Oscillation collective electronic

For the p-type substrate a significant number of electrons are collected at the backside, as shown in the top part of Fig. 3.2. This is true not only for the illuminated p-type electrode but also if the electrode is kept in the dark, which indicates that electrons are injected during the tetravalent dissolution reaction. In the regime of oscillations the electron injection current is found to oscillate, too [CalO]. 

As is specified in work [17], the system of conductivity electrons in a nanocrystal is similar to the resonator. Thereof electromagnetic field in a crystal can give rise to collective electronic excitations [17], which refer to as surface plasmons as they are caused by charges on a nanocrystal surface in dielectric [16]. At a>v dielectric permeability e (oj) according to the formula (5) is negative. It means that conductivity electrons in a nanocrystal oscillate out of phase with electric oscillations of an external field [16]. 

Surface plasmon resonance (SPR) biosensors exploit special electromagnetic waves-surface plasmon-polaritons-to probe interactions between an analyte in solution and a biomolecular recognition element immobilized on the SPR sensor surface. A surface plasmon wave can be described as a light-induced collective oscillation in electron density at the interface between a metal and a dielectric. At SPR, most incident photons are either absorbed or scattered at the metal/dielectric interface and, consequently, reflected light is greatly attenuated. The resonance wavelength and angle of incidence depend upon the permittivity of the metal and dielectric. 

In Fig. 8 we also provide electronic couplings reported by Tretiak et al. [11] for the LH2 of Rs. moUschianum. These couplings were calculated using the collective electronic oscillators (CEO) method [86, 87]. Note that the B800-carotenoid couplings differ between the two species, owing to the 90° difference in orientation of the B800 Bchls. 

In quite another area of physics, the discovery of magic numbers in alkali and other metallic clusters [7] has provided a fresh example of the significance of electronic shell closure. These much larger shells have been shown to oscillate collectively, and the resulting oscillations are of great significance as an example of a many-body resonance. They are discussed at some length in chapter 12. 

The shell model for metal clusters, described above, has an important implication which will not have escaped the reader if electrons become delocalised from individual atoms and can roam freely over the whole cluster to form a closed shell, then this shell should be able to oscillate collectively, and should therefore exhibit giant dipole resonances analogous to those which were described in chapter 5 for free atoms. 

Electron energy-loss spectroscopy at low excitation energies is a surface sensitive technique to study the electronic structure by exciting collective oscillations or electrons from occupied into unoccupied states. In metals with a high density of states arising from d electrons, the excitation of plasmon losses has a relatively low probability. Therefore, the spectra are dominated by interband or intraband transitions. In rare earth metals, excitations of the partially filled/shell are observed that are assigned to be dipole-forbidden 4/4/transitions. These transitions are enhanced near the 4ii-4/threshold [56]. 

Optical properties of copper nanoparticles are quite remarkable because the energy of the dipolar mode of surface collective electron plasma oscillations (surface plasmon resonance or SPR) coincides with the onset of interband transition. Therefore, optical spectroscopy gives an opportunity to study the particle-size dependence of both valence and conduction electrons. The intrinsic size effect in metal nanoparticles, caused by size and interface damping of the SPR, is revealed experimentally by two prominent effects a red shift of the surface plasmon band and its broadening. 

The electronic excitations caused by electron energy loss processes can be divided into two main classes collective electronic excitations and single electron excitation. The collective excitations may be regarded as plasma oscillations of a free or nearly free electron gas embedded in a homogeneously distributed positive charge. An extensive treatment of collective excitation losses is given by Lucas and Sunjic (1972). In a classical treatment the electron plasma frequency a>p is given by... 

The ultraviolet (UV) - visible spectrophotometer is another important tool in the characterisation of vegetable oil-based polymer nanocomposites and is particularly effective for metal nanocomposites. The formation of metal nanoparticles in the matrix can be easily detected by UV-visible spectroscopy. Every metal nanoparticle has its own characteristic surface plasmon resonance value. This band is attributed to the collective oscillation of electron gas in the nanoparticles, with a periodic change in the electronic density at the surface. Parameters such as particle size, shape and dielectric constant of the medium and surface adsorbed species determine the position and shape of the plasmon absorption. When the particles become significantly smaller than the mean free path of electrons in the bulk metal, the plasmon oscillation is dampened. The plasmon absorption peak shifts to a higher wavelength than expected with an increase in aggregation of the nanoparticles. The sharpness of the peak indicates the narrow size distribution. 

Figure 1.1 Optical absorption of small silver particles Ag embedded in argon at low temperatures, according to Ref. [11]. The huge absorption hump is a collective electronic oscillation localized at the interface Ag/Ar. This figure historically gave the impact for the development of the super-atom model for metal clusters. For large clusters a broad, damped peak is observed, whereas for small clusters the line is fragmented...

If metal particles become very small, reaching the nanometer-size scale, a color may occur. This is a typical phenomenon of nanometric metals. Actually, optical absorption may result in the ultraviolet or visible part of the spectrum, and this arises from a surface plasmon resonance. This is due to a collective electron plasma oscillation (plasmon) that is coupled to an external transverse electromagnetic field through the particle surface. It is possible to quantitatively relate the absorption coefficient to the wavelength of the exciting radiation by the Mie theory for spherical inclnsions in a dielectric matrix (34). Far-IR Inminescence is another optical phenomenon frequently observed with nanosized metals (46). 

Plasmon Collective oscillation of electrons in a solid, often of characteristic frequency. 

Collective oscillation of electron gas in metal is known as plasmon. Generally, plasmon refers to the longitudinally collective oscillation of electron gas with respect to the crystal lattice. Plasmon can be crudely categorized as bulk, surface, and particle (Mie) plasmons. The bulk plasmon denotes a collective excitation of the electron gas in the bulk of the metal, which propagates as a longitudinal charge density fluctuation at a resonance frequency ( pj) as mentioned in Equation 13.1. 

The metal NPs which have both size and optical penetration-depth smaller than wavelength of light, all atoms in those particles can be collectively excited. Hence, collective electronic oscillations are known as Mie plasmon. For Mie Plasmon, prominent optical resonance is observed in UV-Vis range. The resonance frequency of the oscillation depends on dielectric properties of the metal, surrounding medium. 

C. Wang, S. Poliak, and M.M. Kappes, Molecular Excited States versus Collective Electronic Oscillations Optical Absorption Probes of Na4 and Nas , Chem. Phys. Lett. 166, 26 (1990). 

In another example of energy transfer, an individual electron can transfer energy to several electrons. Such an example is the excitation of a plasmon, a collective oscillation of electrons in the conduction band, known as a plasmon-loss. The shape of the plasmon-loss feature is directly related to the electron structure of the solid. Examples of plasmon-loss structures for silicon with a native oxide and SiOa are shown in Fig. 3.11. Silicon, being semi-metallic exhibits very sharp plasmon-loss features with a smaller separation (hcai) as compared to the broader features of SiOa, which is a wide bandgap insulator. 

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