Electronic bandgap conditions

The conductivity in perovskites is a direct function of the concentration of ionic species, especially vacancies, Q, and the concentration of mobile electrons, n, and holes, p, available in the material. The concentration of vacancies is often related to the crystal structure of the material, whereas the electronic bandgap conditions to the concentration of mobile electrons and holes. Overall, the total conductivity of a perovskite, <7t> can be expressed as follows ... 

Although it is required to refine the above condition I in actuality, this rather simple but impressive prediction seems to have much stimulated the experiments on the electrical-conductivity measurement and the related solid-state properties in spite of technological difficulties in purification of the CNT sample and in direct measurement of its electrical conductivity (see Chap. 10). For instance, for MWCNT, a direct conductivity measurement has proved the existence of metallic sample [7]. The electron spin resonance (ESR) (see Chap. 8) [8] and the C nuclear magnetic resonance (NMR) [9] measurements have also proved that MWCNT can show metallic property based on the Pauli susceptibility and Korringa-like relation, respectively. On the other hand, existence of semiconductive MWCNT sample has also been shown by the ESR measurement [ 10], For SWCNT, a combination of direct electrical conductivity and the ESR measurements has confirmed the metallic property of the sample employed therein [11]. More recently, bandgap values of several SWCNT... 

When semicondnctors are irradiated with photons of high energy, electron photoemission is possible, as in the case of metals. When the photon energy is lower than the electron work function in the solution, under given conditions, but is still higher than the semiconductor s bandgap W, ... 

Related Polymer Systems and Synthetic Methods. Figure 12A shows a hypothetical synthesis of poly (p-phenylene methide) (PPM) from polybenzyl by redox-induced elimination. In principle, it should be possible to accomplish this experimentally under similar chemical and electrochemical redox conditions as those used here for the related polythiophenes. The electronic properties of PPM have recently been theoretically calculated by Boudreaux et al (16), including bandgap (1.17 eV) bandwidth (0.44 eV) ionization potential (4.2 eV) electron affinity (3.03 eV) oxidation potential (-0.20 vs SCE) reduction potential (-1.37 eV vs SCE). PPM has recently been synthesized and doped to a semiconductor (24). 

Fig. 3.5 Band position of anatase Ti02, bandgap = 3.2 eV, in the presence of a pH = 1 aqueous electrolyte. The energy scale is indicated in electron volts (eV) using either normal hydrogen electrode (NHE) or vacuum level as reference showing the condition for water splitting.

It is important to emphasize that the photocatalytic reactivity of the metal ion-implanted titanium oxides under UV light (A < 280 nm) retained the same photocatalytic efficiency as the unimplanted original pure titanium oxides under the same UV light irradiation conditions. When metal ions were chemically dopec into the titanium oxide photocatalyst, the photocatalytic efficiency decreased dramatically under UV irradiation due to the effective recombination of the photo-formec electrons and holes through the impurity energy levels formed by the doped metal ions within the bandgap of the photocatalyst (in the case of Fig. 10.3).14) These results clearly suggest that metal ions physically implanted do not work as electron and hole recombination centers but only work tc modify the... 

There are three basic types of semiconductor materials depending on their ability to conduct hole (p-type), electrons (n-type), or both (ambipo-lar) under different gate bias conditions. In semiconductor materials, reduction of the bandgap (Eg) will enhance the thermal population of the conduction band and thus increase the number of intrinsic charge carriers. The decrease of Eg can led to true organic metals showing intrinsic electrical conductivity. 

Figure 2.11 Electron relaxation dynamics in 2-D-layered materials, (a) The electron relaxation dynamics for SnSi are shown for two different excess energy conditions within 0.1 eV of the CBM (the bandgap is 2.1 eV) and approximately 1 eV above the CBM. In both cases, the relaxation is extremely fast and occurs on 10 fs time scales. The Unes running through the data are best fits to single relaxation times of 40 fs and 60 fs for the 1 eV and 0.1 eV case, (b) The excess energy dependence corresponds to predictions where coupling to the broad plasmon band of these layered systems opens a new channel that significantly increases carrier relaxation above 3-D materials (compare Fig. 2.9).

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