Excitation conduction electrons

The photophysical origin of plasmonic heating can be attributed to the nonradia-tive relaxation of the optically excited conduction electrons, whose energies then diffuse into the metal lattice (electron-phonon coupling) and are dissipated as heat into the surrounding medium (phonon-phonon interactions).71-73 Under ideal conditions, the nanoparticle temperature can be estimated by the following equation 73... 

In an intrinsic semiconductor, tlie conductivity is limited by tlie tlieniial excitation of electrons from a filled valence band (VB) into an empty conduction band (CB), across a forbidden energy gap of widtli E. The process... 

In a defect-free, undoped, semiconductor, tliere are no energy states witliin tire gap. At 7"= 0 K, all of tire VB states are occupied by electrons and all of the CB states are empty, resulting in zero conductivity. The tliennal excitation of electrons across tire gap becomes possible at T > 0 and a net electron concentration in tire CB is established. The electrons excited into tire CB leave empty states in tire VB. These holes behave like positively charged electrons. Botli tire electrons in the CB and holes in tire VB participate in tire electrical conductivity. 

All teclmologically important properties of semiconductors are detennined by defect-associated energy levels in the gap. The conductivity of pure semiconductors varies as g expf-A CgT), where is the gap. In most semiconductors with practical applications, the size of the gap, E 1-2 eV, makes the thennal excitation of electrons across the gap a relatively unimportant process. The introduction of shallow states into the gap through doping, with either donors or acceptors, allows for large changes in conductivity (figure C2.16.1). The donor and acceptor levels are typically a few meV below the CB and a few tens of meV above the VB, respectively. The depth of these levels usually scales with the size of the gap (see below). 

Consider Figure la, which shows the electronic energy states of a solid having broadened valence and conduction bands as well as sharp core-level states X, Y, and Z. An incoming electron with energy Eq may excite an electron ftom any occupied state to any unoccupied state, where the Fermi energy Ap separates the two... 

For a material to be a good conductor it must be possible to excite an electron from the valence band (the states below the Fermi level) to the conduction band (an empty state above the Fermi level) in which it can move freely through the solid. The Pauli principle forbids this in a state below the Fermi level, where all states are occupied. In the free-electron metal of Fig. 6.14 there will be plenty of electrons in the conduction band at any nonzero temperature - just as there will be holes in the valence band - that can undertake the transport necessary for conduction. This is the case for metals such as sodium, potassium, calcium, magnesium and aluminium. 

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 ... 

First we consider the origin of band gaps and characters of the valence and conduction electron states in 3d transition-metal compounds [104]. Band structure calculations using effective one-particle potentials predict often either metallic behavior or gaps which are much too small. This is due to the fact that the electron-electron interactions are underestimated. In the Mott-Hubbard theory excited states which are essentially MMCT states are taken into account dfd -y The subscripts i and] label the transition-metal sites. These... 

Much of the Pt Mossbauer work performed so far has been devoted to studies of platinum metal and alloys in regard to nuclear properties (magnetic moments and lifetimes) of the excited Mossbauer states of Pt, lattice dynamics, electron density, and internal magnetic field at the nuclei of Pt atoms placed in various magnetic hosts. The observed changes in the latter two quantities, li/ (o)P and within a series of platinum alloys are particularly informative about the conduction electron delocalization and polarization. 

Image plates use stimulated luminescence from storage phosphor materials. The commercially available plates are composed of extremely fine crystals of BaFBrEu2+. X-rays excite an electron of Eu2+ into the conduction band, where it is trapped in an F-center of the barium halide with a subsequent oxidation of Eu2+ to Eu3+. By exposing the BaFBrEu" complex to light from a HeNe laser the electrons are liberated with the emission of a photon at 390 nm [38]. 

Semiconductors are materials that contain a relatively small number of current carriers compared to conductors such as metals. Intrinsic semiconductors are materials in which electrons can be excited across a forbidden zone (bandgap) so that there are carriers in both the valence (holes, p-type) and conduction (electrons, ra-type) bands. The crucial difference between a semiconductor and an insulator is the magnitude of the energy separation between the bands, called the bandgap (Eg). In the majority of useful semiconducting materials this is of the order of 1 eV some common semiconductors are listed in Table 1. 

One source of EM enhancement may be attributed to the excitation of surface plasmons (SP) in the metal. A plasmon is a collective excitation in which all of the conduction electrons in a metal oscillate in phase. In the bulk, there is essentially only one allowed fundamental plasmon frequency. 

Quantum detectors are based on semiconductors. The absorption of a photon excites an electron from the valence band into the conduction band. This can be measured either through a change in resistance (photoconductive... 

The electronic structure and hence optical properties of nanomaterials depend on the core size. For example, nanoparticles of core size >3 nm show surface plasmon resonance, which is due to the excitation of surface plasmons of nanoparticles by light. When the size of gold nanoparticles comes down to around 1 nm, which is equal to the de Broglie wavelength of the conduction electrons, the electronic bands... 

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