SPR absorption

In this Section we want to present one of the fingerprints of noble-metal cluster formation, that is the development of a well-defined absorption band in the visible or near UV spectrum which is called the surface plasma resonance (SPR) absorption. SPR is typical of s-type metals like noble and alkali metals and it is due to a collective excitation of the delocalized conduction electrons confined within the cluster volume [15]. The theory developed by G. Mie in 1908 [22], for spherical non-interacting nanoparticles of radius R embedded in a non-absorbing medium with dielectric constant s i (i.e. with a refractive index n = Sm ) gives the extinction cross-section a(o),R) in the dipolar approximation as ... 

Theoretically, SPR absorption can be estimated by solving Maxwell s equations. Gustav Mie rationalized this for spherical particles in 1908. Nowadays these equations can be solved to predict the corresponding SPR bands for spheres, concentric spherical shells, spheroids and infinite cylinders, and an approximation is required for other geometries. The routine measurement of the SPR absorption of most reported processes of synthesis of Au NPs is, indeed, one of the key points for the characterization of new nanomaterials [183]. 

The SPR is then also called Mie resonance. For simple metals, the SPR absorption band has a Lorentzian shape peaked at oi p, the width of which is directly proportional to the collision constant E introduced in the Drude description of the metal dielectiic constant (Eq. 2). Of course, for noble metals the absorption due to interband transitions has to be taken into account in order to obtain the complete spectrum. 

Whereas Eq. (8) succeeds in explaining qualitatively the broadening and damping of the SPR absorption band with decreasing nanoparticle size, it presents some major drawbacks. First, the parameter A takes different values, from tenths to few units, depending on the theory. The value A = 1 is arbitrarily the most often used. Secondly, the introduction of such a 1/7 dependence in the Drude model results in the red-shift of the SPR with decreasing size, whereas a blue-shift is observed for noble metal nanoparticles [19]. This is due to the influence of bound d electrons which is ignored in the size-dependence considerations that we have described until now [24-27]. However - and even if it cannot of course explain on its own all the size effects - the 1 /7 dependence of different factors is an attractive intuitive... 

The size-dependence of the width and energy of surface plasmon absorption band at 293 K and 77 K we reported earlier [1]. In this paper we discuss the temperature dependence of SPR absorption bandwidth and its energy. 

Fig. 2 depicts the temperature dependence of SPR absorption bandwidth for small particles (< 20 nm), where the monotonic increase of plasmon peak width with temperature takes place. This is similar to the dependences observed for SP in small (1.6-20 nm) gold nanoparticles [2]. 

Such a charge static redistribution due to the deposition of an adsorbate on the particle surface and the respective change in the electron concentration in the MNPs were also observed in the SPR absorption spectra [2, 59]. In metals (silver, sodium, aluminum, etc.), where free conduction electrons dominate, the SPR spectral maximum depends on the concentrations of electrons, N, in nanoparticles as... 

Another example of visible light-driven reaction through non-band-gap excitation is a photocatalytic reaction that uses surface-plasmon resonance (SPR) absorption of small metal particles loaded on base metal oxides. For example, gold particles of the size of ten to several ten nanometers, presenting purplish red color by the SPR absorption, loaded on titania particles have been used for photocatalytic reactions under visible-light irradiation at the wavelength of ca. 600 nm. Based on the results that titania or a related material is necessary for this visible fight-driven reactimi and that SPR absorption cannot induce electnaiic excitation of electrons, the mechanism of this kind of reaction seems complicated and is now under discussimi. 

UV-vis spectra of synthesized sols at pH = 7-7.3 revealed a broad surface plasmon resonance (SPR) absorption with a maximum at 400-450 nm. The position, shape, and intensity of the plasmon resonance depend on the silver particle size, their concentration and nanoparticles (NPs) size distribution. The absorption at 400 nm corresponds to Ag nanoparticles of 10-20 nm in size. Red shift of SPR and broad absorption bands testify the formation of the particles larger 20 nm and also demonstrate high... 

This is usually used to measure the optical absorption spectra of active species (such as rare earth ions, noble metal nanoparticles, and quantum dots) in sol-gel films, between 190 and 2500 nm, with a resolution of 1 nm. From the surface plasmon resonance (SPR) absorption bands of noble metal NPs in sol-gel hosts, the particle sizes can also be evaluated, since the concentration of NPs such as Au and Ag in sol-gel hosts is often too low to be detected by XRD. 

Figure 22.18 depicts the experimental and Mie-simulated absorption spectra for composites Ag 0.02 and Ag 0.04, with Ag/Si molar ratios of 2 and 4%, respectively. Sample Ag 0.04 exhibits a strong absorption peak at 410 nm, due to the well-known SPR absorption of Ag NPs in silica [64], with some broadening compared with the calculated one. A slight red shift in peak position was observed for Ag 0.02, again compared with the calculated spectrum. The SPR interaction is a result of the collective oscillations of all free electrons at the surface of the silver particles, upon interaction with the electromagnetic radiation, whose electric field induces the formation of a dipole in the NP. A restoring force in the... 

Figure 22.18 Mie simulation and experimental SPR absorption spectra for nanocomposites with Ag/Si molar ratios of 0.02 and 0.04. (After Ref. [63].)...

---