Strongly localized defect states

In PPV a PI A peak at 1.4 eV has been assigned to a triplet-triplet transition from the triplet exciton [148,150], as in PDA (it is not known if the near equality in the energies is accidental). But, in addition, two other induced absorptions are observed near 0.6 and 1.6 eV, and since they are associated with the characteristic IR bands (such as the 0.45-eV band in PA), they should be due to a charged state. The absence of ODMR signal suggests that they have no spin and would then be bipolarons. In improved PPV (see Fig. 15), the PIA spectrum contains only the triplet peak [151], suggesting that the presence of the other features is a consequence of strong localization in a defective polymer. Similar results are found in other CPs, but up to now evidence for PIA due to polarons is elusive. 

A defect in the photonic crystal leads to allowed states for particular frequencies and a strong localized mode in the band gap, as shown in Fig. 4. Point defects have great importance for the control of spontaneous emission and light localization events [13]. These can be created by removing one unit structure, by changing the local refractive index, or by modifying the size of the structures. 

The knowledge of the dispersion curves, of the spectroscopic activity of the k = 0 phonons and their precise frequencies has allowed us to locate the energy gaps, where to expect localized defect modes, and to predict where the activation of the density of states singularities (strong peaks in g(v) with no activity in IR and/or Raman) due to lack of symmetry because of disorder could generate extra absorption or scattering. 

Specific Electric Conductivity The specific electric conductivities a of LGS, LGN and LGT were measured using a four-point technique at temperatures between 795 and 1000 K on crystal slices cut perpendicular to the Z-axes. The temperature dependence of the conductivities in dielectric materials with a limited defect density are caused by localized electronic states, and thus the data yield a straight line in a plot of In c versus 1/T, the slope of which is a measure of the activation energy AE of the charge carriers in a Mott-type transition model. These activation energies were found to be 1.1 (LGS), 1.0 (LGN) and 0.9 (LGT) eV, respectively. The rather strong increase in electric conductivity (LGS 10 Q cm at 530°C, 5 X lO- Q- cm- at 730 °C LGT lO- fi- cm" at 600 °C, 8 x 10-"Q- cm- at 730 °C) appears to limit the high-temperature applications of these materials. 

Electron polarization frequently happens at sites surrounding atoms with even lower atomic CNs than that of a flat surface, namely four. As a consequence of the local polarization and entrapment, transition from conductor to semiconductor happens to small clusters such as 3-nm-sized A1 nanoislands deposited on Si substrate [1]. The broken-bond-induced densification and localization of electrons with lowered binding energy in the traps have been observed as defect states [2], chain end states [3, 4], terrace edge states [5-7], and surface states [8-10]. Strong localization of excess electrons also happens to the surface of ice [11] and metal films [12]. 

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