Absorption spectra 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. 

The optical absorption properties of noble metal nanoparticles in the visible range of the electromagnetic spectrum are determined by the effect of the boundary condition of the coherent electron oscillations as well as by d ip electronic transitions. Very small gold nanoparticles (d<2 nm), as well as bulk gold do not show a localized surface plasmon absorption band As discussed earlier (see Fig. 16.5), gold nanorods... 

Figure 9.3 (A) Schematic illustration of PPy nanoparticles prepared in an aqueous dispersion of water-soluble polymer/metal cation complexes. (B) Transmission electron microscopy (TEM) image of the as-prepared PPy nanoparticles. (C) UV-Vis-NIR absorption spectrum of PVA stabilized PPy nanoparticles dispersed in water and stored at 4 °C for 6 months (inset photograph is the as-prepared PPy sample). Heating curves of PPy at various concentrations (D) and the comparison between PPy NPs and Au nanorods over five cycles of NIR laser irradiation (E). Reproduced from ref. 27 with permission from John Wiley and Sons. Copyright 2013 Wiley-VCH Verlag GmbH Co. KGaA, Weinheim.

Noble-metal nanoparticles embedded in a dielectric medium show strong absorption peaks in the visible region of the UV-vis spectrum due to the collective motion of free electrons (surface plasmon resonance). 

The nanoparticles of precious metals possess the unique optical properties connected with the presence of one or several resonance peaks in visible and near IR-area in absorption spectrum. These peaks are caused by so-called localized plasmon resonances. Plasmon resonance peaks are the result of excitation of... 

As the simplest nanoantennas, plasmonic nanoparticles can be utilized to enhance the absorption within thin-film solar cells [243]. They couple incoming waves with the localized SPP field, have increased scattering cross-section and strongly localize electromagnetic field just in the thin active region of the detector. Fig. 2.62. The same principle is applicable for infrared detection [321]. This cannot be done with pure noble metal nanoparticles since their surface plasmon resonance is in ultraviolet or visible part of the spectrum. Because of that their response must be redshifted. In this part, two approaches to such redshifting are described. 

A similar oxidation process was observed for the other metal nanoparticles produced via RESOLV. For example, when a PVP polymer-stabilized suspension of Ag nanoparticles was purifled via dialysis against freshwater, the UV-vis absorption spectrum was signiflcanfly altered (Figure 34). Gradual disappearance of the plasmon absorption band was probably due to the oxidation of the Ag nanoparticles (263). Similarly, oxidation of Cu nanoparticles in a suspension was evidenced by the suspension color changing gradually from dark yellow to blue (263). 

As mentioned above. Ere/ (and thus Emoz) can be highly amplified when the incident field has a frequency in resonance with a plasmon excitation. For metal nanoparticle small with respect to the wavelength, only the dipolar plasmon can be excited. It is educative to recall (see Sec. 1.4.1) the very simple case (a small spherical metal nanoparticle described by the Drude dielectric constant in the vacuum) and compare its absorption spectrum (dominated by the plasmon band) and the intensity of Ere/ at a fixed point along the direction of oscillation of the dipolar plasmon. The absorption cross-section Cabs is given by (see Eq. (1.299)) ... 

A typical UV—visible spectrum of the GNP obtained by the reduction of HAuCLi 3H20 is shown in Figure 2.12. Spherical particles with diameters less than 100 nm exhibit a single strong absorption peak in the visible region of the spectmm. The peak intensity and position of the surface plasmon absorption band are dependent on the size and shape of the metal nanoparticles as well as the surrounding medium of the particle. Thus, the observed peak at 525 nm clearly revealed the formation of GNP (Rajesh et al., 2010). 

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