Plasmon peak

The degree of surface cleanliness or even ordering can be determined by REELS, especially from the intense VEELS signals. The relative intensity of the surface and bulk plasmon peaks is often more sensitive to surface contamination than AES, especially for elements like Al, which have intense plasmon peaks. Semiconductor surfaces often have surface states due to dangling bonds that are unique to each crystal orientation, which have been used in the case of Si and GaAs to follow in situ the formation of metal contacts and to resolve such issues as Fermi-level pinning and its role in Schottky barrier heights. 

C. J. Powell. Opt. Soc. Amer. 59, 738, 1969. Excellent presentation of the interaction between interband and plasmon peaks that is often overlooked in REELS spectral analysis. 

Figure 2.36 A shows a typical low-loss spectrum taken from boron nitride (BN). The structure of BN is similar to that of graphite, i. e. sp -hybridized carbon. For this reason the low-loss features are quite similar and comprise a distinct plasmon peak at approximately 27 eV attributed to collective excitations of both n and a electrons, whereas the small peak at 7 eV comes from n electrons only. Besides the original spectrum the zero-loss peak and the low-loss part derived by deconvolution are also drawn. By calculating the ratio of the signal intensities hot and Iq a relative specimen thickness t/2 pi of approximately unity was found. Owing to this specimen thickness there is slight indication of a second plasmon.

When the silver nanocrystals are organized in a 2D superlattice, the plasmon peak is shifted toward an energy lower than that obtained in solution (Fig. 6). The covered support is washed with hexane, and the nanoparticles are dispersed again in the solvent. The absorption spectrum of the latter solution is similar to that used to cover the support (free particles in hexane). This clearly indicates that the shift in the absorption spectrum of nanosized silver particles is due to their self-organization on the support. The bandwidth of the plasmon peak (1.3 eV) obtained after deposition is larger than that in solution (0.9 eV). This can be attributed to a change in the dielectric constant of the composite medium. Similar behavior is observed for various nanocrystal sizes (from 3 to 8 nm). 

The UV-visible spectrum (Fig. 6) of the aggregates described earlier shows a 0.25-eV shift toward lower energy of the plasmon peak with a slight decrease in the bandwidth (0.8 eV) compared to that observed in solution (0.9 eV). As observed earlier with monolayers, by washing the support, the particles are redispersed in hexane and the absorption spectrum remains similar to that of the colloidal solution used to make the self-assemblies. 

Figure 3 shows the UV-vis DRS spectra of the three groups of catalysts In all cases, a prominent Au plasmon peak around 525 run was observed. This peak was sharper for catalysts of both groups A and B, and broader for catalysts of group C. That is, catalysts of lower C.F. s had broader peaks. In addition, there were three peaks at 270, 230, and 200 nm. These bands were related to the hydroxyls on AljOj, since they were observed on pure AljO, also, and their intensities changed with the moisture content of the sample. 

Remarkably the position of the final plasmon peak of the alloy particles is dependent on the molar ratio of gold to silver nanoparticles. When the ratio is shifted favoring either metal, an alloy of any desired composition can be formed. This alloying phenomenon indicates that it is possible for true tuneability of the properties of a set of nanoparticles. 

This condition predicts that N.A. = 1.4 would capture the surface plasmon peak from an aluminum film surface but not from a silver-film surface. Therefore, since objectives with aperture higher than 1.4 are rather rare, an aluminum film is a better choice. 

We must also take into account two further factors. First, the fact that the transmission efficiency of the analyzer is a fimction of the kinetic energy (K.E.) of the photoelectrons in the ESCA-3 Vacumn Generators instrument the transmission is inversely proportional to the K.E. of the electrons (3a). Second, photoelectron yields must refer to total yield from a particular ionization process and this need not, for example, be just the area of the relevant peak. Account must be taken of all processes that divert electrons from the primary peak, e.g., shake-up, shake-oflF, and plasmon peaks. In some cases, e.g., emission from the Cu 2P3/2 level, the contribution of additional processes is small but in others, and emission from the Al(2p) shell is an example, the no-loss peak is substantially less than the true Al(2p) emission. 

Fig. 4.18 The different degree to which electrons move collectively in various forms of carbon material as evidenced by distinct intensity of the plasmon peak located about 6 eV in EELS spectra (arrow). Hydrogen atoms can make less strong covalent bonds with participation of n electrons if the interplanar distance is increased in layered graphitic nanocrystals as seen in carbon nanosheUs (frame in Fig. 4.17) and in disordered graphitic carbons (Sect. 4.3.1). After [60]...

For isolated particles, the increase in the dielectric constant induces a shift to the lower energy and an increase in the bandwidth of the plasmon peak. For particles... 

EELS study of this interface was carried out by Batson [5], It revealed the presence of Si2+ (SiO like layer) and possible defect states at the boundary (Figure 2a). LELS can also give information about the interface. We showed that the use of relativistic formula was essential to explain the shift of the interface plasmon peak (IPP) as the probe was moved away from the interface (Figure 2b). The agreement with the experiment allowed us to conclude that a 1 nm thick SiO layer had to be introduced between Si and Si02 to fit the IPP position precisely [6],... 

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