Plasmon bands

The physical properties of metal nanoparticles are very size-dependent. This is clear for their magnetic properties, for which the shape anisotropy term is very important. This is also true for the optical properties of nanoparticles displaying plasmon bands in the visible range (Cu, Ag, Au) and for 111-V... 

Thus, we see that the digestive ripening process leads to highly monodispersed nanoparticles that can come together to form ordered superstructures similar to atoms or molecules that form crystals from a supersaturated solution. Then if the superstructure formation can indeed be related to atomic/molecular crystallization, it should also be possible to make these supercrystals more soluble in the solvent with a change of temperature. Indeed, the optical spectra of the three colloids prepared by the different thiols discussed above exhibit only the gold plasmon band at 80 °C suggesting the solubilization of these superlattices at the elevated temperatures [49]. 

The reaction was studied for all coinage metal nanoparticles. In the case of GMEs the rate follows zero-order kinetics with IT for all the coinage metal cases. The observed IT for the Cu catalyzed reaction was maximum but its rate of reduction was found to be minimum. Just the reverse was the case for Au and an intermediate value was obtained for the Ag catalyzed reaction (Figure 7). The adsorption of substrates is driven by chemical interaction between the particle surface and the substrates. Here phe-nolate ions get adsorbed onto the particle surface when present in the aqueous medium. This caused a blue shift of the plasmon band. A strong nucleophile such as NaBH4, because of its diffusive nature and high electron injection capability, transfers electrons to the substrate via metal particles. This helps to overcome the kinetic barrier of the reaction. 

The final silver cluster diameter increases, at given initial Ag and PVA concentrations, for example from 15 to 50 nm (n 50 times larger), when the part of reduction is increasingly achieved by the donor SPV, rather than by radiolytic radicals [31]. A red shift correlates with the growth in size in the final optical surface plasmon band. Nonirradiated solutions of EDTA silver complex are stable because EDTA does not reduce the ions directly. However, after the appearance of the 400-nm spectrum of silver... 

Figure 12 Top Maximum wavelength of the plasmon band of alloyed gold-silver clusters as a function of the mole fraction x of gold in alloyed gold-silver clusters, produced at the dose rate 7.9 MGy hr and the dose 20 kGy. Experiments, calculated values by Mie model. Bottom Extinction coefficient at the maximum of the plasmon band as a function of the mole fraction x of gold in alloyed gold-silver clusters. Experiments, calculated values from Kreibig equation [74] with r = 5 nm A with r = 3 nm. (From Ref 102.)...

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... 

The detection of sharp plasmon absorption signifies the onset of metallic character. This phenomenon occurs in the presence of a conduction band intersected by the Fermi level, which enables electron-hole pairs of all energies, no matter how small, to be excited. A metal, of course, conducts current electrically and its resistivity has a positive temperature coefficient. On the basis of these definitions, aqueous 5-10 nm colloidal silver particles, in the millimolar concentration range, can be considered to be metallic. Smaller particles in the 100-A > D > 20-A size domain, which exhibit absorption spectra blue-shifted from the plasmon band (Fig. 80), have been suggested to be quasi-metallic [513] these particles are size-quantized [8-11]. Still smaller particles, having distinct absorption bands in the ultraviolet region, are non-metallic silver clusters. 

Longer irradiation than that shown in Fig. 81 resulted in decreased absorption of AgT at 275 nm and in an increase at 380 nm, which are characteristic of a developing silver plasmon band [525, 526]. 

Non-metallic gold [527], copper [528], and lead [529] clusters have also been generated by pulse radiolysis. Extension of the irradiation period resulted in cluster growth and in the concomitant development of absorption maxima at 520 nm, 580 nm, and 220 nm, corresponding to the plasmon bands of metallic gold, copper, and lead colloids, respectively [527-529],... 

Alteration of the silver plasmon band spectrum upon electron and hole injection has been rationalized in terms of changes in the density, Ne, and conductivity, o, of the electron gas in the metal particles as described by Eqs. (16)—(18) [506]. Thus, a decrease in Ne by electron extraction from the metallic silver particles increases Xc (Eq. 16) and thereby shifting the absorption maximum (Eq. 15) of the plasmon band to a longer wavelength (Fig. 83). A decrease in Ne also decreases a (Eq. 18), which leads, in turn, to an increase of w (Eq. 17) that is, to an increase in the bandwidth of the plasmon band absorption (Fig. 83). Similarly, the increase in Ne by electron transfer to the silver colloids is paralleled by a decrease in Xc (Eq. 16) and, hence, by a decrease in Xm (Eq. 15), as seen by the shift of the plasmon absorption band to a shorter wavelength (Fig. 83). Electron donation to the silver particles also causes an increase in cr (Eq. 18)... 

Mixing equal concentrations of two separately prepared solutions of colloidal lead and silver particles (each degassed by repeated freeze-pump-thaw cycles on a high-vacuum line and mixed under vacuum) resulted in a slow blue shift of the silver plasmon band from 390 nm to 337 nm and a concomitant broadening... 

Silver particles in the 10()A > D > 20 A size domain having their absorption bands blue shifted from the plasmon band position in the bulk metal. 

Polyethylene glycol has been used as reducing and stabilizing agent for Au NPs. The stability of the resulting Au colloids and the reaction rates are dependent on polymer molar mass. The Au NPs are characterized using UV-Vis, analyzing the plasmon bands [108]. 

Chitosan-stabilized Au NPs can be selectively synthesized on surfaces like poly (dimethylsiloxane) (PDMS) films using HAuC14 as precursor. The computation of surface plasmon bands (SPBs) based on Mie theory and experimental results indicates that the particles are partially coated by chitosan. The proposed mechanism implies that chitosan acts as a reducing/stabilizing agent. Furthermore, PDMS films patterned with chitosan could induce localized synthesis of gold nanoparticles in regions capped with chitosan only [110]. 

Another plasmon resonance approach for detection of mercury vapour is based on localized plasmon resonance in gold nanoparticles deposited on transparent support (Fig. 12.4, right). Changes of the refractive index of gold nanoparticles due to adsorption of mercury should lead to modification of the gold plasmon band of optical adsorption spectra. This approach has been applied successfully for investigation of interaction of biomolecules however, to our knowledge there is still no report on its applications for detection of mercury vapour. 

A relatively recent development is the exploitation of w/c microemulsions for the synthesis of metallic and semiconductor nanoparticles. By reducing silver nitrate, Ji et al. (1999) were able to harvest silver nanoparticles from a w/c microemulsion. Analysis of the plasmon resonance peak at 400 nm indicated that samples collected at intervals of 20 and 10 min were 4 nm in diameter. A subsequent decrease in the intensity of the plasmon band, over a period of 1 h, was attributed to the slow flocculation of nanoparticles. 

St Moores, A. and Goettmann, F. The plasmon band in noble metal nanoparticles an introduction to theory and applications , New J. Chem., 2006, 30, 1121-1132. 

Coinage metal nanoparticles (Au, Ag and Cu) have been particularly popular in nanoparticle research because of their easy synthesis, the high intensity of their surface plasmon band and the ease of functionalising the metal surface with ligands such as thiolates. Gold nanoparticles are prepared... 

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