Defect polarization-polarons

A polaron is a fermion quasi-particle consisting of an anion (or cation) defect with an associated polarized Gegenion (= counterion) atmosphere or polarization this is an excited state of the system, with energy intermediate between the valence band and the conduction band. Its mobility within the lattice is due to the fact that there is a low energy barrier for the polarization to move from one site to the next. Polarons are the dominant excitations in conducting polymers. 

Polaron — Polarons are charged quasiparticles with spin lf. This term has been introduced by physicists as one of the possible solutions to the equations of the relevant defect model of solids in order to describe an electron in a dielectric polarizing its environment (electron-phonon coupling), electrically situated below the conduction band, and transported together with its polarized environment. Polarons and -> bipolarons are the charge carriers in oxidized or reduced (doped) -> conducting polymers. A polaron is defined as a neutral and a charged -> soliton in the same... 

The mobility of electrons will only be briefly touched upon as there is extensive solid physics literature on this (see e.g., Ref.203). In many cases electrons or holes behave as polarons, i.e., similar as the ionic defects. Then, they substantially polarize their environment,... 

While experimental evidence for polaronic relaxation is extensive, other experiments render the polaron models problematic (i) the use of the Arrhenius relation to describe the temperature dependence of the mobility (see above) leads to pre-factor mobilities well in excess of unity, and (ii) the polaron models cannot account for the dispersive transport observed at low temperatures. In high fields the electrons moving along the fully conjugated segments of PPV may reach drift velocities well above the sound velocity in PPV.124 In this case, the lattice relaxation cannot follow the carriers, and they move as bare particles, not carrying a lattice polarization cloud with them. In the other limit, creation of an orderly system free of structural defects, like that proposed by recently developed self-assembly techniques, may lead to polaron destabilization and inorganic semiconductor-type transport of the h+,s and e s in the HOMO and LUMO bands, respectively. 

However, in most experimental systems, the manifestations of the polaronic character of the charge carriers are masked by the effects of disorder. In any solution-deposited thin him, disorder is present and causes the energy of a polaronic charge carrier on a particular site to vary across the polymer network. Variations of the local conformation of the polymer backbone, presence of chemical impurities or structural defects of the polymer backbone, or dipolar disorder due to random orientation of polar groups of the polymer semiconductor or the gate dielectric result in a signihcant broadening of the electronic density of states. 

There are mainly four defects that can have an important role on several properties of an oxide oxygen vacancy, metal vacancy, interstitial oxygen, and a Frenkel defect (interstitial chromium coupled to a chromium vacancy in the case of chromium oxide). Some properties depend on the experimental method of synthesis and so can it be for defects. Some charged defects induce a polaron made of electrons and holes and a polarization field. The local structural deformation extends to 3 A around the defect. For chromium oxide, a neutral chromium... 

Polarization of the non-bonding electrons is responsible for the size emergence of defect and nanostructures with properties that the bulk material does not demonstrate. This kind properties include dilute magnetism, catalysis, Dirac-Fermi polaron, magnetoresistance, etc. 

Densely entrapped bonding and core electrons polarize the nonbonding electrons (NEP) of chain ends, defects, defect edges, and nanocrystals, which yield properties that a bulk substance never demonstrates, such as Dirac-Fermi polaron creation, dilute magnetism, catalytic ability, conductor-insulator transition, hydrophobicity of liquid and solid surfaces. 

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