission of a photon, or non-radiative (Figure 7.5). Within the Drude-Sommerfeld model the plasmon is a superposition of many independent electron oscillations. The non-radiative decay is thus due to a dephasing of the oscillation of individual electrons. In terms of the Drude-Sommerfeld model this is described by scattering events with phonons, lattice ions, other conduction or core electrons, the metal surface, impurities, etc. As a result of the Pauli exclusion principle, the electrons can be excited into empty states only in the CB, which in turn results in electron-hole pair generation. These excitations can be divided into inter- and intraband excitations by the origin of the electron either in the d-band or the CB (Figure 7.5) [15]. 

Schaadt DM, Feng B, Yu ET (2005) Enhanced semiconductor optical absorption via surface plasmon excitation in metal nanoparticles. Appl Phys Lett 86(6) 063106... 

An LSPR bio-sensor based on particle plasmons uses plasmon excitation in small metal particles that are often sized at a few tens of nanometers. Metallic nanoparticles... 

Nano-objects made out of noble metal atoms have proved to present specific physicochemical properties linked to their dimensions. In metal nanoparticles, collective modes of motion of the electron gas can be excited. They are referred to as surface plasmons. Metal nanoparticles exhibit surface plasmon spectra which depend not only on the metal itself and on its environment, but also on the size and the shape of the particles. Pulse radiolysis experiments enabled to follow the evolution of the absorption spectrum during the growth process of metal clusters. Inversely, this spectral signature made it possible to estimate the metal nanoparticles size and shape as a function of the dose in steady-state radiolysis. 

As briefly mentioned in the Introduction the MEF phenomenon is a result of the interactions between the excited states of the fluorophores and the induced surface plasmons of metal nanopaiticles or roughened surfaces. Figure 1.2A depicts our laboratory s interpretation of the processes involved in the interactions of fluorophores with metal nanoparticles in close proximity to one another. There are two main processes thought responsible in MEF 1) non-radiative coupling from the excited state of the fluorescent species to surface plasmons of the metallic... 

The detected fluorescence can be significantly enhanced, however, by exploiting the plasmonic enhancement which can occur when a metal nanoparticle (NP) is placed in the vicinity of a fluorescent label or dye [1-3]. This effect is due to the localised surface plasmon resonance (LSPR) associated with the metal NP, which modifies the intensity of the electromagnetic (EM) field around the dye and which, under certain conditions, increases the emitted fluorescence signal. The effect is dependent on a number of parameters such as metal type, NP size and shape, NP-fluorophore separation and fluorophore quantum efficiency. There are two principal enhancement mechanisms an increase in the excitation rate of the fluorophore and an increase in the fluorophore quantum efficiency. The first effect occurs because the excitation rate is directly proportional to the square of the electric field amplitude, and the maximum enhancement occurs when the LSPR wavelength, coincides with the peak of the fluorophore absorption band [4, 5]. The second effect involves an increase in the quantum efficiency and is maximised when the coincides with the peak of the fluorophore emission band [6]. 

Highly enhanced local fields can be generated in the vicinity of a metallic spherical nanoparticle due to localized surface plasmon excitation. Using the result in Eq. (10b), the electric field components at the sur ce of the small sphere can be... 

Surface plasmons (SPs) are collective electronic excitations near the surfaces of metallic structures. They can usually be described well with classical electromagnetic theory and correspond to electromagnetic fields that are localized and relatively intense near the metallic surfaces [1, 2]. These properties make them potentially useful for a variety of applications in optoelectronics, chemical and biological sensing, and other areas. Metallic nanostructures such as metal nanoparticles and nanostructured thin metal films, particularly those composed of noble metals such as silver or gold, are of special interest because often their SPs can be excited with visible-UV light and are relatively robust. 

In addition to dissociation and isomerization, the process of forming new chemical bonds and larger structures can also be enhanced or controlled with surface plasmon excitations. Here I would like to highlight the plasmon-assisted formation of rather larger structures than the simple molecular systems discussed so far metal nanoparticles themselves and polymeric materials. 

Figure 11.13 Schematic representation of radiative (left) and non-radiative (right) decay of particle plasmons in noble-metal nanoparticles. The non-radiative decay occurs via excitation of electron-hole pairs either vrithin the conduction band (intraband excitation) or between the d band and the conduction band (interband excitation). Reprinted with permission from reference [34]. (2002) American Physical Society.

Excitation and the relaxation (radiative and non radiative) processes of the Tryptophan solution and the colloids are represented in figure 18.7. The new relaxation pathways introduced by the metal nanoparticles (nonradiative decay rate, K p) are shown in figure 18.7(b). Although, one photon at 270nm and two-photons at 532nm are resonant with the excited states of the molecule, these wavelengths are not in resonance with the Plasmon energy level. 

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. 

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